Journal of Biological Chemistry
Volume 273, Issue 41, 9 October 1998, Pages 26462-26469
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ENZYMOLOGY
Electrochemical and Spectroscopic Properties of the Iron-Sulfur Flavoprotein from Methanosarcina thermophila *

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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 mvalue 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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*

This work was supported in part by National Institutes of Health Grant 1-R15-GM52666-01 (to K. K. S.) and by Department of Energy Basic Energy Sciences Grants DE-FG02-ER20053 (to S. W. R.) and DE-FG02-95ER20198 (to J. G. F.).The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

The nucleotide sequence(s) reported in this paper has been submitted to the GenBank™/EMBL Data Bank with accession number(s) U50189 (corrected nucleotide sequence of isf); MTH135, MTH1473, and MTH1595 (for M. thermoautotrophicum); and MJ1083 and MJ10731 (for M. jannaschii).

Supported by a Ministry of Science, Technology, and Energy grant from Thailand.