Amino Acid Catabolism

1. Amino Acid Catabolism • Amino Acids • essential - excess are converted to metabolic intermediates such as: pyruvate, acetyl-CoA and are precursors for fatty acids, glucose, ketone bodies • 2. nonessential - biosynthesis • Amino Acid Breakdown (3 Stages) • 1. Deamination – remove an amino group • 2. Incorporation of ammonia and aspartate nitrogen atoms into urea for excretion • 3. Conversion of amino acid carbon skeletons to common metabolic intermediates

3. -ketoglutarate and glutamate play central roles in amino acid catabolism

4. Stage 1 – Deamination -Remove a-amino group Purpose: excrete excess nitrogen and degrade the remaining carbon skeleton or convert it to glucose Main way = Transamination (2 Stages) i.AA + enzyme →a-keto acid + enzyme-NH2 ii.a-ketoglutarate + enzyme-NH2→ enzyme + glutamate Enzyme requires PLP which acts as a temporary store of amino groups until the next substrate comes in. (“ping-pong” mechanism)

8. Amino transferases

9. Now… How do we get the amine group off of glutamate?!?!?Oxidative deamination via Glutamate dehydrogenase (GDH) -first committed step on the pathway to nitrogen excretion - instead of just “swapping” amino groups, there is a NET LOSS of nitrogen ***GDH is unique because it is one of the only enzymes that does not discriminate between NAD+ and NADP+

11. Stage 2 of Amino Acid Breakdown • Incorporation of ammonia and aspartate nitrogen atoms into urea for excretion (The Urea Cycle) • Living organisms excrete excess nitrogen in 1 of 3 ways. • as ammonia – ex) aquatic animals • as urea – ex) people • as uric acid – ex) birds and reptiles • Urea is synthesized in your liver, secreted into the bloodstream and sequestered by the kidneys for excretion into the urine. Urea is very soluble, but still requires an appreciable amount of water to remove it from the kidneys. Why is it so vital to get rid of? Ammonium salts are toxic and can cause vomiting, convulsions and ultimately coma and death when the blood concentration exceeds 0.25 mM.

13. 2

16. Net Reactions 1Glutamate + H2O + NADP+ ---> -KG + NH4+ + NADPH + H+ Aspartate + NH3 + 3ATP + H2O + HCO3- ---> 2ADP + AMP + PPi + 2Pi + fumarate + urea fumarate + NAD+ ---> oxaloacetate + NADH 1Glutamate + oxaloacetate ---> -KG + aspartate 2 glutamate (NH3) + HCO3- + 3ATP + NAD+ ---> 2 -KG + urea + 2ADP + AMP + 4Pi + NADH NADH = 3ATP

17. Stage 3: Recycling the skeletons

18. Alanine: the simplest case Ala is glucogenic and pyruvogenic Alanine aminotransferase Per 2NH3 4 ATP are consumed by the urea cycle Pyruvate Degradation of alanine costs 2ATP!!

19. How then do we get energy out of pyruvate?

20. Pyruvate ---> acetyl-CoA + CO2 = 1 NADH Isocitrate ---> -ketoglutarate + CO2 = 1 NADH -ketoglutarate ---> succinyl-CoA + CO2 = 1 NADH Succinyl-CoA ---> succinate = 1 GTP Succinate ---> fumarate = 1 FADH2 Malate ---> oxaloacetate = 1 NADH

21. So: Pyruvate = 4 NADH + 1 FADH2 + GTP 1 NADH = 3 ATP 1 FADH2 = 2 ATP 1 GTP = 1 ATP 1 NH3 = -2 ATP Then: Pyruvate = 15 ATP If: Ala = pyruvate + NH3 Then Ala = 15 ATP - 2 ATP = 13 ATP

22. Asparagine asparaginase aspartate aminotransferase Aspartate Oxaloacetate

23. Asp = oxaloacetate + NH3 Asn = oxaloacetate + 2NH3 How then do we get energy out of oxaloacetate? It is not degraded by the TCA cycle!

