I

MetabolismofCarbolrydrates

HO-C-H I H-C-OH I HO*C-H I H-C-OH I h-u I cH2-oH Glucose

energy Major pathways f arbohydratesare the major sourceof t \.-for the living cells.As such,carbohydrates of carbohydrate are the firstcellular constituents, synthesized by The important pathways of carbohydrate green plantsduring photosynthesisfrom carbon metabolismare listed dioxide and water, on absorptionof light. Thus, (Embden-Meyerhofpathway) : { light is the ultimate source of energy for all 1. Glycolysis I biologicalprocesses. The oxidationof glucoseto pyruvateand lactate. I The monosaccharide glucose is the central 2. Citric acid cycle (Krebs cycle or molecule in since all tricarboxylicacid cycle) : The oxidationof acetyl the major pathwaysof carbohydratemetabolism CoA to CO2. Krebs cycle is the final common are connected with it (Fig.l3,1). Clucose is oxidative pathway for carbohydrates,fats or utilizedas a sourceof energy,it is synthesized amino acids, through acetYlCoA. from non-carbohydrateprecursors and storedas : The synthesis of glycogen to releaseglucose as and when the 3. Gluconeogenesis glucosefrom non-carbohydrateprecursors (e.9. need arises. The other monosaccharides glyceroletc.). important in carbohydrate metabolism are amino acids, , and . 4. Glycogenesis: The formation of glycogen The fasting blood glucose level in normal from glucose. individualsis 70-100 mg/dl (+.5-5.5mmol/l) and 5. Glycogenolysis : The breakdown of is very efficientlymaintained at this level (for it glycogento glucose. details relerChapter 35). Liver plays a key role in monitoring and stabilizing blood glucose 6. Hexose monophosphate shunt (pentose fevels. Thus liver may be appropriately phosphatepathway or directoxidative pathway) : considered as glucostat monitor. This pathway is an alternativeto glycolysisand Chapter 13: METABOLISMOF CABBOHYDRATES 245

OXDATIVE PATHWAYS SYNTHETIC PATHWAYS 2. Insulin-dependenttransport system : This occurs in muscle and adiposetissue. Othercarbohydrates Glycolysis (galactose,fructose) Glucose transporters : In recent years,at least six glucosetransporters (CLUT-l to CLUT-5 and Glycogenesis Hexosemono- CLUT-7) in the cell membranes have been phosphateshunt identified. They Lipogenesis exhibit tissue specificity. For synthesisof fat) instance,CLUT-I is abundant in erythrocytes whereas Uronicacid Non-essential GLUT-4 is abundant in skeletalmuscle pathway aminoacids and adiposetissue. Insulin increasesthe number and oromotes Flg. 13.1: Overuiewof glucosemetabolism. the activitv of CLUT-4 in skeletalmuscle and (Note : For majoity of the pathways, glucose adipose tissue. In type 2 diabetes mellitus, insulin resistanceis observed in these tissues. This is due to the reduction in the quantity of CLUT-4 in insulindeficiency. TCA cyclefor the oxidationof glucose(directly to carbon dioxide and water).

Z. Uronic acid pathway : Clucose is convertedto glucuronic acid, pentosesand, in someanimals, to ascorbicacid (notin man).This pathwayis also an alternativeoxidative pathway Clycolysis is derived from the Creek words (glycose-sweet for glucose. or sugar;lysis-dissolution). lt is a universal pathway in the living cells. The 8. Galactose metabolism : The pathways complete pathway of glycolysiswas elucidated concernedwith the conversionof galactoseto in 1940. This pathway is often referred to as glucoseand the synthesisof . Embden-Meyerhof pathway (E.M, pathway) in 9. Fructose metabolism : The oxidation of honour of the two biochemistswho made a fructose to pyruvate and the relation between major contribution to the knowledge of glycolysis. fructoseand glucosemetabolism. Glycolysis 10. Amino sugar and mu.opotyr"ccharide is defined as the sequence of reactions (or metabolism: The synthesisof amino sugarsand converting glucose glycogen) to pyruvate other sugars for the formation of mucopoly- or lactate, with the production of ATP. saccharidesand glycoproteins. Salient features

Entry of glucose into cells 1. Clycolysistakes place in all cells of the body. The of this pathway are present Clucoseconcentration is very low in the cells in the closomal fraction of the cell. comparedto plasma (for humans< 100 mg/dl). However, glucose does not enter the cells by 2. Clycolysisoccurs in the absenceof oxygen simple diffusion.Two specifictransport systems (anaerobic) or in the presence of oxygen are recognizedfor the entry of glucoseinto the (aerobic). Lactate is the end product under cells anaerobic condition. In the aerobic condition, .l pyruvate is formed, which is then oxidized to . Insulin-independent transport system of CO2 and H2O. glucose : This is a carrier mediated uptake of glucosewhich is not dependenton the hormone 3. Clycolysis is a major pathway for ATP insulin.This is operativein hepatocytes,erythro- synthesisin tissues lacking mitochondria, e.g. cltes and brain. erythrocytes,cornea, lens etc. 246 BIOCHEMISTFIY

4. Clycofysis is very essentialfor brainwhich acts only at higher levels of glucose is dependenton glucosefor energy.The glucose i.e., after a meal when blood glucose in brain has to undergo glycolysisbefore it is concentrationis above 100 mg/dl. oxidized to CO2 and H2O. Glucose i-phosphate is impermeable 5. Clycolysis(anaerobic) may be summarized to the cell membrane. lt is a central by the net reaction molecule with a variety of metabolic glycogenesis,gluco- Clucose + 2ADP + 2Pi ------+2lactate + 2ATP fates-glycolysis, neogenesis and pentose phpsphate pathway 6. Clycolysisis a central metabolic pathway. with many of its intermediatesproviding branch point to other pathways.Thus, the intermediates 2. Clucose6-phosphate undergoes isome- give in of glycolysisare usefulfor the synthesisof amino rization to fructose 6-phosphate phospho- acids and fat. the presenceof the hexosdisomerase and Mg2*. 7. Reversal of glycolysis along with the alternate arrangements at the irreversible 3. Fructose6-phosphate is phosphorylated steps, will result in the synthesisof glucose to fructose1,6-bisphosphate by phospho- (gluconeogenesis). (PFK). This is an irreversible and a regulatorystep in glycolysis. Reactions of glycolysis B. Splitting phase The sequence of reactions of glycolysis is 4. The six carbon fructose 1,6- given in Fig.l3.2. The pathway can be divided bisphosphateis split (hencethe name into three distinct phases glycolysis) to two three-carbon A. Energyinvestment phase or priming stage compounds, glyceraldehyde 3-phos- B. Splittingphase phate and di hydroxyacetonephosphate by the enzyme aldolase(fructose 1,6- C. Energygeneration phase. bisphosphatealdolase). The sequence of reactions are discussed 5. The enzyme phosphotrioseisomerase below. catalysesthe reversibleinterconversion A. Energy investment phase of glyceraldehyde 3-phosphate and 1. Glucose is phosphorylatedto glucose dihydroxyacetone phosphate. Thus, 6-phosphate by or tvvo molecules of glyceraldehyde (both are isoenzymes). 3-phosphate are obtained from one This is an irreversible reaction, moleculeof glucose. dependent on ATP and Mg2+. The C. Energy generation phase enzymehexokinase is presentin almost 6. Glyceraldehyde3-phosphate dehydro- all the tissues. lt catalyses the genase converts glyceraldehyde phosphorylation of various hexoses 3-phosphateto 1,3-bisphosphoglycerate. (fructose, mannose etc.), has low K,.n This stepis importantas it is involvedin for substrates(about 0.1 mM) and is the formationof NADH + H+ and a high inhibited by glucose6-phosphate. energy compound 1,3-bisphospho- Clucokinasepresent in liver, catalyses gfycerate. lodoacetate and arsenate the phosphorylationof only glucose, inhibit the enzyme glyceraldehyde has high K. for glucose(10 mM) and is 3-phosphatedehydrogenase. ln aerobic not inhibited by glucose6-phosphate. condition, NADH passesthrough the Due to high affinity (low K.), glucose electron transport chain and 6 ATP is utilized by hexokinaseeven at low (2 x 3 ATP)are synthesizedby oxidative concentration, whereas glucokinase phosphorylation. *irapEer "?*8: METABOLISMOF CARBOHYDBATES 247

HO-C\ H-c-oH I HO-C-H O H-J-oHI tl H-C ' Phosphotriose H-C:O ?H'-o-(P I 6nr-or-r H-C-OH Glucose F=o cH20H cHr-o-@ Dihydroxyacetone . Glyceraldehye phosphate I 3-phosphate Pi. NAD*

NADH+ H ? ?-o-

Cur-o-@ I Fructose 6-phosphate J coo- H-E-o-O Phosohofructokinase t- cH2-oH 2-Phosphoglycerate I Mg2* l---,---tsnolase ?H'-o-O Hro4 ?-or j Ho-c-H , I coo- H-C-OH O I J-o-l>v *-C 8*, Jirr-o-@ Phosphoenolpyruvate Fructose 1,6-bisphosphate . ADP- I Mg' I J ATP+

Fig 13.2 contd, next column Fig 13,2 contd, next page 248 E|IOCHEMISTFIY

coo- 10. The enzyme pyruvatekinase catalyses I phosphate c-oH the transferof high energy tl from phosphoenol PYruvateto ADR CHe leading to the formation of ATP.This Pyruvate(enol) step also is a substrate level phosphorylation. (Pyruvate I SPontaneous Mn2*.) + t requiresK+ and either Mg2+ or coo- This reaction is irreversible. I C:O I Gonversion of pyruvate to CHg Pyruvate(keto) lactate-signilicance f pyruvate produced in glycolysis NADH+ H-\ The fate of Lacrare presence absenceof oxygen dehydrogenas€ dependson the or NAD++,j + in the cells.Under anaerobic conditions (lack of coo- Oz), pyruvateis reducedby NADH to lactatein I presenceof the enzyme lactatedehydrogenase H-Q-OH I (competitive inhibitor-oxamate). The NADH cHg utilized in this step is obtainedfrom the reaction L-Lactate catalysed by glyceraldehyde 3-phosphate The formation of lactateallows Fig. 13.2: The reactionsin thepathway of dehydrogenase. glycolysis(The three stepscatalysed by hexokinase, the regenerationof NAD+ which can be reused phosphofructo4naseand pyruvatekinase, by glyceraldehyde3-phosphate dehydrogenase shownin thick linesare irreversible). so that glycolysisproceeds even in the absence of oxygen to supply ATP.

7. The enzyme phosphoglyceratekinase The occurrenceof uninterruptedglycolysis is acts on 1,3-bisphosphoglyceratevery essentialin skeletalmuscle during strenous resulting in the synthesisof ATP and exercisewhere oxygen supply is very limited. formation of 3-phosphoglycerate.This Glycolysis in the erythrocytes leads to lactate step is a good example of substrate production, since mitochondria-the centresfor Ievel phosphorylation, since ATP is aerobic oxidation-are absent. Brain, retina, synthesizedfrom the substratewithout skin, renal medulla and gastrointestinaltract the involvement of electron transport derive most of their energyfrom glycolysis. chain. reaction is reversible, a rare example Lactic acidosis the kinase reactions. among Lactic acid is a three carbon hydroxy acid. 8. 3-Phosphoglycerate is converted to Elevationof lacticacid in the circulation(normal 2-phosphoglycerateby phosphoglycerate plasma 4-1 5 mg/dl) may occur due to its mutase.This is an isomerizationreaction. increasedproduction or decreasedutilization. Mild formsof lacticacidosis (not life-threatening) 9. The high energy compound phos- are associatedwith strenuousexercise, shock, phoenol pyruvate is generated from respiratory diseases, cancers/ low pyruvate 2-phosphoglycerateby the enzyme dehydrogenaseactivity, von Gierke'sdisease etc. enolase.This enzyme requiresMg2* or Mn2* and is inhihited by fluoride. For Severeforms of lactic acidosisare observed bfood glucose estimation in the due to impairmen/collapse of circulatory system laboratory,fluoride is addedto the blood which is often encountered in myocardial to preventglycolysis by the cells,so that infarction,pulmonary embolism, uncontrolled blood glucose is correctly estimated. hemorrhageand severeshock. This type of lactic Ghapter 13 : METABOLISMOF CAFIBOHYDHATES 249

acidosisis due to inadequatesupply of 02 to the which forms glucose 6-phosphate).Thus, in tissueswith a drasticreduction in ATP synthesis anaerobicglycolysis, 3 ATP are producedfrom (since the cells have to survive in anaerobic glycogen. conditions)which may even lead to death.The term oxygen debf refersto the excessamount of Glycolysis and shuttle pathways 02 required to recover. In clinical practice, In the presenceof mitochondriaand oxygen, measurementof plasma lactic acid is useful to the NADH producedin glycolysiscan participate know about the oxygen debt, and monitor the in the shuttle pathways (Refer Chapter 1l) lor patient'srecovery. the synthesisof ATP. lf the cytosolicNADH uses malate-aspartateshuttle, 3 ATP are generated Production of ATP in glycolysis from eachmolecule of NADH. This is in contrast The details of ATP generationin glycolysis to glycerolphosphate shuttle that produces (from glucose)are given in Table 13.1. Under only 2 ATP. anaerobic conditions, 2 ATP are synthesized while, under aerobic conditions, 8 or 6 ATP are Gancer and glycolysis synthesized-dependingon the shuttlepathway Cancer cells display increased uptake of that operates. glucose, and glycolysis.As the tumors grow When the glycolysisoccurs from glycogen, rapidly, the blood vesselsare unable to supply one more ATP is generated.This is becauseno adequate oxygen, and thus a condition of ATP is consumedfor the activationof glucose hypoxiaexists. Due to this,anaerobic glycolysis (glycogendirectly producesglucose 1 -phosphate predominantlyoccurs to supply energy. The

