THE QUINONE ACCEPTOR A(1) IN PHOTOSYSTEM-I - BINDING-SITE, AND COMPARISON TO Q(A) IN PURPLE BACTERIA REACTION CENTERS

The nature of the binding site of the quinone accepter A(1) in Photosy stem I (PSI) is studied by modeling the protein and cofactor on the ba sis of structural data derived from the intermediate resolution 4 Angs trom X-ray diffraction electron density map, the position and orientat ion of A(1) as evaluated from EPR data, and the histidine ligation of P-700 as deduced from mutation experiments. Several models art: constr ucted within the degrees of freedom allowed by the experimental constr aints. In all cases a close interaction between the A(1) headgroup and the side chain of PsaA-Trp697 (PsaB-Trp677) is found. The model is co mpared to the known binding site of QA in bacterial reaction centers ( bRC) in which a similar quinone-tryptophan arrangement has been establ ished. The results are also compared for consistency with published ma gnetic resonance data. The influences of the protein environment on th e semiquinone g-tensor and hyperfine couplings are considerably differ ent in PSI and bRC. It is argued that this is mainly a result of diffe rences in the hydrogen bonding to the protein, in the strength of the pi-pi interactions with the tryptophan, and in the protein induced asy mmetry in the spin density of the respective quinone radical anion.