On the anomalous temperature behaviour of the EPR signal of monovalent nickel in hydrogenase

Abstract

The dependence on temperature in the range between 4.2 K and 20 K was measured for the EPR signal of monovalent nickel in H2-reduced hydrogenase from Chromatium vinosum and from Methanobacterium thermoautotrophicum. In accordance with measurements on the hydrogenase from Desulfovibrio gigas [Teixeira, M., Moura, I., Xavier, A. V., Huynh, B. H., DerVartanian, D. V., Peck, H. D., Jr, LeGall, J. and Moura, J. J. G. (1985) J. Biol. Chem. 260, 8942-8950; Biochem. Soc. Trans. 13, 572 -5781, the enzyme from C. vinosum showed a distinct transformation of the EPR signal of nickel in this temperature region. The light sensitivity did not change. EPR spectra recorded at 9 GHz and at 35 GHz showed that the transformation of the spectrum at 4.2 K is caused by spin coupling to an unknown paramagnet. No coupling was apparent at temperatures above 20 K. At 4.2 K, additional, very broad signals in the region g = 1.2 -3, as well as a signal around g = 5, were detected in the enzyme from C. Vinosum, both in the H2-reduced state and in the Ar-reoxidised state. The possible origin of the paramagnetic species responsible for these signals is discussed. The EPR signal of monovalent nickel in the enzyme from M . thermoautotropicum showed no significant changes in line shape between 4.2 K and 70 K, nor were any additional signals detected.This suggests that in the reduced form of this enzyme similar paramagnetic species might be absent or not reduced. In contrast, experimental evidence has recently been published for a catalytic role of nickel in hydrogenase from Chromatium vinosum [12, 131 and Desulfovihrio gigas [14]. Detection of nickel in hydrogenase was first reported in a purified preparation of the enzyme from M . thermoautotrophicum, strain AH [15], but was at that time considered to be a contamination. Later, nickel was rediscovered as an essential component for activity in the enzyme from M .