Ubiquinone (coenzyme Q₁₀) binding sites: Low dielectric constant of the gate prevents the escape of the semiquinone

Abstract 

The photosynthetic reaction center (RC) from purple bacteria is frequently used as a model for the interaction of ubiquinones (coenzyme Q) with membrane proteins. Single-turnover flash activation of RC leads to formation of the semiquinone (SQ) of the secondary acceptor quinone after odd flashes and quinol after even flashes. The ubiquinol escapes the binding site in ⩽1ms, while the SQ does not leave the binding site for at least 5min. Observed difference between these times suggests a large energetic barrier for the SQ. However, high apparent dielectric constant in the vicinity of the quinone ring (⩾25) results in a relatively small electrostatic energy of SQ stabilization. To resolve this apparent contradiction I suggest that a significant part of the kinetic stabilization of the SQ is achieved by the special topology of the binding site in which quinone can exit the binding site only by moving its headgroup toward the center of the membrane. The large energetic penalty of transferring the charged headgroup to the membrane dielectric can explain the observed kinetic stability of the SQ.

Abbreviations: Cyt, cytochrome, E_h, redox potential of the medium, P870, primary electron donor, bacteriochlorophyll dimer in the RC, Q_A and Q_B, the primary and secondary quinone-type electron acceptors in the RC, Q, coenzyme Q (ubiquinone), Q₁₀ (Q₁₀), coenzyme Q with 10 isoprene units, QH₂, dihydroquinone (quinol), RC, photosynthetic reaction center, Rb., Rhodobacter, SQ, semiquinone

Keywords: Coenzyme Q₁₀, Semiquinone, Ubiquinone, Lipid membrane, Electron transfer, Born energy