Carbohydrates [CHO]

Chemistry of Carbohydrates All carbohydrates contain C O &OH-functional groups & are classified into:

1-Monosaccharides... Simple sugar that can not hydrolyzed to a simpler form, it may contain three, four, five, six or more carbon atoms known respectively as trioses, tetroses, pentoses, hexoses & so on. Monosaccharides may be aldoses or ketoses depending upon whether they have an aldehyde or ketone group respectively. Most important monosaccharides are hexoses like glucose, galactose & fructose which are reducing substances because it contains aldehyde or ketone groups. Monosaccharides have stereoisomer property which could be D (common) or of its mirror image L (D on right & L on left) depending on position of hydroxyl group at carbon atom adjacent to the terminal alcohol carbon ((C5 in glucose)).

2-Disaccharides... They are products of chemical reaction between two monosaccharides with loss of a molecule of water (can be hydrolyzed), the linkage between two monosaccharides known as glycosidic link. Examples of disaccharides are maltose, lactose & sucrose. If the glycosidic link between aldehyde or ketone group of one monosaccharide & the hydroxyl group of another monosaccharide the produced disaccharide have reducing property as in maltose (glucose + glucose) & lactose (glucose + galactose ) while if the glycosidic link between aldehyde or ketone group of the two molecules of monosaccharide the produced disaccharide have no reducing property as in sucrose (glucose + fructose).

3-Oligosaccharides...They are products of condensation of 3- 10 monosaccharide units as in maltotriose.

4-Polysaccharides… They are products of condensation of more than 10 monosaccharide units; examples are:-

2 A-Starch- Polysaccharide of plant origin consist of amylose (one unbranched chain of glucose molecules linked by α-1, 4- glucosidic linkages with only terminal aldehyde is free) & amylopectin (contain α-1, 4-glucosidic linkages + α-1, 6- branched glucosidic linkages of glucose molecules). B-Glycogen-Polysaccharide of animal origin, it has structure similar to amylopectin except that branching is more extensive.

Fate of Carbohydrates Carbohydrates account for a large proportion of daily intake, dietary digestible carbohydrates include mainly starch , sucrose and to less extent lactose. In order of carbohydrates to be absorbed it should be converted to monosaccharides by digestion. The absorbed monosaccharides (glucose, fructose & galactose) from small intestine reach the liver through portal vein, glucose is the only carbohydrate to be directly used for energy or stored as glycogen while galactose & fructose are mainly converted to glucose in the liver before they can be used. Pentoses as xylose, arabinose & ribose are important in nucleotides, nucleic acids & several coenzymes. Carbohydrate (mainly glucose) is a main source of human energy & it is a unique source of energy to some tissues as nervous system including the brain & in RBC, therefore, we concern with glucose. After absorption of glucose it converted to glucose-6 phosphate inside the cells which may follow one of the following pathways depending on energy requirement, type of tissue & state of glycogen storage. 1-Glycolysis… Produce energy. 2-Hexose monophosphate shunt {phosphogluconate oxidative pathway, pentose phosphate pathway}...Nucleotide synthesis. 3-Glycogenesis... Storage.

Glycolysis Glycolysis is the major pathway for glucose metabolism, occurs in the cytosol of all cells ((outside the mitochondria)) through Embden-Meyerhof pathway. It is unique in that it can function either aerobically or anaerobically, however , anaerobic conditions limit the amount of energy liberated /mole of glucose , therefore , more glucose are needed.. To oxidize glucose beyond pyruvate (the end product of glycolysis) requires oxygen, mitochondrial enzyme system, the citric acid cycle & the respiratory chain. The ability of glycolysis to provide ATP in the anaerobic conditions are especially important in RBC which lack mitochondria & completely depend on glucose as their metabolic fuel, also in skeletal muscle in anoxic episodes. However, in heart muscle, which is adapted for aerobic performance, has relatively low glycolytic activity & poor survival under conditions of ischemia.

4 The steps of glycolysis ((Embden-Meyerhof pathway)) are the followings: Reaction 1: Phosphate Ester Synthesis In all body tissues except the liver, brain & pancreatic β islet cells, the transport of glucose into the cell is regulated by insulin. Following entry of glucose into the cells , phosphate is added to the glucose present in the cytoplasm at the C-6 position using ATP as the phosphate donor in the presence of magnesium ion, this reaction is irreversible inhibited by its product ((glucose 6- phosphate)) & it's catalyzed by the enzymes hexokinase or glucokinase.

Glucose + ATP Mg++ Glucose 6-P+ADP Hexokinase Glucokinase

Hexokinase has a high affinity for its substrate (glucose) & its even act at lower speed on other hexoses, in the liver & pancreatic β islet cells hexokinase is saturated under all normal conditions, therefore, both the liver & pancreatic β islet cells also contain an isoenzyme of hexokinase called glucokinase, which has lower affinity for its substrate (specific on glucose) so it acts at a higher glucose concentration. The function of glucokinase in the liver is to remove glucose from the blood following a meal, providing glucose 6-phosphate in excess of requirements for glycolysis, which will be used for glycogen synthesis and lipogenesis. In the pancreas, the glucose 6-phosphate formed by glucokinase signals increased glucose availability & leads to the secretion of insulin.

5 Glucose 6-phosphate is an important compound at the junction of several metabolic pathways (glycolysis, gluconeogenesis, pentose phosphate pathway, glycogenesis & glycogenolysis). Reaction 2: Isomerization The glucose-6-phosphate is changed into an isomer, fructose-6- phosphate. This means that the number of atoms is unchanged, but their positions have changed (aldose to ketose conversion) .This reversible reaction is catalyzed by phosphohexose isomerase (phosphoglucoisomerase) enzyme.

Glucose-6-P Phosphohexose Fructose-6-p Isomerase

Reaction 3: Phosphate Ester Synthesis This reaction is virtually identical to reaction 1. The fructose-6- phosphate is reacted with phosphate from ATP to make fructose-1, 6 -bisphosphate again this reaction using ATP as the phosphate donor in the presence of magnesium ion. This reaction is irreversible, catalyzed by phosphofructokinase enzyme & it has a major role in regulating the rate of glycolysis.

Fructose-6-P + ATP

++ Mg Phosphofructokinase

Fructose-1, 6-bisphosphate + ADP

Reaction 4: Split Molecule in half The six carbon fructose-1, 6-bisphosphate is split into two trioses phosphate carbon compounds which are dihydroxyacetone-phosphate & glyceraldehyde 3-phosphate. The split is made between the C-3 and C-4 of the fructose. This reversible reaction is catalyzed by Aldolase enzyme. The dihydroxyacetone phosphate & glyceraldehyde-3-phosphate are interconverted by the enzyme phosphotriose isomerase.

Fructose-1, 6-bisphosphate

Aldolase

Dihydroxyacetone- Glyceraldehyde phosphate 3-phosphate Phosphotriose Isomerase

Reaction 5: Oxidation/Phosphate Ester Synthesis This reversible reaction is first an oxidation involving the coenzyme NAD+. Glyceraldehyde 3-phosphate is oxidized to an acid as an intermediate through the conversion of NAD+ to NADH + H+. Then an inorganic phosphate (Pi) is added in a phosphate ester synthesis to form 1, 3-bisphosphoglycerate. This and all the remaining reactions occur twice for each glucose-6-phosphate (six carbons), since there are now two molecules of 3-carbons each.

