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 acceptor A(1) within the photosystem I reaction center, we have carried out site-directed mutagenesis of two tryptophan residues (W693 and W702) in the PsaA subunit of Chlamydomonas reinhardtii. One of these conserved tryptophans (W693) is predicted to be close to the phylloquinone and has been implicated in the interaction of A(1) with an aromatic residue through pi-pi stacking. We find that replacement of W702 with either histidine or leucine has no effect on the electronic structure of A(1)(radical anion) or on forward electron 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 the 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 PSI.