Geometric and Electronic Structures of the NiI and Methyl-NiIII Intermediates of Methyl-Coenzyme M Reductase

Methyl-coenzyme M reductase (MCR) catalyzes the terminal step in the formation of biological methane from methyl-coenzyme M (Me-SCoM)(center dot) and coenzyme B (CoBSH). The active site in MCR contains a Ni-F-430 cofactor, which can exist in different oxidation states. The catalytic mechanism of methane formation has remained elusive despite intense spectroscopic and theoretical investigations. On the basis of spectroscopic and crystallographic data, the first step of the mechanism is proposed to involve a nucleophilic attack of the Ni-I active state (MCRredI) on Me-SCoM to form a Ni-III-methyl intermediate, while computational studies indicate that the first step involves the attack of Nit on the sulfur of Me-SCoM, forming a CH3 center dot radical and a Ni-II-thiolate species. In this study, a combination of Ni K-edge X-ray absorption spectroscopic (XAS) studies and density functional theory (DFT) calculations have been performed on the Ni-I (MCRredI), Ni-II (MCRredI-silent), and Ni-III-methyl (MCRMe) states of MCR to elucidate the geometric and electronic structures of the different redox states. Ni K-edge EXAFS data are used to reveal a five-coordinate active site with an open upper axial coordination site in MCRredI. Ni K-pre-edge and EXAFS data and time-dependent DFT calculations unambiguously demonstrate the presence of a long Ni-C bond (similar to 2.04 angstrom) in the Ni-III-methyl state of MCR. The formation and stability of this species support mechanism I, and the Ni-C bond length suggests a homolytic cleavage of the Ni-III-methyl bond in the subsequent catalytic step. The XAS data provide insight into the role of the unique F-430 cofactor in tuning the stability of the different redox states of MCR.