Production of cobalamin and sirohaem in Bacillus megaterium:: an investigation into the role of the branchpoint chelatases sirohydrochlorin ferrochelatase (SirB) and sirohydrochlorin cobalt chelatase (CbiX)

One the four operons required for cobalamin biosynthesis in Bacillus megaterium is also associated with sirohaem synthesis, and contains three genes, sirA, sir B and sirC. By undertaking functional complementation experiments and in vitro assays using recombinantly produced enzymes, we have been able to demonstrate that (1) SirA acts as a uroporphyrinogen III methyltransferase, transforming uroporphyrinogen III into pre-corrin-2, (2) SirC acts as an NAD(+) dehydrogenase responsible for the oxidation of precorrin-2 sirohydrochlorin, and (3) SirB acts as a ferrochelatase, responsible for the insertion of a ferrous ion into sirohydrochlorin to give sirohaem. Comparative sequence analysis reveals that the primary structure of SirB is highly similar to that of the cobalt chelatase involved in cobalamin biosynthesis in Bacillus megaterium, CbiX, with the exception that CbIX contains a C-terminal histidine-rich motif. Surprisingly, CbiX has been shown (using EPR) to contain a 4Fe-4S centre, a redox centre that is absent from SirB.