Porphobilinogen deaminase

Porphobilinogen deaminase (hydroxymethylbilane synthase, or uroporphyrinogen I synthase) is an enzyme (EC 2.5.1.61) 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:

4 porphobilinogen + H₂O ${\displaystyle \rightleftharpoons }$ hydroxymethylbilane + 4 NH₃