FADH₂ is produced in the citric acid cycle by a membrane-embedded enzyme complex, called succinate dehydrogenase, that contains bound FAD and carries out the reactions succinate + FAD → fumarate + FADH₂ and FADH₂ → FAD + 2H+ + 2e^– The redox potential of FADH_2, however, is only 220 mV. Referring to Panel 14–1 (p. 472) and Figure 14–22, suggest a plausible mechanism by which its electrons could be fed into the electron-transport chain. Draw a diagram to illustrate your proposed mechanism.

Panel 14–1

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redox potential (mV) -400 -300 -200 -100 0 100 200 300 400 500 600 700 800 NADH NAD+ NADH H+ ubiquinone H+ dehydrogenase complex cytochrome c reductase complex cytochrome c cytochrome c oxidase complex H+ 1/2 O₂ + 2 H+ direction of electron flow H₂O 100 80 60 40 20 0 free energy per electron (kJ/mole)