Number of Pathway Enzyme (method of ATP synthesis) ATP synthesized

Glycolysis Glyceraldehyde3-phosphate dehydrogenase 6* (2NADH, ETC, oxidative phosphorylation) Phosphoglyceratekinase (substrate level phosphorylation) 2 t Pyruvatekinase (substrate level phosphorylation) 2 TwoATP are consumed inthe reactions catalysed byhexokinase and -z NetATP synthesis in glycolysisin aerobiccondition

Pyruvatedehydrogenase (2 NADH, ETC, oxidative phosphorylation)

Citricacid cycle lsocitratedehydrogenase (2 NADH, ETC, oxidative phosphorylation) a-Ketoglutaratedehydrogenase Succinatethiokinase (substrate level phosphorylation) z Succinatedehydrogenase (2 FADH2, ETC, oxidative phosphorylation) 4 Malatedehydrogenase (2 NADH, ETC, oxidative phosphorylation) o TotalATP per moleof glucoseunder aerobic condition 38 TotalATP per mole of glucoseunder anaerobic condition 2

4 6 ATPare produced if NADHuses malate shuttle; only 4 ATPare producedif glycerol-phosphateshuttle operates, in wh:tchcase total ATPsynthesized per nole of glucoseoxidation is 36 and not 38 250 ElIOCHEMISTF|Y

cancer cells get adapted to hypoxic glycolysis cAMP l^ throughthe involvementof a transcriptionfactor lE' namely hypoxia-inducible transcription factor (HIF). HIF increasesthe synthesisof glycolytic enzymesand the glucosetransporters.lHowever, the cancercells cannot grow and survivewithout proper vascularization.lOneof the modalitiesof Fructose Fructose2,6- 6-phosphate bisphosphate cancertreatment is to use drugsthat can inhibit vascularizationof tumors. \/ \L Fructose2,G- lrreversible steps in glyeolysis bisphosphatase-t/ Most of the reactions of glycolysis are f l@ reversible.However, the threesteps catalysed by cAMP the enzymes hexokinase (or glucokinase), Flg.l3.3 : of lru6ose2,Fbisphosphatase. phosphofructokinase and , are legutatioll irreversible.These three stagesmainly regulate glycolysis. The reversal of glycolysis, with (activator)for controlling PFK and, ultimately, alternate arrangements made at the three glycolysis in the liver. F2,6-BP is synthesized irreversible stages, leads to the synthesis.of from fructose 6-phosphate by the enzyme glucosefrom pyruvate(gluconeogenesis). phosphofructokinasecalled PFK-2(PFK-1 is the glycolytic enzyme). F2,6-BP is hydrolysed by Regulation of glycolysis fructose 2,6-bisphosphatase.The function of synthesisand degradationof F2,6-BPis brought The three enzymes namely hexokinase(and out by a single enzyme (samepolypeptide with glucokinase),phosphofructokinase and pyruvate two active sites) which is referred to as kinase, catalysing the irreversible reactions (Fig.13.3). regulateglycolysis. bifunctional enzyme In fact, the combined name of phosphofructokinase-2/ Hexokinase is inhibited by glucose fructose2,6-bisphosphatase is used to referto the 6-phosphate. This enzyme prevents the enzymethat synthesizesand degradesF2,6-BP. accumulation of glucose 6-phosphatedue to The activity of PFK-2 and fructose 2,6- product inhibition. Glucokinase, which bisphosphatase is controlled by covalent specifically phosphorylates glucose, is an modification which, in turn, is regulated by inducible enzyme. The substrate glucose, cyclic AMP (cAMP is the second messengerfor probably through the involvementof insulin, certain hormones).Cyclic AMP brings about inducesglucokinase. dephosphorylationof the bifunctional enzyme, Phosphofruclokinase (PFK) is the most resultingin inactivationof activesite responsible important regulatoryenzyme in glycolysis.This for the synthesisof F2,6-BPbut activationof the enzyme catalyses the rafe limiting committed responsible for the hydrolysis of step. PFK is an allostericenzyme regulatedby F2,6-BP. allostericeffectors. ATP, citrate and H+ ions (low Pyruvate kinasealso regulatesglycolysis. This pH) are the most importantallosteric inhibitors, enzyme is inhibited by ATP and activated by whereas,fructose 2,6-bisphosphate, ADP, AMP F1,6-BP. Pyruvate kinase is active (a) in and Pi are the allostericactivators. dephosphorylated state and inactive (b) in phosphorylated state. Inactivation of pyruvate Role of fructose Zr6-bisphosphate kinase by phosphorylationis brought about by in glycolysis cAMP-dependentprotein kinase. The hormone- Fructose2,6-bisphosphate (F2,6-BP) is consi- glucagon inhibits hepatic glycolysis by this dered to be the most important regulatory factor mechanism(Fig.l3.a). Ghapten 13 : METABOLISMOF CARBOHYDFATES 251

A Glucagon - ) cAMir and other mammals.Rapaport-Leubering cycle is l@ mainly concerned with the synthesisof 2,3- I Y bisphosphoglycerate(2,3-BPG) in the RBC. 1,3- Proteinkinase Bisphosphoglycerate(1,3-BPC) produced in glycolysis is converted to 2,3-BPC by the enzyme 2,3-bisphosphoglycerate mutase (Fig.l3.5).2,3-BPC is hydrolysedto 3-phospho- glycerate by bisphosphoglyceratephosphatase. lNote : There is a difference between the usages-bisphosphate and diphosphate. A bisphosphatehas two phosphatesheld separately (e.g. 2,3-BPC),in contrastto diphosphate(e.g. Fig. 13.4 : Regulationof pyruvate kinase. ADP) where the phosphatesare linked togethed. It is now believed that bisphosphoglycerate mutase is a bifunctional enzvme with mutase Pasteur effect and phosphataseactivities catalysed by two The inhibition of glycolysis by oxygen different sites present on the same enzyme. (aerobiccondition) is known as Pasteureffect. About'15-25o/o of the glucose that gets This effect was discovered by Louis Pasteur, convertedto lactatein erythrocytesgoes via 2,3- more than a century a1o, while studying BPG synthesis. fermentationby yeast. He observedthat when anaerobic yeast cultures (metabolizing yeast) Signifieanee of 2,3-BFG were exposedto air, the utiliziation of glucose decreasedby nearly sevenfold. 1. Production of 2,3-BPG allows the glycolysisto proceed without the synthesisof In the aerobic condition, the levels of ATP. This is advantageousto erythrocytessince glycolytic intermediates from fructose 1,6- glycolysis occurs when the need for ATP is bisphosphateonwards decrease while the earlier minimal. Rapaport-Leuberingcycle is, therefore, intermediatesaccumulate. This clearly indicates regarded as a shunt pathway of glycolysis to that Pasteureffect is due to the inhibition of the dissipate or waste the energy not needed by enzyme phosphofructokinase.The inhibitory erythrocytes. effect of citrate and ATP (produced in the presence of oxygen) on . 2,3-BPC, however, is not a waste explainsthe Pasteureffect. moleculein RBC.lt combineswith hemoglobin (Hb) and reduces Hb affinity with oxygen. Grabtree effect Therefore, in the presence of 2,3-BPG, oxyhemoglobin unloads more oxygen to the The phenomenon of inhibition of oxygen fissues. consumptionby the additionof glucoseto tissues having high aerobic glycolysis is known as lncreasein erythrocyte2,3-BPC is observed Crabtreeeffect. Basically,this is opposite to that in hypoxic condition, high altitude,fetal tissues, of Pasteur effect. Crabtree effect is due to anemic conditionsetc. in all these cases,2,3- increasedcompetition of glycolysisfor inorganic BPG will enhancethe supply of oxygen to the phosphate(Pi) and NAD+ which limits their tissues. availabilityfor phosphorylationand oxidation. 3. Glycolysisin the erythrocytesis linkedwith RAFAPORT"LEUBERINGCYCLE 2,3-BPCproduction and oxygentransport. In the deficiencyof the enzyme hexokinase,glucose is This is a supplementarypathway to glycolysis not phosphorylated,hence the synthesisand which is ooerative in the ervthrocvtesof man concentrationof 2,3-BPG are low in RBC. The BIOCHEMISTFIY

Glucose known as pyruvate dehydrogenase complex + (PDH),which is found only in the mitochondria. t High activitiesof PDH are found in cardiac Y H-C=O muscleand kidney.The enzyme PDH requires I five cofactors (coenzymes), namely-TPP, H-C-OH l^ lipoamide, FAD, coenzyme A and NAD+ cH2-o-(7 (lipoamidecontains lipoic acid linkedto e-amino Glyceraldehyde group of lysine).The overall reactionof PDH is 3-phosphate FDFI Pyruvate + NAD+ + CoA ) Acetyl CoA + CO2+NADH+H+

Reactions of PDH complex

o The sequenceof reactionsbrought about by different enzymes of PDH complex in associationwith the coenzymesis depicted in Fig.l3.6. Pyruvate is decarboxylated to give hydroxyethyl TPP, catalysed by PDH (decarboxylaseactivity). Dihydrolipoyl trans- acetylasebrings about the formation of acetyl lipoamide (from hydroxethyl-TPP)and then catalyses the transfer of acetyl group to coenzymeA to produceacetyl CoA. The cycle is coo- completewhen reduced lipoamide is converted I to oxidizedlipoamide by dihydrolipoyldehydro- H-C-OH genase,transferring the reducing equivalentsto cH2-o-e FAD. FADH2, in turn, transfersthe reducing 3-Phosphoglycerate equivalentsto NAD+ to give NADH + H+, which + can pass through the respiratorychain to give Y 3 ATP (6 ATP from 2 moles of pyruvate formed Pyruvate from glucose)by oxidative phosphorylation. Fig. 13.5 : Rapaport-Leuberingcycle for the synthesis of 2,s-bisphosphoglycerate (2,3-BPG). The intermediatesof PDH catalysedreaction are not free but bound with enzymecomplex. In mammals, the PDH complex hasan approximate hemoglobin exhibits high oxygen affinity in molecular weight of 9 x |N. lt contains 60 hexokinase-defectivepatients. On the other hand, molecules of dihydrolipoyltransacetylase and in the patientswith pyruvatekinase deficiency, about 20-30 molecules each of the other the level of 2,3-BPG in erythrocytesis high, two enzymes (pyruvate dehydrogenase and resultingin low oxygen affinity. dihydrolipoyl dehydrogenase). For a more detaileddiscussion on 2,3-BPC, A comparable enzyme with PDH is reler Chapter 10. a-ketoglutarate dehydrogenase complex of citric acid cycle which catalyses the oxidative CONVERSION OF decarboxylationof a-ketoglutarateto succinyl PYRUVATE TO AGETYL GoA CoA. Pyruvate is converted to acetyl CoA by Arsenic poisoning : The enzymes PDH and oxidative decarboxylafion.This is an irreversible a-ketoglutaratedehydrogenase are inhibited by reaction,catalysed by a multienzymecomplex, arsenite.Arsenite binds to thiol (-SH) groupsof 253 Ghapter 13 : METABOLIqT4_Q|CAFIBOHYDRATES

o o cH3-c-coo\ CH3-C-S-Lip-SH Pyruvate

OH I cH3-cH-TPP Lip<3 HydroryethYl-TPP NADH+ H' \erc + 1+ 3 ATP

(Note The reactioninvolving the Fig. 13.6: The mechanismof actionof pyruvatedehydrogenase c9!!l?I. : FADand NA}')' conversionol pyruvateto acetytCoA requiresfive coenzymes-TPP,tipoamide, CoASH'

patients with inherited deficiency of lipoic acid and makesit unavailableto serveas 3. ln (usuallyafter glucose load) . PDH, lactic acidosis is observed. PDH Regulation of 4. PDH activitv can be inhibited by arsenic about by Pyruvatedehydrogenase is a good examplefor and mercuric ions. This is brought -SH groupsof lipoic end producf (acetyl CoA, NADH\ inhibition' bindingof theseions with Besides this, PDH is also regulated by acid. phosphoryIati on and dephosphorylation (Fig' l 3'V PDH is active as a dephosphoenzymewhile it is Metabolic imPortance of Pyruvate inactiveas a phosphoenzyme.PDH phosphatase Pyruvate is a key metabolite. Besides its activityis promotedby Ca2*,Mg+ and insulin(in conversiorito acetyl CoA (utilized in a wide adipose tissue). lt is of interest to note that range of metabolic reactions-citricacid cycle, calcium released during muscle contraction fatty acid synthesisetc.), pyruvate is a good stimulates PDH (by increasing phosphatase substratefor gluconeogenesis. activity)for energYProduction. PDH kinase (responsibleto form inactive PDH) is promoted by ATP, NADH and acetyl ) CoA, while it is inhibited by NAD+, CoA and pyruvate.The net resultis that in the presenceof Plex! high energysignals (ATP, NADH), the PDH is rhosphoenzyme turned off. \ Y@--f;,11.. PDHkinase pDHphosphatase 't Biochemical imPortance of PDH f\ tn .4\ / \ytnsulin 1. Lackof TPP(due to deficiencyof thiamine) nrp, r.rnoH\ A. (adipose the Aceiycon ( 'pi tissue) inhibits PDH activity resulting in o]r\ eon complex accumulationof PYruvate. activedePhosphoenzyme 2. ln the thiamine deficient alcoholics, of pyruvate dihydrogenase (PDH) resulting Fig. 13.7 : Regulation pyruvateis rapidlyconverted to lactate, comaex' in lactic acidosis. 254 BIOCHEMISTRY

