Methyl-coenzyme M reductase (MCR), the enzyme responsible for the microbial formation of methane, is a 300-kilodalton protein organized as a hexamer in an alpha2beta2gamma2 arrangement. The crystal structure of the enzyme from Methanobacterium thermoautotrophicum, determined at 1.45 angstrom resolution for the inactive enzyme state MCRox1-silent, reveals that two molecules of the nickel porphinoid coenzyme F430 are embedded between the subunits alpha, alpha', beta, and gamma and alpha', alpha, beta', and gamma', forming two identical active sites. Each site is accessible for the substrate methyl-coenzyme M through a narrow channel locked after binding of the second substrate coenzyme B. Together with a second structurally characterized enzyme state (MCRsilent) containing the heterodisulfide of coenzymes M and B, a reaction mechanism is proposed that uses a radical intermediate and a nickel organic compound.

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The nickel porphinoid, coenzyme F430, is the prosthetic group of methyl-coenzyme M reductase. The active form of the enzyme exhibits Ni-EPR signals designated as MCR-red1 and MCR-red2. The inactive form of the enzyme is either EPR silent or it exhibits a distinct Ni-EPR signal designated MCR-0x1. Evidence is presented here that the MCR-ox1 form of the enzyme can be converted in vitro to the MCRred1 form by reduction with titanium(II1) citrate at pH 9. During conversion, the specific activity increases with increasing MCR-red1 spin concentration from 2 U/mg to approximately 100 U/mg at spin concentrations higher than 80 %. The reduced methyl-coenzyme-M reductase shows an ultraviolethisible spectrum characteristic for coenzyme F430 in the Ni(1) oxidation state, with maxima at 386 nm and at 750 nm. The results indicate that methyl-coenzyme-M reductase is activated when the enzyme-bound coenzyme F430 is reduced to the Ni(1) oxidation state. The experiments were performed with purified methylcoenzyme-M reductase isoenzyme I of Methanobacterium thermoautotrophicum (strain Marburg).