Porphobilinogen deaminase

Porphobilinogen deaminase (EC 2.5.1.61, Older sources categorize it under EC4.3.1.8), also known as hydroxymethylbilane synthase or uroporphyrinogen I synthase is an enzyme that in humans is encoded by the HMBS gene. Porphobilinogen deaminase is involved in the third step of the heme biosynthetic pathway. It catalyzes the head to tail condensation of four porphobilinogen molecules into the linear hydroxymethylbilane while releasing four ammonia molecules.