Transamination and Aminotransferases

Core Idea

Transamination is the reversible transfer of an amino group from an amino acid to a keto acid, catalyzed by aminotransferases. The reaction requires pyridoxal phosphate (PLP) as a cofactor and is the primary mechanism for both amino acid synthesis and degradation. The amino group typically transfers to α-ketoglutarate, forming glutamate.

How It's Best Learned

Draw the PLP-mediated mechanism showing Schiff base formation. Compare ALT and AST in serum—when and why they are elevated in disease. Calculate amino acid pools using transamination.

Common Misconceptions

Transamination removes ammonia directly; it transfers the amino group to another keto acid. The reaction is freely reversible, not unidirectional.

Explainer

Amino acids are unique among biomolecules because they carry nitrogen — and managing that nitrogen is one of metabolism's central challenges. Transamination is the reaction that shuttles amino groups between molecules, and it is the entry point for both amino acid synthesis and degradation. If you understand amino acid structure (an amino group, a carboxyl group, and a variable R group on a central carbon) and the basics of enzyme kinetics, transamination is where those concepts converge in a single, elegant reaction.

The reaction itself is conceptually simple: an amino acid donates its amino group to a keto acid (an α-keto acid, which has a carbonyl where the amino group would be). The amino acid becomes a keto acid, and the keto acid becomes an amino acid. It is a molecular swap — nitrogen moves from one carbon skeleton to another, and neither molecule is destroyed. For example, alanine (amino acid) + α-ketoglutarate (keto acid) → pyruvate (keto acid) + glutamate (amino acid). The enzyme catalyzing this particular reaction is alanine aminotransferase (ALT), and its counterpart aspartate aminotransferase (AST) transfers the amino group from aspartate to α-ketoglutarate. Both are clinically measured in blood tests — elevated ALT and AST indicate liver damage because these enzymes leak from injured hepatocytes.

What makes transamination mechanistically fascinating is its absolute dependence on the cofactor pyridoxal phosphate (PLP), the active form of vitamin B₆. PLP acts as a molecular intermediary: first, it forms a Schiff base (a covalent bond between its aldehyde group and the amino acid's amino group), then facilitates the transfer of the amino group through a series of electron rearrangements. Midway through the reaction, PLP temporarily carries the amino group as pyridoxamine phosphate (PMP), then donates it to the incoming keto acid. This ping-pong mechanism means the enzyme cycles between two forms — PLP-bound and PMP-bound — with each half-reaction handling one substrate.

The metabolic significance of transamination lies in its role as a nitrogen funnel. Most amino acids cannot be directly deaminated (have their nitrogen removed as free ammonia). Instead, their amino groups are first transaminated onto α-ketoglutarate, producing glutamate — the universal nitrogen collector. Glutamate can then be oxidatively deaminated by glutamate dehydrogenase to release free NH₄⁺, which enters the urea cycle for excretion. This two-step process (transamination → oxidative deamination) is how the body safely handles the nitrogen from protein breakdown. Because the reaction is freely reversible, transamination also works in the biosynthetic direction — cells can synthesize nonessential amino acids by transferring amino groups onto available carbon skeletons.