Bacterial sulfur metabolism depends on the iron-containing porphinoid sirohaem. CysG, S-adenosyl-L-methionine (SAM)-dependent bis-methyltransferase, dehydrogenase and ferrochelatase, 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 dimer of two structurally similar protomers held together asymmetrically through a number of salt-bridges across complementary residues in the CysG_dimeriser region to produce a series of active sites, accounting for CysG's multifunctionality, catalysing four diverse reactions: two SAM-dependent methylations, NAD+-dependent tetrapyrrole dehydrogenation and metal chelation. The CysG_dimeriser region holding the two protomers together is of 74 residues [1].

1: CysG structure reveals tetrapyrrole-binding features and novel regulation of siroheme biosynthesis. Stroupe ME, Leech HK, Daniels DS, Warren MJ, Getzoff ED; Nat Struct Biol. 2003;10:1064-1073. PMID:14595395