The enzyme works by stepwise addition of pyrrolylmethyl groups until a hexapyrrole is present at the active centre. The terminal tetrapyrrole is then hydrolysed to yield the product, leaving a cysteine-bound dipyrrole on which assembly continues. In the presence of a second enzyme, EC 18.104.22.168 uroporphyrinogen-III synthase, which is often called cosynthase, the product is cyclized to form uroporphyrinogen-III. If EC 22.214.171.124 is absent, the hydroxymethylbilane cyclizes spontaneously to form uroporphyrinogen I.
EC 126.96.36.199 created 1972 as EC 188.8.131.52, transferred 2003 to EC 184.108.40.206