This family consists of uroporphyrinogen-III synthase HemD EC:126.96.36.199 also known as Hydroxymethylbilane hydrolyase (cyclizing) from eukaryotes, bacteria and archaea. This enzyme catalyzes the reaction: Hydroxymethylbilane <=> uroporphyrinogen-III + H(2)O. Some members of this family are multi-functional proteins possessing other enzyme activities related to porphyrin biosynthesis, such as HemD with pfam00590, however the aligned region corresponds with the uroporphyrinogen-III synthase EC:188.8.131.52 activity only. Uroporphyrinogen-III synthase is the fourth enzyme in the heme pathway. Mutant forms of the Uroporphyrinogen-III synthase gene cause congenital erythropoietic porphyria in humans a recessive inborn error of metabolism also known as Gunther disease.