INFLUENCE OF HYDROCARBON TAIL STRUCTURE ON QUINONE BINDING AND ELECTRON-TRANSFER PERFORMANCE AT THE Q(A) AND Q(B) SITES OF THE PHOTOSYNTHETIC REACTION-CENTER PROTEIN

Binding free energies of 37 functional replacement quinone cofactors w ith systematically altered hydrocarbon tail structures have been deter mined for the Q(A) and Q(B) redox catalytic sites of the reaction cent er protein isolated from Rhodobacter sphaeroides and solubilized in aq ueous and in hexane solutions. The first two and part of the third iso prene units of the 10-unit tail of the native ubiquinone-10 cofactor i nteract with the protein interior at each site. Contributions of the s ame tail structures to the binding free energies of quinones at the Q( A) and Q(B) sites are comparable, suggesting that the binding domains share common features. Comparison of the affinities of a homologous se ries of 10 n-alkyl-substituted ubiquinones resolves the binding forces along the length of the tail binding domain and shows that strong ste ric constraints oppose accommodation of the tail in its extended confo rmation. Differences in the contributions of identical tail substituen ts to ubiquinone- and menaquinone-Q(A) site affinities, and tail-induc ed changes of up to 5-fold in the rates of Q(A) site-mediated electron -transfer reactions, suggest that the tail adjusts the position of the quinone ring. Substitution of ubiquinone with the native 10-unit isop rene tail does not alter the affinity for the sites as determined in h exane solution. However, one- and two-isoprene-substituted quinones bi nd more tightly than analogs substituted with saturated-alkyl tail sub stituents. The sites therefore exhibit binding specificity for the nat ive isoprene tail structure. Calculations indicate that the binding sp ecificity arises primarily from a lower integrated torsion potential e nergy in the bound isoprene tails. The results suggest that the in viv o tail-protein interaction is designed to deter competitive interferen ce of quinone function by amphiphilic species present in the native me mbrane.