-!- This enzyme catalyzes the third of three steps leading to the formation of siroheme from uroporphyrinogen III. -!- The first step involves the donation of two S-adenosyl-L-methionine- derived methyl groups to carbons 2 and 7 of uroporphyrinogen III to form precorrin-2 (EC 2.1.1.107) and the second step involves an NAD(+)-dependent dehydrogenation to form sirohydrochlorin from precorrin-2 (EC 1.3.1.76). -!- In Saccharomyces cerevisiae, the last two steps are carried out by a single bifunctional enzyme, Met8p. -!- In some bacteria, steps 1-3 are catalyzed by a single multifunctional protein called CysG, whereas in Bacillus megaterium, three separate enzymes carry out each of the steps, with SirB being responsible for the above reaction.

-!- This enzyme is a type II chelatase, being either a monomer (CbiX) or a homodimer (CibK) and being ATP-independent. -!- CbiK from Salmonella enterica uses precorrin-2 as the substrate to yield cobalt-precorrin-2. -!- The enzyme contains two histidines at the active site that are thought to be involved in the deprotonation of the tetrapyrrole substrate as well as in metal binding. -!- CbiX from Bacillus megaterium inserts cobalt at the level of sirohydrochlorin (factor-II) rather than precorrin-2.