| Abstract: | Ferredoxin from Methanosarcina thermophila is an electron acceptor for the CO dehydrogenase complex which decarbonylates acetyl-coenzyme A and oxidizes the carbonyl group to carbon dioxide in the pathway for conversion of the methyl group of acetate to methane (K. C. Terlesky and J. G. Ferry, J. Biol. Chem. 263:4080-4082, 1988). Resonance Raman spectroscopy and electron paramagnetic resonance spectroelectrochemistry indicated that the ferredoxin contained two 4Fe-4S] clusters per monomer of 6,790 Da, each with a midpoint potential of -407 mV. A 3Fe-4S] species, with a midpoint potential of +103 mV, was also detected in the protein at high redox potentials. Quantitation of the 3Fe-4S] and 4Fe-4S] centers revealed 0.4 and 2.1 spins per monomer, respectively. The iron-sulfur clusters were unstable in the presence of air, and the rate of cluster loss increased with increasing temperature. A ferredoxin preparation, with a low spin quantitation of 4Fe-4S] centers, was treated with Fe2+ and S2-, which resulted in an increase in 4Fe-4S] and a decrease in 3Fe-4S] clusters. The results of these studies suggest the 3Fe-4S] species may be an artifact formed from degradation of 4Fe-4S] clusters. |
