1. February 14 Chapter 26 Amino Acid Metabolism
Biochemistry 432/832
February 14
Chapter 26
Amino Acid Metabolism

2. Announcements: -

3. Outline 26.1 The Two Major Pathways of N Acquisition
26.2 The Fate of Ammonium
26.3 Glutamine Synthetase
26.4 Amino Acid Biosynthesis
26.5 Metabolic Degradation of Amino Acids

4. The nitrogen cycle

5. The glutamate dehydrogenase reaction

6. The glutamine synthetase reaction

7. The glutamate dehydrogenase/glutamine synthase pathway
Two N fixing steps - one inefficient
One each

8. The glutamate synthase reaction

9. The glutamine synthase/GOGAT pathway
One N fixing step - inefficient but expensive
One NADPH
Two ATP

10. Glutamine Synthetase A Case Study in Regulation
GS in E. coli is regulated in three ways:
Feedback inhibition
Covalent modification (interconverts between inactive and active forms)
Regulation of gene expression and protein synthesis - - control the amount of GS in cells

11. Allosteric regulation of glutamine synthase activity by feedback inhibition

12. Covalent Modification of Glutamine Synthetase
Each subunit is adenylylated at Tyr-397
Adenylylation inactivates GS
Adenylyl transferase catalyzes both the adenylylation and deadenylylation
PII (regulatory protein) controls both activities
AT:PIIA catalyzes adenylylation
AT:PIID catalyzes deadenylylation
-ketoglutarate and Gln regulate PII

13. Covalent modification of glutamine synthase - adenylylation of Tyr397

14. The cyclic cascade system that regulates the covalent modification of GS

15. Gene Expression regulates GS
Gene GlnA is actively transcribed only if transcriptional enhancer NRI is in its phosphorylated form, NRI-P
NRI is phosphorylated by NRII, a protein kinase
If NRII is complexed with PIIA (inactivator) it acts as a phosphatase, not a kinase

16. Transcriptional regulation of GlnA expression through the reversible phosphorylation of NR1
Activated by glutamine, inactivated by a-ketoglutarate

17. Amino Acid Biosynthesis
Plants and microorganisms can make all 20 amino acids and all other organisms need N metabolites
In these organisms, glutamate is the source of N, via transamination (aminotransferase) reactions
Mammals can make only 10 of the 20 amino acids
The others are classed as "essential" amino acids and must be obtained in the diet
All amino acids are grouped into families according to the intermediates that they are made from

18. Glutamate-dependent transamination - primary mechanism for amino acid synthesis

21. The -Ketoglutarate Family
Glu, Gln, Pro, Arg, and sometimes Lys

22. Proline biosynthesis from glutamate

23. Synthesis of ornithine from glutamate - a step in arginine biosynthesis

24. The Urea Cycle
N and C in the guanidino group of Arg come from NH4+, HCO3- (carbamoyl-P), and the -NH2 of Glu and Asp
Breakdown of Arg in the urea cycle releases two N and one C as urea
Important N excretion mechanism in livers of terrestrial vertebrates
Urea cycle is linked to TCA by fumarate

25. The urea cycle

26. The Aspartate Family Asp, Asn, Lys, Met, Thr, Ile
Transamination of oxaloacetate gives Asp
Amidation of Asp gives Asn
Thr, Met, and Lys are made from Asp

27. Transamination of oxaloacetate yields aspartate

28. Asp + Gln --> Asn + Glu

29. The Pyruvate Family Ala, Val, Leu
Transamination of pyruvate gives Ala
Val is derived from pyruvate
Leu synthesis, like that of Ile and Val, begins with an -keto acid
Transaminations from Glu complete each of these pathways

30. 3-Phosphoglycerate Family Ser, Gly, Cys
3-Phosphoglycerate dehydrogenase diverts 3-PG from glycolysis to amino acid paths
Transamination by Glu gives 3-P-serine
Phosphatase yields serine
A PLP-dependent enzyme makes Cys

31. Biosynthesis of serine from 3-phosphoglycerate

32. Glycine biosynthesis from serine

33. Cysteine biosynthesis
Sulfhydration of serine by sulfide
Sulfhydration of O-acetylserine

34. Sulfate assimilation

35. Degradation of Amino Acids
The 20 amino acids are degraded to produce TCA and glycolytic intermediates

36. Degradation of amino acids

37. Degradation of Ala, Ser, Cys, Gly and Trp and Thr to pyruvate