Fig. 4.

Energy levels for sequential electron transfer from a primary semiquinone to a secondary quinone. A Q_AS:Q_BS indicate a complex of two quinones which have the same electrochemistry as two isolated ubiquinones in solution at pH 7 (Fig. 1). To start the cycle one quinone is reduced to the semiquinone forming Q_AS^•−:Q_BS. Given a solution E_h of 0 mV or a chemical electron donor with an E_m of 0 mV, the single reduction of quinone with an E_m of −145 mV to Q^•− is uphill by 145 meV. Since these quinones are identical, electron transfer from Q_AS^•− to Q_BS will be isoenergetic. Given the semiquinone pK_a of 4.9, protonating either semiquinone would be uphill by ≈120 meV. Thus, the lowest energy, singly reduced state will be a 50:50 mixture of Q_AS^•−:Q_BS and Q_AS:Q_BS^•−. The second turnover starts with the second reduction of Q_AS forming Q_AS^•−:Q_BS^•−. The thermodynamically preferred pathway has the electron transfer occurring before the first proton is bound. The formation of Q_AS:Q_BS⁼ requires only 55 meV because the second reduction of Q_BS is coupled to the favorable oxidation of Q_AS. The reaction path where Q_BS^•− is protonated to form Q_AS^•−:Q_BSH before it is reduced is 120 meV uphill. Q_AS:Q_BSH⁻ where one quinone has two electrons and one proton is 290 meV lower in energy then Q_AS^•−:Q_BS^•− and is essentially isoenergetic with the initial Q_AS:Q_BS state at pH 7. Once Q_AS:Q_BSH⁻ is formed the second protonation to form Q_AS:Q_BSH₂ is downhill by −220 meV. B The ubiquinone energy levels in R. sphaeroides RCs at pH 7 and E_h 0. In the initial reaction Q_A is reduced to the semiquinone forming Q_A^•−:Q_B. Calculations put this state near 0 mV, close to the measured values between −45 and −70 mV (all calculated values are from Zhu and Gunner (2005)). Q_B^•− is stabilized by 30 (calculation) to ≈70 meV more then Q_A^•−. The second turnover starts with formation of Q_A^•−:Q_B^•−. Calculations and the experiments of Graige and Okamura show that in the protein Q_A^•−:Q_B^•H is lower in energy then Q_A:Q_B⁼ (Graige et al. 1998). The calculations place the energy of Q_A^•−:Q_B^•H 260 meV above Q_A^•−:Q_B^•−, while the kinetics of forward electron transfer support a value of 160 meV (grey text; Graige et al. 1999). The second reduction of Q_BH⁻ is favorable. The anionic Q_AQ_BH⁻ is 110 meV more stable in the protein then in solution, while Q_AQ_BH₂ is 50 meV less stable favoring quinol dissociation