This family consists of uroporphyrinogen-III synthase HemD EC:4.2.1.75 also known as Hydroxymethylbilane hydrolyase (cyclizing) from eukaryotes, bacteria and archaea. This enzyme catalyses 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 Swiss:Q59294 with Pfam:PF00590, however the aligned region corresponds with the uroporphyrinogen-III synthase EC:4.2.1.75 activity only. Uroporphyrinogen-III synthase is the fourth enzyme in the heme pathway [2]. 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 [1].

1: Molecular basis of congenital erythropoietic porphyria: mutations in the human uroporphyrinogen III synthase gene. Xu W, Astrin KH, Desnick RJ; Hum Mutat 1996;7:187-192. PMID:8829650

2: Isolation of the gene HEM4 encoding uroporphyrinogen III synthase in Saccharomyces cerevisiae. Amillet JM, Labbe-Bois R; Yeast 1995;11:419-424. PMID:7597845