Ni(II) and Ni(I) forms of pentaalkylamide derivatives of cofactor F430 of Methanobacterium thermoautotrophicum

Spectroscopic investigation of the nickel-containing porphinoid cofactor F430. Comparison of the free cofactor in the +1, +2 and +3 oxidation states with the cofactor bound to methyl-coenzyme M reductase in the silent, red and ox forms

Methyl-coenzyme M reductase (MCR) catalyzes the methane-forming step in methanogenic archaea. It contains the nickel porphinoid F(430), a prosthetic group that has been proposed to be directly involved in the catalytic cycle by the direct binding and subsequent reduction of the substrate methyl-coenzyme M. The active enzyme (MCRred1) can be generated in vivo and in vitro by reduction from MCRox1, which is an inactive form of the enzyme. Both the MCRred1 and MCRox1 forms have been proposed to contain F(430) in the Ni(I) oxidation state on the basis of EPR and ENDOR data. In order to further address the oxidation state of the Ni center in F(430), variable-temperature, variable-field magnetic circular dichroism (VTVH MCD), coupled with parallel absorption and EPR studies, have been used to compare the electronic and magnetic properties of MCRred1, MCRox1, and various EPR silent forms of MCR, with those of the isolated penta-methylated cofactor (F(430)M) in the (+)1, (+)2 and (+)3 oxidation states. The results confirm Ni(I) assignments for MCRred1 and MCRred2 forms of MCR and reveal charge transfer transitions involving the Ni d orbitals and the macrocycle pi orbitals that are unique to Ni(I) forms of F(430). Ligand field transitions associated with S=1 Ni(II) centers are assigned in the near-IR MCD spectra of MCRox1-silent and MCR-silent, and the splitting in the lowest energy d-d transition is shown to correlate qualitatively with assessments of the zero-field splitting parameters determined by analysis of VTVH MCD saturation magnetization data. The MCD studies also support rationalization of MCRox1 as a tetragonally compressed Ni(III) center with an axial thiolate ligand or a coupled Ni(II)-thiyl radical species, with the reality probably lying between these two extremes. The reinterpretation of MCRox1 as a formal Ni(III) species rather than an Ni(I) species obviates the need to invoke a two-electron reduction of the F(430) macrocyclic ligand on reductive activation of MCRox1 to yield MCRred1.

Spectroscopic and computational characterization of the nickel-containing F430 cofactor of methyl-coenzyme M reductase

Methyl-coenzyme M reductase (MCR) catalyzes the terminal reaction in methanogenesis, the formation of methane from methyl-coenzyme M and coenzyme B. The active site of MCR binds the prosthetic group F(430), a unique nickel hydrocorphin cofactor. Here, spectroscopy and computations are employed in developing detailed electronic descriptions of the Ni(II) and Ni(I) forms of the free cofactor. Absorption, magnetic circular dichroism (MCD), and variable-temperature variable-field MCD data are analyzed within the framework of time-dependent DFT computations to assign key electronic transitions. DFT calculations are further employed to evaluate possible reduced F(430) models-a one-electron reduced Ni(I)F(430) model and a three-electron reduced Ni(I)F(red430) model (possessing a reduced hydrocorphin ligand)-on the basis of excited-state spectra and published EPR/ENDOR parameters. While calculations on both models yield spectroscopic parameters that are consistent with most experimental data, overall better agreement is achieved using the Ni(I)F(430) model, particularly with respect to electronic absorption and (1)H ENDOR. The experimentally validated bonding descriptions generated herein show that in Ni(II)F(430) the occupied Ni 3d orbitals are too low in energy to significantly perturb the dominant electronic transition involving the pi and pi* frontier MOs of the macrocycle (i.e., the HOMO-->LUMO transition). Upon one-electron reduction of the Ni(II) ion, the occupied Ni 3d orbitals are raised in energy, shifting between the HOMO and the LUMO of the oxidized cofactor. These ground-state changes have a dramatic effect on the excited-state structure, causing a blue shift of the dominant pi-->pi* transition and the appearance of numerous Ni 3d-->hydrocorphin pi* charge-transfer features in the vis/near-IR region.

Biochemistry of methanogenesis

Methane is a product of the energy-yielding pathways of the largest and most phylogenetically diverse group in the Archaea. These organisms have evolved three pathways that entail a novel and remarkable biochemistry. All of the pathways have in common a reduction of the methyl group of methyl-coenzyme M (CH3-S-CoM) to CH4. Seminal studies on the CO2-reduction pathway have revealed new cofactors and enzymes that catalyze the reduction of CO2 to the methyl level (CH3-S-CoM) with electrons from H2 or formate. Most of the methane produced in nature originates from the methyl group of acetate. CO dehydrogenase is a key enzyme catalyzing the decarbonylation of acetyl-CoA; the resulting methyl group is transferred to CH3-S-CoM, followed by reduction to methane using electrons derived from oxidation of the carbonyl group to CO2 by the CO dehydrogenase. Some organisms transfer the methyl group of methanol and methylamines to CH3-S-CoM; electrons for reduction of CH3-S-CoM to CH4 are provided by the oxidation of methyl groups to CO2.