Five new EPR signals assigned to nickel in methyl-coenzyme M reductase from Methanobacterium thermoautotrophicum, strain Marburg

EPR spectroscopy of intact cells and of cell extracts from Methanobacterium thermoautotrophicum, strain Marburg, revealed the existence of five new EPR signals assigned to factor F430 in methyl-coenzyme M reductase, in addition to the one discovered recently in the purified enzyme (Albracht, S.P.J., Ankel-Fuchs, D., Van der Zwaan, J.W., Fontijn, R.D. and Thauer, R.K. (1986) Biochim. Biophys. Acta 870, 50–57), which is further referred to as MCR-ox1. Two of the new signals, a strongly axial one, termed MCR-red1, and a rhombic one, termed MCR-red 2, could be evoked by incubation of intact cells with H2 at 60°C, but not at 4°C. Subsequent incubation under N2 did not change these signals, but they quickly disappeared on anaerobic contact with CO2 at 60°C, but not at 4°C. This behaviour indicates that under physiological conditions nickel in factor F430 of methyl-coenzyme M reductase is redox active and, the the reduced state, can occur in two forms with strongly different ligand fields. The possible significance of these findings for the mechanism of action of methyl-coenzyme M reductase in intact cells is discussed. A third new signal, termed MCR-ox2, was observed after oxidation by air of intact cells previously reduced by H2 at 60°C. In cell extract and isolated methyl-coenzyme M reductase a fourth new signal, called MCR-dark, could be evoked under several conditions. The species responsible for this signal was highly sensitive to light. Illumination below 100 K converted the signal to yet another one of rhombic nature (termed MCR-light) which was similar but not identical to the rhombic signal (MCR-red2) observed in intact cells. At 170 K the change was reversible. From the latter results it is concluded that nickel in factor F430 of methyl-coenzyme M reductase can exist in a form where its coordination can be greatly changed by illumination below 100 K.