Methyl-coenzyme M reductase (MCR) from methanogenic archaea catalyzes the final step of methane formation, in which methyl-coenzyme M (2- methylthioethanesulfonate, methyl-SCoM) is reduced with coenzyme B (N-(7-mercaptoheptanoyl)threonine phosphate, CoBSH) to form methane and the heterodisulfide CoBS-SCoM. The active dimeric form of MCR contains two Ni(I)-F430 prosthetic groups, one in each monomer. This report describes studies of the reaction of the active Ni(I) state of MCR (MCR red1) with BES (2-bromoethanesulfonate) and CoBSH or its analogue, CoB6SH (N-(6-mercaptohexanoyl)threonine phosphate), by transient kinetic measurements using EPR and UV-visible spectroscopy and by global fits of the data. This reaction is shown to lead to the formation of three intermediates, the first of which is assigned as an alkyl-Ni(III) species that forms as the active Ni(I)-MCRred1 state of the enzyme decays. Subsequently, a radical (MCRBES radical) is formed that was characterized by multifrequency electron paramagnetic resonance (EPR) studies at X- (∼9 GHz), Q- (∼35 GHz), and D- (∼130 GHz) bands and by electron-nuclear double resonance (ENDOR) spectroscopy. The MCRBES radical is characterized by g-values at 2.00340 and 1.99832 and includes a strongly coupled nonexchangeable proton with a hyperfine coupling constant of 50 MHz. Based on transient kinetic measurements, the formation and decay of the radical coincide with a species that exhibits absorption peaks at 426 and 575 nm. Isotopic substitution, multifrequency EPR, and ENDOR spectroscopic experiments rule out the possibility that MCRBES is a tyrosyl radical and indicate that if a tyrosyl radical is formed during the reaction, it does not accumulate to detectable levels. The results provide support for a hybrid mechanism of methanogenesis by MCR that includes both alkyl-Ni and radical intermediates.
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