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

Abstract

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.

Expression and Association of Group IV Nitrogenase NifD and NifH Homologs in the Non-Nitrogen-Fixing ArchaeonMethanocaldococcus jannaschii

“…The active site of methyl-coenzyme M reductase contains F430, a nickel-containing heterocyclic macrocycle cofactor with external cyloketone and lactam rings (10,49). However, very little is known about the biosynthetic pathway for F430 (50).…”

Coordination and binding geometry of methyl-coenzyme M in the red1m state of methyl-coenzyme M reductase

“…1) at the nickel center is widely disputed [5][6][7][8][9][10][11][12][13]. In essence, two mechanisms mainly differing in the nature of the initial cleavage of the sulfur-carbon bond of CH 3 -S-CoM are currently discussed.…”

“…According to mechanism ''B,'' the Ni(I) center initially acts as a nucleophile, attacking methyl-coenzyme M at the carbon of the CH 3 -S group, generating a CH 3 -Ni(III)F 430 intermediate and HS-CoM [1,[9][10][11][12]. Recent findings indicate that such a species can exist: 3-bromopropane sulfonate was shown to react with the active enzyme MCR red1 to give -O 3 S(CH 2 ) 3 …”

Overview of ligand versus metal centered redox reactions in tetraaza macrocyclic complexes of nickel with a focus on electron paramagnetic resonance studies

“…An extensive discussion of MCR, with its many forms, both EPR-active and EPR-silent, [24][25][26][99][100][101] is outside the scope of this study. We present here EPR spectra only of the form that is correlated with enzyme activity, MCR red1 , 22 -in 2-Methf.…”