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

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, F 430, in which the Ni is in the 1+ oxidation state in the active form, MCR red1 . We describe the preparation and spectroscopic characterization of a Ni−methyl species, denoted MCR Me, generated from MCR red1 by reaction with CH 3 I. EPR and 13 C, 1,2 H pulsed ENDOR spectra of methyl isotopologues (CH 3, CD 3, 13 CH 3 ) umambiguously establish the presence of CH 3 −Ni(III) moiety. They explain why both MCR red1 and MCR Me have d x 2 - y 2 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 MCR Me species undergoes conversion to methane and to methyl−SCoM, indicating its catalytic competence as an intermediate in methanogenesis.