Amino Acid Metabolism: Overview and Fate in Human Biochemistry

1. BC368 Biochemistry of the Cell II Nitrogen Metabolism I (Ch 18) Amino Acid Oxidation and the Production of Urea May 1, 2015

2. Overview of amino acid metabolism • Proteins constantly undergo turnover.

3. Overview of amino acid metabolism • Proteins constantly undergo turnover. • Amino acids are also used to synthesize some non-protein metabolites. • No protein stores, so essential amino acids must come from diet.

4. Amino acid catabolism in humans • Proteins are broken down in stomach and small intestine to constituent amino acids. • Amino acids are either used as building blocks or burned for energy (~10% of our energy needs). • Catabolism of amino acids increases • for use in gluconeogenesis when glucose is unavailable (e.g., starvation/diabetes) • when protein content of diet exceeds need for building blocks • during times of stress

5. Overview of amino acid catabolism • Special role for four amino acids:

6. Digestion and Absorption • Digestive events are triggered of the hormone gastrin, released when food enters the stomach.

7. Digestion and Absorption • Low pH activates digestive enzymes; e.g., pepsin. • Resulting amino acids are absorbed by the intestinal mucosa, enter the capillaries, travel to the liver. • Liver can degrade all amino acids but Leu, Ile, Val.

8. Divergent pathways of NH3 groups and carbon skeletons Fig 18-1 Fig 18-1

9. Removal of amino group via transamination • Amino groups can be removed by transamination. • In liver cytosol, amino groups are passed to α-KG, forming glutamate. • Transaminases (aka aminotransferases) require pyridoxal phosphate cofactor.

10. Pyridoxal phosphate and transamination

11. Removal of amino group via oxidative deamination Fig 18-1 • Glutamate in the liver cytosol enters the mitochondrial matrix, where its amino group is removed by glutamate dehydrogenase. • Amino group must be processed for excretion or recycled. Fig 18-7

12. Transport of amino groups as glutamine • Peripheral tissues may send their amino groups as glutamine through the bloodstream to the liver for processing. To liver via bloodstream Fig 18-8

13. Transport of amino groups as alanine Fig 18-1 • In concert with the Cori cycle, skeletal muscle may send pyruvate through bloodstream as alanine (the glucose-alanine cycle). • Operates when muscle proteins are undergoing catabolism. Fig 18-9

14. Summary of paths of amino groups Fig 18-1 Fig 18-2

15. Fate of NH4+ Fig 18-1 excreted as NH3 (ammonotelic)

16. Fate of NH4+ excreted as uric acid (uricotelic)

17. Fate of NH4+

18. Fate of NH4+ excreted as urea in H2O (ureotelic)

19. Case Study EM, the third child of parents unrelated by blood, had one healthy sister and one brother who demonstrated spasticity. EM appeared normal at birth with good Apgar scores. Hypotonia was observed after the third month of life. At 7 months of age (weight, 6.0 kg; height, 67 cm), he was admitted for evaluation of painful swollen joints. Neurological examination revealed hyperreflexia and an inability to lift his head. Laboratory tests revealed the following: • What is wrong with EM? • What treatment would you recommend?

20. Alternate fate of NH4+ = uric acid from purines The Disease of Kings and the King of Diseases… Gout, the evil demon • Podagra (swelling of the big toe) results from crystals of uric acid in the synovial fluid The Gout by James Gilray, 1799

21. Normal pathway of purine degradation AMP GMP XO= xanthine oxidase

22. Treatment for gout Trojan horse inhibitor of xanthine oxidase Fig 22-47

23. Gertrude Elion, 1918-1999 Treatment for gout Trojan horse inhibitor of xanthine oxidase Fig 22-47

24. Treatment for gout Trojan horse inhibitor of xanthine oxidase Fig 22-47

25. Purine Salvage Defect • Purines are recycled through the purine salvage pathway. • Key enzyme is HGPRT (hypoxanthine-guanine phosphoribosyltransferase). • Defect in HGPRT leads to Lesch-Nyhan syndrome.

26. Purine Salvage Defect • Purines are recycled through the purine salvage pathway. • Key enzyme is HGPRT (hypoxanthine-guanine phosphoribosyltransferase). • Defect in HGPRT leads to Lesch-Nyhan syndrome.

27. Urea cycle • Occurs in the liver • Spans two compartments: matrix and cytosol

28. Preparatory step: carbamoyl phosphate synthetase I Occurs in the matrix First N of urea Fig 18-11

29. Step 1: Ornithine transcarbamoylase Ornithine is analogous to OA Also occurs in the matrix, but citrulline is transported to cytosol

30. Step 2: Argininosuccinate synthetase Second N of urea!

31. Step 3: Argininosuccinase

32. Step 4: Arginase Ornithine is transported back to the matrix.

33. Krebs’ bicycle Fig 18-12

34. Twenty-four hours after birth, a formula-fed male infant becomes somnolent and feeds poorly. Soon he begins to vomit and then goes into a coma. It looks like sepsis, but he has no risk factors and his sepsis work-up is negative. His serum ammonia and ornithine are elevated while his citrulline levels are undetectable. The maternal grandmother tells you that she had a son who died as a baby from the same symptoms. Which of the following enzymes is defective? • Carbamoyl phosphate synthetase • Ornithine transcarbamoylase • Arginase • Alanine aminotransferase • Pyruvate carboxylase

35. Fig 18-15 Fates of carbon skeletons • Glucogenic amino acids are degraded to pyruvate or TCA intermediate. • Ketogenic amino acids are degraded to acetoacetyl-CoA or acetyl-CoA. • Some amino acids are both.