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Home , Lactase

Ahmed Ali Hussein BVMS, PhD Scientific degree (Lecture ), Department of Physiology, , and Pharmacology College of Veterinary Medicine, University of Mosul, Mosul, Iraq https://orcid.org/ 0000-0001-7680-4791 https://www.researchgate.net/profile/ Ahmed Hussein Carbohydrate | Part I | 2nd year 2019

Carbohydrates Carbohydrates are the most abundant organic molecules in nature. The empiric formula for many of the simpler carbohydrates is (CH2O)n , hence the name “hydrate of carbon.” Example: (C6 H12O6).

Monosaccharides (simple ) containing an aldehyde group are called aldoses and those with a keto group are called ketoses.

Examples of an aldose (A) and a ketose (B) .

Metabolism:- which is the sum of all the chemical changes occurring in a cell, a tissue, or the body.

Most pathways can be classified as either catabolic (degradative) or anabolic (synthetic). Catabolic reactions break down complex molecules, such as proteins, polysaccharides, and lipids, to a few simple molecules, for example, CO2, NH3 (ammonia), and water. Anabolic pathways form complex end products from simple molecules, for example, the synthesis of the polysaccharide, glycogen, from glucose.

Biochemistry | | Dr. Ahmed Ali Hussein Page | 1 Functions of carbohydrates :-

They have a wide range of functions, including providing a significant fraction of the energy in the diet of most organisms, acting as a storage form of energy in the body, and serving as components that mediate some forms of intercellular communication.

Carbohydrates also serve as a structural component of many organisms, including the cell walls of bacteria, the exoskeleton of many insects, and the fibrous cellulose of plants.

Classification and Structure of Carbohydrates :-

1. Monosaccharides .

2. .

3. Oligosaccharides

4. Polysaccharides.

1. Monosaccharides :-

Monosaccharides (simple sugars) can be classified according to the number of carbon atoms they contain. Examples of some monosaccharides commonly found in humans.

2. Disaccharides.

Disaccharides contain two monosaccharide units;

Important disaccharides include

( + glucose),  sucrose (glucose + ),  maltose (glucose + glucose).

Biochemistry | Carbohydrate Metabolism | Dr. Ahmed Ali Hussein Page | 2

A glycosidic bond between two hexoses producing a .

3. Oligosaccharides contain from three to about ten monosaccharide units,

4. Polysaccharides.

Polysaccharides contain more than ten monosaccharide units, and can be hundreds of sugar units in length.

Important polysaccharides include branched glycogen (from animal sources) and starch (plant sources) and unbranched cellulose (plant sources); each is a polymer of glucose. The bonds that link sugars are called glycosidic bonds.

Biochemistry | Carbohydrate Metabolism | Dr. Ahmed Ali Hussein Page | 3 and absorption of Carbohydrates :-

The mechanical and chemical digestion of carbohydrates begins in the mouth. Chewing, also known as mastication, crumbles the carbohydrate foods into smaller pieces. The salivary glands in the oral cavity secrete saliva that coats the food particles. Saliva contains the , salivary . This enzyme breaks the bonds between the monomeric sugar units of disaccharides, oligosaccharides, and starches. The salivary amylase breaks down amylose and amylopectin into smaller chains of glucose, called dextrins and maltose. Amylase is sensitive to pH and thus is inhibited in the acidic environment of the stomach.

The pancreas releases pancreatic enzyme called pancreatic amylase, which starts again the breakdown of dextrins into shorter and shorter carbohydrate chains. This , known collectively as disaccharides, are sucrose, maltose and lactose. breaks sucrose into glucose and fructose molecules. breaks the bond between the two glucose units of maltose, and lactase breaks the bond between galactose and glucose. Once carbohydrates are chemically broken down into single sugar units they are then transported into inside of intestinal cells. Then the cells in the have membranes that contain many transport proteins in order to get the monosaccharides and other nutrients into the blood where they can be distributed to the rest of the body. The first organ to receive glucose, fructose and galactose is the liver. The liver takes them up and converted galactose to glucose, breaks fructose into even smaller carbon- containing unites, and either stores glucose as glycogen or exports it back to the blood.

