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

The nature of the binding site of the quinone acceptor A₁ in Photosystem I (PSI) is studied by modeling the protein and cofactor on the basis of structural data derived from the intermediate resolution 4 Å X-ray diffraction electron density map, the position and orientation of A₁ as evaluated from EPR data, and the histidine ligation of P₇₀₀ as deduced from mutation experiments. Several models are constructed within the degrees of freedom allowed by the experimental constraints. In all cases a close interaction between the A₁ headgroup and the side chain of PsaA-Trp697 (PsaB-Trp677) is found. The model is compared to the known binding site of Q_A 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 π-π interactions with the tryptophan, and in the protein induced asymmetry in the spin density of the respective quinone radical anion.