A transglutaminase is an enzyme that catalyzes the formation of an isopeptide bond between a free amine group (e.g., protein- or peptide-bound lysine) and the acyl group at the end of the side chain of protein- or peptide-bound glutamine. The reaction also produces a molecule of ammonia. Such an enzyme is classified as EC 18.104.22.168. Bonds formed by transglutaminase exhibit high resistance to proteolytic degradation (proteolysis).
Transglutaminases were first described in 1959. The exact biochemical activity of transglutaminases was discovered in blood coagulation protein factor XIII in 1968.