Spectroscopic studies of the nature of the iron clusters in the soluble hydrogenase from Desulfovibrio desulfuricans (strain Norway 4)

Abstract

⁵⁷Fe‐enriched samples of the soluble hydrogenase from Desulfovibrio desulfuricans (Norway) have been investigated in both the native (oxidized) and the dithionite‐reduced states using Mössbauer spectroscopy. The data clearly show that the iron in this enzyme is predominantly in the form of iron‐sulphur clusters which are closely similar to the [4Fe‐4S] clusters found in a large number of ferredoxins, such as that from Bacillus stearothermophilus. There appear to be two [4Fe‐4S] clusters. The iron‐sulphur clusters in the oxidized protein are virtually diamagnetic, as indicated by Mössbauer, electron spin resonance and magnetic circular dichroic spectroscopy. On reduction by dithionite + methyl viologen, Mössbauer spectroscopy showed that only 50% of the [4Fe‐4S] clusters were reduced. Even reduction with hydrogen up to a pressure of 23 GPa did not reduce the iron‐sulphur clusters completely. An ESR signal due to a rapidly relaxing species with g = 2. 03, 1.89 was observed in the reduced protein, together with a weaker spectrum from a slower‐relaxing spcies at g= 2.34, 2.12.