4 porphobilinogen + H2O <=> hydroxymethylbilane + 4 NH4(+)
4 porphobilinogen + h2o <=> hydroxymethylbilane + 4 nh4(+)
4 porphobilinogen + H2O <=> hydroxymethylbilane + 4 NH4(+)
4 porphobilinogen + h2o <=> hydroxymethylbilane + 4 nh4(+)