A new EPR signal of nickel in Methanobacterium thermoautotrophicum

Abstract The two main EPR signals detectable in suspensions of whole cells of Methanobacterium thermoautotrophicum strain Marburg have both been assigned to nickel by growth of the bacterium on a medium enriched in 61 Ni ( I = 3/2). One of the signals could be identified as the soluble hydrogenase, that has earlier been characterized (Albracht, S.P.J.; Graf, E.-G. and Thauer, R.K. (1982) FEBS Lett. 140, 311–313). The other signal is new. Its g values and its hyperfine structure indicate that it represents nickel in a tetragonally distorted octahedral ligand field with 4 equivalent N atoms in the equatorial plane. From the available EPR data, it could not be established whether the nickel ion is in the monovalent or trivalent state. The nickel is attached to a soluble protein. Purified methyl-coenzyme M reductase displays the same signal. The spin concentration of the species in various preparations of the enzyme accounted for at most one-fifth of the concentration of the protein-bound factor F 430 . It is proposed that the signal represents F 430 in intact, active methyl-coenzyme M reductase molecules.