29.9 Catabolism of Proteins: Deamination

Protein catabolism breaks down proteins into amino acids, then removes their amino groups. This process, called deamination, is crucial for recycling amino acids and managing nitrogen in the body.

Transamination transfers amino groups between amino acids and keto acids, using enzymes and cofactors. The resulting products feed into various metabolic pathways, while excess nitrogen is converted to urea for safe excretion.

Protein Catabolism: Deamination

Process of transamination in catabolism

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• Transfers amino group from amino acid to α-keto acid catalyzed by transaminases (aminotransferases)
• Pyridoxal phosphate (PLP) cofactor stabilizes negative charge on α-carbon of amino acid during reaction as electron sink
• Amino group transferred from amino acid to PLP forms pyridoxamine phosphate (PMP) and α-keto acid
• PMP transfers amino group to another α-keto acid regenerates PLP and produces new amino acid (alanine and glutamate)

Conversion of amino acids to keto acids

1. α-amino group of amino acid forms Schiff base with aldehyde group of PLP
2. Schiff base tautomerizes resulting in quinonoid intermediate formation
3. Quinonoid intermediate hydrolyzes releasing α-keto acid product and forming PMP
4. PMP forms Schiff base with another α-keto acid substrate (pyruvate and α-ketoglutarate)
5. Schiff base tautomerizes forming second quinonoid intermediate
6. Second quinonoid intermediate hydrolyzes releasing new amino acid product and regenerating PLP

Regeneration of pyridoxal phosphate

• PMP formed during first half-reaction contains amino group from original amino acid substrate
• PMP forms Schiff base with new α-keto acid substrate
• Schiff base tautomerizes forming quinonoid intermediate
• Quinonoid intermediate hydrolyzes releasing new amino acid product and regenerating PLP
• Regenerated PLP available to participate in another round of transamination completing catalytic cycle

Protein Breakdown and Nitrogen Metabolism

• Proteolysis initiates protein catabolism by breaking down proteins into individual amino acids
• Oxidative deamination removes amino groups from amino acids, catalyzed by glutamate dehydrogenase
• Ammonia produced during deamination is toxic and must be eliminated
• Urea cycle converts ammonia into urea for excretion, maintaining nitrogen balance in the body