Spectroscopic characterization of ferredoxins I and II from Desulfovibrio africanus

Abstract

Ferredoxins I and II isolated from Desulfovibrio africanus strain Benghazi (NCIB 8401) were examined by electron spin resonance (ESR) and magnetic circular dichroism (MCD) spectroscopy. The ferredoxins were essentially ESR-silent in the oxidized state. The spectra of the reduced ferredoxins indicated the presence of single [4Fe-4S] clusters, with intensities of approx. 1 spin per protein subunit. The spectra of reduced ferredoxin I showed two components which were interpreted as due to different aggregated forms of the protein. The midpoint reduction potentials, determined by the mediator-titration technique monitored by ESR spectroscopy, were estimated to be −385 ± 15 mV for both proteins. The MCD spectra of the oxidized ferredoxins confirmed that the ferredoxins were essentially diamagnetic, and were typical of [4Fe-4S] clusters in the 2+ oxidation level. MCD magnetization curves of the reduced ferredoxin I, constructed from all the major features of the MCD spectra, consistently gave intercept values of 0.5 ± 0.01 corresponding to a paramagnet with a g value near to 2.0 and a spin of S = 1 2 . This is taken as confirmation that the proteins are exceptionally free of oxidized paramagnetic species which interfere with the spectra of other [4Fe-4S] 1+:2+ ferredoxins.