Structure and function of SirC from Bacillus megaterium - a metal binding precorrin-2 dehydrogenase

Heidi L Schubert , Ruth S Rose , Helen K Leech , Amanda A Brindley , Christopher P Hill , Stephen E. J. Rigby , Martin J Warren (1)

1 Department of Biosciences

In Bacillus megaterium, the synthesis of vitamin B12 (cobalamin) and siroheme diverge at sirohydrochlorin along the branched modified tetrapyrrole biosynthetic pathway. This key intermediate is made by the action of SirC, a precorrin-2 dehydrogenase that requires NAD+ as a cofactor. The structure of SirC has now been solved by X-ray crystallography to 2.8 Å resolution. The protein is shown to consist of three domains and has a similar topology to the multifunctional siroheme synthases Met8p and the N-terminal region of CysG, both of which catalyse not only the dehydrogenation of precorrin-2 but also the ferrochelation of sirohydrochlorin to give siroheme. Guided by the structure, a number of active site residues within SirC were investigated by site-directed mutagenesis. No active site general base was identified, although surprisingly some of the resulting protein variants were found to have significantly enhanced catalytic activity. Unexpectedly, SirC was found to bind metal ions such as cobalt and copper, and to bind them in an identical fashion to that observed in Met8p. It is suggested that SirC may have evolved from a Met8p-like protein by loss of its chelatase activity. It is proposed that the ability of SirC to act as a single monofunctional enzyme, in conjunction with an independent chelatase, may provide greater control over the intermediate at this branchpoint in the synthesis of siroheme and cobalamin.

Mots clés: Life Sciences

Heidi L Schubert, Ruth S Rose, Helen K Leech, Amanda A Brindley, Christopher P Hill, et al.. Structure and function of SirC from Bacillus megaterium - a metal binding precorrin-2 dehydrogenase. Biochemical Journal, 2008, 415 (2), pp.257-263. ⟨10.1042/BJ20080785⟩. ⟨hal-00479012⟩