Investigation by EPR and ENDOR spectroscopy of the nickel(I) form of cofactor F{sub 430} of Methanobacterium thermoautorophicum and of nickel(I) octaethylisobacteriochlorin
- Northwestern Univ., Evanston, IL (United States)
- Brookhaven National Lab., Upton, NY (United States)
- Univ. of Georgia, Athens, GA (United States)
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, F{sub 430}, at the active site in the resting state. A Ni(I) state (Ni{sup I}F{sub 430}) has been proposed as the active form of the cofactor. Nickel isobacteriochlorins have been used to model F{sub 430}. We have investigated both Ni{sup I}F{sub 430} and Ni{sup 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{sup I}F{sub 430} appears axial and {sup 1,2}H ENDOR of Ni{sup I}F{sub 430} in H{sub 2}O versus D{sub 2}O 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{sup I}F{sub 430} and Ni{sup I}OEiBC give {sup 14}N ENDOR signals arising from the four pyrrole nitrogen ligands to Ni(I). Previous EXAFS studies of Ni{sup I}OEiBC and Ni{sup I}F{sub 430}Me{sub 5} showed two sets of Ni(I)-N distances differing by nearly 8%, and in agreement with this, the {sup 14}N ENDOR signals for both Ni{sup I}OEiBC and Ni{sup I}F{sub 430} can be analyzed in terms of superimposed signals from two distinct types of nitrogen ligand. 86 refs., 5 figs., 3 tabs.
- Research Organization:
- Brookhaven National Laboratory (BNL), Upton, NY
- DOE Contract Number:
- AC02-76CH00016
- OSTI ID:
- 458612
- Journal Information:
- Journal of the American Chemical Society, Journal Name: Journal of the American Chemical Society Journal Issue: 4 Vol. 119; ISSN JACSAT; ISSN 0002-7863
- Country of Publication:
- United States
- Language:
- English
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