8 This reaction is catalyzed by glyceraldehyde-3-phosphate dehydrogenase enzyme which is a tetramer (consist of four monomers) containing SH-groups , therefore , this dehydrogenase enzyme may be inactivated by SH poison iodoacetate which stop glycolysis at this point , therefore , iodoacetate used as a preservative of blood sample to prevent in vitro glycolysis .

Glyceraldehyde 3-phosphate +NAD+ Pi Glyceraldehyde-3- phosphate dehydrogenase

1, 3-bisphosphoglycerate+ NADH + H+

Reaction 6: Hydrolysis of Phosphate; Synthesis of ATP One of the phosphate groups of 1, 3-bisphosphoglycerate undergoes hydrolysis to form 3-phosphoglycerate and a phosphate ion is transferred directly to an ADP to make ATP, therefore, at this stage two molecules of ATP are produced / molecule of glucose undergo glycolysis, this reversible reaction is catalyzed by Phosphoglycerate kinase enzyme in the presence of magnesium ion.

1, 3-bisphosphoglycerate + ADP

Mg++ Phosphoglycerate kinase

3-phosphoglycerate + ATP

The toxicity of arsenic is due to competition of arsenate with inorganic phosphate in the above reactions to give l-arseno-3-phosphoglycerate, which hydrolyzes spontaneously to give 3-phosphoglycerate + heat, without generating ATP. Reaction 7: Isomerization In this reaction the phosphate group moves from the 3 position of 3- phosphoglycerate to the 2 position in an isomerization reaction producing 2-phosphoglycerate. This reversible reaction is catalyzed by Phosphoglycerate mutase enzyme.

3-phosphoglycerate

Phosphoglycerate mutase.

2-phosphoglycerate

Reaction 8: Alcohol Dehydration (Enolation) In this reversible reaction, dehydration of 2-phosphoglycerate forming phosphoenolpyruvate, this reaction is catalyzed by Enolase enzyme which is dependent on the presence of either Mg++ or Mn++ ions & inhibited by fluoride which is used as a preservative of blood sample to prevent in vitro glycolysis in the estimation of glucose.

Mg++/ Mn++ 2-phosphoglycerate Phosphoenolpyruvate

Enolase + H2O

Reaction 9: Phosphate Ester Hydrolysis, Synthesis of ATP This is the final reaction in glycolysis. phosphate group of phosphoenolpyruvate is transferred to ADP forming ATP while Phosphoenolpyruvate is converted into enolpyruvate which undergoes spontaneous (nonenzymic) isomerization to pyruvate ,therefore , at this stage two molecules of ATP are produced / molecule of glucose undergo glycolysis. This irreversible reaction is catalyzed by pyruvate kinase enzyme in the presence of magnesium.

Phosphoenolpyruvate + ADP

Mg++ Pyruvate kinase

Pyruvate + ATP

Fate of pyruvate depending on availability of aerobic or anaerobic conditions: 1-Aerobic condition: pyruvate is transported from the cytoplasm to the mitochondria via special pyruvate transporter & within the mitochondria it's oxidatively decarboxylated into acetyl Co-A (active acetate) by several different enzymes working sequentially in a multienzyme complex called collectively as pyruvate dehydrogenase complex system.

Pyruvate + NAD+ + CoA-SH

Pyruvate dehydrogenase complex

+ Acetyl CoA+NADH+H +CO2

13 The presence of arsenate or deficiency of thiamin inhibit pyruvate dehydrogenase allowing pyruvate to accumulate , also it is inhibited by the product (Acetyl CoA) , therefore , any source that give rise to acetyl Co-A can inhibit pyruvate dehydrogenase system as amino acids & fatty acids. Acetyl Co-A enter citric acid cycle for further energy production. 2- Anaerobic condition: pyruvate is reduced to lactate by + NADH 2 produced in reaction( 5) of glycolysis, this reversible reaction catalyzed by lactate dehydrogenase enzyme with production of NAD+ allowing glycolysis to proceed in anaerobic conditions by regenerating sufficient NAD+ for reaction( 5) to continue.

Pyruvate+NADH+H+ lactate Lactate+NAD+ dehydrogenase

Therefore, tissues that can function under hypoxic condition can produce lactate as in skeletal muscle & RBC (even under aerobic condition because it has no mitochondria).

Conclusion 1-Glycolysis is represented simply as: glucose + 2 NAD+ + 2 ADP + 2 P 2 pyruvate + 2 ATP + 2 NADH + 2 H+ 2-Glucose with six carbons is converted into two pyruvate molecules with three carbons each. The ATP produced is as follow:-

14 A-Anaerobic conditions:-2 ATP are produced (2ATP produced at reaction 6 + 2ATP at reaction 9 - 2ATP consumed at reactions 1 &3). B-Aerobic conditions:- 2 ATP are produced as in anaerobic conditions. + + 5 ATP are produced from entrance of two NADH2 molecules + + produced at reaction 5 to respiratory chain (each NADH2 ++molecule produce 2.5 ATP molecules) + +5 ATP molecules generated by entrance of two NADH2 molecules (produced from conversion of two molecules of pyruvate into two molecules of acetyl-CoA / one molecule of glucose) to respiratory chain. + 20 ATP are produced from the citric acid cycle. Therefore the total ATP molecules that produced at aerobic conditions are 2+5+5+20= 32 ATP molecules. 3-Three reactions ((1, 3, and 9)) are irreversible regulating glycolysis while the rest of reactions are reversible. 4-Glycolysis reaction can be blocked at reaction 5 by iodoacetate & at reaction 8 by fluoride, therefore, iodoacetate & fluoride are used as preservative of blood sample for glucose estimation.

Clinical Aspects 1-Inhibition of Pyruvate Metabolism Leads to Lactic Acidosis, the main causes of this inhibition are:- A-Arsenite inhibit pyruvate dehydrogenase complex. B-Thiamin is a coenzyme for pyruvate dehydrogenase, therefore, its deficiency lead to lactic acidosis..

16 C-Inherited pyruvate dehydrogenase deficiency presented with lactic acidosis, particularly after a glucose load. Because of its dependence on glucose as a fuel. , brain is a prominent tissue where these metabolic defects manifest themselves in neurological disturbances. 2- Inherited aldolase deficiency & pyruvate kinase deficiency in erythrocytes cause hemolytic anemia. 3- The exercise capacity of patients with muscle phosphofructokinase deficiency is low, particularly on high- carbohydrate diets. 4- Competition of arsenate with inorganic phosphate to give l- arseno-3-phosphoglycerate, which hydrolyzes spontaneously to give 3-phosphoglycerate + heat, without generating ATP.

17 Citric Acid Cycle {Krebs cycle , Tricarboxylic Acid Cycle } It is a series of reactions discovered by Hans Krebs in 1937 occur in the mitochondria that oxidize acetyl-CoA to CO2 +H2O in addition to the production of reducing equivalents (NADH2 and FADH2) that upon reoxidation in the respiratory chain ATP are formed. It is dependant on oxygen availability, therefore, absence (anoxia) or deficiency (hypoxia) of oxygen leads to total or partial inhibition of the cycle respectively.

