Biochemical and Spectroscopic Studies of the Electronic Structure and Reactivity of a Methyl−Ni Species Formed on Methyl-Coenzyme M Reductase

Abstract

The enzyme methyl coenzyme M reductase (MCR) catalyzes the final step of methane production by methanogenic organisms. The active site contains a Ni−macrocyclic complex, F430, in which the Ni is in the 1+ oxidation state in the active form, MCRred1. We describe the preparation and spectroscopic characterization of a Ni−methyl species, denoted MCRMe, generated from MCRred1 by reaction with CH3I. EPR and 13C, 1,2H pulsed ENDOR spectra of methyl isotopologues (CH3, CD3, 13CH3) umambiguously establish the presence of CH3−Ni(III) moiety. They explain why both MCRred1 and MCRMe have dx2-y2 odd-electrons although formally having Ni(I) in the former and Ni(III) in the latter. The simple MO description further gives a simple explanation to the small transfer of spin density (∼1%) from Ni to methyl. The MCRMe species undergoes conversion to methane and to methyl−SCoM, indicating its catalytic competence as an intermediate in methanogenesis.