CHARACTERIZATION AND DETERMINATION OF THE REDOX PROPERTIES OF THE 2[4FE-4S] FERREDOXIN FROM METHANOSARCINA-BARKERI STRAIN MS

Ferredoxin was purified from methanol-grown Methanosarcina barkeri str ain MS. It was isolated as a dimer with a subunit molecular weight of 6,200. The protein contained 7.4 mol iron and 7.2 mol acid-labile sulf ur per monomer. In the reduced state the ferredoxin exhibited an EPR s pectrum characteristic of two spin-coupled [4Fe-4S](1+) clusters. The E(m) of the [4Fe-4S](2+;1+) couple was -322 mV +/- mV vs. NHE at 21 de grees C and pH 7.0. The midpoint potential wits temperature but not pH dependent. At the physiological temperature of 37 degrees C the E(m) was -340 mV.