Summary

Mouth :- Salivary amylase converts polysaccharide to smaller saccharides . Stomach :- Low PH stops action of salivary amylase. Duodenum :- Pancreatic amylase converts polysaccharide to maltose and isomaltose. Small intestine :- Disaccharides are digested and absorbed.

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 The three principle monosaccarides resulting from the digestion are :- Glucose, Fructose and Galactose.

Absorption: Once carbohydrates are digested, the products must be absorbed and transported to the portal circulation. Digestion and absorption are typically coupled. Glucose absorption occurs in the small intestine via the SGLT-1 transporter (sodium glucose co-transporter). Fructose absorption is completed via the GLUT5 transporter by facilitated diffusion. Glucose and galactose are actively transported from the small intestine lumen by the sodium glucose transporter (SGLT-1) located in the brush border of the small intestine.

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Major Pathways in carbohydrate metabolism

Glucose is oxidized by glycolysis, an energy-generating pathway that converts is to pyruvate. In the absence of oxygen, pyruvate is converted (reduced) to lactate, so it can allow the production of ATP in tissues that lack mitochondria example RBC or in exersizing skeletal muscules. When oxygen is present, pyruvate is further degraded to form acetyl-CoA. it occurs in cells with mitochondria and the end is pyruvate.

Significant amounts of energy in the form of ATP can be extracted from acetyl-CoA by the citric acid cycle and the electron transport system.

Excess glucose is converted to its storage form, glycogen, by glycogenesis. When glucose is needed as a source of energy or as a precursor molecule in biosynthetic processes, glycogen is degraded by glycogenolysis.

Glucose can be converted to ribose-5-phosphate (a component of nucleotides) and NADPH (a powerful reducing agent) by means of the pentose phosphate pathway.

 Metabolism of CHO can be subdivide as following :-

1- Glycolysis pathway .

2- Citric acid cycle . ( oxidation of pyruvate to Acetyl-CoA) .

3- Glycogenesis .

4- Glycogenolysis .

5- Gluconeogenesis .

6- Hexose monophosphate shunt .

Glycolysis pathway :-

Found in the cytosol of all mammalian cells (to provide energy) for the metabolism of glucose to pyruvate and lactate. Glycolysis also called Embden-Meyerhof Pathway.

Glycolysis is consists of 10 reactions, occurs in tow stages :

1. Glucose is phosphorylated twice and cleaved to form tow molecules of glyceraldehyde-3-phosphate (G-3-P). The tow ATP molecules consumed during this stage are like an investment, because this stage creates the actual substrates for oxidation in form that is trapped inside the cell.

2. Glyceraldehyde-3-phosphate is converted to pyruvate. Four ATP and tow NADH molecules are produced. Because tow ATP were consumed in stage 1, the net production of ATP per glucose molecule is 2.and (+2 NADH)

Biochemistry | Carbohydrate Metabolism | Dr. Ahmed Ali Hussein Page | 8 The glycolytic pathway can be summed up in the following equation :

+ + D-Glucose + 2 ADP + 2 Pi + 2 NAD 2 Pyruvate + 2 ATP + 2 NADH + 2H + 2 H2O

 Glycolysis is controlled by 3 enzymes catalyzing non- equilibrium reactions :

1- Hexokinase or glucokinase . (1)

2- Phosphofructokinase (PFK) . (3)

3- Pyruvate kinase . (10)

All these enzymes found in the cytoplasm.

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The reactions of the Glycolytic pathway

1. Synthesis of glucose-6-phosphate. Immediately after entering a cell, glucose and other sugar molecules are phosphorylated. Phosphorylation prevents transport of glucose out of the cell and increases the reactivity of the oxygen in the resulting phosphate ester. Hexokinases (glucokinase) catalyze the phosphorylation of hexoses in all cells in the body. ATP, a cosubstrate in the reaction, is complexed with Mg 2+ . The reaction is irreversible.

