1. METABOLISM OF CARBOHYDRATES: DIGESTION OF CARBOHYDRATES. SYNTHESIS AND DEGRADATION OF GLYCOGEN

2. DIGESTION OF CARBOHYDRATES Glycogen, starch and disaccharides (sucrose, lactose and maltose) are hydrolyzed to monosaccharide units in the gastrointestinal tract. The process of digestion starts in the mouth by the salivary enzyme  –amilase. The time for digestion in mouth is limited. Salivary  - amilase is inhibited in stomach due to the action of hydrochloric acid. Another  - amilase is produced in pancreas and is available in the intestine.

3.  -amilase hydrolyzes the  -1-4-glycosidic bonds randomly to produce smaller subunits like maltose, dextrines and unbranched oligosaccharides.  -amilase

4. The intestinal juice contains enzymes hydrolyzing disaccharides into monosaccharides (they are produced in the intestinal wall) Sucrase hydrolyses sucrose into glucose and fructose Sucrose sucrase Fructose Glucose

5. Lactose lactase Maltase hydrolyses maltose into two glucose molecules Lactase hydrolyses lactose into glucose and galactose Maltose maltase Galactose Glucose

6. ABSORPTION OF CARBOHYDRATES Only monosaccharides are absorbed The rate of absorption: galactose > glucose > fructose Glucose and galactose from the intestine into endothelial cells are absorbed by secondary active transport Na + Glucose Protein

7. Carrier protein is specific for D-glucose or D-galactose. L-forms are not transported. There are competition between glucose and galactose for the same carrier molecule; thus glucose can inhibit absorption of galactose. Fructose is absorbed from intestine into intestinal cells by facilitated diffusion. Absorption of glucose from intestinal cells into bloodstream is by facilitated diffusion.

8. Transport of glucose from blood into cells of different organs is mainly by facilitated diffusion. The protein facilitating the glucose transport is called glucose transporter (GluT). GluT are of 5 types. GluT 2 is located mainly in hepatocytes membranes (it transport glucose into cells when blood sugar is high); GluT 1 is seen in erythrocytes and endothelial cells; GluT 3 is located in neuronal cells (has higher affinity to glucose); GluT 5 – in intestine and kidneys; GluT 4 - in muscles and fat cells.

9. In the well-fed state the glucose after absorption is taken by liver and deposited as a glycogen Glycogen is a very large, branched polymer of glucose residues that can be broken down to yields glucose molecules when energy is needed Most glucose residues in glycogen are linked by a-1,4-glyco- sidic bonds, branches are created by a-1,6-glycosidic bonds GLYCOGEN SYNTHESIS AND DEGRADATION

10. Liver (10 % of weight) and skeletal muscles (2 %) – two major sites of glycogen storage Glycogen is stored in cytosolic granules in muscle and liver cells of vertebrates Glycogen serves as a buffer to maintain blood-glucose level. Stable blood glucose level is especially important for brain where it is the only fuel. The glucose from glycogen is readily mobilized and is therefore a good source of energy for sudden, strenuous activity.

11. Glucose-6-phosphate is the central metabolite in the synthesis and decomposition of glycogen. In the well-fed state glucose is converted to glucose-6-phosphate, which is the precursor for the glycogen synthesis. The glucose-6-phosphate derived from the breakdown of glycogen has three fates: (1) glycolysis; (2) pentose- phosphate pathway; (3) convertion to free glucose for transport to another organs.

12. Glycogenolysis - degradation of glycogen The reaction to release glucose from polysaccharide is not simple hydrolysis as with dietary polysaccharides but cleavage by inorganic phosphate – phosphorolytic cleavage Phosphorolytic cleavage or phosphorolysis is catalyzed by enzyme glycogen phosphorylase There are two ends on the molecules of starch or glycogen: a nonreducing end (the end glucose has free hydroxyl group on C4) and a reducing end (the end glucose has an anomeric carbon center (free hydroxyl group on C1) DEGRADATION OF GLYCOGEN

13. Glycogen phosphorylase removes glucose residues from the nonreducing ends of glycogen Acts only on a-1-4 linkages of glycogen polymer Product is a-D-glucose 1-phosphate (G1P) Cleavage of a glucose residue from the nonreducing end of glycogen

