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Fumarate reductase from Escherichia coli functions both as an anaerobic fumarate reductase and as an aerobic succinate dehydrogenase. A site-directed mutation of E. coli fumarate reductase in which FrdB Pro-159 was replaced with a glutamine or histidine residue was constructed and overexpressed in a strain of E. coli lacking a functional copy of the fumarate reductase or succinate dehydrogenase complex. The consequences of these mutations on bacterial growth, assembly of the enzyme complex, and enzymatic activity were investigated. Both mutations were found to have no effect on anaerobic bacterial growth or on the ability of the enzyme to reduce fumarate compared with the wild-type enzyme. The FrdB Pro-159-to-histidine substitution was normal in its ability to oxidize succinate. In contrast, however, the FrdB Pro-159-to-Gln substitution was found to inhibit aerobic growth of E. coli under conditions requiring a functional succinate dehydrogenase, and furthermore, the aerobic activity of the enzyme was severely inhibited upon incubation in the presence of its substrate, succinate. This inactivation could be prevented by incubating the mutant enzyme complex in an anaerobic environment, separating the catalytic subunits of the fumarate reductase complex from their membrane anchors, or blocking the transfer of electrons from the enzyme to quinones. The results of these studies suggest that the succinate-induced inactivation occurs by the production of hydroxyl radicals generated by a Fenton-type reaction following introduction of this mutation into the [3Fe-4S] binding domain. Additional evidence shows that the substrate-induced inactivation requires quinones, which are the membrane-bound electron acceptors and donors for the succinate dehydrogenase and fumarate reductase activities. These data suggest that the [3Fe-4S] cluster is intimately associated with one of the quinone binding sites found n fumarate reductase and succinate dehydrogenase.

Aerobic inactivation of fumarate reductase from Escherichia coli by mutation of the [3Fe-4S]-quinone binding domain