Purification and characterization of a ferredoxin from acetate-grown Methanosarcina thermophila

A ferredoxin, which functions as an electron acceptor for the CO dehydrogenase complex from Methanosarcina thermophila, was purified from acetate-grown cells. It was isolated as a trimer having a native molecular weight of approximately 16,400 and monomer molecular weight of 4,888 calculated from the amino acid composition. The ferredoxin contained 2.80 ± 0.56 Fe atoms and 1.98 ± 0.12 acid-labile sulfide. UV-visible absorption maxima were 395 and 295 nm with monomeric extinction coefficients of ε₃₉₅ = 12,800 M^-1 cm^-a1 and ε₂₉₅ = 14,460 M^-a1 cm^-1. The A₃₉₅/A₂₉₅ ratio ranged from 0.80 to 0.88. There were 5 cysteines per monomer but no methionine, histidine, arginine, or aromatic amino acids. The N-terminal amino acid sequence showed a 4-cysteine cluster with potential to coordinate a Fe:S center. The protein was stable for 30 min at 70°C, but denatured during incubation at 85°C.