Respiratory Chain Complex II and Succinate: Quinone Oxidoreductases

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Succinate:quinone oxidoreductases (SQORs; EC 1.3.5.1) are enzymes that couple the two-electron oxidation of succinate to fumarate (eqn [1]) to the two-electron reduction of quinone to quinol (eqn [2]):1succinate⇄fumarate+2H++2e−2quinone+2H++2e−⇄quinolThey can also catalyze the opposite reaction, the coupling of quinol oxidation to the reduction of fumarate. Depending on the direction of the reaction catalyzed in vivo, the members of the superfamily of SQORs can be classified as either succinate: quinone reductases (SQRs) or quinol:fumarate reductases (QFRs). SQR and QFR complexes are anchored in the cytoplasmic membranes of archaebacteria and eubacteria, and in the inner mitochondrial membrane of eukaryotes with the hydrophilic domain canonically extending into the cytoplasm and the mitochondrial matrix, respectively. SQR (respiratory complex II) is involved in aerobic metabolism as part of the citric acid cycle (Krebs cycle) and of the aerobic respiratory chain. QFR participates in anaerobic respiration with fumarate as the terminal electron acceptor, and is part of the electron transport chain catalyzing the oxidation of various donor substrates (e.g., H2 or formate) by fumarate. These reactions are coupled via an electrochemical proton potential (Δp) to adenosine diphosphate phosphorylation with inorganic phosphate by adenosine triphosphate synthase.