Overview of Acetyl Co- A Metabolism 1-Acetyl Co-A is at the confluence of the major metabolic pathways of carbohydrate, lipid & protein. 2-Acetyl Co-A serves as source of acetyl units in the anabolic processes responsible for synthesis of long chain fatty acids, cholesterol, steroid & ketone bodies. 3-Catabolism of acetyl Co-A in the citric acid cycle.

Importance of Citric Acid Cycle 1-Final common pathway for the aerobic oxidation of carbohydrate, lipid & protein because glucose, fatty acids & most amino acids are metabolized to acetyl-CoA or intermediates of the cycle. 2-Central role in gluconeogenesis, lipogenesis & interconversion of amino acids. 3-Liberation of much free energy from oxidation of carbohydrate, lipid & protein. 4-Formation of reducing equivalents which enter the respiratory chain for energy production.

Reactions of Citric Acid Cycle Reaction 1: Synthesis of Citrate Condensation of acetyl Co-A & oxaloacetate to form citrate & release of CoA-SH, this irreversible reaction is catalyzed by citrate synthase enzyme.

Acetyl Co-A + Oxaloacetate + H2O

Citrate synthase

Citrate + CoA-SH

Reaction 2: Dehydration & Rehydration Citrate is isomerized to isocitrate by the enzyme aconitase ((aconitate hydratase)) in the presence of iron in the Fe++ state. This reversible reaction takes place in two steps : Step 1/dehydration of citrate to cis-aconitate (intermediate) which is enzyme bound. Step 2 /rehydration of cis-aconitate to isocitrate. The poison fluoroacetate is toxic because fluoroacetyl-CoA condenses with oxaloacetate to form fluorocitrate, which inhibits aconitase, causing citrate to accumulate.

Citrate H2O Aconitase Fe++

Cis-aconitate H2O Aconitase Fe++

Isocitrate

Reaction 3: Dehydrogenation & Decarboxylation Isocitrate in the presence of NAD+ is converted to α-ketoglutarate .Although this reaction is reversible its directed more toward the production of α-ketoglutarate & is catalyzed by the enzyme isocitrate dehydrogenase & it occurs in two steps:

21 Step 1/dehydrogenation of isocitrate in the presence of NAD+ to oxalosuccinate ((intermediate& enzyme bound)) +NADH+H+ Step2/decarboxylation of oxalosuccinate to α-ketoglutarate + ++ ++ +CO2, Mn or Mg is an important component of the decarboxylation reaction.

Isocitrate + NAD+

Isocitrate Dehydrogenase

Oxalosuccinate +NADH +H+

An Isocitrate Mn++ /Mg++ Dehydrogenase

α-ketoglutarate +CO2

21 Reaction 4:Oxidation&Decarboxylation (Oxidative decarboxylation) α-ketoglutarate undergoes oxidative decarboxylation (similar to conversion of pyruvate into acetyl Co-A) .This irreversible reaction is catalyzed by α-ketoglutarate dehydrogenase complex which require also identical cofactors to that of pyruvate dehydrogenase as thiamin diphosphate& is inhibited by arsenate causing accumulation of α-ketoglutarate. In this reaction α-ketoglutarate in the presence of NAD+ & CoA-SH result in the formation of succinyl -CoA (contain high + energy bond) +NADH + H + CO2

α-ketoglutarate + NAD+ + CoA-SH α-ketoglutarate dehydrogenase complex

+ Succinyl –CoA + CO2 + NADH + H

Reaction 5: Hydrolysis of Succinyl-CoA, Synthesis of ATP Succinyl-CoA in the presence of ADP & inorganic phosphate (Pi) is converted into succinate +ATP + CoA-SH. This is the only reaction in the citric acid cycle include the generation of ATP at substrate-level. This reversible reaction is catalyzed by succinate thiokinase enzyme in the presence of Mg++. Succinate thiokinase also share in gluconeogenesis

Succinyl-CoA + Pi + ADP

Mg++ Succinate thiokinase

Succinate + ATP +CoA-SH

Reaction 6: Dehydrogenation Dehydrogenation of succinate into fumarate by transfer of hydrogen directly from substrate into a flavoprotein it is the only dehydrogenation reaction in the cycle involving direct transfer of hydrogen from substrate into a flavoprotein, without participation of NAD, this reversible reaction is catalyzed by succinate dehydrogenase enzyme which is inhibited by malonate resulting in succinate accumulation.

Succinate+FAD Fumarate+FADH2 Succinate dehydrogenase

23 Reaction 7: Hydration Hydration of the double bond of fumarate to form L-Malate. This reversible reaction is catalyzed by fumarase (fumarate hydratase) enzyme.

Fumarate + H2O L-Malate Fumarase

Reaction 8: Dehydrogenation This is the final reaction of the citric acid cycle where dehydrogenation of malate in the presence of NAD+ into oxaloacetate + NADH + H+ .This reversible reaction is catalyzed by the enzyme Malate dehydrogenase. The oxaloacetate produced in this reaction condense with acetyl-CoA (Reaction 1: Synthesis of Citrate) & so the cycle continue again.

L-Malate +NAD+

Malate dehydrogenase

Oxaloacetate + NADH + H+

24 Energetic of Citric Acid Cycle + Reaction 3-produce oneNADH2 molecule which enter the respiratory chain producing 2.5 ATP molecules . + Reaction 4- produce oneNADH2 molecule which enter the respiratory chain producing 2.5 ATP molecules . Reaction 5-produce 1 ATP molecule. + Reaction 6-produce one FADH2 molecule which enter the respiratory chain producing 1.5 ATP molecules. + Reaction 8- produce oneNADH2 molecule which enter the respiratory chain producing 2.5 ATP molecules. Therefore , 10 ATP molecules are produced for each cycle from one molecule of acetyl-CoA. Energy produced of one molecule of glucose under aerobic conditions when enter citric acid cycle is as follow: 7 ATP molecules up to pyruvate synthesis. + +5 ATP molecules generated by two NADH2 molecules produced from conversion of two molecules of pyruvate into two molecules of acetyl-CoA / one molecule of glucose +20 ATP molecules from entrance of two molecules of acetyl- CoA / one molecule of glucose to citric acid cycle. Therefore , 32 ATP molecules are produced.

Regulation of Citric Acid Cycle 1-Proper function of respiratory chain which depend mainly on the availability of oxygen , ADP & NAD+. 2-Regulatory reactions ((irreversible or nearly irreversible)) in the cycle which are: Reaction 1: Synthesis of Citrate. Reaction 3: Dehydrogenation & Decarboxylation. Reaction 4:Oxidation&Decarboxylation (Oxidative decarboxylation).

Role of Vitamins in Citric Acid Cycle Vitamins play a key role in the citric acid cycle. (1) Riboflavin, in the form of flavin adenine dinucleotide (FAD) in reaction 6 (Dehydrogenation). (2) Niacin, in the form of nicotinamide adenine dinucleotide (NAD) in three dehydrogenation reactions in the cycle (Reactions 3, 4&8). (3) Thiamin, as thiamin diphosphate, the coenzyme for α-ketoglutarate dehydrogenase reaction. (4) Pantothenic acid, as part of coenzyme A.

