Modulation of the fumarases of Escherichia coli in response to oxidative stress.

The [4Fe-4S]-containing fumarases A + B of Escherichia coli are susceptible to oxidative inactivation, while fumarase C, which is not an iron-sulfur protein, is induced under oxidative conditions. Thus, imposition of oxidative stress, whether by addition of paraquat or by mutational deletion of superoxide dismutases, diminished fumarases A + B while elevating fumarase C. H2O2 appeared to be one cause of the inactivation of fumarases A + B, but was not involved in the induction of fumarase C. Thus lack of hydroperoxidases I and II did increase the paraquat-elicited inactivation of fumarases A + B without affecting the induction of fumarase C by paraquat. The importance of Fe(II) for the unstable fumarases was exposed by alpha,alpha'-dipyridyl, which decreased fumarases A + B without affecting fumarase C or the inductive effect thereon of paraquat. The oxidative inactivation and the subsequent reactivation of fumarase A was examined in extracts. Under air there was a first-order inactivation of fumarase A, which was rapidly reversed when O2 was excluded. The role of iron loss and restitution, in this inactivation and reactivation, was clarified by EDTA, which was without effect on the aerobic inactivation, but blocked the anaerobic reactivation. These results are consistent with a predominantly ferric state of the available iron in the presence of O2 and of the ferrous state in the absence of O2.