Cofactor 430 of methyl-coenzyme M reductase from Methanobacterium thermoautotrophicum was studied in both the extracted form in aqueous solution and protein-bound by using low-temperature magnetic-circular-dichroism spectroscopy. In both forms the nickel was present as high-spin paramagnetic nickel(II), spin S = 1, subject to almost equal zero-field splitting (cofactor F430, D = +9.0 cm-1, E/D = 0; methyl-coenzyme M reductase, D = +8.5 cm-1, [E/D[ = 0.2). This suggests identical axial co-ordination by oxygen ligand(s) both in aqueous cofactor F430 and in the investigated state of the protein.

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