The quinone acceptor A1 in photosystem I:: Binding site, and comparison to QA in purple bacteria reaction centers

The nature of the binding site of the quinone accepter A(1) in Photosystem I (PSI) is studied by modeling the protein and cofactor on the basis of structural data derived from the intermediate resolution 4 Angstrom X-ray diffraction electron density map, the position and orientation of A(1) as evaluated from EPR data, and the histidine ligation of P-700 as deduced from mutation experiments. Several models art: constructed within the degrees of freedom allowed by the experimental constraints. 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 compared to the known binding site of QA in bacterial reaction centers (bRC) in which a similar quinone-tryptophan arrangement has been established. The results are also compared for consistency with published magnetic resonance data. The influences of the protein environment on the semiquinone g-tensor and hyperfine couplings are considerably different in PSI and bRC. It is argued that this is mainly a result of differences in the hydrogen bonding to the protein, in the strength of the pi-pi interactions with the tryptophan, and in the protein induced asymmetry in the spin density of the respective quinone radical anion.