Is There a Ni-Methyl Intermediate in the Mechanism of Methyl-Coenzyme M Reductase?

The formation of methyl-Ni(F(430)) species in methyl-coenzyme M reductase (MCR) has been investigated using the B3LYP hybrid density functional method and an active-site model built on the basis of X-ray crystal structure. CH(3)-I, CH(3)-Br, CH(3)-Cl, and CH(3)-S-CH(3) were chosen as the substrates, the last one regarded as a model of the native substrate (methyl-coenzyme M, CH(3)-SCoM). The calculations indicate that the formation of CH(3)-Ni(F(430)) in MCR is dependent on the acidity of the substrate leaving group. A CH(3)-Ni(F(430)) species has been observed with methyl halides as substrates, while the formation of CH(3)-Ni(F(430)) from the native substrate is demonstrated to be inaccessible energetically. These results agree well with the current experiments.