Nitrogen metabolism — recognizing oxidative deamination vs transamination Which reaction is a correct example of oxidative deamination (removal of an amino group with concurrent oxidation), rather than transamination?

Introduction / Context:
Nitrogen removal from amino acids occurs by two major mechanisms: transamination (amino group transfer) and oxidative deamination (amino group release as free ammonia with oxidation of the carbon skeleton). Distinguishing these pathways is fundamental for understanding the urea cycle and hepatic nitrogen handling.

Given Data / Assumptions:

• Glutamate dehydrogenase catalyzes oxidative deamination of glutamate to alpha-ketoglutarate + NH3 with NAD+ or NADP+ as cofactor.
• Transaminases swap amino groups between amino acids and alpha-keto acids without releasing free ammonia.
• Non-physiologic substrate pairings are implausible in core amino acid pathways.

Concept / Approach:

Scan each reaction: if NH3 is released and the amino acid carbon becomes an alpha-keto acid, it is oxidative deamination. If amino groups are exchanged between partners with no free ammonia, it is transamination. Only the glutamate → alpha-ketoglutarate + NH3 reaction matches oxidative deamination.

Step-by-Step Solution:

Verification / Alternative check:

Standard hepatic nitrogen flow: most amino acids first undergo transamination to glutamate; glutamate then undergoes oxidative deamination, releasing NH3 for urea synthesis.

Why Other Options Are Wrong:

Option B and E are transamination reactions. Options A and D include hexanoic acid, a fatty acid, not an alpha-keto acid partner in amino transfer, making them incorrect.

Common Pitfalls:

Assuming any ammonia release equals oxidative “deamination” without checking the carbon skeleton product, or accepting unrealistic reaction partners.

Final Answer:

Glutamate → alpha-ketoglutarate + NH3