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J Biol Chem, Vol. 273, Issue 41, 26462-26469, October 9, 1998

Electrochemical and Spectroscopic Properties of the Iron-Sulfur Flavoprotein from Methanosarcina thermophila

Donald F. Becker, Ubolsree Leartsakulpanich^§, Kristene K. Surerus, James G. Ferry^§, and Stephen W. Ragsdale

From the  Department of Biochemistry, The Beadle Center, University of Nebraska, Lincoln, Nebraska 68588-0664, the ^§ Department of Biochemistry and Molecular Biology, Eberly College of Science, Pennsylvania State University, University Park, Pennsylvania 16802-4500, and the  Department of Chemistry, University of Wisconsin, Milwaukee, Wisconsin 53201

An iron-sulfur flavoprotein (Isf) from the methanoarchaeaon Methanosarcina thermophila, which participates in electron transfer reactions required for the fermentation of acetate to methane, was characterized by electrochemistry and EPR and Mössbauer spectroscopy. The midpoint potential (E_m) of the FMN/FMNH₂ couple was 0.277 V. No flavin semiquinone was observed during potentiometric titrations; however, low amounts of the radical were observed when Isf was quickly frozen after reaction with CO and the CO dehydrogenase/acetyl-CoA synthase complex from M. thermophila. Isf contained a [4Fe-4S]^2+/1+ cluster with g values of 2.06 and 1.93 and an unusual split signal with g values at 1.86 and 1.82. The unusual morphology was attributed to microheterogeneity among Isf molecules. The E_m value for the 2+/1+ redox couple of the cluster was 0.394 V. Extracts from H₂-CO₂-grown Methanobacterium thermoautotrophicum cells catalyzed either the H₂- or CO-dependent reduction of M. thermophila Isf. In addition, Isf homologs were found in the genomic sequences of the CO₂-reducing methanoarchaea M. thermoautotrophicum and Methanococcus jannaschii. These results support a general role for Isf in electron transfer reactions of both acetate-fermenting and CO₂-reducing methanoarchaea. It is suggested that Isf functions to couple electron transfer from ferredoxin to membrane-bound electron carriers, such as methanophenazine and/or b-type cytochromes.

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