24. Run malate dehydrogenase in reverse Malate Oxaloacetate Loss of one NADH

25. Malic enzyme: malate dehydrogenase, decarboxylating Malate NADPH NADP+ Pyruvate Malate dehydrogenase + Malic enzyme = Net conversion of one NADH to one NADPH Oxaloacetate + NADH + NADP+ ---> pyruvate + CO2 + NAD+ + NADPH

26. So…oxaloacetate = pyruvate - NADH + NADPH If: pyruvate = 4 NADH + 1 GTP + 1 FADH2 Then: oxaloacetate = 3NADH + GTP + FADH2 + NADPH If: Asp = oxaloacetate + NH3 and Asn = oxaloacetate + 2 NH3 Then: Asp = 3 NADH + GTP + FADH2 + NH3 + NADPH and Asn = 3 NADH + GTP + FADH2 + 2 NH3 + NADPH If: NADH = 3 ATP GTP = ATP FADH2 = 2 ATP NH3 = -2 ATP Then: Asp = 10 ATP + NADPH and Asn = 8 ATP + NADPH

27. Glutamine Glutaminase Glutamate dehydrogenase -ketoglutarate Glutamate

28. How do we get energy from -ketoglutarate?

29. If: -ketoglutarate ---> succinyl-CoA + CO2 = 1 NADH If: Succinyl-CoA ---> succinate = 1 GTP If: Succinate ---> fumarate = 1 FADH2 Then: -ketoglutarate to malate = 1 NADH, 1 GTP, 1 FADH2 If: malate to pyruvate converts NADH to NADPH If: Pyruvate = 4 NADH + 1 GTP + 1 FADH2 Then: -ketoglutarate = 4NADH + 2 GTP + 2 FADH2 + NADPH

30. So… -ketoglutarate = 4NADH + 2 GTP + 2 FADH2 + NADPH If: Glu = -ketoglutarate + NH3 + NAPDH and Gln = -ketoglutarate + 2 NH3 + NAPDH Then: Glu = 4 NADH + 2GTP + 2FADH2 + NH3 + 2 NAPDH and Gln =4 NADH + 2GTP + 2FADH2 + 2NH3 + 2 NAPDH If: NADH = 3 ATP GTP = ATP FADH2 = 2 ATP NH3 = -2 ATP Then: Glu = 16 ATP + 2 NADPH and Gln = 14 ATP + 2 NADPH

31. Alanine Pyruvate + NH3 Cysteine Pyruvate + S2- + NH3 Serine Pyruvate + NH3

32. 2 Glycine Pyruvate + CO2 + 2 NH3 + NADH Threonine + glycine Pyruvate + Acetyl-CoA + 2 NH3 + CO2 + 2NADH

33. Aspartate Oxaloacetate + NH3 Asparagine Oxaloacetate + 2 NH3 Glutamate -ketoglutarate + NH3 + NADPH Glutamine -ketoglutarate + 2 NH3 + NADPH

34. Arginine -ketoglutarate + urea + 2 NH3 + 2 NADPH Proline -ketoglutarate + NH3 + 2 NADPH Histidine -ketoglutarate + N5,N10-methenyl THF + 3 NH3 + NADPH

35. Methionine + serine (Costs 3 extra ATP) methylated acceptor + 2 NH3 + pyruvate + S2- + NADH + succinyl-CoA Isoleucine (Costs 1 extra ATP) Acetyl-CoA + succinyl-CoA + NH3 + 2 NADH + FADH2 Valine (Costs 1 extra ATP) Succinyl-CoA + NH3 + 3 NADH + FADH2 Leucine (Costs 1 extra ATP) 3 Acetyl-CoA + NH3 + NADH + FADH2

36. Lysine 2 Acetyl-CoA + 3 NADH + 2 CO2 + 2 NH3 Tryptophan (Costs 2 NADPH) Formate + pyruvate + 2 acetyl-CoA + 2 NH3 + 3 CO2 + NADH Tyrosine (Costs ascorbate) Fumarate + 2 acetyl-CoA + NH3 + CO2 Phenylalanine (Costs ascorbate and NADPH) Fumarate + 2 acetyl-CoA + NH3 + CO2

39. When there is lots of glucose

40. When there is a glucose deficiency but plenty of protein

41. High fat diet