Tetrapolymerization of the monopyrrole PBG into the hydroxymethylbilane pre-uroporphyrinogen in several discrete steps. H2O + 4 porphobilinogen = hydroxymethylbilane + 4 NH4(+) Binds 1 dipyrromethane group covalently. Monomer. The porphobilinogen subunits are added to the dipyrromethane group. Belongs to the HMBS family.