Carotenoid Oxidation in Photosystem II

Abstract

The oxidation of carotenoid upon illumination at low temperature has been studied in Mn-depleted photosystem II (PSII) using EPR and electronic absorption spectroscopy. Illumination of PSII at 20 K results in carotenoid cation radical (Car^+•) formation in essentially all of the centers. When a sample which was preilluminated at 20 K was warmed in darkness to 120 K, Car^+• was replaced by a chlorophyll cation radical. This suggests that carotenoid functions as an electron carrier between P680, the photooxidizable chlorophyll in PSII, and Chl_Z, the monomeric chlorophyll which acts as a secondary electron donor under some conditions. By correlating with the absorption spectra at different temperatures, specific EPR signals from Car^+• and Chl_Z^+• are distinguished in terms of their g-values and widths. When cytochrome b₅₅₉ (Cyt b₅₅₉) is prereduced, illumination at 20 K results in the oxidation of Cyt b₅₅₉ without the prior formation of a stable Car^+•. Although these results can be reconciled with a linear pathway, they are more straightforwardly explained in terms of a branched electron-transfer pathway, where Car is a direct electron donor to P680⁺, while Cyt b₅₅₉ and Chl_Z are both capable of donating electrons to Car^+•, and where the Chl_Z donates electrons when Cyt b₅₅₉ is oxidized prior to illumination. These results have significant repercussions on the current thinking concerning the protective role of the Cyt b₅₅₉/Chl_Z electron-transfer pathways and on structural models of PSII.