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 61 ms, while the SQ does not leave the binding site for at least 5 min. 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 ( P 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.

Photosystem II: Structure and mechanism of the water:plastoquinone oxidoreductase

“…This drastic difference in affinity can be explained by the high Born energy that is required for the Q À B exchange if the topology of the binding site permits a release of the head group only through the central part of the membrane (see above) with its low dielectric constant (Shinkarev 2006).…”

Distinct properties of semiquinone species detected at the ubiquinol oxidation Q o site of cytochrome bc 1 and their mechanistic implications

“…However, the destabilized SQ o is proposed to conserve sufficient redox energy to reduce haem b L which seems difficult to reconcile with the statement that SQ o interacts paramagnetically with the oxidized haem b L . Furthermore, it is important to bear in mind that in photosynthetic reaction centres a similar concept of low dielectric gate around the SQ binding site was introduced to rationalize high stability of SQ, because the contributions from electrostatic energy and hydrogen bonds were not enough to explain SQ stabilization [ 49 ].…”

A Caged, Destabilized, Free Radical Intermediate in the Q‐Cycle

“…[38] The deprotonated SQ anion is then trapped by conformational changes in SQ o intermediates and protein components, consistent with several proposed models. [12, 26b, 39] The SQ o is trapped because the surrounding low dielectric of the Q o site forms a hydrophobic “cage”, [40] but without thermodynamic stabilization.…”