Section 8. Amino Acid Metabolism

1. Section 8. Amino Acid Metabolism One-carbon metabolism, purine metabolism 11/29/05

2. Nucleotide Roles • Activated precursors for DNA and RNA synthesis: dNTPs and NTPs. • Activation of other moieties for synthesis: UDP-glucuronate, CDP-diacylglycerol, etc. • Energy transduction: ATP for muscle contraction and ion transport. • Control reactions: GDP GTP of G-proteins. • Control of metabolism: cAMP, ADP/ATP ratio, enzyme phosphorylation. • Constituents of other small molecules: NADH, FAD, CoA, etc. 1

3. Purines • There are two pathways for nucleotide biosynthesis: de novo and salvage. • Hypoxanthine is an intermediate on the de novo pathway to AMP and GMP and guanine, and on the salvage pathway to AMP. 2

4. Purine Biosynthesis: the committed step • Addition of an amino group from glutamine to PRPP to form 5-phosphoribosyl-1-amine is the committed step. 3

5. Purine Biosynthesis 4

6. AMP and GMP • GMP and AMP are phosphorylated to di- and triphosphates. see Fig. 25.9 5

7. Control Mechanisms • All purine nucleotides inhibit amidophosphoribosyl transferase activity. • For balance: • AMP inhibits the synthesis of adenylosuccinate. • GMP inhibits the synthesis of xanthylate. 6

8. AMP inhibits upper pathway GMP inhibits lower pathway 7

9. Single Carbon Carriers • Biotin -CO2- • S-adenosyl methionine (SAM) -CH3 • Tetrahydrofolate (THF) -CH3 -CH2- =CH- -HC=O -C=NH • Single carbons for: • carnitine • choline • creatine • epinephrine • purines • thymine • others 8

10. Biotin • Biotin carries a carboxyl group, activated by ATP hydrolysis. • The coenzyme is covalently attached to a flexible lysine sidechain in the enzyme active site. 9

11. S-Adenosylmethionine (SAM) • Homocysteine is methylated by N5-methyl-THF. • B12 is required. 10

12. Tetrahydrofolate • Dietary folic acid is reduced to the coenzyme form THF. • The 5- and 10- nitrogens can carry single carbons in all oxidation states except carboxylate. 11

13. THF Carbon States • The carbon comes from serine or formate. • Oxidation, reduction and transamination occur while bound to THF. Fig. 24.13 12

14. Cobalamine • Derived from vitamin B12. • Required for methylation of homocysteine. Styer 4th 13

15. Single Carbon Scheme 14

16. Purine Degradation • Xanthine oxidase contains molybdenum (Mo) and iron-flavoprotein. • Urate is an antioxidant. 15

17. Gout • Urate can precipitate as a sodium salt, causing gout. • Allopurinol is oxidized by xanthine oxidase to alloxanthine, a potent inhibitor of xanthine oxidase. • Less urate is produced. • The use of the salvage pathway increases. 16

18. Salvage Pathway • Nucleotides can also be made from intact bases, either from dietary sources or from degraded nucleotides that were made by the de novo pathway. • The salvage pathway is prevalent in the brain. 17

19. Lesch-Nyhan Syndrome • Caused by very low levels of hypoxanthine-guanine phophoribosyltransferase. • This disrupts the salvage pathway for purine nucleotide biosynthesis. • High [PRPP] & [urate] and increased de novo purine biosynthesis. • Symptoms are gout, mental deficiency, spasticity, compulsive self-mutilation, aggression. 18

20. Next topic: Pyrimidines, catecholamines, serotonin & histamine.