26 Clinical Aspects The few genetic defects of citric acid cycle enzymes that have been reported are associated with severe neurological damage as a result of very considerably impaired ATP formation in the central nervous system.

27 Glycogen metabolism Glycogen is the major carbohydrate storage in animals including human, it present mainly in: 1-liver: glycogen represent up to 5% of liver weight, its concern with maintenance of blood glucose level between the meals. After 12-18 hours of fasting the liver glycogen is almost totally depleted. 2-Muscle: glycogen represent up to 0.7% of muscle weight but because of great muscle mass its about 3-4 times to that of liver , its concern as a source of glucose for glycolysis within the muscle itself. Metabolisms of glycogen include synthesis (glycogenesis) & degradation (glycogenolysis); these two processes are not simply the reversal of one series of reactions but they are separated reactions. Glycogenesis Glycogenesis is the process of glycogen synthesis, in which glucose molecules are added to chains of glycogen. Glycogen synthesis depends on the demand for glucose and ATP (energy). If both are present in relatively high amounts, then the excess of insulin promotes the glucose conversion into glycogen for storage in liver and muscle cells.

Reaction 1 Glucose is phosphorylated to glucose 6-phosphate in irreversible reaction catalyzed by hexokinase in muscle & glucokinase in liver in the presence of magnesium ion.

++ Mg Glucose+ATP Hexokinase Glucose 6-P+ADP Glucokinase

28 Reaction 2 Glucose 6-phosphate is isomerized to glucose 1-phosphate in a reversible reaction catalyzed by phosphoglucomutase enzyme in the presence of magnesium ion.

Mg++ Glucose6-p Glucose1-p phosphoglucomutase

Reaction 3 Glucose 1-phosphate reacts with uridine triphosphate (UTP) to form the active nucleotide uridine diphosphate glucose (UDPGlc) & inorganic pyrophosphate (PPi); this reaction is catalyzed by UDPGlc Pyrophosphorylase enzyme.

UDPGlc UTP+Glucose1-p UDPGlc+PPi Pyrophosphorylase

Although this reaction is reversible but subsequent hydrolysis of PPi pull the reaction to right. Reaction 4 Glycogen synthase catalyzes the formation of a glucosidic bond between C1 of the activated glucose of UDPGlc and C4 of a terminal glucose residue of glycogen primer producing 1 4 Glucosyl units with liberation of uridine diphosphate (UDP). Therefore the preexisting glycogen primer must be present to initiate this reaction. This glycogen primer may in turn be formed on a primer known as glycogenin, which is a protein that is glycosylated on a specific tyrosine residue by UDPGlc. This reaction is irreversible.

29 UDPGlc +Glycogenin

Glycogen primer

Glycogen synthetase + UDPGlc

UDP + 1 4 Glucosyl units

Reaction 5 In reaction 4 the branches of the glycogen (tree) become elongated as successive 1, 4 linkages occur & when the chain has been lengthened to at least11 glucose residue a second enzyme called branching enzyme transfer a part of 1,4-chain (minimum 6 glucose residues) to a neighboring chain to form 1,6 -linkage thus establishing a branch point in the molecule then the branches grow by further addition of 1 4 glucosyl units producing glycogen. This reaction is irreversible.

1 4 Glucosyl units

Branching enzyme

1 4 &1 6 Glucosyl units ((Glycogen))

The rate limiting reaction of glycogenesis is that reaction catalyzed by glycogen synthase enzyme. The incorporation of 1 mol of glucose into glycogen requires two high energy phosphates (ATP in reaction 1 & UTP in reaction 3).

31 Glycogenolysis Reaction 1 Glycogenolysis is initiated by the reaction catalyzed by glycogen phosphorylase enzyme which cause phosphoroylytic cleavage of 1,4 link of glycogen until approximately 4 glucose residues remain on either side of 1,6-branch where another enzyme called (α-[1 4] α-[1 4] glucan transferase) transfers a trisaccharide unit from one branch to the other, exposing the 1 6 branch point. Hydrolysis of the 1 6 linkages requires the debranching enzyme. Further phosphorylase action can then proceed. The rate limiting reaction in glycolysis is that catalyzed by phosphorylase enzyme. Therefore, the combined action of irreversible reactions catalyzed by these three enzymes convert glycogen to glucose 1- phosphate.

Reaction 2 Glucose 1-phosphate is converted to glucose 6-phosphate by reversible reaction catalyzed by phosphoglucomutase enzyme in the presence of magnesium ion. Mg++ Glucose1-p Glucose6-p phosphoglucomutase

Reaction 3 This irreversible reaction occurs only in the liver & kidney (not in muscle) to remove phosphate from glucose 6-phosphate enabling glucose to diffuse from the cell to blood, this reaction is catalyzed by glucose 6-phosphatase enzyme.

Glucose 6-phosphatase Glucose6-p+H2O Glucose + Pi

33 Regulation of Glycogen Metabolism

Glycogenesis Glycogenesis is controlled by glycogen synthetase enzyme, there are two forms of glycogen synthetase which are: 1- Synthetase-a:-Unphosphorylated & most active form. 2- Synthetase-b: - Phosphorylated & inactive. Synthetase-a is converted to synthetase-b (inactivation) by a phosphorylation reaction catalyzed by protein kinase enzyme while conversion of synthetase-b to synthetase-a (activation) by dephosphorylation reaction catalyzed by protein phosphatase enzyme which is under the positive control of glucose-6- phosphate & negative control of cAMP. Two forms of the protein kinases are present which are:- A-Ca++ dependent protein kinase: Stimulated by calcium. B-cAMP-dependent protein kinase: Stimulated by cAMP.

34 Glycogenolysis Glycogenolysis is controlled by glycogen phosphorylase enzyme there are two forms of glycogen phosphorylase which are:- 1-Phosphorylase-a: Phosphorylated & active , inhibited by glucose 6-phosphate. 2-phosphorylase-b: Unphosphorylated & inactive. Conversion from a to b forms (inactivation) require dephosphorylation reaction catalyzed by protein phosphatase enzyme while conversion of b to a forms (activation) require phosphorylation reaction catalyzed by phosphorylase kinase enzyme. Two forms of phosphorylase kinases are present which are:- A-cAMP-dependent phosphorylase kinase: Stimulated by cAMP. B-Ca-dependant phosphorylase kinase: Stimulated by calcium. In muscle glycogen phosphorylase enzyme is immunologically distinct form & it’s more sensitive to c-AMP. Therefore; cAMP which is stimulated by certain hormones(( as epinephrine in muscle & glucagon in liver)) inhibit glycogenesis synchronously with the activation of glycogenolysis through the following mechanism:- I)-cAMP inhibits glycogenesis by:- 1-Inhibition of protein phosphatase enzyme. 2-Stimulation of cAMP-dependent protein kinase enzyme. II)-cAMP stimulates glycogenolysis by:- 1-Inhibition of protein phosphatase enzyme. 2-Stimulation of cAMP-dependent phosphorylase kinase enzyme. Calcium ion which may follow muscle contraction inhibit glycogenesis synchronously with the activation of glycogenolysis through the following mechanism:- I)-Stimulate glycogenolysis through the activation of Ca- dependant phosphorylase kinase. II)- Inhibit glycogenesis through the activation of Ca dependent protein kinase. Insulin inhibit glycogenolysis synchronously with the activation of glycogenesis through the following mechanism:-

35 Insulin increasing cellular glucose uptake leads to elevation of glucose 6-phosphate which inhibit the activity of phosphorylase a enzyme in glycogenolysis together with stimulation protein phosphatase enzyme in glycogenesis .

