Electron transfer within complex II: Succinate:ubiquinone oxidoreductase of Escherichia coli

Electron transfer within Escherichia coli succinate: ubiquinone oxidoreductase has been examined by the pulse radiolysis technique using spectrophotometric detection. Electrons have been introduced into the protein by the bimolecular reaction with quantified concentrations of the low potential N-methylnicotinamide radical at a rate constant of 7 × 10⁸ M^-1 S^-1. Two redox-active centers in the protein are initially reduced, assigned as the high potential [3Fe-4S] center and the bound ubiquinone, followed by intramolecular equilibration with the b heme in both cases. Electron equilibration at 25 °C from the ubisemiquinone proceeds with an observed rate constant of 7,200 s^-1 and from the more distant [3Fe-4S] reduced center at a rate constant of 1,200 s^-1. Temperature dependence studies have revealed that both reactions have large free energies of activation, with ΔG‡ values of +0.53 and +0.58 eV, respectively. Cumulative spectral changes, as well as accompanying decreases in the rates of intramolecular electron transfer, observed upon adding electrons to progressively reduced protein, indicate that 4 electrons must be introduced into the protein before the heme center is fully reduced. Overall, evidence is presented that the heme, far from being a bystander in the efficient transfer of reducing equivalents from succinate to the ubiquinone via the flavin-Fe/S centers, plays a pivotal role in providing a lower energy pathway for the transfer of an electron from the high potential [3Fe-4S] center to ubiquinone.