Threonine ammonia-lyase, also commonly referred to as threonine deaminase or threonine dehydratase, is an enzyme responsible for catalyzing the conversion of L-threonine into alpha-ketobutyrate and ammonia. Alpha-ketobutyrate can be converted into L-isoleucine, so threonine ammonia-lyase functions as a key enzyme in BCAA synthesis. It employs a pyridoxal-5'-phosphate cofactor, similar to many enzymes involved in amino acid metabolism. It is found in bacteria, yeast, and plants, though most research to date has focused on forms of the enzyme in bacteria. This enzyme was one of the first in which negative feedback inhibition by the end product of a metabolic pathway was directly observed and studied. The enzyme serves as an excellent example of the regulatory strategies used in amino acid homeostasis.

Threonine ammonia-lyase is a tetramer of identical subunits, and is arranged as a dimer of dimers. Each subunit has two domains: a domain containing the catalytic active site and a domain with allosteric regulatory sites. The two have been shown to be distinct regions, but the regulatory site of one subunit actually interacts with the catalytic site of another subunit. Both domains contain the repeating structural motif of beta sheets surrounded by alpha helices. While the threonine binding site is not perfectly understood, structural studies do reveal how the pyridoxal phosphate cofactor is bound. The PLP cofactor is bonded to a lysine residue by means of a Schiff base, and the phosphate group of PLP is held in place by amine groups derived from a repeating sequence of glycine residues. The aromatic ring is bound to phenylalanine, and the nitrogen on the ring is hydrogen bonded to hydroxyl group-containing residues.

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