Acetyl CoA

The citric acid cycle (Krebs cycle or tricarboxylic acid-TCA cycle) is the most important metabolic pathway for the energy Oxaloacetate Citrate (4c) (6c) supplyto the body. About 65-70% of the ATP is synthesized in Krebs cycle. Citric acid cycle \ f+co, SuccinylCoA cr-Ketoglutarate essentially involves the oxidation of acetyl CoA (4c) (5c) to CO2 and H2O. This cycle utilizesabout two- thirds of total oxygen consumed by the body. The name TCA cycle is used,since, at the outset Fig. 13.8: An overuiewol Krebscycle. of the cycle, tricarboxylic acids (citrate, cis- aconitateand isocitrate)participate. T$A egeEe*mm e*weru*eww TGA eyeie-*ftfuer eecB€raB metahmlfrc pff{$t}&rffiy Krebs cycle basically involves the combinationof a two carbon acetyl CoA with a The citric acid cycle is the final common four carbonoxaloacetate to producea six carbon oxidative pathway for carbohydrates,fats and tricarboxvlic acid, citrate. In the reactionsthat amino acids.This cycle not only suppliesenergy follow, the two carbonsare oxidizedto CO2 and but also provides many intermediatesrequired oxaloacetate is regenerated and recycled. for the synthesisof amino acids, glucose,heme Oxaloacetate is considered to play a catalytic etc. Krebs cycle is the most important role in citric acid cycle. An overview of Krebs central pathway connecting almost all the cycle is depicted in Fig.l3.8. individual metabolicpathways (either directly or indirectly). TGA eyeEe.-an @BeE?#S#Ee ffirfref hEsfspr-_v Krebscycle is a cyclic process.However, it should not be viewed as a closed circle, since The citric acid cycle was proposedby Hans many compoundsenter the cycle and leave.TCA Adolf Krebs in 1937, based on the studiesof cycle is comparableto a heavytraffic circle in a oxygen consumption in pigeon breast muscle. national highway with many connectingroads. The cycle is named in his honour (Nobel Prize Each intermediate of the cycle connecting for Physiologyand Medicine in 1953.) another pathway is a road! -r [Note : It is of interestto note that the original ReactBoms of citrie a€id eycBe manuscripton TCA cycle submittedby Krebsto the journal 'Nature' was not accepted. He Oxidative decarboxylation of pyruvate to published it in another journal Enzymoligia. acetyl CoA by pyruvatedehydrogenase complex Krebsused to carrythe rejectionletter (of Nature) is discussedabove. This step is a connectinglink with him, and advisethe researchesnever to De between glycolysis and TCA cycle. A few discouragedby researchpaper rejectionl. authors, however, describe the conversion of pyruvate to acetyl CoA along with citric acid fr-*aati+er {Eg'P'$;:.1+,{.}-VsiE+ cycle. The events of TCA cycle are described hereunder(Fig.l3.9). The enzymes of TCA cycle are located in mitochondrial matrix, in close proximity to the 1. Formation of citrate : Krebscycle proper electron transport chain. This enables the startswith the condensationof acetyl CoA and synthesisof ATP by oxidative phosphorylation oxaloacetate, catalysed by the enzyme citrate without any hindrance. synthase. Ghapter 13 : METABOLISM 2s5

(i II ci-t3-o-coo- Pyruv#

o ll CH3-C-SCoA AcetylCoA

o c-coo-tl I cH2-coo- NADH+ H* Oxaloacetate NADl Aconitase

nn- HO-CH-COO_ I I c cH2-coo- 8t-.oo- L-Malate ClsAconitate t \,rHzO Aconitasd\ + cii2__coo- I

II A AAA_ cH-ctf'l- fi-tr-lr\Ju -ooc-c-Hti Ho-dH-coo- lsocitrate Fumarate + t lsocitrat€ f runo* FADH,{ dehydlqgenase- I o"frtHSlL /*NADH+H- FAD/\ cH2-c00- I cH2-cCIo- cH-coo- cH2-cOo- I O:C-COO- ,,,,, Oxalosuccinate

Fig_1g.g : The citricacid (Krebs)cycte. (trreersible reactions shown by thickarrows) 256 BIOCHEMISTRY

2. and 3. Citrate is isomerized to isocitrate Acetyl CoA + 3 NAD+ + FAD + CDP + Pi + by the enzyme aconitase.This is achieved in a 2H2O ------>2CO2 + 3NADH + 3H+ + FADH2 + two stage reaction of dehydration followed by GTP + CoA hydration through the formation of an intermed iate-crs-acon itate. Regeneration of oxaloacetate 4. and 5. Formation of a-ketoglutarate : in TGA cycle (lCD) The enzyme isocitrate dehydrogenase The TCA cycle basically involves the (oxidative catalysesthe conversion decarboxy- oxidation of acetyl CoA to COz with lation)of isocitrateto oxalosuccinateand then to simultaneousregeneration of oxaloacetate.As o-ketoglutarate.The formationof NADH and the such,there is no net consumptionof oxaloacetate liberationof CO2 occur at this stage. or any other intermediatein the cycle. 6. Conversion of cl-ketoglutarateto succinyl CoA occurs through oxidative decarboxylation, Requirement of O" by TGA cycle catalysed by o-ketoglutarate dehydrogenase There is no direct participation of oxygen in complex. This enzyme is dependent on five Krebs cycle. However, the cycle operatesonly cofactors-TPP, lipoamide, NAD+, FAD and under aerobic conditions.This is due to the fact CoA. The mechanism of the reaction is that NAD+ and FAD (from NADH and FADH2, analogousto the conversionof pyruvateto acetyl respectively)required for the operation of the CoA (See Fig.l3.6). At this stage of the TCA cycle can be regeneratedin the respiratorychain cycle, second NADH is produced and the only in the presenceof 02. Therefore,citric acid second CO2 is liberated. cycle is strictlyaerobic in contrastto glycolysis 7. Formation of succinate: SuccinylCoA is which operatesin both aerobic and anaerobic convertedto succinateby succinatethiokinase. conditions. This reaction is coupled with the phosphorylation of CDP to CTP. This is a Energetics of citric acid cycle substrate level phosphorylation. GTP is converted to ATP bv the enzvme nucleoside Duringthe processof oxidationof acetylCoA diphosphatekinase. via citric acid cycle,4 reducingequivalents (3 as NADH and one as FADH2) are produced. CTP + ADP <+ ATP + CDP Oxidation of 3 NADH by electron transport 8. Conversion of succinate to fumarate : chain coupled with oxidative phosphorylation Succinate is oxidized by succinate dehydro- resultsin the synthesisof 9 ATP,whereas FADH2 genase to fumarate.This reaction resultsin the leadsto the formationof 2 ATP.Besides, there is production of FADH2 and not NADH. one substratelevel phosphorylation.Thus, a total 9. Formation of malate : The enzyme of twelve ATP areproduced from one acetyl CoA. fumarasecatalvses the conversionof fumarateto malate with the addition of H2O. Inhibitors of Krebs cycle

10. Conversion of malate to oxaloacetate : The important enzymes of TCA cycle Malate is then oxidized to oxaloacetate by inhibited by the respectiveinhibitors are listed malate dehydrogenase.The third and final synthesisof NADH occurs at this stage. The oxaloacetateis regeneratedwhich can combine Aconitase Fluoroacetate with another molecule of acetyl CoA, and (non-competitive) continuethe cycle. o,-Ketoglutarate Arsenite Summary of TCA cycle dehydrogenase (non-competitive) Succinate Malonate The eventsof Krebscycle may be summarized dehydrogenase (competitive) as given in the next column Ghapter 13: METABOLISMOF CARBOHYDRATES 257

Fluoroacetate-a suicide substrate : The The most importantsynthetic (anabolic) reactions inhibitor fluoroacetate is first activated to connectedwith TCA cycle are given (Fig.l3.l0) fluoroacetvl CoA which then condenseswith 1. Oxaloacetate and o-ketoglutarate,respec- oxaloacetate to form fluorocitrate. TCA cycle tively, serve as precursorsfor the synthesisof (enzyme-aconitase)is inhibited by fluorocitrate. aspartateand glutamate which, in turn, are The compound fluoroacetate,as such, is a requiredfor the synthesisof other non-essential harmlesssubstrate. But it is convertedto a toxic amino acids,purines and pyrimidines. compound (fluoroc itrate) by celI u lar metabolism. This is a suicidereaction committed by the cell, 2. Succinyl CoA is used for the synthesisof and thus fluoroacetateis regardedas a suicide porphyrinsand heme. substrate. 3. Mitochondrialcitrate is transportedto the cytosol, where it is cleaved to provide acetyl Regulation of citric acid cycle CoA for the biosynthesisof fatty acids, sterols The cellulardemands of ATP are crucial in etc. controlling the rate of citric acid cycle. The regulationis broughtabout eitherby enzymesor Anaplerosis or anaplerotic reactions the levels of ADP. Three enzymes-namely The synthetic reactions described above citrate synthase, isocitrate dehydrogenase and depletethe intermediatesof citric acid cycle.The a-ketoglutarate dehydrogenase-regu Iate c itric cycle will cease to operate unless the acid cycle. intermediatesdrawn out are replenished.Ihe 1. Citrate synthase is inhibited by ATP, reactions concerned to replenish or to fill up NADH, acetyl CoA and succinyl CoA. the intermediates of citric acid cycle are called anaplerotic reactions or anaplerosis (Creek : fill 2. lsocitrate dehydrogenase is activated by up). ln Fig.l3.10, the important synthetic ADP, and inhibitedby ATP and NADH. pathways that draw the intermediates of TCA 3. o-Ketoglutaratedehydrogenase is inhibited cycle and the anapleroticreactions to fill them by succinylCoA and NADH. up are grven. 4. Availability of ADP is very important for The salientfeatures of important anaplerotic the citric acid cycle to proceed.This is due to reactionsare described the fact that unlesssufficient levels of ADP are available, oxidation (coupled with phospho- 1. Pyruvate carboxylase catalyses the pyruvate rylation of ADP to ATP) of NADH and FADH2 conversionof to oxaloacetate.This is through electron transport chain stops. The an ATP dependentcarboxylation reaction. accumulationof NADH and FADH2will leadto Pyruvate + CO2 + ATP ------+ inhibition of the enzymes(as statedabove) and Oxaloacetate+ADP+Pi also limitsthe supplyof NAD+ and FAD which The details of the above reaction are are essentialfor TCA cycle to proceed. describedunder gluconeogenesis. Amphibolic nature 2. Pyruvateis convertedto malate by NADP+ of the citric acid cycle dependentmalate dehydrogenase (malic enzyme).

The citric acid cycle provides various Pyruvate+ CO2+ NADPH+ H+$ intermediates for the synthesis of many Malate+NADPH++HrO compoundsneeded by the body. Krebscycle is 3. Transaminationis a orocesswherein an and anaholic in nature, hence both cataholic amino acid transfersits amino group to a keto regarded as amphibolic. acid and itself gets converted to a keto acid. The TCA cycle is actively involved in gluco- formation of a-ketoglutarateand oxaloacetate neogenesis,transamination and deamination. occursby this mechanism. 258 BIOCHEMISTF|Y

Non-essential AsDartate aminoacids, punnes, pyrimidines TransaminationAcetyl CoA

Pyruvate Citrate.-'....-..-.-fFatty acids, sterois

Citric acld cycle

d,-Ketoglutarate

Glutamate I J Non-essential aminoacids, purines

Fig. 13.10 : Major synthetic and anaplerotic pathways of the intetmediates of citric acid cycle.

