Transamination: ALT, AST, PLP and Amino Group Transfer

Transamination moves an amino group from an amino acid to an alpha-keto acid, producing a new amino acid and a new keto acid. The enzymes are transaminases (aminotransferases), which belong to the transferase class (EC 2), and all of them use pyridoxal phosphate (PLP), the active form of vitamin B6.

The general reaction

amino acid 1 + alpha-keto acid 2 ↔ alpha-keto acid 1 + amino acid 2

The reaction is freely reversible and the net number of amino groups does not change. Nothing is lost as ammonia here; the amino group is only relocated.

Why PLP

PLP binds the enzyme and then the incoming amino group in a two-step "ping-pong" mechanism. The amino group is first passed to PLP (making pyridoxamine phosphate, PMP) as the first keto acid leaves. PMP then hands the amino group to the second keto acid, regenerating PLP. PLP is a cofactor, so it is regenerated and not consumed.

Glutamate is the collector

Most transaminases use alpha-ketoglutarate / glutamate as one of the two pairs. Amino groups from many amino acids end up on alpha-ketoglutarate to make glutamate. Glutamate can then release its nitrogen as ammonium (glutamate dehydrogenase, oxidative deamination), which feeds the urea cycle. Alpha-ketoglutarate is also a citric acid cycle intermediate, so transamination links amino acid breakdown to the cycle.

ALT and AST

Because ALT is more liver-specific, a raised ALT points to liver cell damage. AST rises in liver injury and also in heart and muscle injury. Both enzymes leak into blood when cells are damaged.

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Transamination versus deamination

• Transamination: swaps an amino group; no free ammonia; reversible.
• Oxidative deamination: glutamate dehydrogenase removes the amino group from glutamate as NH₄⁺ and makes alpha-ketoglutarate; uses NAD⁺ or NADP⁺.

Common traps

• Calling transaminases oxidoreductases. They are transferases (EC 2). The group moved is an amino group.
• Thinking ammonia is released. Not in transamination; that happens at glutamate dehydrogenase.
• Forgetting the cofactor. Transaminase = PLP = vitamin B6.
• Mixing the substrates. ALT uses alanine and pyruvate; AST uses aspartate and oxaloacetate.

One-minute recap

• Transaminase: transferase (EC 2), PLP-dependent, reversible.
• Amino groups funnel onto alpha-ketoglutarate to make glutamate.
• ALT: alanine/pyruvate, liver. AST: aspartate/oxaloacetate, liver, heart, muscle.