Probing the Reaction Mechanism of Methyl Coenzyme M Reductase

Abstract

Methyl coenzyme M reductase (MCR) is the key enzyme in both microbial methane production and anaerobic methane oxidation. MCR catalyzes the reaction of methyl-coenzyme M (CH3SCoM) and coenzyme B (HSCoB) to methane and the corresponding heterodisulfide CoMSSCoB. MCR is a large heterohexameric protein complex (aß?)2 containing two 50 Å long active sites channels. Central to this activity is the nickel-containing tetrapyrrole factor 430 (F430). Coenzyme F430 is embedded at the channel bottom and the substrates CH3S-CoM and HSCoB bind in front of F430 into a solvent free and hydrophobic channel. Two principally different catalytic mechanisms are currently discussed. Mechanism I is based on a nucleophilic attack of Ni(I) onto the methyl group of CH3SCoM yielding methylNi(III) and mechanism II on an attack of vi Ni(I) onto the thioether sulfur of CH3SCoM generating a Ni(II)SCoM intermediate. However, both mechanisms have been criticized because the forming Ni-Me bond is much weaker than the breaking S-Me bond (in CH3-SCoM-), which would make that step in the cycle unrealistic. Recently there is a new DFT-based mechanism proposed. This mechanism the full F430 cofactor of MCR along with a coordinated O=CH2CH2C(H)NH2C(H)O (surrogate for glutamine) as a model of the active site for conversion of CH3SCoM- (CH3SCH2CH2SO3-) and HSCoB to methane plus the corresponding heterodisulfide. We studied on the interaction of MCR with bromo-alkyl compounds that inhibit the enzyme. X-ray absorption and ENDOR studies show the presence of a Ni-C bond after incubation with bromomethane (BrMe), bromomethane (BES) and 3-bromopropane sulfonate (BPS). In addition we showed that the geometry around the nickel becomes highly asymmetric when the propyl-sulfonate group is bound to the nickel. We studied on heterodisulfide. Data shows that back reaction is present. These are in line with the new DFT-based mechanism that predicts that the nickel atom protrudes from the tetrapyrrole plane during the reaction.