hydroxymethylbilane synthase;
HMB-synthase;
porphobilinogen deaminase;
pre-uroporphyrinogen synthase;
uroporphyrinogen I synthase;
uroporphyrinogen I synthetase;
uroporphyrinogen synthase;
uroporphyrinogen synthetase;
porphobilinogen ammonia-lyase (polymerizing);
(4-[2-carboxyethyl]-3-[carboxymethyl]pyrrol-2-yl)methyltransferase (hydrolysing)

Transferases;
Transferring alkyl or aryl groups, other than methyl groups;
Transferring alkyl or aryl groups, other than methyl groups (only sub-subclass identified to date)

porphobilinogen:(4-[2-carboxyethyl]-3-[carboxymethyl]pyrrol-2-yl)methyltransferase (hydrolysing)

4 porphobilinogen + H2O = hydroxymethylbilane + 4 NH3 [RN:R00084]

R00084

porphobilinogen [CPD:C00931];
H2O [CPD:C00001]

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 4.2.1.75 uroporphyrinogen-III synthase, which is often called cosynthase, the product is cyclized to form uroporphyrinogen-III. If EC 4.2.1.75 is absent, the hydroxymethylbilane cyclizes spontaneously to form uroporphyrinogen I.