Characterization of carotenoid and chlorophyll photooxidation in photosystem II

Photosystem II (PSII) contains two accessory chlorophylls (Chl_Z, ligated to D1-His118, and Chl_D, ligated to D2-His117), carotenoid (Car), and heme (cytochrome b₅₅₉) cofactors that function as alternate electron donors under conditions in which the primary electron-donation pathway from the O₂-evolving complex to P680⁺ is inhibited. The photooxidation of the redox-active accessory chlorophylls and Car has been characterized by near-infrared (near-IR) absorbance, shifted-excitation Raman difference spectroscopy (SERDS), and electron paramagnetic resonance (EPR) spectroscopy over a range of cryogenic temperatures from 6 to 120 K in both Synechocystis PSII core complexes and spinach PSII membranes. The following key observations were made: (1) only one Chl⁺ near-IR band is observed at 814 nm in Synechocystis PSII core complexes, which is assigned to Chl_Z⁺ based on previous spectroscopic studies of the D1-H118Q and D2-H117Q mutants [Stewart, D. H., Cua, A., Chisholm, D. A., Diner, B. A., Bocian, D. F., and Brudvig, G. W. (1998) Biochemistry 37, 10040-10046]; (2) two Chl⁺ near-IR bands are observed at 817 and 850 nm in spinach PSII membranes which are formed with variable relative yields depending on the illumination temperature and are assigned to Chl_Z⁺, and Chl_D⁺, respectively; (3) the Chl and Car cation radicals have significantly different stabilities at reduced temperatures with Car⁺ decaying much faster; (4) in Synechocystis PSII core complexes, Car⁺ decays by recombination with Q_A^- and not by Chl_Z/Chl_D oxidation, with multiphasic kinetics that are attributed to an ensemble of protein conformers that are trapped as the protein is frozen; and (5) in spinach PSII membranes, Car⁺ decays mainly by recombination with Q_A^-, but also partly by formation of the 850 nm Chl cation radical. The greater stability of Chl_Z⁺ at low temperatures enabled us to confirm that resonance Raman bands previously assigned to Chl_Z⁺ are correctly assigned. In addition, the formation and decay of these cations provide insight into the alternate electron-donation pathways to P680⁺.