INVESTIGATION BY EPR AND ENDOR SPECTROSCOPY OF THE NICKEL(I) FORM OF COFACTOR F-430 OF METHANOBACTERIUM-THERMOAUTOTROPHICUM AND OF NICKEL(I) OCTAETHYLISOBACTERIOCHLORIN

The terminal step in methane generation by the archaeon Methanobacteri um thermoautotrophicum is catalyzed by the enzyme S-methyl coenzyme M reductase (methylreductase). This enzyme contains a Ni(II) tetrapyrrol e cofactor, F-430, at the active site in the resting state. A Ni(I) st ate ((NiF430)-F-I) has been proposed as the active form of the cofacto r. Nickel isobacteriochlorins have been used to model F-430. We have i nvestigated both (NiF430)-F-I and Ni(I)OEiBC using CW and pulsed EPR a nd ENDOR spectroscopy at X-band and Q-band microwave frequencies. In a greement with a previous X-band EPR and ESEEM study, at Q-band, the g tensor of (NiF430)-F-I appears axial and H-1,H-2 ENDOR of (NiF430)-F-I in H2O versus D2O solvent shows no evidence for strongly coupled, sol vent-exchangeable hydrogens, and this indicates that there is no water axially coordinated to Ni(I) in contrast to the Ni(LI) resting state. Both (NiF430)-F-I and Ni(I)OEiBC give N-14 ENDOR signals arising from the four pyrrole nitrogen ligands to Ni(I). Previous EXAFS studies of Ni(I)OEiBC and Ni(I)F(430)Me(5) showed two sets of NL(I)-N distances differing by similar to 8, and in agreement with this, the N-14 ENDOR signals for both Ni(I)OEiBC and (NiF430)-F-I can be analyzed in terms of superimposed signals from two distinct types of nitrogen ligand. Th e difference in bond lengths determined by EXAFS is reflected in diffe rent hyperfine and quadrupole coupling constants as determined by ENDO R. Comparison of these magnetic parameters for Ni(I) complexes with th ose for related species, such as (CuTPP)-T-II, shows that one Set of n itrogen ligands resembles porphyrin pyrrole, while the other set is di stinctively different.