4. a-Ketoglutaratecan also be synthesized living system,energy is trapped leading to the from glutamate by glutamate dehydrogenase synthesisof 38 ATPwhich is equivalentto 1,159 action. KJ (1 ATP has high energy bond equivalentto Clutamate+ NAD(P)++ H2O <+ 30.5 KJ).That is, about 48% of the energy in cx,-Ketoglutarate+ NAD(P)H + H+ + NHf glucosecombustion is actuallycaptured for ATP generation. Energetics of glucose oxidation When a molecule of glucose (6 carbon) undergoesglycolysis, 2 moleculesof pyruvateor lactate (3 carbon) are produced. Pyruvate is oxidatively decarboxylatedto acetyl CoA (2 The synthesis of glucose from non- carbon) which entersthe citric acid cycle and carbohydratecompounds is known as gluco- gets completelyoxidized to CO2 and H2O. The neogenesis. The major substrates/precursors overall process of glucose being completely for gluconeogenesis are lactate, pyruvate, oxidized to CO2 and H2O via glycolysis and glucogenic amino acids,propionate and glycerol. citric acid cycle is as follows C6H1206+ 602 + 38ADP + 38Pi ------+ Location of gluconeogenesis 6CO2+6H2O+38ATP Cluconeogenesisoccurs mainly in the The enzlimes of glucose metabolism cytosol, although some precursorsare produced responsiblefor generating ATP are given in in the mitochondria.Cluconeogenesis mostly Table 13.1. takes place in liver (about 1 kg glucose When a molecule of glucose is burnt in a synthesized everyday) and, to some extent, in calorimeter,2,780 K) of heat is liberated.In the kidney matrix (aboutone-tenth of liver capacity). Chrpter'l 3 : METABOLISMOF CAFBOHYDHATES 259 rirf?p*ffiance of gluecn€0genesis 4. Certainmetabolites produced in the tissues accumulatein the blood, e.g. lactate,glycerol, Clucose occupies a key position in the propionate etc. Cluconeogenesis effectively metabolism and its continuous supply is clearsthem from the blood. absolutelyessential to the body for a variety of functions Reactions of gluconeogenesHs 1. Brain and central nervous system/ Cluconeogenesis closely resembles the erythrocytes,testes and kidney medulla are reversedpathway of glycolysis,although it is not dependenton glucosefor continuoussupply of the completereversal of glycolysis.Essentially, 3 energy.Human brainalone requires about 120 g (outof 10) reactionsof glycolysisare irreversible. of glucoseper day, out of about 160 g needed The seven reactions are common for both by the entire body. glycolysisand gluconeogenesis(Fi9.13.11). The three irreversihle sfeps of glycolysis are 2. Clucose is the only sourcethat supplies catalysedby the enzymes,namely hexokinase, energy to the skeletalmuscle, under anaerobic phosphofructokinaseand pyruvatekinase. These conditions. three stages-bypassed by alternate enzymes 3. ln fasting even more than a day, specificto gluconeogenesis-arediscussed gluconeogenesismust occur to meet the basal 1. Conversionof pyruvate to phosphoenol- requirementsof the body for glucose and to pyruvate : This takes place in two steps maintainthe intermediatesof citric acid cycle. (Fig.l3.12). Pyruvate carboxylase is a biotin- This is essentialfor the survivalof humansand dependentmitochondrial enzyme that converts other animals. pyruvateto oxaloacetatein presenceof ATP and

BtoMEDtCAt / CLtNtCAt CONCEPTS

ES G/ycolysisis an important sourceof energysupply for brain, retina, skin and renal medulla. B€ The crucial signilicanceof glycolysisis its obility to generateATP in the absenceof oxygen. € Skeletal muscle, during strenous exercise, requires the occurrence ol uninterrupted glycolysis.This is due to the limited supply of oxygen. The cardiac muscle cqnnot suruiuefor long in the obsenceof oxygen sinceit is not well odapted for glycolysisunder anoerobic conditions. Glycolysis in erythrocytes is associoted with 2, 3-bisphosphoglycerate(2,3-BPG) produc- tion. In the presenceof 2, S-BPG,oxyhemoglobin unloads more oxygen to the tissues. tr-g- The occurrenceof glycolysisis uerg much eleuated in rapidly growing concer cells. ss Lactic acldosis is olso obserued in patients with deficiency oJ the enzgme pgruuate dehydrogenase.It could also be due to collapse of circulatory systemencountered in myocardial infarction and pulmonary embolism. Citric acid cycle is the final common oxidatiue pathway lor carbohydrates,t'ots and amino ocids.lt utilizes (indirectly)about 2/3 of the total oxygen consumed by the body ond generatesobout U3 of the total energy (ATP). Unlike the other metabolic pathways/cycles,uery few genetic abnormalities of Krebs cycle are known. This may be due to the uital importance ol thts metabolic cycle for the suruiual of liJe 260 BIOCHEMISTFIY I Phosphoenolpyruvate Glucose6- phosphate

Oxaloacetate

Glyceraldehyde . Dihydroxyacetone 3-phosphate I_ phosfihate

Oxaloacetate+-@ I 1,3-Bisphosphoglycerate Glycerol3-phosphate +i ADP

Fig. 13.11: The pathway of gluconeogenesis.[The enzymes catalysingineversible steps in glycolysis arc shown in red. The important enzymes participating in gluconeogenesis are shown in shaded green. The substrates for gluconeogenesis are in blue. The numbers represent the entry of glucogenic amino acids : (1) Alanine, glycine, serine, cysteine, threonine and tryptophan; (2) Aspartate and asparagine; (3) Arginine, glutamate, glutamine, histidine, proline; (4) lsoleucine, methionine, valine; (5) Phenylalanine, tyrosine].

CO2. This enzyme regulatesgluconeogenesis malateis catalysedby malatedehydrogenase, an and requiresacetyl CoA for its activity. enzyme present in both mitochondria and cvtosol. Oxaloacetate is synthesized in the mitochondrialmatrix. lt has to be transportedto ln the cytosol, phosphoenolpyruvate carboxy- the cytosolto be usedin gluconeogenesis,where kinase converts oxaloacetate to phosphoenol- the rest of the pathway occurs. Due to pyruvate.CTP or ITP (not ATP) is used in this membraneimpermeability, oxaloacetate cannot reaction and the CO2 (fixed by carboxylase)is diffuseout of the mitochondria.lt is convertedto liberated. For the conversion of pyruvate to malate and then transported to the cytosol. phosphoenol pyruvate, 2 ATP equivalentsare Within the cytosol,oxaloacetate is regenerated. utilized.This is in contrastto only one ATP that The reversibleconversion of oxaloacetateand is liberatedin glycolysisfor this reaction. Chapter 13 : METABOLISMOF CARBOHYDRATES 261

o Gluconeogenesis from amino acids cH3-c-coo- The carbon skeletonof glucogenic amino Pyruvate acids(all except leucine and lysine)results in the ATF formation of pyruvate or the intermediatesof co; tte citric acid cycle (Fig.l3.ll) which, ultimately, ase result in the synthesisof glucose. ADP+ Pi o Gluconeogenesis from gtycerol -ooc-cH2-c-coo- Clycerol is liberated mostly in the Oxaloacetate adipose tissueby the hydrolysis of fats (triacylglycerols). GTP. The enzyme glycerokinase(found in liver and renolpyruvate kidney, absent in GDP rxykihise adipose tissue) activates glycerol COz{ to glycerol 3-phosphate. The latter is converted to dihydroxyacetone phosphate by glycerol 3-phosphate dehydrogenase. Dihydroxyacetonephosphate is an intermediate in glycolysiswhich can be convenientlyused for cH2-c-coo- glucoseproduction. Phosphoenolpyruvate Ftg. 13.12: Conversionof pyruvateto Gluconeogenesis from propionate phosphoenolpyruvate. Oxidation of odd chain fatty acids and the breakdown of some amino acids (methionine, 2. Conversion of fructose 1,6-bisphosphate isoleucine)yields a three carbon propionyl CoA. to fructose 6-phosphate : Phosphoenolpyruvate Propionyl CoA carboxylase acts on this in undergoesthe reversalof glycolysisuntil fructose presence of ATP and biotin and converts to 1,6-bisphosphateis produced. The enzyme methyl malonyl CoA which is then convertedto fructose lr6-bisphosphatase converts fructose succinyl CoA in presence of 812 coenzyme 1,6-bisphosphateto fructose 6-phosphate.This (Refer Fig.7.38). Succinyl CoA formed from enzyme requires Mg2* ions. Fructose 1,6- propionyl CoA entersgluconeogenesis via citric bisphosphataseis absentin smooth muscle and acid cycle. heart muscle.This enzyme is also regulatoryin gluconeogenesi* Gluconeogenesis from lactate (Cori cyclel 3. Conversion of glucose 6-phosphate to gfucose : Glucose 5-phosphatasecatalyses the Lactateproduced by active skeletalmuscle is conversionof glucose 6-phosphateto glucose. a major precursorfor gluconeogenesis.Under The presenceor absenceof this enzyme in a anaerobic conditions, pyruvate is reduced to tissuedetermines whether the tissue is capable lactateby lactatedehydrogenase (LDH) of contributingglucose to the blood or not. lt is mostly presentin liver and kidney but absentin Pyruvate + NADH * H* *\ Lactate+ NAD+ muscletbrain and adiposetissue. Lactate is a dead end in glycolysis, since it The overall summaryof gluconeogenesisfor must be reconverted to pyruvate for its further the conversion of pyruvate to glucose is metabolism. The very purpose of lactate shown below production is to regenerate NADH so that 2 Pyruvate+ 4ATP + 2CTP + 2NADH + 2H+ glycolysis proceeds uninterrupted in skeletal + 6H2O ------+Clucose + 2NAD+ + 4ADP -r muscle. Lactate or pyruvate produced in the 2GDP+6Pi +6H+ muscle cannot be utilized for the svnthesisof 262 BIOCHEMISTF|Y

Glucose Glucose LrvEB Glycogenf GtucassJ 1 Glucose6-phosphate i Glycogen + I Pyruvate I Pyruvate I

Flg. 13.13: TheCori cycle (blue)and glucose-alanine(red) cycle (other reactions @mmon for both cycles). glucose due to the absence of the key enzymes Regulation of gluconeogenesis of gluconeogenesis(glucose 6-phosphatase and The hormoneglucagon and the availabilityof fructose1,6-bisphosphatase). substratesmainly regulate gluconeogenesis,as The plasmamembrane is freely permeableto discussedhereunder. lactate. Lactate is carried from the skeletal Influence of glucagon : This is a hormone, musclethrough blood and handedover to liver, secreted by a-cells of the pancreatic islets. where it is oxidized to pyruvate. Pyruvate, so Clucagon stimulates gluconeogenesisby two produced, is converted to glucose by mechanisms gluconeogenesis,which is then transportedto the skeletalmuscle. 1. Active form of pyruvate kinase is convertedto inactive form through the mediation The cycle involving the syntfiesis of glucose of cyclic AMP, brought abofit by glucagon. in liver from the skeletal muscle lactate and the Decreasedpyruvate kinase results in the reduced reuseof glucosethus synthesizedby the muscle conversion of phosphoenol pyruvate to pyruvate for energy purpose is known as Cori cycle and the former is diverted for the synthesisof (Fig.t3.t A. glucose. Gfle.ee@cie"alanime cycle 2. Clucagon reduces the concentration of fructose2,6-bisphosphate. This compound allos- There is a continuoustransport of amino acids terically inhibits phosphofructokinase and from muscle to liver, which predominantly activates fructose 1,6-bisphosphatase,both occurs during starvation.Alanine dominates favour increasedgluconeogenesis. among the transported amino acids. lt is postulated that pyruvate in skeletal muscle Availability of substrates: Among the various undergoestransamination to produce alanine. substrates, glucogenic amino acids have Alanine is transportedto liver and used for stimulatinginfluence on gluconeogenesis.This is gluconeogenesis.This cycle is referred to as particularly important in a condition like gfucose-alanine cycle (Fig.l3.1 3). diabetesmellitus (decreased insulin level)where 'iih*ryter 13 : METABOLISMOF CARBOHYDBATES amino acidsare mobilizedfrom muscleprotein for the purposeof gluconeogenesis. Acetyl promotes CoA gluconeogenesis : Clycogen is the storage form of glucose in During starvation--dueto excessivelipolysis in animals,as is starchin plants.lt is storedmostly adiposetissue-acetyl CoA accumulatesin the in Iiver (6-8%)and muscle(1-2"/"'). Due to more liver.Acetyl CoA allostericallyactivates pyruvate musclemass, the quantityof glycogenin muscle carboxylase resulting in enhanced glucose (250 S) is about three times higher than that in production. the liver (75 g). Glycogenis storedas granulesin the cytosol, where most of the enzymes of Alceilrel inhihits gtu*ofiGesgenesis glycogensynthesis and breakdownare present. Ethanoloxidation in the liver to acetaldehvde by the enzyme dehydrogenaseutilizes Functians of glycogen NAD+. produced The excessNADH in the liver The prime function of liver glycogen is to interfereswith gluconeogenesisas illustrated maintain the blood glucose levels, particularly below. betweenmeals. Liver glycogen stores increase in Ethanol+ NAD+--+ Acetaldehyde+ NADH + H+ a well-fed state which are depleted during fasting.Muscle glycogenserves as a fuel reserve Pyruvate + NADH + H+ <-+Lactate + NAD. for the supplyof ATP during musclecontraction. Oxaloacetate+ NADH + H+ e+ Malate + NAD+ Why store glycogen It is evident from the above reactionsthat pyruvate and oxaloacetate,the predominant as a fuel re$*rve? substrates for gluconeogenesis, are made As such, fat is the fuel reserve of the bodv. unavailable by alcohol intoxication. This However, fat is not preferred,instead glycogen is happensdue to overconsumptionof NAD+ and chosen for a routine, and day to day use of excessiveproduction of NADH by alcohol. energy for the following reasons Alcohol consumption increasesthe risk of . Glycogen can be rapidly mobilized hypoglycemia(reduced plasma glucose)due to . Clycogen can generateenergy in the absence reduced gluconeogenesis. particularly This is of oxygen importantin diabeticpatients who are on insulin treatment. . Brain dependson continuousglucose supply (which mostly comes from glycogen.) Glueoneogenesis frorn fat? On the other hand, fat mobilization is slow, It is often stated that glucose cannot be needs 02 for energy production and cannot synthesizedfrom fat. In a sense,it is certainly produce glucose (to a significantextent). Thus, true, since the fatty acids (most of them being fat may be consideredas a fixed depositwhile even chain), on oxidation, produce acetyl CoA glycogen is in the current/savingaccount in a which cannot be convertedto pyruvate.Fufther, bank! the two carbons of acetyl CoA disappearas 2 moles of CO2 in TCA cycle. Therefore,even GTYCOGENESIS chain fatty acids cannot serveas precursorsfor The synfhesis of glycogen from glucose is glucose formation. The prinre reason why glycogenesis (Fig.|3.14). Glycogenesis takes animals cannot convert fat to glucose is the place in the cytosol and requiresATP and UTp, absence of glyoxylate cycle (described later). besidesglucose. However,the glycerol releasedfrom lipolysis 1. Synthesisof UDP-glucose: The enzymes and the propionateobtained from the oxidation hexokinase(in muscle)and glucokinase(in liver) of odd chain fatty acids are good substratesfor convert glucose to glucose 6-phosphate. gluconeogenesis,as discussedabove. Phosphoglucomutasecatalyses the conversionof 264 BIOCHEMISTFIY glucose 6-phosphateto glucose 1-phosphate. Glucose Uridine diphosphate glucose (UDPC) is Al r\l Glucokinase synthesizedfrom glucose 1-phosphateand UTP ,l ADP(I by UDP-glucosepyrophosphorylase. + Glucose6-phosphate 2. Requirement of primer to initiate glyco- I genesis : A small fragment of pre-existing I Phosphoglucomutase J glycogen must act as a 'primel to initiate Glucose1-phosphate glycogen synthesis.lt is recently found that in UTP. I the absence of glycogen primer, a specific \ UDP-glucose protein-namely'glycogenin'---<.an .,4 pyrophosphorylase accept PPi

