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 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 π-π interactions with the tryptophan, and in the protein induced asymmetry in the spin density of the respective quinone radical anion.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Surfaces, Coatings and Films
- Materials Chemistry