Bacterial sulphur metabolism depends on the iron-containing porphinoid sirohaem. CysG is a multi-functional siroheme synthase with S-adenosyl-L-methionine (SAM)-dependent bismethyltransferase, dehydrogenase and ferrochelatase activities. CysG synthesises sirohaem from uroporphyrinogen III via reactions which encompass two branchpoint intermediates in tetrapyrrole biosynthesis, diverting flux first from protoporphyrin IX biosynthesis and then from cobalamin (vitamin B12) biosynthesis.

CysG is a homodimeric gene fusion product, and this superfamily entry represents the dimerisation domain of CysG. The dimerisation region acts to hold the two structurally similar protomers held together asymmetrically through a number of salt-bridges across complementary residues within the dimerisation region. CysG dimerisation produces a series of active sites, accounting for CysG's multi-functionality, catalysing four diverse reactions, such as SAM-dependent methylation, NAD+-dependent tetrapyrrole dehydrogenation and metal chelation.

PFAM:PF10414, INTERPRO:IPR037115,DOI:10.1038/nsb1007PMID:14595395