2. Conversion of glucose-6-phosphate to fructose-6-phosphate. During reaction 2 of glycolysis, the open chain form of the aldose glucose-6-phosphate is converted to the open chain form of the ketose fructose-6-phosphate by phosphoglucose (PGI) in a readily reversible reaction:

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3.The phosphorylation of fructose-6-phosphate. Phosphofructokinase-1 (PFK-1) irreversibly catalyzes the phosphorylation of fructose-6-phosphate to form fructose-1,6-bisphosphoate:

4. Cleavage of fructose-1,6-bisphosphate. Stage 1 of glycolysis ends with the cleavage of fructose- 1,6-bisphosphate into tow three-carbon molecules: glyceraldehydes-3-phosphate (G-3-P) and dihydroxyacetone phosphate (DHAP).the name of enzyme is aldolase.

5. The interconversion of glyceraldehyde-3-phosphate and dihydroxy-acetone phosphate. Of the tow products of the aldolase reaction, only G-3-P serves as a substrate for the next reaction in glycolysis. To prevent the loss of the other three-carbon unit from the glycolytic pathway, triose phosphate isomerase catalyzes the reversible conversion of DHAP to G-3-P: After this reaction, the original molecule of glucose has been converted to tow molecules of G-3-P.

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6. Oxidation of glyceraldehydes-3-phosphate. During reaction 6 of glycolysis, G-3-P undergoes oxidation and phosphorylation. The product, glycerate-1,3-bisphosphate, contains a high-energy phosphoanhydride bond, which may be used in the next reaction to generate ATP: This complex process is catalyzed by glyceraldehydes-3-phosphate dehydrogenase.

7. Phosphoryl group transfer. In this reaction ATP is synthesized as phosphoglycerate kinase catalyzes the transfer of the high-energy phosphoryl group of glycerate-1,3-bisphosphate to ADP:

8. Shift of the phosphate group from carbon 3 to carbon . The shift of the phosphate group from carbon 3 to carbon 2 of phosphoglycerate by phosphoglycerate mutase is freely reversible.

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9. Dehydration of 2-phosphoglycerate. Enolase catalyzes the dehydration of glycerate-2-phosphate to form phosphoenolpyruvate (PEP), the reaction is reversible:

10. Synthesis of pyruvate. In the final reaction of glycolysis, pyruvate kinase catalyzes the transfer of a phosphoryl group from PEP to ADP. Tow molecules ATP are formed for each molecule of glucose.

Reduction of pyruvate to lactate

Lactate, formed by the action of lactate dehydrogenase, is the final product of anaerobic glycolysis in eukaryotic cells. So it can allow the production of ATP in tissues that lack mitochondria example: leukocytes, red blood cells, lens and cornea of the eye, kidney medulla and testes.

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Energy yield from glycolysis:

1- Anaerobic glycolysis: Tow molecules of ATP are generated for each molecules of glucose converted to tow molecules of lactate.

2- Aerobic glycolysis: Tow ATP per molecules of glucose, tow molecules of NADH are also produced per molecule of glucose.

Citric Acid Cycle

Citric acid cycle, also called the Krebs cycle or the Tricarboxylic acid cycle (TCA) plays several roles in metabolism. It is the final pathway where the oxidative metabolism of carbohydrates, amino acids and fatty acids converge, their carbon skeletons being converted to Co2 . the oxidation provides energy for the production of the majority of ATP. The cycle occurs totally in the mitochondria.

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Reaction of the TCA cycle

Oxidative decarboxylation of pyruvate :

Pyruvate, the end product of aerobic glycolysis, must be transported into the mitochondria before it can enter the TCA cycle. Pyruvate is converted to acetyl CoA by the pyruvate dehydrogenase complex.