3. Glycogen synthesisby glycogen synthase: Clycogen synthase is responsible for the formation of 1,4-glycosidic linkages. This enzyme transfersthe glucosefrom UDP-glucose 13(UDP--)\l Glycogensynthase to the non-reducingend of glycogento form cr- I 1,4 linkages. 13uDP{/J

4. Formation of branches in glycogen : Clycogen synthasecan catalysethe synthesisof '1 a linear unbranched molecule with ,4 u- glycosidic linkages. Glycogen, however, is a branched tree-like structure.The formation of branches is brought about by the action of a 710 branching enzyme, namely glucosyl a-4-6 +-- a 1-6-Bond . (amylo o 1,4 -+ 1,6 trans- glucosidase).This enzyme transfers a small fragmentof five to eight glucose residuesfrom the non-reducing end of glycogen chain (by breaking a-1,4 linkages) to another glucose residuewhere it is linked by ct-1,6 bond. This leads to the formation of a new non-reducing end, besidesthe existingone. Clycogenis further elongated and branched, respectively,by the enzymes glycogen synthaseand glucosyl 4-6 transferase.

The overall reactionof the glycogensynthesis for the addition of each glucoseresidue is

(Clucose)n+ Glucose + 2ATP ------> Fig. 13.14 : Glycogen synthesis from glucose (glycogenesis). (Clucose)n*1+2ADP+Pi Ghapter 13 : MEIABOLISMOF CAFIBOHYDBATES 265

Of the two ATP utilized, one is requiredfor the phosphorylationof glucosewhile the other is neededfor conversionof UDP to UTP.

GLYCOGENOLYSIS The degradation of stored glycogen in liver and muscle constitutes glycogenolysis. The pathwaysfor the synthesisand degradationof glycogenare not reversible.An independentset Glycogen of enzymes present in the cytosol carry out Pi(-)- glycogenolysis. Clycogen is degraded by Glycogen breaking d.-'l,4- and a-1,6-9lycosidic bonds F (Fig.t3J A. ]1 Gl cose 1- 1. Action of glycogen phosphorylase: The a- phosphate a 1,4-glycosidicbonds (from the non-reducing ends) are cleaved sequentiallyby the enzyme glycogen phosphorylase to yield glucose 1-phosphate. This process-called phospho- rolysis---<.ontinuesuntil four glucose residues ffi remainon eitherside of branchingpoint (a-1,6- Limit deldrin glycosidic link). The glycogen so formed is known as limit dextrin which cannot be further I o"nr"n.i,ingenzyme (transferaseactivity) degraded by phosphorylase. Clycogen phosphorylasepossesses a moleculeof pyridoxal J phosphate,covalently bound to the enzyme. ijr 2. Action of debranching enzyme : The branches of glycogen are cleaved by two I o"br"n.hingemyme enzymeactivities present on a singlepolypeptide Glucosesy'^.# (cr1-+6 giucosttaCJbciivityy (rree) called debranching enzyme, hence it is a | bifunctional enzyme. +

Clycosyl 4 : 4 translerase(oligo rl..-1,4 --> 'l ,4 glucan transferase)activity removes a fragment | ,rnn"r, actionof of three or four glucose residues attached at a PhosPhovlase I branch and transfersthem to another chain. + Here, one c-1,4-bond is cleaved and the same Glucose1-phosphate cr-1,4bond is made,but the placesare different. I Amylo cr-1,6-9lucosidasebreaks the o,-1,6 I enosptrogtucomutase bond at the branchwith a singleglucose residue J and releases a free glucose. Glycolysis{- Glucose6-phosphate I The glycogenis remainingmolecule of again I GlucoseSphosphatase available for the action of phosphorylaseand J (intiver) debranching enzyme to repeat the reactions GLUCOSE statedin 1 and 2. 3. Formation of glucose 6-phosphate and Fig. 13.15 : Glycogen degradation to glucose- glucose : Through the combined action of glycogenolysis. (The ratio of glucose 1-phosphate to glucose is 8: t). glycogen phosphorylase and debranching 266 BIOCHEMISTRY enzymetglucose 1-phosphateand free glucose in a ratio of 8 : 1 are produced. Clucose 1-phosphate is convertedto glucose6-phosphate Glucose6- ATP phosphate I by the enzyme phosphoglucomutase. I I The fate of glucose6-phosphate depends on (J @CI / ,/- the tissue.The liver, kidney and intestinecontain I I r''1 the enzyme glucose 6-phosphatasethat cleaves glucose6-phosphate to glucose.This enzyme is absentin muscle and brain, hence free glucose cannot be produced from glucose 6-phosphate Glycogen in these tissues.Therefore, liver is the major glycogenstorage organ to provide glucose into U'on"t" the circulationto be utilised by various tissues. Glyic.ogen.syn-th t In the peripheraltissues, glucose 6-phosphate o I produced by glycogenolysiswill be used for Glucose6- glycolysis.lt may be noted that though glucose phosphate 6-phosphataseis absentin muscle,some amount of free glucose(8-10% of glycogen)is produced in glycogenolysis due to the action. of debranchingenzyme (a-1,6-9lucosidase activity). l. of glycogen meta- Degradation of glycogen bolism : There are certain metabolites that by lysosornal acid maltase allostericallyregulate the activitiesof glycogen synthase and glycogen phosphorylase. The Acid maltase or a-1,4-glucosidase is a control is carriedout in sucha way that glycogen lysosomal enzyme. This enzyme continuously synthesisis increasedwhen substrateavailability degrades a small quantity of glycogen. The and energy levelsare high. On the other hand, significanceof this pathway is not very clear. glycogenbreakdown is enhancedwhen glucose However, it has been observed that the concentrationand energy levels are low. The deficiency of lysosomal enzyme o,-1,4 allostericregulation of glycogen metabolismis glucosidaseresults in glycogen accumulation, depicted in Fig.l3.l6. In a well-fed state, the causinga seriousglycogen storage disease type availabilityof glucose6-phosphate is high which ll (i.e. Pompe'sdisease). allosterically activates glycogen synthase for glycogen synthesis.On the other hand, Regerlation of glycogenesis more glucose 6-phosphate and ATP allosterically and glycoqenolysis inhibit glycogenphosphorylase. Free glucosein A good coordination and regulation of liver also acts as an allosteric inhibitor of glycogen synthesis and its degradation are glycagen phosphorylase. essential to maintain the blood glucose 2. Hormonal regulation of glycogen metabo- levels. Glycogenesisand glycogenolysisare, lisrn : The hormones,through a complex series respectively, controlled by the enzymes of reactions,bring about covalent modification, glycogen synthase and glycogen phosphorylase. namely phosphorylationand dephosphorylation Regulationof theseenzymes is accomplishedby of enzyme proteins which, ultimately control three mechanisms glycogensynthesis or its degradation. 1. Allostericregulation cAMP as second messengerfor hormones : 2. Hormonalregulation The hormones like epinephrine and 3. lnfluenceof calcium. norepinephrine,and glucagon(in liver) activate Chapter 13 : METABOLISMOF CAFIBOHYDRATES 267 adenylatecyclase to increasethe productionof ln the Fig.l3.17, the inhibition of glycogen cAMP. The enzyme phosphodiesterasebreaks synthesis brought by epinephrine (also down cAMP. The hormone insulin increasesthe norepinephrine)and glucagonthrough cAMP by phosphodiesteraseactivity in liver and lowersthe converting active glycogen synthase 'a' to cAMP levels. inactivesynthase 'b', is given.

Regulation of glycogen synthesis by cAMP : Regulation of glycogen degradation by The glycogenesis is regulated by glycogen cAMP : The hormones like epinephrineand glucagon glycogenolysis synthase.This enzyme exists in two forms- bring about by their action on glycogen phosphorylase through glycogen synthase 'a'-which is not phosphorylatedand most active, and secondly, cAMP as illustrated in Fig.l3.18. Glycogen phosphorylase glycogen synthase'b' as phosphoryIated inacti ve existsin two forms, an active 'a' form and inactiveform'b'. form. Clycogen synthase'a' can be converted to 'b' form (inactive) by phsophorylation. The The cAMP-formed due to hormonal degreeof phosphorylationis proportionalto the stimulus-activates cAMP dependent protein inactive state of enzyme. The process of kinase.This active proteinkinase phosphorylates phosphorylation is catalysed by a cAMP- inactive form of glycogen phsophorylase dependent . The protein kinase kinase to active form. (The enzyme protein phosphorylates and inactivates glycogen phosphataseremoves phosphate and inactivates synthaseby converting'a' form to 'b' form. The phosphorylase kinase). The active phospho- glycogen synthase'b' can be converted back to rylase kinase phosphorylatesinactive glycogen synthase'a' by protein phosphatasel. phosphorylase'b' to active glycogen phospho-

Glucagon Epinephrine

Adenylate cycrase PLASMA MEMBRANE (inactive)

5'AMP 268 BIOCHEMISTF|Y

Glucagon Epinephrine (liver) (liver,muscle)

Adenylate cyclase PLASMA MEMBRANE (inactive)

ATP--Q----+ cAMp I I cAMP-dependent ; protein kinase - ) (inactive)

ua- -ca2*

phosphorylaseb (inactive)

rylase'a' which degradesglycogen. The enzyme protein phosphataseI can dephosphorylateand convert active glycogen phosphorylase'a' to inactive 'b' torm. Th'e synthesis and degradative pathways of metabolism (particularly reactions involving 3. Effect of Ca2+ ions on glycogenolysis : phosphorylationand dephosphorylationutilizing When the muscle contracts, Ca2+ ions are ATP) are well regulatedand subjectedto fine releasedfrom the sarcoplasmicreticulum. Ca2+ tuning to meet the body demands,with minimal bi nds to calmodulin- calci u m modulati ng p rotein wastage of energy and metabolites. Thus, and directly activates phosphorylase kinase glycolysis and gluconeogenesis(breakdown of without the involvement of cAMP-deoendent glucoseto pyruvate,and conversionof pyruvate protein kinase. to glucose), glycogenolysisand glycogenesis The overall effect of hormones on glycogen oDeratein a selectivefashion to suit the cellular metabolism is that an elevated glucagon demands. ff on the other hand, the synthesisand or epinephrine level increases glycogen degradative metabolic pathways of a particular degradation whereas an elevated insulin results substance(say gluconeogenesisand glycolysis in increased glycogen synthesis. refated to glucose) operate to the same extent Chapter 13 : METABOLISMOF CAFIBOHYDFATES 269 simultaneously,this would resultin futile cycles. enzymes in the glycogen storagedisorders is However,futile cycles, consuming energy (ATP) depicted in Fig.l3.19. The biochemical lesions are wasteful metabolic exercises.They are and the characteristicfeatures of the disorders minimallyoperative due to a well coordinated are given in Table 13.2. metabolicmachinery. von Gierke's disease (type ll The incidence of type I glycogen storage diseaseis 1 per 200,000 persons.lt is transmitted by autosomalrecessive trait. This disorderresults The metabolic defects concerned with the in various biochemical manifestations. glycogen synthesis and degradation are 1. Fasting hypoglycemia : Due to the defect collectively referred to as glycogen storage in the enzyme glucose 6-phosphatase,enough diseases.These disorders are due to defects in free glucose is not releasedfrom the liver into the enzymeswhich may be either generalized blood. (affecting all tissues) or tissue-specific.The inherited disorders are characterized by 2. Lactic acidemia : Clucose is not deposition of normal or abnormal type of synthesizedfrom lactateproduced in muscleand glycogenin one or more tissues.A summaryof liver. Lactatelevel in blood increasesand the oH glycogen metabolism along with the defective is lowered (acidosis).