Conclusion Inhibition of glycogenolysis enhances net glycogenesis & vice versa inhibition of glycogenesis enhances net glycogenolysis. This occurs by many factors as cAMP , calcium ion & insulin.

Glycogen Storage Diseases((Glycogenosis)) The term ((Glycogen Storage Disease)) is a generic term that describe a group of inherited disorders (Types) characterized by deposition of abnormal type or quantity of glycogen due to partial or complete absence of certain enzymes. Since glycogen molecules can become enormously large, an inability to degrade glycogen can cause cells to become pathologically engorged; it can also lead to the functional loss of glycogen as a source of cell energy and as a blood glucose buffer. The following table summarizes the types of glycogen storage diseases.

37 Hexose monophosphate shunt {phosphogluconate oxidative pathway, pentose phosphate pathway}

The pentose phosphate pathway is an alternative route for the metabolism of glucose. It does not generate ATP but has two major functions: (1) Formation of NADPH for reduction processes & for synthesis of fatty acids & steroids. (2) Synthesis of ribose 5-phosphate for nucleotide & nucleic acid formation.

Reactions of Pentose Phosphate Pathway The reactions of the pathway occur in the cytoplasm. The sequences of reactions are divided into two phases:- 1-Oxidative nonreversible phase. 2-Nonoxidative reversible phase.

Oxidative Nonreversible Phase Oxidation (dehydrogenation) of glucose 6-phosphate into ribulose 5-phosphate is achieved through the following steps:- Step 1 Dehydrogenation (oxidation) of glucose 6-phosphate into 6-phosphogluconate occurs via the formation of 6-phosphogluconolactone. -Dehydrogenation (oxidation) of glucose 6-phosphate into 6- phosphogluconolactone is catalyzed by glucose-6-phosphate dehydrogenase enzyme in the presence of NADP+ which is converted into NADPH + H+. The hydration of 6-phosphogluconolactone into 6-phosphogluconate is accomplished by the enzyme gluconolactone hydrolase. Both reactions of this step require cofactors which are calcium or magnesium ions.

38 Step 2 Dehydrogenation (oxidation) with decarboxylation of 6- Phosphogluconate into a ketopentose known as ribulose 5- phosphate. This reaction is catalyzed by the enzyme 6-phosphogluconate dehydrogenase in the presence of NADP+ which is converted into NADPH + H+. This reaction requires cofactors which are calcium or magnesium ions.

Nonoxidative Reversible Phase All reactions of this phase are reversible except that reaction catalyzed by fructose 1, 6-bisphosphatase enzyme which is irreversible. By this phase ribulose 5-phosphate is end in the formation of glucose 6-phosphate through the following steps. Step 1 Ribulose 5-phosphate is the substrate for two enzymes which are: - A-Ribulose 5-phosphate 3-epimerase alters the configuration about carbon 3, forming another ketopentose known as Xylulose 5-phosphate. B-Ribose 5-phosphate ketoisomerase converts ribulose 5- phosphate which is a ketopentose to the corresponding

39 aldopentose known as ribose 5-phosphate, which is the precursor of the ribose required for nucleotide & nucleic acid synthesis.

Step 2 Transketolase enzyme catalyze the conversion of a ketose sugar into an aldose with two carbons less & simultaneously converts an aldose sugar into a ketose with two carbons more. 2+ The reaction requires Mg & thiamin diphosphate (vitamin B1) as coenzyme. Therefore, transketolase catalyzes the transfer of the two-carbon unit from the five-carbon ketose (xylulose 5-phosphate) to the five- carbon aldose (ribose 5-phosphate), producing the seven-carbon ketose (sedoheptulose 7-phosphate) & the three-carbon aldose (glyceraldehyde 3-phosphate).

Step 3 Transaldolase allows the transfer of three-carbons (carbons1-3) from the ketose (sedoheptulose 7-phosphate) onto the aldose (glyceraldehyde 3-phosphate) to form the six-carbon ketose (fructose 6-phosphate) & the four-carbon aldose (erythrose 4- phosphate).

Step 4 Transketolase catalyze reaction that involve xylulose 5- phosphate to donates a two-carbon unit to erythrose 4-phosphate to form glyceraldehyde 3-phosphate & fructose 6-phosphate. 2+ The reaction requires Mg & thiamin diphosphate (vitamin B1) as coenzyme.

Step 5 .In order to oxidize glucose completely, there must be:- A- Conversion of two molecules of glyceraldehyde 3-phosphate into one molecule of glucose 6-phosphate .This involves reversal of glycolysis in addition to the enzyme known as fructose 1, 6- bisphosphatase. In tissues that lack fructose 1, 6-bisphosphatase, glyceraldehyde 3-phosphate follows the normal pathway of glycolysis to pyruvate.

B-Conversion of fructose 6-phosphate to glucose 6-phosphate by phosphohexose isomerase. Therefore, the net result of pentose phosphate pathway is that three molecules of glucose 6-phosphate give rise to three molecules of CO2 , two molecules of glucose 6-phosphate & one molecule of glyceraldehyde 3-phosphate. Since two molecules of glyceraldehyde 3-phosphate can regenerate glucose 6-phosphate, the pathway can account for the complete oxidation of glucose. The rate limiting step of pentose phosphate pathway is mainly controlled by glucose-6-phosphate dehydrogenase enzyme.

Although glucose 6-phosphate is common to both pentose phosphate pathway & glycolysis, the pentose phosphate pathway is markedly different from glycolysis by:- 1-Oxidation utilizes NADP rather than NAD. 2-CO2, which is not produced in glycolysis, is a characteristic product of pentose phosphate pathway. 3- No ATP is generated in the pentose phosphate pathway, whereas ATP is a major product of glycolysis.

44 Reductive function of NADPH The main reductive function of NADPH is found in erythrocytes, in which NADPH +H+ share in the reduction of oxidized glutathione in a reaction catalyzed by glutathione reductase enzyme. The reduced glutathione remove H2O2 in a reaction catalyzed by glutathione peroxidase enzyme to be converted back into oxidized form. This reaction is important, since H2O2 decrease the life span of the erythrocyte by causing oxidative damage to the cell leading to hemolysis.

Clinical Aspect of Pentose Phosphate Pathway Glucose-6-Phosphate Dehydrogenase Deficiency ((G6PD Deficiency)) or Favism :- It’s the main clinical aspect of pentose phosphate pathway in which there is a genetic deficiency of glucose-6-phosphate dehydrogenase enzyme with consequent impairment of the generation of NADPH + H+ ,this disease is common in populations of Mediterranean & Afro-Caribbean origin. The defect is manifested as red cell hemolysis (hemolytic anemia) when susceptible individuals are subjected to oxidants that contains H2O2 because glutathione peroxidase which remove H2O2 is dependent upon a supply of NADPH, which in erythrocytes can be formed only via the pentose phosphate pathway. These oxidants that contains H2O2 include mainly drugs as primaquine, aspirin, sulfonamides or when they have eaten fava beans[hence the term favism].

