Resonance Raman studies of the [4Fe-4S] to [2Fe-2S] cluster conversion in the iron protein of nitrogenase

Abstract

Resonance Raman spectroscopy has been used to investigate the Fe-S stretching modes of the [4Fe-4S] 2+ cluster in the oxidized iron protein of Clostridium pasteurianum nitrogenase. The results are consistent with a cubane [4Fe-4S] cluster having effective T d symmetry with cysteinyl coordination for each iron. In accord with previous optical and EPR studies [(1984) Biochemistry 23, 2118–2122], treatment with the iron chelator α,α′-dipyridyl in the presence of MgATP is shown to effect cluster conversion to a [2Fe-2S] 2+ cluster. Resonance Raman data also indicate that partial conversion to a [2Fe-2S] 2+ cluster is induced by thionine-oxidation in the presence of MgATP in the absence of an iron chelator. This result suggests new explanations for the dramatic change in the CD spectrum that accompanies MgATP-binding to the oxidized Fe protein and the anomalous resonance Raman spectra of thionine-oxidized Clostridium pasteurianum bidirectional hydrogenase.