Abstract
Cytochrome b562 (Cyt b562) reconstituted with nickel didehydrocorrin (NiII(DDHC)), a protein-based functional model of methyl-coenzyme M reductase (MCR), was investigated to demonstrate methane generation via intraprotein cleavage of a C–S bond. NiII(DDHC) was synthesized as a model complex of an MCR cofactor known as F430 and found to show NiII/NiI redox behavior with a potential of −0.61 V vs. Ag|AgCl. This potential is slightly positive-shifted compared to that of F430 without protein. Conjugation of NiII(DDHC) with the apo-form of Cyt b562 provides reconstituted Cyt b562 (rCyt b562(NiII(DDHC))) which was characterized by spectroscopic measurements. Photoirradiation of rCyt b562(NiII(DDHC)) generates methane gas in the presence of tris(2,2′-bipyridine)ruthenium(II) chloride as a photosensitizer and sodium ascorbate as a sacrificial reagent. Further experiments using Cyt b562 mutants indicate that methane is derived from the CH3S group of the methionine residue in the heme-binding site where thioether, thiol and the nickel center are precisely arranged. The present study demonstrates the first example of methane generation via intraprotein cleavage of a C–S bond using a functional model of MCR.
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