CARBOHYDRATE METABOLISM

1. CARBOHYDRATE METABOLISM Mia Kusmiati Departemen Biokimia FK UNISBA

3. Carbohydrates • Carbohydrates are called carbohydrates because they are essentially hydrates of carbon (i.e. they are composed of carbon and water and have a composition of (CH2O)n. • The major nutritional role of carbohydrates is to provide energy and digestible carbohydrates provide 4 kilocalories per gram. No single carbohydrate is essential, but carbohydrates do participate in many required functions in the body.

5. Carbohydrate Metabolism

8. Glucose transport

9. Glycolysis What is glycolysis?  Ten step metabolic pathway to convert glucose into two molecules of pyruvate and two molecules each of NADH and ATP.  All carbohydrates to be catabolized must enter the glycolytic pathway. - Glycolysis is central in generating both energy and metabolic intermediaries.

10. The Glycolysis Pathway • Major anaerobic pathway in all cells • NAD+ is the major oxidant • Requires PO4 • Generates 2 ATP’s per glucose oxidized • End product is lactate (mammals) or ethanol (yeast) • Connects with Krebs cycle via pyruvate

11. Phase I. Energy Investment. 1- Glucose is phosphorylated. Glucose enters a cell through a specific glucose transport process. It is quickly phosphorylated at the expense of an ATP. The investment of an ATP here is called “priming.” Enzymes = hexokinase or glucokinase 2. Isomerization of glucose 6-phosphate Enzyme = phosphoglucoisomerase

12. glucose 6-phosphate fructose 6-phosphate aldose to ketose isomerization reversible, G= 1.7 kJ/mole

13. 3- Second phosphorylation Enzyme = phosphofructokinase ATP ADP fructose 1,6 bisphosphate • second ATP investment • highly exergonic, essentially • irreversible, G°´= -14.2 kJ/mole • - highly regulated, modulating carbon • flux through glycolysis in response • to energy and carbon requirements

14. 4- Cleavage to two triose phosphates Enzyme = aldolase HC=O H2COP HCOH O=C HCOP + CH2OH H glyceraldehydedihydroxyacetone 3-phosphate phosphate where P = phosphate cleaves a 6C sugar to 2 3C sugars G°´= +23.8 kJ/mole, driven by next Rx.

15. 5-Isomerization of Dihydroxyacetone phosphate Enzyme = triose-phosphate isomerase H2C-OH C=O CH2-O- P dihydroxyacetone glyceraldehyde phosphate 3-phosphate

16. 5-Isomerization of Dihydroxyacetone phosphate Enzyme = triose-phosphate isomerase H2C-OH C=O CH2-O- P dihydroxyacetone glyceraldehyde phosphate 3-phosphate

17. allows interconversion of two triosephosphate products of aldolase cleavage only glyceraldehyde 3-phosphate canbe used further in glycolysis.  aldose-ketose isomerization similar to phosphoglucoisomerase rxn allows dihydroxyacetone phosphateto be metabolized asglyceraldehyde 3-phosphate reversible,G°´= +7.5 kJ/mole. This is important in gluconeogenesis

18. **************************************** End of First Phase:  Production of two glyceraldehyde 3-phosphate molecules from one glucose molecule with the expenditure of two ATPs.  Therefore: the energy yields of the following steps are multipled by two. ***************************************** Second Phase:

19. 6- Oxidation of glyceraldehyde 3-phosphate Enzyme= glyceraldehyde-3-phosphate dehydrogenase O HOPO OH NAD NADH O OPOH C=O O- HCOH H2C O- P + glyceraldehyde 3-phosphate 1,3 bisphosphoglycerate -addition of phosphate, oxidation, production of NADH, formation of high energy compound

20. 7- Transfer of phosphate to make ATP Enzyme = phosphoglycerate kinase O=C-O- P O=C-OH P HC-OH + P HC-OH +P H2C-O-P P H2C-O-P P Adenosine Adenosine 1,3PG ADP 3-phosphoglycerate ATP - first substrate level phosphorylation, yielding ATP - 2 1,3 bis PG yield 2 ATPs, thus so far ATP yield = ATP input - high free energy yield, G°´=-18.8kJ/mole drives several of the previous steps.

21. 8- Phosphate shift setup Enzyme= phosphoglycerate mutase - shifts phosphate from position 3 to 2 - reversible, ΔG = + 4.6 kJ/mole

22. 9- Generation of second very highenergy compound Enzyme = enolase -- little energy change in this reaction, ΔG = +1.7 kJ/mole because the energy is locked into enolphosphate

23. 10- Final generation of ATP • Enzyme = pyruvate kinase • P • O H ADP ATP O • -OOC-C=CH -OOC-C-CH3 • phosphoenolpyruvate pyruvate • second substrate levelphosphorylation • yielding ATP • - highly exergonic reaction, • irreversible, ΔG = -31.4 kJ/mole.

28. Regulation of Glycolysis • 6-phosphofructokinase-1 Allosteric enzyme negative allosteric effectors Citrate , ATP Positive allosteric effectors AMP, fructose1,6-bisphosphate, fructose2,6-bisphosphate Changes in energy state of the cell (ATP and AMP)

29. Regulation of Glycolysisfig.6-4

30. Pyruvate Kinase Allosteric enzyme Inhibited by ATP. Isoenzyme in liver activated by fructose 1,6 bisphosphate inhibited by alanine Regulated by phosphorylation and dephosphorylation Hexokinase Different isoenzymes Hexokinase IV glucose 6-phosphate is an allosteric inhibitor promote biosynthesis Regulation of Glycolysis

32. The Significance of Glycolysis • Glycolysis is the emergency energy-yielding pathway • Main way to produce ATP in some tissues red blood cells, retina, testis, skin, medulla of kidney • In clinical practice

33. Aerobic Oxidation of Glucose • Glucose oxidation • Oxidation of glucose to pyruvate in cytosol • Oxidation of pyruvate to acetylCoA in mitochondria • Tricarboxylic acid cycle and oxidative phosphorylation