45 Uronic Acid Pathway Uronic acid pathway is an alternative pathway for glucose metabolism, but like the pentose pathway it does not lead to the generation of ATP. Uronic acid pathway catalyzes the conversion of glucose into glucuronic acid, pentoses& in animal ascorbic acid. -Glucuronic acid (glucuronate) is important in: A-Incorporated into proteoglycans. B- Steroid hormones, bilirubin & number of drugs are conjugated with glucuronate to be excreted in urine or bile. -Pentoses produced from uronic acid pathway mainly xylulose 5- phosphate which enter the pentose phosphate pathway. -Vitamin C is produced in animal from uronic acid pathway while in human & other primates can not synthesized due to lack of certain enzyme involved in this synthesis.

Metabolism of other hexoses They include mainly fructose & galactose metabolism.

Fructose Metabolism Fructose enters the liver from the small intestine through the hepatic portal vein. Inside the body ((mainly in the liver)) fructose is converted mainly into glucose or to a less extent into the intermediates of glycolysis & in minor amounts is converted into fatty acids ((increase in excess fructose intake)), this process occurs through the following pathways:- Major pathway: - It’s the main pathway for fructose metabolism, it occurs in the liver & to less extent in the kidney &intestine through the following steps:- (1)- Pathway is initiated by a specific kinase enzyme present in the liver & to less extent in the kidney &intestine known as fructokinase, which catalyzes the irreversible phosphorylation reaction of fructose to fructose 1-phosphate in the presence of ATP which is converted into ADP. This enzyme (fructokinase) not acts on glucose & unlike glucokinase; its activity is not affected by

46 fasting or by insulin, which may explain why fructose is cleared from the blood of diabetics at a normal rate. (2)-Fructose 1-phosphate is cleaved into glyceraldehyde & dihydroxyacetone phosphate by reversible reaction catalyzed by hepatic fructose 1-phosphate aldolase enzyme. (3)-Phosphorylation of glyceraldehyde to glyceraldehyde 3- phosphate by irreversible reaction catalyzed by triokinase enzyme in the presence of ATP which is converted into ADP. (4)-The two triose phosphates ((dihydroxyacetone phosphate & glyceraldehyde 3-phosphate)), may be:- A-Substrates for aldolase & hence gluconeogenesis, which is the fate of much of the fructose metabolized in the liver. B-Degraded by glycolysis. Fructose undergoes more rapid glycolysis in the liver than does glucose because it bypasses the regulatory step of glycolysis catalyzed by phosphofructokinase enzyme.

Minor pathway: - In extrahepatic tissues, hexokinase enzyme catalyzes the phosphorylation of fructose into fructose 6- phosphate which enter glycolysis pathway. However, glucose inhibits this phosphorylation of fructose since it’s a better substrate for hexokinase; therefore, this pathway is consider as a minor pathway.

Note:- Minor amount of pyruvate ((produced from the entering of fructose metabolites into the glycolysis)) is converted into fatty acids, this conversion is increase by excess fructose intake.

Clinical Aspects of Fructose Metabolism 1) - Loading of the liver with fructose: - as occur with intravenous infusion or following very high fructose intakes, may cause the followings:- A-Hyperlipidemia because excess fructose in the liver increases fatty acid synthesis & so VLDL secretion, leading to increased LDL cholesterol which can be regarded as potentially atherogenic.

48 B-Hyperuricemia because excess fructose intakes, causes sequestration of inorganic phosphate in fructose 1-phosphate & so diminished ATP. As a result there is less inhibition of denovo purine synthesis by ATP & so uric acid formation is increased, causing hyperuricemia, which is a cause of gout. 2)-Essential fructosuria: - Inborn error of metabolism due to lack of hepatic fructokinase, the condition is benign. 3)-Hereditary fructose intolerance:- Inborn error of metabolism due to absence of hepatic fructose 1-phosphate aldolase lead to the following presentations that usually occur during infancy when baby start to eat fructose containing diets. A- Fructose-induced hypoglycemia because accumulation of fructose 1-phosphate inhibits the activity of liver phosphorylase causing hypoglycemia despite the presence of high glycogen reserves, this especially evident after fructose administration. B-Hyperuricemia because sequestration of inorganic phosphate by the accumulated fructose 1-phosphate leads to depletion of ATP ,therefore, there is less inhibition of denovo purine synthesis by ATP & so uric acid formation is increased causing hyperuricemia which is a cause of gout. C-Liver impairment due to the accumulation of fructose 1- phosphate in the liver. D-Failure to thrive. Diagnosis by detecting fructose in urine after administration of fructose, more precise diagnosis by measurement of hepatic fructose 1-phosphate aldolase activity. Treatment by diets low in fructose, sorbitol & sucrose (because they converted into fructose).

Galactose Metabolism Galactose is derived from the intestinal hydrolysis of the disaccharide known as lactose which is the sugar of milk. It’s readily converted in the liver to glucose by the following steps:- (1)-Galactose is phosphorylated to galactose 1-phosphate in the presence of ATP which is converted into ADP in the irreversible reaction catalyzed by galactokinase enzyme, this reaction need magnesium ion as a cofactor.

49 (2)-Galactose 1-phosphate reacts with uridine diphosphate glucose (UDPGlc) to form uridine diphosphate galactose (UDPGal) & glucose 1-phosphate, in the reversible reaction catalyzed by galactose 1-phosphate uridyl transferase enzyme. (3)-The conversion of UDPGal to UDPGlc is by reversible reaction catalyzed by UDPGal 4-epimerase enzyme. (4)-Finally, glucose is liberated from UDPGlc after it enters the glycogenesis pathway followed by the glycogenolysis. Note:- Since the epimerase reaction is reversible. Therefore; glucose can be converted to galactose, so that galactose is not a dietary essential. However, galactose is required in the body not only in the formation of lactose but also as a constituent of glycolipids, proteoglycans & glycoproteins.

Clinical Aspect of Galactose Metabolism Galactosemia: - Inborn error of metabolism characterize by the inability to metabolize galactose , its caused mainly by inherited defects of galactose 1-phosphate uridyl transferase & to less extent by defect in galactokinase or UDPGal 4-epimerase, the clinical features of galactosemia usually start to appear after the

51 baby start sucking milk which contain lactose, these clinical features include mainly:- 1-Cataract:-The excess galactose concentration in the eye is reduced to galacticol, which accumulates, causing cataract due to its osmotic effect. 2-Failure to thrive. 3-Liver impairment:-especially in galactose 1-phosphate uridyl transferase deficiency because of the accumulation of galactose 1- phosphate in the liver. 4-Hypoglycemia. Diagnosis by detection of galactose in urine. Treatment by galactose-free diets. As the UDPGal 4-epimerase is present in adequate amounts in most cases of galactosemia, the galactosemic individual can still form UDPGal from glucose so that normal growth & development can occur regardless of these galactose-free diets.

51 Gluconeogenesis Gluconeogenesis is the term used to include all the pathways responsible for converting noncarbohydrate precursors to glucose or glycogen. These noncarbohydrate precursors include glucogenic amino acids, lactate & glycerol. Liver & kidney are the major gluconeogenic tissues.

