• Korean Journal of Microbiology
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• v.55 no.2
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• pp.123-130
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• 2019

Chelatase catalyzes the insertion of divalent metal into tetrapyrrole and plays a key role in the biosynthesis of metallated tetrapyrroles, such as cobalamin, siroheme, heme, and chlorophyll. SirB is a sirohydrochlorin (SHC) chelatase that generates cobalt-SHC or iron-SHC by inserting cobalt or iron into the center of sirohydrochlorin tetrapyrrole. To provide structural insights into the metal-binding and SHC-recognition mechanisms of SirB, we determined the crystal structure of SirB from Bacillus subtilis subsp. spizizenii (bssSirB) in complex with cobalt ions. bssSirB forms a monomeric ${\alpha}/{\beta}$ structure that consists of two domains, an N-terminal domain (NTD) and a C-terminal domain (CTD). The NTD and CTD of bssSirB adopt similar structures with a four-stranded ${\beta}-sheet$ that is decorated by ${\alpha}-helices$. bssSirB presents a highly conserved cavity that is generated between the NTD and CTD and interacts with a cobalt ion on top of the cavity using two histidine residues of the NTD. Moreover, our comparative structural analysis suggests that bssSirB would accommodate an SHC molecule into the interdomain cavity. Based on these structural findings, we propose that the cavity of bssSirB functions as the active site where cobalt insertion into SHC occurs.