 Pyruvate dehydrogenase complex consist of 3 enzymes ( E1; also called decarboxylase, E2; dihydrolipoyl transacetylase, E 3; dihydrolipoyl degydrogenase) and 5 coenzymes ( TPP thiamine pyrophosphate, Lipoic acid, CoA-SH, FAD and NAD+).  Pyruvate dehydrogenase deficiency causes lactic acidosis. Symptoms are varied and include developmental defects (especially of the brain and nervous system), muscular spasticity and early death.

1. Synthesis of citrate from acetyle CoA and oxaloacetate. 2. Isomerization of citrate. 3. Oxidation and decarboxylation of isocitrate 4. Oxidation decarboxylation of α-ketoglutrate. 5. Cleavage of succinyl CoA . 6. Oxidation of succinate . 7. Hydration of fumarate . 8. Oxidation of malate .

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Biochemistry | Carbohydrate Metabolism | Dr. Ahmed Ali Hussein Page | 16 Energy of TCA cycle:

Tow carbon atoms enter the cycle as Acetyl CoA and leave as CO2. Oxidation of one NADH by the electron transport chain leads to formation three ATP, where as oxidation of FADH2 yileds approximately tow ATP. The total yiled of ATP on the oxidation of one acetyl CoA is 12 ATP .

 No. of ATP gained each turn of TCA cycle 2*12= 24 ATP.  For complete oxidation of one glucose molecule (Glycolysis) 2NADH 2*3 = 6ATP 2ATP = 8ATP  Pyruvate dehydrogenase reaction (2*3 = 6ATP)  The sum of ATP gained = 38 ATP

Regulation of the TCA cycle The TCA cycle is controlled by the regulation of several enzyme activities like: citrate synthase, isocitrate dehydrogenase and α-ketoglutarate dehydrogenase complex.l

Biochemistry | Carbohydrate Metabolism | Dr. Ahmed Ali Hussein Page | 17 Gluconeogenesis

Gluconeogenesis, the formation of new glucose molecules from noncarbohydrate precursors, occurs primarily in the liver. Precursor molecules include lactate, pyruvate, glycerol and certain α-keto acids(molecules derived from amino acids). Under certain condition (i.e. metabolic acidosis or starvation) the kidney can make small amounts of new glucose. Between meals adequate blood glucose levels are maintained by the of liver glycogen. When liver glycogen is depleted (e.g. owing to prolonged fasting or vigorous exercise), the gluconeogenesis pathway provides the body with adequate glucose.

Gluconeogenesis reaction :

In Gluconeogenesis, which occurs when blood sugar levels are low and liver glycogen is depleted, 7 of the 10 reactions of glycolysis are reversed. Three irreversible glycolytic reactions are bypassed by alternative reactions. The major substrate for Gluconeogenesis are certain amino acid (derived from muscle), lactate (formed in muscle and red blood cells) and glycerol (produced from the degradation of triacylglycerols).

Biochemistry | Carbohydrate Metabolism | Dr. Ahmed Ali Hussein Page | 18 Gluconeogenesis Regulation

The four key enzymes in gluconeogenesis (Pyruvate carboxylase, PEP carboxykinase, Fructose-1,6- bisphosphatase and Glucose-6-phosphatase) .

Glycogen Metabolism Glycogen is the storge from of glucose. The synthesis and degradation of glycogen are carefully regulated so that sufficient glucose is available for the bodys energy needs. Both glycogenesis and glycogenolysis are controlled primarily by three hormones : insulin, glucagon and epinephrine.

Glycogenesis (Glycogen synthesis) Glycogen synthesis occurs after a meal, when blood glucose levels are high. The synthesis of glycogen from glucose-6-phosphate. The process occur in cytosol.

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 Tow enzymes are required to complete the synthesis :- 1. Glycogen synthase Which catalyze the transfer of glucose to glycogen primer via α (1→4) glycosidic chain of glycogen. This enzyme only synthesize α (1→4) linkage.

2. Branching enzyme :- Which catalyze the formation of α (1→6) linkage. After that glycogen synthase reactivated again and glucose will be added to the α (1→4) chain. The result is a highly branched glycogen structure.

