Investigations on the influence of headgroup substitution and isoprene side-chain length in the function of primary and secondary quinones of bacterial reaction centers

Abstract

The contributions of headgroup and side-chain in the binding and function of the primary (Q A) and secondary (Q B) quinones of isolated reaction centers (RCs) from Rhodobacter sphaeroides were investigated. Various ubiquinones and structurally similar quinones were reconstituted into RCs depleted of one (1Q-RCs) or both (0Q-RCs) quinones. The influence of partition coefficients on the apparent binding affinities was minimized by expressing dissociation constants in terms of the mole fraction of quinone partitioned into the detergent. It was then apparent that the size of the isoprenyl side-chain was of little consequence in determining the binding affinity or the functional competence of either Q A or Q B, although an alkyl chain of equivalent size was a poor substitute. The degree of substitution of the headgroup, however, was a sensitive determinant of binding. For both quinone sites, the trisubstituted plastoquinones bound more weakly than the fully substituted ubiquinones. Similarly, for binding to the Q A site, duroquinone (tetramethylbenzoquinone) bound much more strongly than trimethylbenzoquinone. The affinity of the Q A site for ubiquinones was about 20-times stronger than the Q B site, but the Q B site is probably not more specific than the Q A site. However, Q B function depends on a suitable redox free-energy drop from Q A as well as binding, and of all the quinones tested only the ubiquinones simultaneously supported full Q A and Q B activity. Even plastoquinone-A, which fills both roles in Photosystem II, was unable to do so in bacterial RCs, although it did bind. The unique ability of ubiquinones to both bind and provide the appropriate redox span is discussed. The temperature dependence of binding of the isoprenyl ubiquinones at the Q A site changed markedly with chain length. For Q-10-Q-7, the binding enthalpy was positive and net binding was entirely driven by entropic factors. For the shorter-chain ubiquinones, Q-6-Q-1, both entropy and enthalpy of binding were favorable. This strong entropy-enthalpy compensation is suggested to arise from antagonistic interactions (anticooperativity) between headgroup and tail binding. For Q B function by hydrophobic quinones, the temperature dependence of the micelle properties prevented easy access to thermodynamic parameters. However, for water-soluble Q-0, binding to the Q B site was determined to be enthalpically driven. In the isoprenyl ubiquinone series, the electron transfer equilibrium for Q − AQ B ↔ Q AQ − B declined gradually as the side-chain was shortened from 7 to 1 isoprene unit, in both 0Q and 1Q-RCs, implying that the in situ redox midpoint potential in the Q B site decreased with decreasing side-chain length.