Site-directed mutagenesis of PsaA residue W693 affects phylloquinone binding and function in the photosystem I reaction center of Chlamydomonas reinhardtii

To investigate the environment of the phylloquinone secondary electron acce ptor A(1) within the photosystem I reaction center, we have carried out sit e-directed mutagenesis of two tryptophan residues (W693 and W702) in the Ps aA subunit of Chlamydomonas reinhardtii. One of these conserved tryptophans (W693) is predicted to be close to the phylloquinone and has been implicat ed in the interaction of A(1) with an aromatic residue through pi-pi stacki ng. We find that replacement of W702 with either histidine or leucine has n o effect on the electronic structure of A(1)(radical anion) or on forward e lectron transfer from A(1)(radical anion) to the iron-sulfur center F-x. In contrast, the same mutations of W693 alter the electronic structure of the photoaccumulated A(1)(radical anion) and slow forward electron transfer as measured by the decay of the electron spin-polarized signal arising from t he P700(.+)/A(1)(radical anion) radical pair. These results provide support for the hypothesis that W693 has a role in poising the redox potential of A(1)/A(1)(radical anion) so it can reduce F-x, and they indirectly provide evidence for electron transfer along the PsaA-side branch of cofactors in P SI.