Using site-directed mutagenesis to probe the role of the D2 carotenoid in the secondary electron-transfer pathway of photosystem II

摘要

Secondary electron transfer in photosystem II (PSII), which occurs when water oxidation is inhibited, involves redox-active carotenoids (Car), as well as chlorophylls (Chl), and cytochrome b (559) (Cyt b (559)), and is believed to play a role in photoprotection. Car(D2) may be the initial point of secondary electron transfer because it is the closest cofactor to both P-680, the initial oxidant, and to Cyt b (559), the terminal secondary electron donor within PSII. In order to characterize the role of Car(D2) and to determine the effects of perturbing Car(D2) on both the electron-transfer events and on the identity of the redox-active cofactors, it is necessary to vary the properties of Car(D2) selectively without affecting the ten other Car per PSII. To this end, site-directed mutations around the binding pocket of Car(D2) (D2-G47W, D2-G47F, and D2-T50F) have been generated in Synechocystis sp. PCC 6803. Characterization by near-IR and EPR spectroscopy provides the first experimental evidence that Car(D2) is one of the redox-active carotenoids in PSII. There is a specific perturbation of the Car(a (TM)+) near-IR spectrum in all three mutated PSII samples, allowing the assignment of the spectral signature of Car (D2) (a (TM)+) ; Car (D2) (a (TM)+) exhibits a near-IR peak at 980 nm and is the predominant secondary donor oxidized in a charge separation at low temperature in ferricyanide-treated wild-type PSII. The yield of secondary donor radicals is substantially decreased in PSII complexes isolated from each mutant. In addition, the kinetics of radical formation are altered in the mutated PSII samples. These results are consistent with oxidation of Car(D2) being the initial step in secondary electron transfer. Furthermore, normal light levels during mutant cell growth perturb the shape of the Chl(a (TM)+) near-IR absorption peak and generate a dark-stable radical observable in the EPR spectra, indicating a higher susceptibility to photodamage further linking the secondary electron-transfer pathway to photoprotection.