Notes on Amino Acid Metabolism

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Amino Acid Metabolism

• Amino acids are also energy metabolites and precursors of many biologically active products.

• Amino acids are classified as essential and nonessential.

• Essential: required by diet; nonessential: synthesized by mammals using other molecules.

• Excess dietary amino acids are neither stored nor excreted, instead, they are converted to intermediates like pyruvate, oxaloacetate, acetyl-CoA, and alpha-ketoglutarate.

• Amino acids are broken down in three stages: deamination, incorporation of ammonia and aspartate nitrogen atoms into urea for excretion, and conversion of amino acid carbon skeletons to common metabolic intermediates.

Amino Acid Deamination

• Removal of alpha-amino group with the object of excreting excess nitrogen and degrading the remaining carbon skeleton of converting it to glucose.

• Transamination: Aminotransferase reactions occur in two stages: amino group of amino acid is transferred to the enzyme, producing the corresponding keto acid and aminated enzyme, and the amino group is transferred to the keto acceptor, forming the amino acid product and regenerating the enzyme.

• Oxidative Deamination: Glutamate Dehydrogenase: Glutamate is oxidatively deaminated in the mitochondrion by glutamate dehydrogenase (the only known enzyme that can accept NAD+ and NADP+ as its redox coenzyme).

• Two nonspecific amino acid oxidases, L-amino acid oxidases and D-amino acid oxidases, catalyzes the oxidation of L- and D- amino acids, utilizing FAD as their redox coenzyme.

Urea Cycle

• Metabolic breakdown of amino acids results in excess nitrogen.

• Mammals (ureotelic beings) excrete the nitrogen as urea, a less toxic product of nitrogen.

• Urea is synthesized by the liver by using enzymes of the urea cycle.

Paths of Degradation

• Alanine, cysteine, glycine, serine and threonine are degraded to pyruvate.

• Asparagine and aspartate are degraded to oxaloacetate.

• Arginine, glutamate, glutamine, histidine and praline are degraded to alpha-ketoglutarate.

• Isoleucine, methionine, and valine are degraded to succinyl-CoA.

• Tryptophan is degraded to alanine and acetoacetate.

• Phenylalanine and tyrosine are degraded to fumarate and acetocetate.

Additional Readings:

Basic Biochemistry

1. Nucleic Acid Structure and Organization
2. DNA Replication and Repair
3. Transcription and RNA Processing
4. Genetic Code, Mutations, and Translation
5. Genetic Regulation
6. Recombinant DNA
7. Amino Acids, Proteins, Enzymes
8. Hormones
9. Vitamins
10. Energy Metabolism
11. Glycolysis and Pyruvate Dehydrogenase
12. Citric Acid Cycle and Oxidative Phosphorylation
13. Glycogen, Gluconeogenesis, and Hexose Monophosphate Shunt
14. Lipid Synthesis and Storage
15. Lipid Mobilization and Catabolism
16. Amino Acid Metabolism Disorders
17. Purine and Pyrimidine Metabolism
18. Electron Transport
19. Citric Acid Cycle and Glyoxylate Cycle
20. Glycolysis
21. Pyruvate Metabolism
22. Mitochondrial ATP formation
23. Gluconeogenesis
24. Glycogen Metabolism
25. Nitrogen Fixation (Metabolism) reactions, and Heme Metabolism
26. Amino Acid Metabolism
27. What is Medium Chain Acyl-CoA Dehydrogenase Deficiency (MCADD)?

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