Molecular interactions of the quinone electron acceptors Q(A), Q(B), and Q(C) in photosystem II as studied by the fragment molecular orbital method

Molecular interactions of the three plastoquinone electron acceptors, Q(A), Q(B), and Q(C), in photosystem II (PSII) were studied by fragment molecular orbital (FMO) calculations. Calculations at the FMO-MP2/6-31G level using PSII models deduced from the X-ray structure of the PSII complexes from Thermosynechococcus elongatus provided the binding energies of Q(A), Q(B), and Q(C) as -56.1, -37.9, and -30.1 kcal/mol, respectively. The interaction energies with surrounding fragments showed that the contributions of lipids and cofactors were 0, 24 and 45 % of the total interaction energies for Q(A), Q(B), and Q(C), respectively. These results are consistent with the fact that Q(A) is strongly bound to the PSII protein, whereas Q(B) functions as a substrate and is exchangeable with other quinones and herbicides, and the presence of Q(C) is highly dependent on PSII preparations. It was further shown that the isoprenoid tail is more responsible for the binding than the head group in all the three quinones, and that dispersion forces rather than electrostatic interactions mainly contribute to the stabilization. The relevance of the stability and molecular interactions of Q(A), Q(B), and Q(C) to their physiological functions is discussed.