Enzvme defecl Organ(s)involved Characteristic features vonGierke's disease Glucose6-phosphatase Liver,kidney and Gfcogenaccumulates in hepatocytes and renal cells, (typeI glycogenosis) intesline enhrgedliver and kidney, tasting hypoglycemia, lactic acidemia;hyperlipidemia; ketosis; gouty arthritis. ll Pomoe'sdisease Lysosomalcr-1,4 gluco- All organs Glycogenaccumulates in lysosomes in almost all the sidase(acid maltase) tissues;heart is mostlyinvolved; enlarged liver and heart,nervous system is alsoafiected; death occurs at anearly age due to heartfailure. lll Cori'sdisease Amyloa-1,6-glucosidase Liver,muscle, Branchedchain glycogen accumulates; liver enlarged; (limitdextrinosis, (debranchingenzyme) heart,leucocyles clinical manifestations aresimilar but milder compared to Forbe'sdisease) vonGierke's disease.

lV Anderson'sdisease Glucosyl4-6 transferase . Mosttissues A rare disease,glycogen with onlyfew branches (amylopectinosis) (branchingenzyme) accumulate;cinhosis ol liver,impairment inliver function. V McArdle'sdisease Muscleglycogen Skeletalmuscle Muscleglycogen stores very high, not available during (typeVglycogenosis)phosphorylase exercise;subiects cannot perform strenous exercise; sufferfrom muscle cramps; blood lactate and pyruvate do not increaseafter exercise; muscles may get damageddue to inadequateenergy supply. Vl Her'sdisease Liverglycogen Liver Liverenlarged; liver glycogen cannot form glucose phosphorylase (pyruvateand lactate can be precursorsfor glucose); mildhypoglycemia andketosis seen, not a veryserious disease. Vll Tarui'sdisease PhosphofructokinaseSkeletal muscle, Musclecramos due to exercise:blood lactate not erythrocytes elevated;hemolysis occurs. Rareglycogen disorders Vlll, lX, X andXl havebeen identified. They are due to defectsin theenzynes concerned with activating and deactivatingliver phosphorylase. 270 BIOCHEMISTF|Y

Limitdextrin UDP-glucose

Glucose1- phosphate

V (musc!ei VI (liver) Glycogen (o 1,4and 1,6-bonds) Glycogenunbranched (a 1,4-bonds) Glucosyl(4-6) IV transferase

Glucose+ oligosaccharides

Fig. 13.19 : Summary of glycogen metabolism with glycogen storage diseases (Red blocks indicate storage disease, l-von Gierke's disease; Il-Pompe's disease; III-Cori's disease; Iv-Anderson's disease; V-ltlc Ardle's disease; W-Her's disease; Wl-Tarui's disease).

3. Hyperlipidemia : There is a blockade in The important featuresof the glycogen storage gluconeogenesis.Hence more fat is mobilizedto diseasesare given in Table 13.2. meet energy requirementsof the body. This resultsin increasedplasma free fatty acids and ketone bodies. 4. Hyperuricemia: Glucose6-phosphate that accumulates is diverted to pentose phosphate Hexose monophosphate pathway or HMP pathway, leading to increased synthesisof ribose shunf is also called pentose phosphatepathway phosphateswhich increasethe cellular levelsof or phosphogluconate pathway. This is an phosphoribosylpyrophosphate and enhancethe alternative pathway to glycolysis and TCA cycle metabolismof purine nucleotidesto uric acid. for the oxidation of glucose. However, HMP Elwated plasma levels of uric acid shunt is more anabolic in nature, since it is (hyperuricemia)are often associatedwith gouty concernedwith the biosvnthesisof NADPH and arthritis (painfuljoints). pentoses. Ghapter'13 : METABOLISMOF CAFIBOHYDHATES 271

HMP shunt-a unique multifunctional pathway

The pathway startswith glucose 6-phosphate. As such, no ATP is directly utilized or produced in HMP pathway. lt is a unique multifunctional pathway,since there are severalinterconvertible produced proceed substances which may in GlucoseFphosphab differentdirections in the metabolic reactions.

Location of the pathway

The enzymesof HMP shuntare locatedin the cytosol. The tissues such as liver, adipose tissue, adrenal gland, erythrocytes, fesfes and lactating o ll mammary gland, are highly active in HMP shunt. Most of these tissues are involved in the biosynthesisof fatty acidsand steroidswhich are dependenton the supply of NADPH.

Heactions of the pathway lr cH2o-(7> The sequence of reactions of HMP shunt 6+ttospttogluconolactone (Fig.l3.2O)is divided into two phases-oxidative and non-oxidative.

1. Oxidative phase : Glucose 6-phosphate dehydrogenase(C6PD) is an NADP-dependent enzyme that converts glucose 6-phosphateto !UL.' 6-phosphogluconolactone.The latter is then hydrolysedby the gluconolactonehydrolase to H-C-OH 6-phosphogluconate.The next reactioninvolving HO-C-H the synthesisof NADPH is catalysedby 6-phos- H-C-OH phogluconatedehydrogenase to produce 3 keto H-C-OH 6-phosphogluconate which then undergoes cH2o-o decarboxylationto give ribuloseS-phosphate. 6-Phosphogluconate G6PD regulates HMP shunt : The first reactioncatalysed by C6PD is most regulatoryin HMP shunt. This enzyme catalyses an irreversible reaction. NADPH competitively inhibits G6PD. lt is the ratio of NADPH/NAD+ cH2oH that ultimatelydetermines the flux of this cycle. I C=O I 2. Non-oxidative phase : The non-oxidative H-C-OH reactionsare concernedwith the interconversion H-C-OH l^ of three, four, five and seven carbon monosac- cH2o-€> charides.Ribulose 5-phosphate is acted upon by RibuloseSphoaphale an epimeraseto produce xylulose 5-phosphate I I while ribose5-phosphate ketoisomerase converts I ribuloseS-phosphate to ribose 5-phosphate. Flg. 13.20 contd. noxt prgo 272 BIOCHEMISTFIY

Ribulose S-phosPhate

XyluloseS-phosphate

cH20H H-C:O C:O H-C-OH I HO-C-H H-C-OH I I H-C-OH H-C-OH l^ cHro-o cH2o-<9> Ribose S-phosphate H2OH XyluloseS-phosphate C:O I HO-C-H H-C-OH I H-C-OH | /^\ cH2o-g> Fructoseo-phosphate

cH20H I C:O Transketolase HO-C-H H-C:O I I H-C-OH H-C-OH I I ,,r H-C-OH cH2o-9> I H-C-OH Glyceraldehyde 3-phosphate cHro-o Sedoheptulose I Reversaro{ 7-phosphate ulvcolvsis +I Fructose6-phosphate

Transaldolase

H-C=O cH20H t- H-C-OH G:O I H-C-OH HO-C-H I H-C-OH cHro-e I H-c-oH Erythrose4-phosphate lr cH2o-9> Fructose6-phosPhate

13.20: The hexosemonophosphate shunt. (TPP -Thiamine pyrophosphate) Ghapter 13 : METABOLISMOF CAFIBOHYDHATES

6 NADP+ 6 NADPH+ 6H+ (6) clucose G-phosphate(OC) I (6) 6-Phosphogluconolactone(6C) l-anro

vr (6)6-Phosphogluconate (6C)

lr6NADP* (5)Glucose 6- u to'*fru*ADPH phosphate(6C) + + 6H+ (6) Ribuloses-phosphate (5C) tI I I (2 + 2) Xylulose5- (5) Fructose6- (2) Ribose5- phosphate(6C)

(2) Fructose6- phosphate(6C)

(2) Glyceraldehyde (2)Sedoheptulose (2) Glyceraldehyde 3-phosphate(3C) 7-phosphate(7C) 3-phosphate(3C) I I Reversalof elvcolvsis J (1)Fructose o-phosphate(6C)

EMhrose4- (2) Fructose phosphate(4C) 6-phosphate(6C)

Fig. 13.21: Overuiewof hexosemonophosphate shunt representing the numberof molecules(pretix in red) and the number of carbon atoms (suffix in blue). Note that of the i-molecules of glucose 6-phosphate that enter HMP shunt, one molecule is oxidized as S-molecules are finally recovered.

The enzyme transketolase catalyses the Transketolaseacts on xylulose 5-phosphateand transfer of two carbon moiety from xylulose transfersa 2-carbon fragment (glyceraldehyde) 5-phosphateto ribose 5-phosphateto give a from it to erythrose 4-phosphate to generate 3-carbon glyceraldehyde 3-phosphate and a fructose 6-phosphate and glyceraldehyde 7-carbon sedoheptulose 7-phosphate. Trans- 3-phosphate. ketolaseis dependenton the coenzyme thiamine Fructose 6-phosphate and glyceraldehyde pyrophosphate (TPP) and Mg2+ ions. 3-phosphatecan be further catabolizedthrough Transaldolasebrings about the transfer o1 a glycolysisand citric acid cycle. Clucose may 3-carbon fragment (active dihydroxyacetone) also be synthesizedfrom thesetwo compounds. from sedoheptulose7-phosphate to glyceral- dehyde 3-phosphateto give fructose6-phosphate An overview of HMP shunt is given in and four carbon erythrose 4-phosphate. Fig.l3.21 . For the completeoxidation of glucose 274 BIOCHEMISTF|Y

6-phosphate to 6CO2, we have to start with 6 2 GSH (reduced)\ moleculesof glucose6-phosphate. Of these6, 5 '*"-.-t molesare regeneratedwith the productionof 'l2 / /'NADP' Glutathione Glutathione NADPH. peroxidase reductase The overall reactionmay be representedas ,/\ /\ H"o( \ o-s-s-ev/ \1rRopH* n- 6 Glucose6-phosphate + 12 NADP++ 6H2O (oxidized) -----s6CO2 +'12 NADPH + 12H+ + 5 Clucose Glutathione(reduced, GSH) detoxifiesH2O2, 6-phosphate. peroxidasecatalyses this reaction. NADPH is responsible for the regeneration of reduced SignifEcance of HMP shunt glutathionefrom the oxidized one. HMP shunt is unique in generatingtwo 4. Microsomal cytochrome P+sosystem (in important products-penfoses and NADPH- liver) brings about the detoxification of drugs neededfor the biosyntheticreactions and other and foreign compounds by hydroxylation functions. reactionsinvolving NADPH. 5. Phagocytosisis the engulfmentof foreign pentoses lnnportance of particles,including microorganisms, carried out In the HMP shunt,hexoses are convertedinto by white blood cells. The processrequires the pentoses, the most important being ribose supplyof NADPH. 5-phosphate.This pentoseor its derivativesare 6. Special functions of NADPH in RBC : useful for the synflresis of nucleic acids (RNA NADPH produced in erythrocyteshas special and DNA) and many nucleotidessuch as ATP, functions to Derform. lt maintains the NAD+, FAD and CoA. concentrationof reduced glutathione (reaction Skeletalmuscle is capable of synthesizing explainedin 3) which is essentiallyrequired to pentoses,although only the first few enzymesof preserve the integrity of RBC membrane. HMP shunt are active. lt, therefore,appears that NADPH is also necessaryto keep the ferrous (Fe2+) the completepathway of HMP shunt may not be iron of hemoglobinin the reducedstate so (Fe3+) requiredfor the synthesisof pentoses. that accumulationof methemoglobin is prevented.

