Chapter 13: Carbohydrate Metabolism

1. Chapter 13:Carbohydrate Metabolism

2. CARBOHYDRATE METABOLISM – DIGESTION The main reaction of carbohydrate digestion to monosaccharides is hydrolysis:

3. BLOOD GLUCOSE LEVELS Regulated by the liver If levels are above the renal threshold, glucose will appear in the urine, a condition called glucosuria.

4. GLYCOLYSIS Glucose (C6) is catabolically oxidized through a many step process to pyruvate (C3) 2 ATP, 2 NADH, 4 H+, and 2 H2O are produced in addition to the 2 molecules of pyruvate

5. GLYCOLYSIS – REGULATION Several steps of the glycolysis pathway are regulated: Glucose-6-phosphate competitively inhibits hexokinase (feedback inhibition). The allosteric enzyme phosphofructokinase is inhibited by ATP and citrate and activated by ADP and AMP. The allosteric enzyme pyruvate kinase is inhibited by ATP.

6. GLYCOLYSIS – REGULATION, cont.

7. After glycolysis, pyruvate can be: Oxidized to acetyl CoA (aerobic conditions). Reduced to lactate (anaerobic conditions). Reduced to ethanol (anaerobic conditions for some prokaryotic organisms). Note: All processes must regenerate NAD+ from NADH so glycolysis can continue.

8. PYRUVATE OXIDATION TO ACETYL CoA Occurs in the mitochondria Most of the acetyl CoA will be completely oxidized to CO2 in the citric acid cycle.

9. PYRUVATE REDUCTION TO LACTATE Occurs in cells after strenuous or long-term muscle activity because the cellular supply of oxygen is not adequate for the reoxidation of NADH to NAD+ Under anaerobic conditions, animals and some microorganism can obtain limited energy through lactate fermentation.

10. PYRUVATE REDUCTION TO ETHANOL Under anaerobic conditions, come microorganisms can obtain limited energy through glycolysis and the two step conversion of pyruvate to ethanol.

11. Pyruvate oxidized to acetyl CoA can enter the citric acid cycle where it will be further oxidized to CO2, producing one molecule of GTP and the reduced forms of NAD+ and FAD which can then enter the electron transport chain to produce ATP. The overall reaction is:

12. THE CITRIC ACID CYCLE Click here to play Chemistry Interactive

13. CITRIC ACID CYCLE – REGULATION There are three main points of regulating citric acid cycle activity: Citrate synthetase – the first step of the cycle is inhibited by ATP and NADH and activated by ADP Isocitrate dehydrogenase – the third step of the cycle is inhibited by NADH and activated by ADP Alpha-ketoglutarate dehydrogenase – the fourth step of the cycle is inhibited by succinyl CoA, NADH, and ATP

14. CITRIC ACID CYCLE – REGULATION, cont.

15. NADH and FADH2 are produced by the citric acid cycle. They enter the electron transport chain where they can be used to supply hydrogen ions and electrons to reduce oxygen to water.

16. THE ELECTRON TRANSPORT CHAIN Found in the inner membrane of the mitochondria and involves iron-containing enzymes (cytochromes) Energy released by the electron transport chain used to drive the oxidative phosphorylation of ADP to ATP Chemiosmotic hypothesis – the electron transport chain pumps H+ across the inner mitochondrial membrane, H+ then flows back across the membrane, causing the formation of ATP by F1-ATPase Click here to play Coached Problem Click here to play Chemistry Interactive

17. OXIDATIVE PHOSPHORYLATION: CHEMIOSMOTIC HYPOTHESIS

18. OXIDATIVE PHOSPHORYLATION FROM ELECTRON TRANSPORT Mitochondrial NADH to NAD+ generates 2.5 ATP from ADP FADH2 to FAD generates 1.5 ATP from ADP NADH produced in cytoplasm does not pass through the mitochondrial membrane to the site of the electron transport chain Brain and muscle NADH employ a transport mechanism that passes electrons from cytoplasm NADH through the membrane to FAD molecules inside mitochondria Cytoplasmic NADH generates only 1.5 molecules of ATP Liver, heart, and kidney cytoplasmic NADH have a more efficient shuttle that results in one molecule mitochondrial NADH (and 2.5 molecules ATP) for every cytoplasmic NADH

19. THE COMPLETE OXIDATION OF GLUCOSE

20. GLYCOGEN METABOLISM – SYNTHESIS The synthesis of glycogen from glucose (glycogenesis) involves UTP. Glycogen is mainly stored in liver and muscle tissue.

21. GLYCOGEN METABOLISM – BREAKDOWN Glycogen can be broken down to glucose monomers via its alpha(1-4) and alpha(1-6) linkages. Glucose 6-phosphate can then be catabolized by the glycolysis pathway.

22. GLUCONEOGENESIS The synthesis of glucose from noncarbohydrate molecules Glucose synthesized from lactate, certain amino acids, and glycerol Anaerobic conditions, lactate produced by muscles reconverted to glucose by the liver through the Cori Cycle

23. THE CORI CYCLE

24. HORMONAL CONTROL OF CARBOHYDRATE METABOLISM Three main hormones control carbohydrate metabolism:1. insulin Decreases blood glucose levels Increases absorption of glucose by cells Increases the synthesis of glycogen, fatty acids, proteins Stimulates glycolysis 2. glucagon Increases blood glucose levels Activates glycogen breakdown in liver 3. epinephrine Increases blood glucose levels Stimulates glycogen breakdown in muscle

25. HORMONAL CONTROL, cont. Biochemical balance is maintained between the stored glycogen level and the blood sugar level by two opposing hormones, insulin and glucagon.