The nickel-sirohydrochlorin formation mechanism of the ancestral class II chelatase CfbA in coenzyme F430 biosynthesis

• Published in: Chemical science (Cambridge) Vol. 12; no. 6; pp. 2172 - 218
• Main Authors: Fujishiro, Takashi, Ogawa, Shoko
• Format: Journal Article
• Language: English
• Published: Cambridge Royal Society of Chemistry 18.02.2021; The Royal Society of Chemistry
• Subjects: Biosynthesis; Chemistry; Cobalt; Crystal structure; Crystallography; Insertion; Nickel; Reaction mechanisms; Substrates
• ISSN: 2041-6520; 2041-6539
• DOI: 10.1039/d0sc05439a

The class II chelatase CfbA catalyzes Ni 2+ insertion into sirohydrochlorin (SHC) to yield the product nickel-sirohydrochlorin (Ni-SHC) during coenzyme F430 biosynthesis. CfbA is an important ancestor of all the class II chelatase family of enzymes, including SirB and CbiK/CbiX, functioning not only as a nickel-chelatase, but also as a cobalt-chelatase in vitro . Thus, CfbA is a key enzyme in terms of diversity and evolution of the chelatases catalyzing formation of metal-SHC-type of cofactors. However, the reaction mechanism of CfbA with Ni 2+ and Co 2+ remains elusive. To understand the structural basis of the underlying mechanisms and evolutionary aspects of the class II chelatases, X-ray crystal structures of Methanocaldococcus jannaschii wild-type CfbA with various ligands, including SHC, Ni 2+ , Ni-SHC, and Co 2+ were determined. Further, X-ray crystallographic snapshot analysis captured a unique Ni 2+ -SHC-His intermediate complex and Co-SHC-bound CfbA, which resulted from a more rapid chelatase reaction for Co 2+ than Ni 2+ . Meanwhile, an in vitro activity assay confirmed the different reaction rates for Ni 2+ and Co 2+ by CfbA. Based on these structural and functional analyses, the following substrate-SHC-assisted Ni 2+ insertion catalytic mechanism was proposed: Ni 2+ insertion to SHC is promoted by the support of an acetate side chain of SHC. The substrate-assisted nickel chelatase mechanism of CfbA in coenzyme F430 biosynthesis was unveiled by X-ray crystal structure analysis.