Understanding Cofactors in Enzyme Activity

1. Enzymes-cofactors Dr. Mamoun Ahram

2. Resources • Biochemistry. 5th edition. Berg JM, Tymoczko JL, Stryer L. New York: W H Freeman; 2002. • 8.1.1 Many Enzymes Require Cofactors for Activity • http://www.ncbi.nlm.nih.gov/books/NBK22380/ • Section 9.2 Making a Fast Reaction Faster: Carbonic Anhydrases • http://www.ncbi.nlm.nih.gov/books/NBK22599/ • The Medical Biochemistry Pages • Introduction to Vitamins and Minerals • http://themedicalbiochemistrypage.org/vitamins.php

3. Cofactors • What are cofactors? • What are coenzymes? • When are coenzymes called prosthetic groups and when are they cosubstrates?

4. Classification of cofactors

5. Apoenzymes vs. holoenzymes

6. Vitamin derivatives as cofactors • What are vitamins? • Why are they important? • What are the two classes of vitamins?

7. Vitamin C • Ascorbic acid • Example: prolyl hydroxylase • synthesizes 4-hydroxyproline (collagen) • An antioxidant

8. Thiamin (vitamin B1) • Active form: thiamin pyrophosphate, TPP

9. Pyruvate dehydrogenase complex • Decarboxylation of pyruvate into acetyl CoA by the pyruvate dehydrogenase complex

10. α-ketoglutarate dehydrogenase • Decarboxylation of α-ketoglutarate into succinyl CoA by α-ketoglutarate dehydrogenase

11. Riboflavin (vitamin B2) • The precursor for the coenzymes flavin adenine mononucleotide (FMD) and flavin adenine dinucleotide (FAD)

12. Flavoproteins • Protiens that require FMN or FAD as cofactors • Redox reactions • FAD and FMN are prosthetic groups

13. Succinate dehydrogenase • Oxidation of succinate into fumarate by succinate dehydrogenase

14. Pyruvate dehydrogenase complex • FAD is a prosthetic group for the pyruvate dehydrogenase complex • It does not change in the overall reaction

15. Niacin (vitamin B3) • Precursor of nicotinamide adenine dinucleotide (NAD+) and nicotinamide adenine dinucleotide phosphate (NADP+) • Cofactors for numerous dehydrogenases • Cosubstrates

17. Pantothenic acid (vitamin B5) • Precursor of alanine and pantoic acid • Synthesis of coenzyme A (CoA) • Metabolism of carbohydrate, fats and proteins

18. Coenzyme A • Coenzyme A transports acetyl groups from one substrate to another • via reactive thioester bond

19. Pyruvate dehydrogenase complex • Decarboxylation of pyruvate into acetyl CoA by the pyruvate dehydrogenase complex

20. Citrate synthase • Condensation of acetyl CoA and oxaloacetate into citrate by citrate synthase

21. Pyridoxal phosphate (vitamin B6) • Pyridoxal, pyridoxamine and pyridoxine are precursors of active form • Metabolism of amino acids

22. Aminotransferases • All aminotransferases contain the prosthetic group pyridoxal phosphate (PLP)

23. Examples • Aspartate aminotransferase • Alanine aminotransferase

24. Biotin • Carboxylation reactions

25. Pyruvate carboxylase • condensation of CO2 to pyruvate forming oxaloacetate by pyruvate carboxylase

26. Acetyl CoA carboxylase • Carboxylation of acetyl CoA to malonyl CoA by acetyl CoA carboxylase initiating fatty acid synthesis • Biotin is a prosthetic group

27. Cobalamin (vitamin B12) • Composed of a tetrapyrrol ring structure and a cobalt ion in the center • Two significant reactions • linking fatty acid metabolism to sugar metabolism by rearranging methylmalonyl CoA to succinyl CoA • the formation of methionine from homocysteine by methylation

28. Folic acid • Reduced within cells to tetrahydrofolate (THF) by dihydrofolate reductase (DHFR), an NADPH-requiring enzyme • THF derivatives carry and transfer of one carbon units during biosynthetic reactions, especially of nucleotides

29. Lipoic acid • Not a dietary requirement in humans • Not a vitamin • A co-factor in pyruvate dehydrogenase and α-ketoglutarate dehydrogenase

30. Metals

31. Example

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