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Glycogenolysis (Degradation of glycogen) Glycogenolysis, that mobilizes stored glycogen in liver and skeletal muscle. In liver it used to maintain glucose level in blood and in muscles it used for contraction.

Glycogen is degraded by tow enzymes : 1- Glycogen phosphorylase. Removal of glucose from the nonreducing ends of glycogen. Glycogen phosphorylase uses inorganic phosphate (Pi) to cleave the α(1,4) linkages of glycogen to yield glucose-1-phosphate until it comes with four glucose residues of a branch point.

Biochemistry | Carbohydrate Metabolism | Dr. Ahmed Ali Hussein Page | 21 2- Debranching enzyme. Hydrolysis of the α(1,6) glycosidic bonds at branch points of glycogen. Debranching enzyme (Amylo- α (1,6)-glucosidase) transfers three of these residues to a nearby nonreducing end and releases the fourth residue as free glucose.

Biochemistry | Carbohydrate Metabolism | Dr. Ahmed Ali Hussein Page | 22 Pentose Phosphate Pathway (PPP) Also called the hexose monophosphate shunt or 6-phosphogluconate pathway, which occurs in the cytosol of the cell. This pathway is an alternative metabolic pathway for glucose oxidation in which no ATP is generated. Its principle products are NADPH, a reducing agent required in several anabolic processes and ribose-5-phosphate a structural component of nucleotides and nuclic acids.

Its divided into two phases :- 1. Oxidative phase . 2. Non- oxidative phase . 1) The oxidative phase:- Converted the glucose-6-phosphate (G-6-P) to Ribulose-5-phosphate with production of tow NADPH.

2) Non-oxidative phase :- Ribulos-5-phosphate is converted to both ribose-5-phosphate and xylulose- 5-phosphate by an isomerase and epimerase respectively. Then to glycolytic intermediates fructose-6- phosphate and glyceraldehydes-3-phosphate.

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Function of the pathway :- 1. Production of NADPH * Which acts as a biochemical reductant * Which is required for the biosynthesis of fatty acid, steroid * In erythrocyted maintain glutathione (GSH) in its reduced state.

2. Produce Ribose-5-phosphate, it required for the biosynthesis of an nucleic acid.

3. It aids in the interconversion of three, four, five, six and seven carbon sugars. (transketolase and transaldotase are tow enzymes important for those inter conversion).

Regulation of glucose level in the blood Glucose level in blood results from tow processes :- 1) Average glucose enter the blood. 2) Average glucose leave the blood.

Biochemistry | Carbohydrate Metabolism | Dr. Ahmed Ali Hussein Page | 24 1) Glucose enter the blood by : a- Absorption of dietary CH2O from intestine to the blood. b- Glycogenolysis (break down) in liver. c- gluconeogenesis 2) Glucose leave the blood by : a- In liver and muscles stored as glycogen. (glycogenesis) b- fat biosynthesis. c- Glycoproteins biosynthesis. d- as urine.

 There are many factors that affect glucose level in blood :- 1- Liver : Is responsible for regulation of blood glucose by : a. Glycogenesis. b. Glycogenolysis. c. Gluconeogenesis.

2- Muscles : Participates through Cori cycle. Lactate (which is produced from glucose in glycolysis) enter the liver and converted to pyruvate and then to glucose.

Cori cycle

3- Kidney : Kidney excret excess glucose by urine and kidney reabsorbed glucose when blood glucose is low.

4- Hormones : A. The hormone insulin (stimulated by hyperglycemia) play acentral role in the regulation of blood glucose. Released by pancreatic β-cells.  The rate of glucose uptake by insulin :-

Biochemistry | Carbohydrate Metabolism | Dr. Ahmed Ali Hussein Page | 25 1. Decrease gluconeogenesis . 2. Decrease glycogenolysis . 3. Activation of glycolysis . 4. Increase transport of glucose through cell membrane . B. Glucagon, stimulated by hypoglycemia, released by pancreatic α-cells.:- 1. Increase glycogenolysis . 2. Increase glyconeogenesis .

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