14. GP is a dimer of identical subunits (97kD each) Catalytic sites are in clefts between the two domains of each subunit Binding sites for glycogen, allosteric effectors and a phosphorylation site Two forms of GP Phosphorylase a (phospho- rylated) active form Phosphorylase b (dephospho- rylated) less active Structure of glycogen phosphorylase (GP)

15. GP catalyzes the sequential removal of glucose residues from the nonreducing ends of glycogen GP stops 4 residues from an a 1-6 branch point Tranferase shifts a block of three residues from one outer branch to the other A glycogen-debranching enzyme or 1,6- glucosidase hydrolyzes the 1-6-glycosidic bond The products are a free glucose-1-phosphate molecule and an elongated unbranched chain

16. Phosphoglucomutase catalyzes the conversion of G1P to glucose 6-phosphate (G6P) Metabolism of Glucose 1-Phosphate (G1P)

17. Glycogen Synthesis Synthesis and degradation of glycogen require separate enzymatic steps Cellular glucose converted to G6P by hexokinase Three separate enzymatic steps are required to incorporate one G6P into glycogen Glycogen synthase is the major regulatory step

18. Phosphoglucomutase catalyzes the conversion of glucose 6-phosphate (G6P) to glucose 1- phosphate (G1P). Glucose 1-Phosphate formation

19. UDP-glucose is activated form of glucose. UDP-glucose is synthesized from glucose- 1-phosphate and uridine triphosphate (UTP) in a reaction catalized by UDP-glucose pyrophosphorylase

20. Glycogen synthase adds glucose to the nonreducing end of glycogen

21. A branching enzyme forms  -1,6-linkages Glycogen synthase catalyzes only  -1,4- linkages. The branching enzyme is required to form  -1,6-linkages. Branching is important because it increases the solubility of glycogen. Branching creates a large number of terminal residues, the sites of action of glycogen phosphorylase and synthase.

22. Muscle glycogen is fuel for muscle contraction Liver glycogen is mostly converted to glucose for bloodstream transport to other tissues Both mobilization and synthesis of glycogen are regulated by hormones Insulin, glucagon and epinephrine regulate mammalian glycogen metabolism Regulation of Glycogen Metabolism

23. Insulin is produced by b-cells of the pancreas (high levels are associated with the fed state) Insulin increases rate of glucose transport into muscle, adipose tissue via GluT 4 transporter Insulin stimulates glycogen synthesis in the liver via the second messenger phosphatidylinositol 3,4,5-triphosphate (PIP 3 ) Hormones Regulate Glycogen Metabolism Insulin

24. Glucagon Secreted by the a cells of the pancreas in response to low blood glucose (elevated glucagon is associated with the fasted state) Stimulates glycogen degradation to restore blood glucose to steady-state levels Only liver cells are rich in glucagon receptors and therefore respond to this hormone

25. Epinephrine (Adrenalin) Released from the adrenal glands in response to sudden energy requirement (“fight or flight”) Stimulates the breakdown of glycogen to G1P (which is converted to G6P) Increased G6P levels increase both the rate of glycolysis in muscle and glucose release to the bloodstream from the liver and muscles Both liver and muscle cells have receptors to epinephrine

26. Effects of hormones on glycogen metabolism

27. Glycogen phosphorylase (GP) and glycogen synthase (GS) control glycogen metabolism in liver and muscle cells GP and GS are reciprocally regulated both covalently and allosterically (when one is active the other is inactive) Covalent regulation by phosphorylation (-P) and dephosphorylation (-OH) Allosteric regulation by glucose-6-phosphate (G6P) Reciprocal Regulation of Glycogen Phosphorylase and Glycogen Synthase

28. COVALENT REGULATION Active form “a” Inactive form “b” Glycogen phosphorylase -P -OH Glycogen synthase -OH -P Reciprocal Regulation of GP and GS ALLOSTERIC REGULATION by G6P GP a (active form) - inhibited by G6P GS b (inactive form) - activated by G6P

29. Activation of GP and inactivation of GS by Epinephrine and Glucagone

30. Activation of GS and inactivation of GP by Insulin