The terminal step in methane generation by the archaeon Methanobacterium thermoautotrophicum is catalyzed by the enzyme S-methyl coenzyme M reductase (methylreductase). This enzyme contains a Ni(II) tetrapyrrole cofactor, F430, at the active site in the resting state. A Ni(I) state (Ni(I)F430) has been proposed as the active form of the cofactor. Nickel isobacteriochlorins have been used to model F430. We have investigated both Ni(I)F430 and Ni(I)OEiBC using CW and pulsed EPR and ENDOR spectroscopy at X-band and Q-band microwave frequencies. In agreement with a previous X-band EPR and ESEEM study, at Q-band, the g tensor of Ni(I)F430 appears axial and 1,2H ENDOR of Ni(I)F430 in H2O versus D2O solvent shows no evidence for strongly coupled, solvent-exchangeable hydrogens, and this indicates that there is no water axially coordinated to Ni(I) in contrast to the Ni(II) resting state. Both Ni(I)F430 and Ni(I)OEiBC give 14N ENDOR signals arising from the four pyrrole nitrogen ligands to Ni(I). Previous EXAFS studies of Ni(I)OEiBC and Ni(I)F430Me5 showed two sets of Ni(I)-N distances differing by ~ 8%, and in agreement with this, the 14N ENDOR signals for both Ni(I)OEiBC and Ni(I)F430 can be analyzed in terms of superimposed signals from two distinct types of nitrogen ligand. The difference in bond lengths determined by EXAFS is reflected in different hyperfine and quadrupole coupling constants as determined by ENDOR. Comparison of these magnetic parameters for Ni(I) complexes with those for related species, such as Cu(II)TPP, shows that one set of nitrogen ligands resembles porphyrin pyrrole, while the other set is distinctively different.
ASJC Scopus subject areas
- Colloid and Surface Chemistry