Coordination chemistry of F430. Axial ligation equilibrium between square-planar and bis-aquo species in aqueous solution.

X-ray absorption spectroscopic characterization of axial ligand coordination to factor F430, the nickel-tetrapyrrole cofactor of the S-methyl-coenzyme M (CH3SCoM) methyl reductase enzyme from methanogenic bacteria, is presented. The nickel of isolated F430 is hexacoordinate at 10 K in aqueous solution (as is the enzyme-bound cofactor), whereas the epimerized and ring-oxidized derivatives of F430 have four-coordinate nickel. Reduction of the ring-oxidized derivative, F560, with dithionite yields F430 in its native configuration, with axial ligands indistinguishable from those present when the cofactor is obtained directly from the holoenzyme. Thus, we conclude that the axial ligands to F430 in aqueous solution are water molecules. Analysis of the nickel extended x-ray absorption fine structure is consistent with this conclusion. Resonance Raman spectra obtained at room temperature contain features characteristic of both 4- and 6-coordinate forms of the cofactor. We have found that the resonance Raman, optical, and x-ray absorption spectra of aqueous solutions of F430 are temperature-dependent due to a ligand-binding equilibrium involving the square-planar and 6-coordinate bis-aquo forms of the cofactor. At low temperatures (less than 250 K) the 6-coordinate form predominates, whereas higher temperature solutions contain both 4- and 6-coordinate species in a dynamic equilibrium. Similar behavior is observed in other weakly coordinating solvents such as methanol and ethanol. The 4-coordinate form is predominant in solvents with strong electron-withdrawing substituents such as 2,2,2-trifluoroethanol and 2-mercaptoethanol. The relevance of this facile ligand exchange to the active site structure and enzymatic mechanism of the parent enzyme is discussed.