Importance of gluconeogenesis 1-Meets the needs of the body for glucose which is important in supplying energy especially for the nervous system& erythrocytes. 2- Clears lactate produced by the muscles & erythrocytes. 3- Clears glycerol produced by adipose tissue. Therefore, failure of gluconeogenesis is usually fatal.

Pathways of gluconeogenesis The pathway of gluconeogenesis involves reversal of glycolysis, the citric acid cycle & some special reactions. Three irreversible reactions of glycolysis ((catalyzed by hexokinase, phosphofructokinase & pyruvate kinase enzymes)) prevent simple reversal of glycolysis for glucose syntheses by gluconeogenesis.They are circumvented as follows:- (1)- The conversion of pyruvate into phosphoenolpyruvate , to achieve a reversal of glycolysis is through the following steps: A)-Mitochondrial pyruvate carboxylase enzyme catalyzes the carboxylation of pyruvate ((present in the mitochondria)) to oxaloacetate, this irreversible reaction require ATP in the presence of the vitamin biotin as a coenzyme. B)-Oxaloacetate does not cross the mitochondrial inner membrane; it should be converted to malate inside the mitochondria by Krebs cycle in reaction catalyzed by malate dehydrogenase enzyme, then malate is transported into the cytosol & in the cytosol is converted back to oxaloacetate by malate dehydrogenase enzyme. C)-Phosphoenolpyruvate carboxykinase enzyme catalyzes the decarboxylation & phosphorylation of oxaloacetate which present in the cytoplasm to phosphoenolpyruvate ,this irreversible

52 reaction require GTP (Guanosine triphosphate) as the phosphate donor which is converted into GDP (Guanosine diphosphate). (2)-The conversion of fructose 1, 6-bisphosphate to fructose 6- phosphate, to achieve a reversal of glycolysis, is catalyzed by fructose- 1, 6-bisphosphatase enzyme which present in the liver, kidney & skeletal muscle but is probably absent from heart & smooth muscle. (3)-The conversion of glucose 6-phosphate to glucose, to achieve a reversal of glycolysis, is catalyzed by glucose-6- phosphatase enzyme which present in the liver & kidney but absent from muscle & adipose tissue, which, therefore, cannot export glucose into the bloodstream. Therefore by reversal of glycolysis & by citric acid cycle as described above glucose can be formed from the following noncarbohydrate precursors:- 1- Glucogenic amino acids after transamination or deamination of these amino acids they yield either pyruvate or intermediates of the citric acid cycle as α-ketoglutarate , oxaloacetate or fumarate which enters the gluconeogenic pathway. 2-Lactate by a reaction catalyzed by lactate dehydrogenase enzyme is converted into pyruvate which enters the gluconeogenic pathway. 3-Glycerol which is converted into dihydroxyacetone phosphate which enters gluconeogenesis through the reverse of glycolysis, the conversion of glycerol into dihydroxyacetone phosphate occurs through the following steps:- Step 1:-Glycerol is released from adipose tissue as a result of lipolysis & only tissues such as liver & kidney that possess glycerol kinase enzyme that catalyzes the irreversible conversion of glycerol to glycerol 3-phosphate in the presence of ATP which is converted into ADP.

Glycerol kinase Glycerol + ATP Glycerol 3-phosphate + ADP

Step 2:-Glycerol 3-phosphate is oxidized into dihydroxyacetone phosphate by reversible reaction catalyzed by glycerol 3-

53 phosphate dehydrogenase enzyme in the presence of NAD+ which is converted into NADH + H+.

Glycerol 3-phosphate + NAD+

Glycerol 3-phosphate dehydrogenase

Dihydroxyacetone phosphate + NADH + H+

Note: - Gluconeogenesis pathway requires energy which is derived mainly from fatty acid oxidation.

Regulation of Gluconeogenesis 1-Pyruvate carboxylase enzyme in gluconeogenesis requires acetyl-CoA as an activator & at the same time acetyl-CoA inhibit pyruvate dehydrogenase complex which convert pyruvate into acetyl-CoA to enter citric acid cycle. Because acetyl-CoA is derived also from the oxidation of fatty acids, this explains the action of fatty acid oxidation in sparing the entering of pyruvate to the citric acid cycle & in stimulating gluconeogenesis.

2-Fructose-2, 6-bisphosphate is most potent positive stimulator of glycolysis ((through its stimulation of phosphofructokinase)) & inhibitor of gluconeogenesis ((through its inhibition of fructose 1, 6 bisphosphatase)). Fructose 2, 6-bisphosphate is formed by phosphorylation of fructose 6-phosphate by phosphofructokinase-2 enzyme which is under the positive control of fructose 6-phosphate Therefore, when glucose is abundant it elevate the concentration of fructose 6-phosphate which increase the concentration of

56 fructose 2, 6-bisphosphate ((stimulate glycolysis & inhibit gluconeogenesis)) while when glucose is low the concentration of fructose 6-phosphate & fructose 2, 6-bisphosphate is reduced lead to stimulation of gluconeogenesis & inhibition of glycolysis. 3-Hormones that regulate gluconeogenesis acting through the enzymes that control the irreversible reactions of gluconeogenesis, these enzymes are pyruvate carboxylase, Phosphoenolpyruvate carboxykinase, fructose- 1, 6-bisphosphatase& glucose-6- phosphatase. These hormones are the following:- A-Glucocorticoids, glucagon & epinephrine stimulate gluconeogenesis through their induction of the enzymes that control the irreversible reactions of gluconeogenesis. B-Insulin inhibits gluconeogenesis through its repression of the enzymes that control the irreversible reactions of gluconeogenesis.

Hormonal Control of Carbohydrate Metabolism 1-Glycolysis is stimulated by insulin & is inhibited by glucagon & epinephrine. 2-Glycogenesis is stimulated by insulin & is inhibited by glucagon & epinephrine. 3-Glycogenolysis is stimulated by glucagon & epinephrine & is inhibited by insulin. 4-Gluconeogenesis is stimulated by glucocorticoids, glucagon & epinephrine & is inhibited by insulin.

Blood Glucose Level The concentration of blood glucose is regulated within narrow limits ranging from 3.3 mmol/L ((60 mg/dL)) in starvation up to 7.2 mmol/L ((130 mg/dL)) after the ingestion of a carbohydrate meal. A sudden decrease in blood glucose will cause convulsions due to the immediate brain dependence on a supply of glucose. However, much lower concentration can be tolerated, provided progressive adaptation is allowed by gluconeogenesis & ketone bodies formation.

57 Sources of Blood Glucose I ))-Diet:- The digestible dietary carbohydrates yield glucose, galactose& fructose that are transported to the liver via the hepatic portal vein. Galactose & fructose are readily converted to glucose in the liver . II ))-Gluconeogenesis:- glucose is formed from the following two groups of compounds that undergo gluconeogenesis . (A) Compounds that involved a direct conversion to glucose within the liver including most of glucogenic amino acids & glycerol. (B) Compounds which are the products of the metabolism of glucose in the tissues as lactate & the amino acid alanine through the following cycles: 1-Lactic Acid (Cori) Cycle:- Lactate which formed by the glycolysis in the skeletal muscles & erythrocytes, is transported to the liver & kidney via circulation where it reforms glucose by gluconeogenesis, this formed glucose reach skeletal muscle & erythrocytes through the circulation again to become available for glycolysis & so on the cycle continue again. This process is known as the Cori cycle or lactic acid cycle. 2- Glucose-Alanine Cycle:- Most important amino acid transported via circulation from skeletal muscle to the liver during fasting state is alanine which forms by transamination of pyruvate. In the liver alanine is converted into glucose by gluconeogenesis, this formed glucose reach skeletal muscle through the circulation again to become available for glycolysis & alanine formation so that the cycle continue again. This process is known as the glucose-alanine cycle. III ))-Glycogenolysis:- Another source of blood glucose is from glycogenolysis of the liver glycogen.

