amino acid catabolism

Meaning: Amino acid catabolism is the breakdown of amino acids for energy, mainly in fasting or with excess dietary protein. The nitrogen and the carbon skeleton are handled separately. Aminotransferases, which use PLP (from vitamin B6) as a cofactor, move the α-amino group to α-ketoglutarate, making glutamate. Glutamate dehydrogenase then releases NH₄⁺, which the liver converts to urea. The leftover α-keto acid enters metabolism: glucogenic amino acids yield pyruvate or citric acid cycle intermediates, while ketogenic ones yield acetyl-CoA or acetoacetate. Leucine and lysine are purely ketogenic.

Equation: amino acid + α-ketoglutarate ⇌ α-keto acid + glutamate. This is a reversible transamination. No SI units apply because it is a reaction, not a quantity.

On the MCAT: Expect questions on where the nitrogen goes (urea cycle, then excretion by the kidney), which carbon skeletons can make glucose versus ketone bodies, and predictions that fasting raises urea production and gluconeogenesis from amino acids.

Common confusion: Transamination only transfers the amino group. Free NH₄⁺ is released by oxidative deamination of glutamate. Also, ketogenic products such as acetyl-CoA cannot give net glucose in animals, because the pyruvate dehydrogenase step is irreversible.