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, F₄₃₀, 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 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 MCR_red1 and MCR_Me have d_x2-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 MCR_ME species undergoes conversion to methane and to methyl-SCoM, indicating its catalytic competence as an intermediate in methanogenesis.