Nickel and iron-sulphur centres in Desulfovibrio gigas hydrogenase: ESR spectra, redox properties and interactions

Abstract

Some properties of a nickel species (NiC) in Desulfovibrio gigas hydrogenase (ferredoxin:H + oxidoreductase, EC 1.18.99.1), which is associated with the activated state, are described. At temperatures above 20 K this species yields an ESR spectrum at g = 2.19, 2.16, 2.01, but at lower temperatures the spectrum changed into a fast-relaxing species with a complex lineshape. Substitution of the enzyme with 61Ni shows that the complex spectrum is associated with nickel. The splitting was correlated with the presence of a broad ESR signal which probably originates from the reduced [4Fe-4S] clusters. It did not correlated with the reduced [3Fe- xS] cluster. These results indicate that the complex spectrum is due to splitting of the NiC spectrum by spin-spin interaction with the [4Fe-4S] cluster or clusters. The NiC species was light-sensitive in frozen samples, and underwent a change in spectrum which was reversed in the dark at temperatures above 200 K. Treatment of the enzyme with carbon monoxide in the presence of hydrogen induced a new type of ESR signal, which disappeared on removal of either hydrogen or carbon monoxide. The NiC species represents an intermediate oxidation state of the enzyme. The midpoint redox potentials, estimated by mediator titrations under controlled hydrogen/argon gas mixtures, were shown to be strongly pH-dependent. The values at pH 7.0 were estimated to be −270 mV (−120 mV/pH unit) for the appearance of the NiC ESR signal and −390 mV (−60 mV/pH unit) for its disappearance. The midpoint potential of the broad ESR signal was estimated to be −350 (−60 mV/pH unit). Possible schemes for redox states of the various nickel species are discussed.