Innportance of NADBH Glucose G-phosphate 1. NADPH is required for the reductive dehydrogenase defieiency biosynthesis of fatty acids and sferoidg hence G6PD deficiency is an inherited sex-linked HMP shunt is more active in the tissues trait. Although the deficiency occurs in all the concernedwith lipogenesis,e.g. adiposetissue, cellsof the affectedindividuals, it is more severe liver etc. in RBC. 2. NADPH is used in the synthesisof certain HMP shunt is the only meansof providing amino acids involving the enzyme glutamate NADPH in the erythrocytes.Decreased activity dehydrogenase. of C6PD impairsthe synthesisof NADPH in RBC. This results in the accumulation of 3. Thereis a continuousproduction of H2C-2 methemoglobin and peroxides in erythrocytes in the livingcells which can chemicallydamage leading to hemolysis. unsaturatedlipids, proteins and DNA. This is, however, prevented to a large extent through Clinical manifestationsin C5PD deficiency : antioxidant reactionsinvolving NADPH. Cluta- Most of the patientswith C6PD deficiency do thione mediatedreduction ol H2O2 is given in not usuallyexhibit clinical symptoms.Some of the next column. them, however, develop hemolytic anemia if Glrapter 13 : METABOLISMOF CAFBOHYDFATES 275 they are administeredoxidant drugs or exposed participate,whereas, in uronic acid pathway,the to a severe infection. The drugs such free sugarsor sugaracids are involved. (antimalarial), as primaquine acetanilide 1. Formation and importance of UDP- (antipyretic), (antibiotic) or sulfamethoxazole glucuronate : Clucose 6-phosphate is first (favism) produce ingestion of fava beans convertedto glucose1-phosphate. UDP-glucose jaundice patients. hemolytic in these Severe is then synthesizedby the enzyme UDP-glucose generation infectionresults in the of free radicals pyrophosphorylase.Till this step, the reactions (in macrophages)which can enter RBC and are the same as described in glycogenesis causehemolysis. (Fig.l3Jg. UDP-glucose dehydrogenaseoxi- G6PD deficiency and resistanceto malaria : dizes UDP-glucoseto UDP-glucuronate. It is interestingto note that G6PD deficiency is UDP-glucuronateis the metabolicallyactive associatedwith resistanceto malaria(caused by form of glucuronate which is utilized for Plasmodium falciparum). This is explained from conjugationwith many substanceslike bilirubin, the fact that the parasitesthat causemalaria are steroid hormones and certain drugs. Several dependent on HMP shunt and reduced insoluble compounds are convertedto soluble glutathionefor their optimum growth in RBC. onesthrough conjugation and, further,the drugs Therefore, G6PD deficiency-which is seen are detoxified.UDP-glucuronate is also required frequently in Africans-protects them from for the synthesisof glycosaminoglycansand malaria,a common diseasein this region.lt is proteoglycans. regardedas an adaptabilityof the people living 2. Conversion of UDP-glucuronate to in malaria-infectedregions of the world. L-gulonate : UDP-glucuronateloses its UDP moiety in a hydrolytic reactionand releasesD- Wernicke-Korsakoff syndrome glucuronatewhich is reduced to L-gulonateby This is a genetic disorder associatedwith an NADPH-dependentreaction. HMP shunt. An alteration in transketolase 3. Synthesis of ascorbic acid in some activitythat reducesits affinity (by tenfold or so) animals : L-Culonate is the precursorfor the with thiamine pyrophosphateis the biochemical synthesisof ascorbic acid (vitamin C) in many lesion. The symptoms of Wernicke-Korsakoff animals. The enzyme L-gulonolactoneoxidase- syndrome include mental disorder, loss of which converts gulonate to ascorbic acid-is memory and partialparalysis. The symptomsare absent in man, other primatesand guinea pigs. manifestedin alcoholicswhose diets are vitamin- Therefore,vitamin C has to be supplementedin deficient. the diet for these animals. pernicious trans- In anemia, erythrocyte 4. Oxidation of L-gulonate : L-Gulonate is ketolaseactivitv is found to increase. oxidized to 3-ketogulonate and then decarboxylated to a pentose, L-xylulose. L-Xyluloseis convertedto D-xylulosevia xylitol by a reduction(NADPH-dependent) followed by an oxidation(NAD+-dependent) reaction. This is This is an alternativeoxidative pathway for necessary since the D-xylulose (and not glucose and is also known as glucuronic acid L-form)-after gettin g phosphorylated-can enter pathway (Fig.l3.22). lt is concerned with the the hexose monophosphateshunt, for further synthesis of glucuronic acid, pentoses and metabolism. vitamin, ascorbic acid (except in primatesand Effect of drugs guinea pigs).Dietary xylulose enters uronic acid on uronic acid pathway pathway through which it can participate in other .In most of the pathwaysof Administration of drugs (barbital, chloro- carbohydrate metabolism, phosphate esters butanol etc.) significantlyincreases the uronic 276 BIOCHEMISTF|Y acid pathwayto achievemore synthesis Glucose6-phosphate I of glucuronate from glucose. Certain Phosphoglucomutase drugs (aminopyrine, antipyrine) were +I found to enhance the svnthesis of Glucose1-phosphate ascorbicacid in rats. rirtr | " ) uDP-gtucose PYroPhosPhorylase Essential pentosuria =;-.r( ,i v| This is a raregenetic disorder related UDP-glucose to the deficiencv of an NADP- dependent enzyme xylitol dehydro- 2NAD-\l \ UDPglucose genase. Due to this enzyme defect, ,i dehydrogenase zNADHr H-Y L-xylulose cannot be converted to I xylitol. The affectedindividuals excrete + UDP-glucuronate large amounts of L-xylulose in urine. Essential pentosuria is asymptomatic Hzo-rl I Ghcuronidase and the individuals suffer from no A ill-effects.lt has been reportedthat the UDPYI administrationof drugsaminopyrine and antipyrine increasesthe excretion,of L-xylulosein pentosuricpatients,

L-Gulonate

The disaccharidelactose, present in L-Gulonolactone milk and milk products,is the principal 'i,,.; L-Gulonolactone dietary source of galactose. I oxidase CO"+'1 (p-galactosidase)of intestinal mucosal -+ 2) cells hydrolyseslactose to galactoseand L-Xylulose 2-Keto-L-gulonolactone glucose. Galactose is also produced I i\AUTnNtAnpl] *+ I within the cells from the lysosomal {tn" + degradation of glycoproteins and deftydrogenasq L-Ascorbicacid glycolipids.As is the case for fructose, NAD 1) galactose entry into the cells is not Xylitol dependenton insulin. NAD*\l I The specific enzyme, namely l ,phosphorylates galactose NADH+ H-y'1 v to galactose 1-phosphate.This reacts D-Xylulose with UDP-glucose in an exchange reaction to form UDP-galactose in + presenceof the enzyme galactose 1- Xylulose5-phosphate phosphate uridyltransferase(Fig.l3.23). +I UDP-galactoseis an active donor of Hexose monophosphate shunt galactose for many synthetic reactions involving the formation of Fig. 13.22 : Uronic acid pathway [UDP-uidine diphosphate); compounds like lactose, glycosamino- (1) Block in essential pentosuria; (2) Enzyme absent in pimates (including man) and guinea pigsl. glycans,glycoproteins, cerebrosides and Ghapter 13: METABOLISMOF CAFBOHYDHATES 277

Galactose

Galactitol i Galactose1-phosphate---.r r-UDP-glucose\ \/\ Galactos phosphate UDp-hexose uridYltransferase 4-ePimerase ,/\ / Glucose1-phosphatey' \ UOe-gatactose/ I I Phosphog mutasl I + Glucose6-phosphate \+ \ Lactose Glycosaminoglycans y J (in mammarygland) Glycolipids Glycolysis Glucose Glycoproteins

glycolipids.UDP-galactose can be converted.to However, with increased levels of galactose UDP-glucoseby UDP hexose4-epimerase. In this (galactosemia), this pathway assumes way, galactosecan enterthe metabolicpathways significance.Galactitol (like , discussed of glucose. lt may be noted that galactoseis not later)has been implicatedin the developmentof an essentialnutrient since UDP-glucosecan be cataract. convertedto UDP-galactoseby the enzymeUDP- 3. The accumulation of galactose hexose4-epimerase. 1-phosphateand galactitolin varioustissues like DISORDERSOF liver, nervoustissue, lens and kidney leads to GALACTOSE METABOLISM impairmentin their function. 4. The accumulation of galactose galactosemia Glassical 1-phosphatein liver resultsin the depletion of Calactosemiais due to the deficiencyof the inorganicphosphate (sequestering of phosphate) enzyme galactose 1-phosphate uridyltrans- for other metabolicfunctions. ferase. lt is a rare congenital disease in 5. The clinical symptoms of galactosemia infants, inherited as an autosomal recessive are-loss of weight (in infants) hepato- galactosemia disorder.The salientfeatures of are splenomegaly,jaundice, mental retardationetc. Iisted. In severe cases,cataract, amino aciduria and 1. Calactosemetabolism is impairedleading albuminuriaare also observed. to increased galactose levels in circulation Diagnosis: Earlydetection of galactosemiais (galactosemia) and urine (galactosuria). possible (biochemicaldiagnosis) by measuring 2. The accumulatedgalactose is divertedfor the activity of galactose 1-phosphateuridyl- the production of galactitol (dulcitol) by the transferasein erythrocytes. enzymealdose reductase (the sameenzyme that Treatment : The therapy includesthe supply convertsglucose to sorbitol).Aldose reductaseis of diet deprived of galactoseand lactose. presentin lens, nervoustissue, seminal vesicles etc. The conversionof galactoseto galactitol is Galactokinase deficiency : The defect in insignificant in routine galactosemetabolism. the enzyme galactokinase, responsible for 278 BIOCHEMISTF|Y

,i,/xt SorbitolQl{^-Glucose...' Glucose 6-phosphate AIOOSe ^

ATP Fructokinase ATP (1) Fructose1-phosphate AldolaseB (2)

Triokinase NADH+ H+ Alcohol dehydrogenase Triosephosphate NAD- Glycolysis tsomerase (pyruvate) Glycerol 3-phosphate . dehvdrooenase Glycerol ciryceror+DHAP 3-phosphate

..J L. TriacylglycerolsPhospholipids

Fig. 13.24 : Metabolism of fructose (Metabolic defects 1-Fructosuria; 2-Fructose intolerance). (Note: The shaded part representsthe polyol pathway) phosphorylationof galactose,will also result in is also found in free form in honey and many galactosemia and galactosuria. Here again fruits.ln the body, entry of fructoseinto the cells galactose is shunted to the formation of is not controlledby the hormoneinsulin. This is galactitol. Cenerally, galactokinase-deficientin contrastto glucosewhich is regulatedfor its individuals do not develop hepatic and renal entry into majority of the tissues. Developmentof cataract complications. occurs Fructoseis mostly phosphorylatedby fructo- at a very early age, sometimeswithin an year kinaseto fructose1-phosphate. Fructokinase has after birth. The treatment is the removal of been identified in liver, kidney and intestine. galactose and lactose from the diet. Hexokinase, which phosphorylates various monosaccharides,can also act on fructose to produce fructose 6-phosphate. However, hexokinase has low affinity (hish K.n) for fructose,hence this is a minor pathway. The major dietary source of fructose is the Fructose1-phosphate is cleaved to glyceral- disaccharide sucrose (cane sugar), containing dehyde and dihydroxyacetone phosphate equimolar quantitiesof fructoseand glucose.lt (DHAP) by (Fi9.13.24. This is in METABOLISMOF CAFBOHYDRATES 279 contrast to fructose 6-phosphate which is Ingestion of large quantities of fructose or convertedto fructose1, 6-bisphosphateand split sucrose is linked with many health by aldolase A (details in glycolysis-See complications. Fig.l3.2). Clyceraldehydeis phosphorylatedby the enzyme to glyceraldehyde 3- Sslrfu[tef I P$lyc]! pathwfiy phosphatewhich, along with DHAP, enters (so glycolysisor gluconeogenesis. Polyol pathway termed since sorbitol is a polyhydroxy sugar) basically involves the more rapidly metabolized(via The fructoseis conversion of glucose to fructose via sorbitol glycolysis) liver than glucose.This is due by the (Fig.l3.2a). This pathway is absent in liver. fact that the rate limiting reaction in to the Sorbitol pathway is directly related to glucose glycolysis by phosphofructokinaseis catalysed concentration, and is higher in uncontrolled Increaseddietary intake of fructose bypassed. diabetes. significantly elevatesthe production of acetyl CoA and lipogenesis(fatty acid, triacylglycerol The enzymealdose reductase reduces glucose and very low density lipoprotein synthesis). to sorbitol (glucitol)in the presenceof NADPH.

