Proton-coupled electron transfer and tyrosine D of photosystem II.

Photosystem II (PSII) is a photosynthetic reaction center that oxidizes water and reduces bound plastoquinone. PSII electron transfer is mediated by two redox-active tyrosine residues. One of these residues, tyrosine D (YD), has been assigned as Tyr160 of the D2 polypeptide by site-directed mutagenesis and isotopic labeling. Previous spectroscopic evidence has established that His189 in the D2 subunit forms a hydrogen bond with YD* and donates a proton to YD* when the radical is reduced. However, the mechanism of this reaction has not been elucidated. In this report, EPR spectroscopy and 2H2O solvent exchange were used to investigate the pL dependence of the YD* reduction rate. The kinetic isotope effect (KIE), induced by solvent exchange, was also measured as a function of pL. Under the conditions employed, the reduction of YD* is attributed to recombination with the QA- plastoquinone acceptor of PSII. The kinetic data were fit with a biexponential function. The majority, slow phase exhibited a pL-dependent rate constant, with a minimum at pL 7.5. Solvent exchange gave significant KIE at values between pL 5.5 and 8.0. In particular, at high pL (> or =7.5), the values of the KIE were determined to be 2.1 +/- 0.6 and 2.4 +/- 0.5. These values are consistent with a coupled electron and proton reaction, which occurs with a single kinetic step at pL values > or =7.5. The lower KIE values and the rate acceleration observed at low pL may be consistent with a change of mechanism in which the protonation of YD* occurs first, followed by rate-limiting electron transfer. The more modest acceleration in rate at high pL values is attributed to a small, pL-induced change in the distance between YD* and QA-.