Role of Carotenoids in Photosystem II (PSII) Reaction Centers

Abstract

A photoprotection mechanism operative in closed reaction centers (RCs) is proposed, where–as a consequence of the negative charge on the quinone QA-triplet 3Chl is formed by the radical pair mechanism on the accessory Chl of the normally inactive D2 branch where it can be subsequently quenched by the spatially close β-carotene in the D2 branch. Whereas β-carotene in the D1 branch is more than 17 A away from the accessory D1-chlorophyll (\({{\rm Chl}_{\rm accD_1})}\) and, therefore, cannot quench the Chl triplet, the D2-carotene is only 13.2 A away from \({{\rm Chl}_{\rm accD_2}}\) . We propose that the D2 branch becomes active in electron transfer and thus plays a photoprotective role when the intact RCs are closed under high photon fluence conditions. This interpretation allows combining many seemingly inconsistent observations in the literature and reveals the so far “elusive” RC triplet quenching mechanism in PSII. Based on laser-induced optoacoustic studies, an important structural role is assigned to the β-carotene in the D1 branch, i.e., this carotene ensures a rigid structure.