BIOMEDICAT/ CLINICAL CONCEPT9

A continuous presence of glucose---+upplied through diet or synthesized in the body (gluconeogenesisFis essential t'or the suruiual of the organism, Alcohol intoxication reducesgluconeogenesis. Human brain consumes about 120 g of glucose per day out of the 760 g needed by the body. lnsufficient supply of glucose to brain may lead to coma and death. Liuer glycogen sen)esas an immediate source for maintaining blood glucose leuels, particularly between the meols. The glycogen sfores in the liuer get depleted after 72- 18 hours ol fasting. Muscle glycogen is primarily concerned with the supply of hexoses thot undergo g/ycolysis to prouide energg during muscle contraction. Glycogen storage diseoses----charocterizedby deposition of normol or abnormal type of glycogen in one or more fissues-result in muscular weakness,or euen death. The occurrence of HMP shunt (NADPH production) tn the RBC is necessaryto rnaintsin the integrity of erythrocyte membrane and to pretlent the occumulation of methemoglobin. Deficiency ol glucose 6-phosphate dehydrogenase results in hemolysis of RBC, cousing hemolgtic anemio. The subjects of G6PD deficiency are, howeuer, resistant to maloria. lJronic acid pathwag is concerned with the production of glucuronic acid (inuoluedin detoxification), pentosesand uitamin C. Man is incapoble of synthesizing uitomin C due to the absenceof a single enzyme-L-gulonolactone oxidase. The conuersion of glucose to fructose is impaired in diabetes mellitus, cousing occumulation ol sorbitol. This compound hos been implicated in the deuelopment of cataract, nephropathy, peripheral neutopathy etc. Seuere casesof golactosemiaare associated with the deuelopment of cataract, believed to be due to the accumulation ol galactitol. 280 BIOCHEMISTRY

Sorbitol is then oxidized to fructose by sorbitol fructose is not convertedto fructose 1-phosphate. dehydrogenaseand NAD+. Aldose reductase is This is an asymptomaticcondition with excretion absent in liver but found in many tissueslike of fructose in urine. Treatment involves the lens and retina of the eye, kidney, placenta, restriction of dietary fructose. Schwann cells of peripheral neryes,efihrocytes 2. Hereditary fructose intolerance : This is and seminal vesicles. The enzyme sorbitol due to the absence of the enzyme aldolase B. dehydrogenaseis present in seminal vesicle, Hereditary fructose intolerance causes spleen and ovaries. Fructose is a preferred .l intracellular accumulation of fructose -phos- carbohydratefor energy needsof sperm cells due phate, severe hypoglycemia,vomiting, hepatic to the presenceof sorbitol pathway. failure and jaundice. Fructose 1-phosphate allostericallyinhibits liver phosphorylaseand Sorbitol pathway in blocks glycogenolysisleading to hypoglycemia. diabetes mellitus Early detection and intake of diet free from In uncontrolled diabetes (hyperglycemia), fructose and sucrose, are advised to overcome large amounts of glucoseenter the cells which fructose intolerance. are not dependenton insulin. Significantly,the fructose Fructoseis cells with increasedintracellular glucose levels 3. Consumption of high : 1-phosphate in diabetes(lens, retina, nerve cells, kidney etc.) rapidly converted to fructose by The the enzymealdolase possess high activity of aldose reductase and fructokinase. activityof to this, fructose sufficient supply of NADPH. This results in a B is relatively less, and, due This leads rapid and efficient conversion of glucose to 1-phosphateaccumulates in the cell. inorganic sorbitol. The enzyme , to the depletion of intracellular phosphate (Pi) phenomenon however, is either low in activity or absent in levels. The of (like these cells, hence sorbitol is not converted to binding of Pi to the organic molecules leads lessavailabilitv fructose. Sorbitol cannot freely passthrough the fructosehere)-that to the functions-is cell membrane, and accumulate in the cells of Pi for the essentialmetabolic phosphate. Due where it is produced. Sorbitol-due to its known as sequestering of to the Pi, which happens in hydrophilic nature-causes strong osmotic decreasedavailability of fructose, the liver effectsleading to swelling of the cells. Some of overconsumption of This includes the pathological changes asmciated with metabolismis adverselyaffected. diabefes (like cataract formation, peripheral the lowered synthesisof ATP from ADP and Pi. period neuropathy,nephropathy etc.) are believed to be High consumptionof fructoseover a long in due to the accumulation of sorbitol, as is associatedwith increaseduric acid blood gout. explainedabove. leading to This is due to the excessive breakdownof ADP and AMP (accumulateddue It is clearly known that in diabetic animals to lack of Pi) to uric acid. sorbitolcontent of lens,nerve, and glomerulusis elevated. This causes damage to tissues.lt thus appears that majority of the complications associated with diabetes share a common pathogenesis as a consequence of polyol pathway. Certain inhibitors of aldose reductase When a hydroxylgroup of a sugaris replaced can prevent the accumulation of sorbitol, and by an amino group, the resultantcompound is thus the associatedcomplications. However, this an amino sugar. approach is still at the experimental stage. The important amino sugarsare glucosamine, galactosamine, mannosamine, sialic acid etc. Defects in fructose metabolism They are essential components of glyco- 1. : Due to the saminoglycans,glycolipids (gangliosides)and deficiency of the enzyme hepatic fructokinase, glycoproteins.They are also found in some Chapter 13 : METABOLISMOF CAFIBOHYDBATES 281

Glucosamine Glucose I I I Glutamine I + Glutamate J Glucosamine Glucosamine UDP- 6-flH3i;:," -l - o5["fJ3fi3," 6-phosphate 1-phosphate glucosamine I ncevrcon I I I N-AcetyF N-Acetyl- Glycosamino-

N-Acetyl- UDP-N-

oligosaccharidesand certain antibiotics. lt is may result in skeletal deformities, and mental estimated that about 2Oo/" of the glucose is retardation.Mucopolysaccharidoses are important utilized for the synthesisof amino sugars,which for elucidatingthe role of lysosomesin health and mostly occurs in the connective tissue. disease. The outline of the pathway for the synthesis CAGs are degraded by a sequentialaction of of amino sugars is given in Fi9.13.25. Fructose Iysosomal acid hydrolasese.g. exoglycosidases, 6-phosphate is the major precursor for sulfatases. Some important mucopoly- glucosamine, N-acetylgalactosamine and saccharidosesand the enzyme defectsare listed. N-acetylneuraminicacid (NANA).The utilization Hurler's syndrome (MPS l)-L-lduronidase of the amino sugars for the formation of glycosaminolgycans, glycoproteins and Hunter's syndrome (MPS Il)-lduronate sulfatase gangliosidesis also indicatedin this figure. Sanfilippo syndrome (MPS lll)-Four differt enzymes (e.g. heparan sulfamidase) Mucopolysaccharidoses SIy syndrome (MPS Vll)-p-Glucuronidase. The lymsomal storage diseases caused by defects in the glycosaminoglycans(GACs) are known as mucopolysaccharidoses.The diseases name is so given since the original name for GACs was mucopolysaccharides(MPS). These are more than a dozen rare genetic diseases. The animals,including man, cannot carry out Mucopolysaccharidosesare characterizedby the the net synthesis of carbohydrate from fat. accumulation of GACs in various tissuesthat However, the planfs and many microorganisms 282 E|IOCHEMISTF|Y are equipped with the metabolic Faityacid machinery-namely the glyoxylate I Y cycle-fo convert fat into carbohydrates. This pathway is very AcetylCoA significant in the germinating seeds where the stored triacylglycerol (fat) is converted to sugarsto meet the energy needs. Location of the cycle : The glyoxylate cycfe occurs in glyoxysomes,specialized cellular organelles, where fatty acid oxidation is also operative. Reactions of the cycle : The glyoxylate cycle (Fig.13.26) is regarded as an anabolic variant of citric acid cycle. Acetyl CoA produced from fatty acid oxidation condenses with oxaloacetateto give citrate which is then converted to isocitrate. At this stage, isocitrate bypassesthe citric acid cycle and is cleaved by isocitrate to succinate and glyoxylate. Another molecule of acetyl CoA is now utilized to combine with glyoxylate to form malate. This reaction is catalysed by Fig. 13.26 : The glyoxylate cycle green malate synthase and the malate so (enzymes reprcsented in shade). formed enters citric acid cycle. The glyoxylate cycle is a cyclic pathway that succinate. The succinate is converted to results in the conversion of two 2-carbon oxaloacetate and then to glucose involving the fragmentsof acetyl CoA to 4-carbon compound, reactions of gluconeogenesis. Ghapter 13 ; METABOLISMOF CARBOHYDRATES 283

7. Csrbohydrates are the major source ol energy for the ltuing cells. Glucose (normal lasting blood leuel 70-100 m7/dl) is the central molecule in carbohydrate metabolism, actiuely participating in a number of metabolic pathways--glycolysis, gluconeo- genesls, glycogenesis, glgcogenolysis, hexose monophosphate shunt, uronic ocid pathway etc.

2. Glucose is oxidized in glycolysis, either in anaerobic (2 ATP formed) or aerobic (8 ATP formed) conditions, resulting in the lormation of 2 moles ol lactate or pyruuate, r*pectioely.

3. Acetyl CoA is produced from pyruuate which is completely oxidized in citric acid cycle, the final common oxidative pathwoy lor all foodstuffs. The complete oxidation of one mole ol glucose generotes 38 ATP.

4. Gluconeogenesis is the synthesis of glucose from noncarbohydrate precursors like amino acids (except leucine and lysine), Iaetate, glgcerol, propionate etc. The reuersal of glycolgsis with olternate arrangements mode ot three irreuersible reactions ol glgcol ysis constitutes gluconeogenesis.

5. Glycogen is the storogeform of glucose. The degradation of glycogen (glgcogenolysis)in musclemeets the immediatefuel requirements,whereas the liuer glgcogenmaintains the blood glucoseleuel. Enzyme defects in synfhesisor degradationol glycogen lead to storage disorders. uon Gierke's diseose (TVpe I) is due to the defect in the enzyme glucose 6-phosphatase.

6. Hexose monophosphate shunt (HMP shunt) is the direct oxidatiue pathway of glucose. HMP shunt ossumes significance since it genercites NADPH and pentoses, respectiuely required for the synthesisot' lipids and nucleic acids.

7. Glucuronate-inuolued in the conjugation of bilirubin, steroid hormones and detoxification of drugs-is synthesizedin uronic acid pathwog. Due to a singleenzyme det'ect (gulonolactone oxidase) in fhis pathwoy, man cannot synthesize ascorbic acid (uitamin C) wheresssome onimals can.

8. Goloctosemio is mostly due to the delect in the enzyme galactose 7-phosphate uridyltranst'erase.This re.sultsin the diuersionot' galactoseto produce galactitol which has been implicated in the deuelopment of cataract.

9. Glucose can be converted to Jructose uia sorbitol pathway. In prolonged hgperglgcemia (uncontrolled diabetes), sorbitol accumulates in the fissues, resulting in cataroct, nephropathy, peripherol neuropathy etc.

10. Amino sugors (glucosomine, galactosamine, mannosamine etc.), synthesized from fructose 6-phosphate are essential components of glgcosaminoglycans,glgcolipids and glycoproteins. 284 BIOGHEMISTRY

I. Essayquestions 1. Describebriefly the metabolismof glucose6-phosphate. 2. Cive an account of glycogenmetabolism. 3. Justify that citric acid cycle is the final common metabolic pathway for the oxidation of foodstuffs. 4. Discussthe synthesisof glucosefrom non-carbohydratesources. 5. Describethe hexosemonophosphate shunt and add a note on its significance. II. Short notes (a) Clycogenolysis,(b) UDPC, (c) Calactosemia,(d) Cori cycle, (e) 2, 3- BPG,(0 Clycogenstorage diseases,(g) Essentialfructosuria, (h) Conversionof pyruvateto acetyl CoA, (i) Energeticsof TCA cycle, (j) TPP in carbohydratemetabolism. III. Fill in the blanks 1. Name the five vitamins required.by pyruvatedehydrogenase or cr-ketoglutaratedehydrogenase complex 2. Muscle glycogendoes not directly contributeto blood glucosedue to absenceof the enyme

3. Ascorbic acid is not synthesizedin man due to lack of the enzyme 4. The compoundimplicated in the developmentof cataractin diabeticpatients is 5. Galactosemiais mostly due to the deficiencyof the enzyme 6. The two amino acids that are never glucogenicare and 7. Substratelevel phosphorylationin citric acid cycle is catalysedby the enzyme 8. The metabolic pathway concerned with the conversion of L-xylulose to D-xylulose is

9. The name of the protein that has been identifiedto serveas a primer for glycogensynthesis is

10. The metabolite among the citric acid cycle intermediatesperforming a catalytic role

IV. Multiple choice questions 11. One of the followingenzymes in glycolysiscatalyses an irreversiblereaction. (a) Hexokinase(b) Phosphofructokinase(c) Pyruvatekinase (d) All of them. 12. Synthesisof 2, 3-bisphosphoglycerateoccurs in the tissuenamely. (a) Liver (b) Kidney (c) Erythrocytes(d) Brain. 13. The hormonethat lowers cAMP concentrationin liver cells is (a) Glucagon(b) Insulin(c) Epinephrine(d) Thyroxine. 14. The number of ATP producedwhen a molecule of acetyl CoA is oxidized through citric acid cycle (a) 12 (b) 24 (c) 38 (d) 1s. 15. The connectinglink betweenHMP shuntand lipid synthesisis (a) Ribose(b) NADPH (c) Sedoheptulose7-phosphate (d) NADH.