Glucosuria Normally glucose is continuously filtered by the glomeruli but its completely reabsorbed in the renal tubules; therefore, normally there is no glucose in urine. This happen when venous blood glucose concentration is below the renal threshold for glucose {171-180 mg/dl (9.5-10.0 mmol/L) } The presence of glucose in urine (glucosuria) suggest: 1-Hyperglycemia when venous blood glucose concentration exceeds the renal threshold for glucose as occurs in diabetes mellitus. 2- Reduction of renal threshold for glucose as occurs in: A-Renal glucosuria which is harmless condition with no obvious cause for it. B-During pregnancy which is due to hypervolemia that occur during pregnancy.

59 Diabetes Mellitus Diabetes mellitus is a family of disorders that is characterized by hyperglycemia. The disorders of diabetes differ in their etiology , symptoms & in the consequences of disease. In Mosul, more than 10% of the population suffers from diabetes. Classification of Diabetes Mellitus Diabetes have been classified into four forms which are:- -Type 1 diabetes mellitus. -Type 2 diabetes mellitus. -Gestational diabetes mellitus. -Other specific types of diabetes mellitus.

Type1 Diabetes Mellitus Type 1 diabetes usually represents about 5-10% of diabetes & it is due to lack of insulin production & secretion by the beta cells of the pancreas. Type 1 diabetes manifests itself usually during childhood & adolescents. Treatment by insulin replacement, diet management & exercise . Type 1 diabetes is subclassified into:- A-Immune mediated : represent the common form of type 1 diabetes in which there is an autoimmune destruction of the beta cells of the pancreas by autoantibodies leading to absolute insulin deficiency. There is a genetic susceptibility for the development of these autoantibodies, with certain histocompatibility antigens predominant (HLA-DR3 &DR4) . However, the development of disease is complex; triggering factors, such as rubella, mumps, & other viral infection & chemical contact may be necessary for progression of disease. B-Idiopathic: represent the rare form of type 1 diabetes in which there is no obvious cause for the development of disease.

61 Type 2 Diabetes Mellitus It forms the most common type of diabetes & it is either due to the body does not produce enough insulin or reduction of cellular effects of insulin ((insulin resistance)). The etiology of type 2 diabetes is polygenic which means that both hereditary & environmental factors (obesity, lack of physical activity & certain racial groups) are important for its appearance. Other factors important in the development of disease are previous history of gestational diabetes, increasing age, dyslipidemia & hypertension. Type 2 diabetes usually affects obese people older than 40 years. Treatment usually by weight reduction, diet management & oral hypoglycemic drugs. Insulin may be prescribed for type 2 diabetics who fail to achieve glycemic control with other measures.

Gestational Diabetes Mellitus (GDM) It’s defined as diabetes that is diagnosed first time during pregnancy , it affects about 4% of all pregnant women. During pregnancy there is reduction of cellular effects of insulin ((insulin resistance)). Most pregnant women will compensate with increased secretion of insulin; those individuals who are unable to compensate may develop gestational diabetes . The hyperglycemia of gestational diabetes diminishes after delivery; however, the individual who has developed gestational diabetes is at higher risk for the development of type 2 diabetes thereafter specially those showing autoantibodies at the time of delivery.

Other Specific Types of Diabetes It’s previously called secondary diabetes. These specific types include mainly:-TABLE 4-1 -Genetic defects of beta cell function. -Genetic defects in insulin action. -Diseases of the exocrine pancreas such as cystic fibrosis. -Endocrinopathies such as Cushing’s syndrome.

61 -Drug or chemical-induced such as glucocorticoids. -Infections. -Uncommon forms of immune-mediated diabetes.

Impaired Glucose Tolerance (Impaired Fasting Glucose, Prediabetes) Impaired glucose tolerance represents a blood glucose levels are higher than normal but not high enough to be characterized as overt diabetes (borderline stage). Persons with impaired glucose tolerance have a higher risk for macroangiopathy & the cardiovascular mortality than those with normal person. 20-30% of people with impaired glucose tolerance will develop clinically overt diabetes mellitus within 10 years. Therefore, persons with impaired glucose tolerance need follow-up & weight reduction.

Diagnosis of Diabetes Mellitus Usually by measuring plasma glucose as follow:-

1-Fasting plasma glucose test:- Measures plasma glucose after at least 8 hours without eating. This test is used to detect diabetes or pre-diabetes as follow:-

Fasting Plasma Glucose Result (mg/dL) Diagnosis

70-99 Normal

Pre-diabetes 100 to 125 (impaired fasting glucose)

126 and above Diabetes

2-Oral glucose tolerance test (OGTT):- Measures plasma glucose after at least 8 hours without eating & 2 hours after drink a liquid containing 75 gm glucose. This test is more precise in diagnose diabetes or especially pre-diabetes as follow:-

62 2-Hour Plasma Glucose Result (mg/dL) Diagnosis

139 and below Normal

Pre-diabetes 140 to 199 (impaired glucose tolerance)

200 and above Diabetes

In gestational diabetes it’s diagnosed by a special form of OGTT in which measures plasma glucose after at least 8 hours without eating then we measures plasma glucose 1 hour , 2 hours & 3 hours after drink a liquid containing 100 gm glucose. If plasma glucose levels in at least two of four reading reach the value found in following table, it means that the pregnant women have gestational diabetes.

When Plasma Glucose Result (mg/dL)

95 or higher Fasting

At 1 hour 180 or higher

At 2 hours 155 or higher

At 3 hours 140 or higher

Long Term Complications of Diabetes Mellitus 1- Microangiopathy resulting in the development of retinopathy, nephropathy & neuropathy. 2-Macroangiopathy resulting in atherosclerosis & coronary heart disease.

HOMA-IR stands for Homeostatic Model Assessment of Insulin Resistance. The meaningful part of the acronym is “insulin resistance”. It marks for both the presence and extent of any insulin resistance that you might currently express. It is a terrific way to reveal the dynamic between your

63 baseline (fasting) blood sugar and the responsive hormone insulin. See The Blood Code book for further insight about your result.

Healthy Range: 1.0 (0.5–1.4) Less than 1.0 means you are insulin-sensitive which is optimal. Above 1.9 indicates early insulin resistance. Above 2.9 indicates significant insulin resistance.

HOMA-IR Blood Code Calculation Insulin Glucose 0 0 0 uIU/mL (mU/L) X (mg/dL) = HOMA-IR

= 0 HOMA-IR The HOMA-IR calculation requires U.S. standard units. To convert from international S.I. units: Insulin: pmol/L to uIU/mL, divide by (÷) 6 Glucose: mmol/L to mg/dL, multiply by (x) 18