Formylmethanofuran dehydrogenase (Fmd) from Methanosarcina barkeri is a molybdenum iron-sulfur protein involved in methanogenesis. The enzyme contains approximately 30 mol non-heme iron/mol and 30 mol acid-labile sulfur/mol. We report here the cloning and sequencing of the encoding genes, and that these genes form a transcription unit fmdEFACDB. Evidence is provided that the subunit FmdB harbours the molybdenum-containing active site and may bind one [4Fe-4S] cluster. fmdF encodes a protein with four tandemly repeated bacterial-ferredoxin-like domains and is predicted to be a polyferredoxin that could contain as many as 32 iron atoms in eight [4Fe-4S] clusters. The other genes code for proteins without sequence motifs characteristic for iron-sulfur proteins. These findings suggest that most of the iron-sulfur clusters present in the purified formylmethanofuran dehydrogenase are associated with the subunit FmdE The finding that FmdF forms a tight complex with the other subunits of formylmethanofuran dehydrogenase indicates a function of the polyferredoxin in the reaction catalyzed by the enzyme. findE encodes a protein not present in the purified enzyme. All six genes of the fmd operon were expressed in Escherichia coli and yielded proteins of expected molecular masses. A malE-fmdF gene fusion was constructed and expressed in E, coli, making the apoprotein of the polyferredoxin available in preparative amounts.
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Vorholt, J. A., Vaupel, M., & Thauer, R. K. (1996). A polyferredoxin with eight [4Fe-4S] clusters as a subunit of molybdenum formylmethanofuran dehydrogenase from Methanosarcina barkeri. European Journal of Biochemistry, 236(1), 309–317. https://doi.org/10.1111/j.1432-1033.1996.t01-1-00309.x