Carotenoid oxidation in photosystem II

The oxidation of carotenoid upon illumination at low temperature has been s tudied in Mn-depleted photosystem II (PSII) using EPR and electronic absorp tion spectroscopy. Illumination of PSII at 20 K results in carotenoid catio n radical (Car(+.)) formation in essentially all of the centers. When a sam ple which was preilluminated at 20 K was warmed in darkness to 120 K, Car(.) was replaced by a chlorophyll cation radical. This suggests that caroten oid functions as an electron carrier between P680, the photooxidizable chlo rophyll in PSII, and Chl(Z), the monomeric chlorophyll which acts as a seco ndary electron donor under some conditions. By correlating with the absorpt ion spectra at different temperatures, specific EPR signals from Car(+.) an d Chl(Z)(+.) are distinguished in terms of their g-values and widths. When cytochrome b(559) (Cyt b(559)) is prereduced, illumination at 20 K results in the oxidation of Cyt b(559) without the prior formation of a stable Car( +.). Although these results can be reconciled with a linear pathway, they a re more straightforwardly explained in terms of a branched electron-transfe r pathway, where Car is a direct electron donor to P680(+), while Cyt b(559 ) and Chl(Z) are both capable of donating electrons to Car(+.) and where th e Chl(Z) donates electrons when Cyt b(559) is oxidized prior to illuminatio n. These results have significant repercussions on the current thinking con cerning the protective role of the Cyt b(559)/Chl(Z) electron-transfer path ways and on structural models of PSII.