Evidence for organometallic intermediates in bacterial methane formation involving the nickel coenzyme F₄₃₀.

Abstract

Bioorganometallic chemistry underlies the reaction mechanisms of metalloenzymes that catalyze key processes in the global carbon cycle. Metal ions that appear well suited for the formation of metal-carbon bonds are nickel, iron, and cobalt. The formation and reactivity of alkylcobalt species (methylcobalamin and adenosylcobalamin) at the active sites of B₁₂-dependent methyltransferases and isomerases have been well studied and serve as models to guide hypothesis for how organometallic reactions occur in other systems. This review focuses on methyl-coenzyme M reductase (MCR), which is responsible for all biologically produced methane on earth. At its active site, this enzyme contains a nickel corphin (F₄₃₀), which bears similarity to the cobalt corrin in cobalamin (B₁₂). Several mechanisms have been proposed for the MCR-catalyzed reaction, and a methylnickel species is a central intermediate in all but one of these mechanisms. After introducing some important concepts of bioorganometallic chemistry and describing methanogenesis and some of the key properties of MCR, this review discusses research that has led to the generation and characterization of alkylnickel species in MCR and in model complexes related to F₄₃₀. Then, the focus shifts to the reactions that these alkylnickel species can undergo both in the enzyme and in bioinspired models: protonolysis to form alkanes and thiolysis to form thioethers, including methyl-SCoM (the natural methyl donor for MCR). Throughout, results are discussed in relation to the proposed models for the MCR mechanism.