SPECTROSCOPIC STUDIES OF THE NATURE OF THE IRON CLUSTERS IN THE SOLUBLE HYDROGENASE FROM DESULFOVIBRIO-DESULFURICANS (STRAIN NORWAY-4)

57Fe-enriched samples of the soluble hydrogenase from Desulfovibrio desulfuricans (Norway) were investigated in both the native (oxidized) and the dithionite-reduced states using Mossbauer spectroscopy. The Fe in this enzyme is predominantly in the form of Fe-S 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 2 [4Fe-4S] clusters. The Fe-S clusters in the oxidized protein are virtually diamagnetic, as indicated by Mossbauer, ESR and magnetic circular dichroic spectroscopy. On reduction by dithionite + methyl viologen, Mossbauer spectroscopy showed that only 50% of the [4F-4S] clusters were reduced. Even reduction with hydrogen up to a pressure of 23 gigapascals GPa did not reduce the Fe-S 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 species at g = 2.34, 2.12.