CHARACTERIZATION OF THE IRON-SULFUR CLUSTERS IN FERREDOXIN FROM ACETATE-GROWN METHANOSARCINA-THERMOPHILA

Ferredoxin from Methanosarcina thermophila is an electron acceptor for the CO dehydrogenase complex which decarbonylates acetyl-coenzyme A a nd oxidizes the carbonyl group to carbon dioxide in the pathway for co nversion of the methyl group of acetate to methane (K. C. Terlesky and J. G. Ferry, J. Biol. Chem. 263:4080-4082, 1988). Resonance Raman spe ctroscopy and electron paramagnetic resonance spectroelectrochemistry indicated that the ferredoxin contained two [4Fe-4S] clusters per mono mer 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 t he 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 th e rate of cluster loss increased with increasing temperature. A ferred oxin preparation, with a low spin quantitation of [4Fe-4S] centers, wa s 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 sug gest the [3Fe-4S] species may be an artifact formed from degradation o f [4Fe-4S] clusters.