ELECTROCHEMICAL AND SPECTROSCOPIC PROPERTIES OF THE IRON-SULFUR FLAVOPROTEIN FROM METHANOSARCINA-THERMOPHILA

An iron-sulfur flavoprotein (Isf) from the methanoarchaeaon Methanosar cina thermophila, which participates in electron transfer reactions re quired for the fermentation of acetate to methane, was characterized b y electrochemistry and EPR and Mossbauer spectroscopy. The midpoint po tential (E-m) of the FMN/FMNH2 couple was -0.277 V. No flavin semiquin one was observed during potentiometric titrations; however, low amount s of the radical were observed when Isf was quickly frozen after react ion 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 amo ng Isf molecules. The E-m value for the 2+/1+ redox couple of the clus ter was -0.394 V, Extracts from H-2-CO2-grown Methanobacterium thermoa utotrophicum cells catalyzed either the H-2- or CO-dependent reduction of M. thermophila Isf, In addition, Isf homologs were found in the ge nomic sequences of the CO2-reducing methanoarchaea M. thermoautotrophi cum 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 co uple electron transfer from ferredoxin to membrane-bound electron carr iers, such as methanophenazine and/or b-type cytochromes.