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An intricate case of charge migration in proteins is represented by the diheme-containing class of succinate:quinone oxidoreductases, where, depending on the species and the direction of the reaction catalyzed in vivo, transmembrane electron transfer is either coupled to transmembrane proton transfer or not. The absence of compensatory transmembrane proton transfer results in an electrogenic overall reaction, as has been demonstrated for diheme-containing succinate:menaquinone reductases. The presence of such a compensatory transmembrane proton transfer renders the overall reaction electroneutral, as has been shown to be the case for diheme-containing menaquinol:fumarate reductases (QFR). The availability of high-resolution X-ray crystal structures of the diheme-containing QFR from Wolinella succinogenes has inspired and enabled several theoretical and experimental studies providing proof and further characterization of these electron and proton transfer events. [ABSTRACT FROM AUTHOR]