A NEW ELECTRON-PARAMAGNETIC-RES SIGNAL OF NICKEL IN METHANOBACTERIUM-THERMOAUTOTROPHICUM

The two main EPR signals detectable in suspensions of whole cells of Methanobacterium thermoautotrophicum strain Marburg have both been assigned to nickle by growth of the bacterium on a medium enriched in 61Ni (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 Thaurer, 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 F430. It is proposed that the signal represents F430 in intact, active methyl-coenzyme M reductase molecules.