Mutations in Both Sides of the Photosystem I Reaction Center Identify the Phylloquinone Observed by Electron Paramagnetic Resonance Spectroscopy

The core of photosystem I (PS1) is composed of the two related integral membrane polypeptides, PsaA and PsaB, which bind two symmetrical branches of cofactors, each consisting of two chlorophylls and a phylloquinone, that potentially link the primary electron donor and the tertiary acceptor. In an effort to identify amino acid residues near the phylloquinone binding sites, all tryptophans and histidines that are conserved between PsaA and PsaB in the region of the 10th and 11th transmembrane α-helices were mutated in Chlamydomonas reinhardtii. The mutant PS1 reaction centers appear to assemble normally and possess photochemical activity. An electron paramagnetic resonance (EPR) signal attributed to the phylloquinone anion radical (A₁ ^-) can be observed either transiently or after illumination of reaction centers with pre-reduced iron-sulfur clusters. Mutation of PsaA-Trp⁶⁹³ to Phe resulted in an inability to photo-accumulate A₁^-, whereas mutation of the analogous tryptophan in PsaB (PsaB-Trp⁶⁷³) did not produce this effect. The PsaA-W693F mutation also produced spectral changes in the time-resolved EPR spectrum of the P ₇₀₀⁺ A₁^- radical pair, whereas the analogous mutation in PsaB had no observable effect. These observations indicate that the A₁^- phylloquinone radical observed by EPR occupies the phylloquinone-binding site containing PsaA-Trp⁶⁹³. However, mutation of either tryptophan accelerated charge recombination from the terminal Fe-S clusters.