Interactions of oxaloacetate with Escherichia coli fumarate reductase

Abstract Fumarate reductase of Escherichia coli is converted to a deactivated state when tightly bound by oxaloacetate (OAA). Incubation of the inhibited enzyme with anions or reduction of the enzyme by substrate restores both the activity of the enzyme and its sensitivity to thiol reagents. In these respects the enzyme behaves like cardiac succinate dehydrogenase. Close to an order of magnitude difference was found to exist between the affinities of OAA for the oxidized ( K D ∼ 0.12 μ M) and reduced ( K D ∼ 0.9 μ M) forms of fumarate reductase. Redox titrations of deactivated fumarate reductase preparations have confirmed that reductive activation, as in cardiac succinate dehydrogenase ( B. A. C. Ackrell, E. B. Kearney, and D. Edmondson (1975) J. Biol. Chem. 250 , 7114–7119 ), is the result of reduction of the covalently bound FAD moiety and not the non-heme iron clusters of the enzyme. However, the processes differed for the two enzymes; activation of fumarate reductase involved 2 e − and 1H + , consistent with reduction of the flavin to the anionic hydroquinone form, whereas the process requires 2 e − and 2H + in cardiac succinate dehydrogenase. The reason for the difference is not known. The redox potential of the FAD FADH 2 couple in FRD ( E m ∼ −55 mV) was also slightly more positive than that in cardiac succinate dehydrogenase (—90 mV).