Topology and photoprotective role of carotenoids in photosystem II of chloroplast: a hypothesis.

On the basis of existing evidence, a model is proposed for the topology of carotenoids in photosystem II (PS II) of chloroplasts. suggesting their possible roles in the photoprotection and stability of PS II complex. The presence of one cis and one trans beta-carotene at reaction centre II (RC II), with different photoprotective functions, is suggested. The core antennae (CP43, CP47) are presumed to contain beta-carotene molecules in clusters. The possible molecular mechanism for formation of a quenching complex in the minor LHC II, involving zeaxanthin, chlorophyll a and the glutamic acid side chain of the light harvesting protein, is worked out. This complex is proposed to be an efficient triplet quencher, in addition to its role as a quencher of singlet chlorophyll energy. The migration of triplet energy from RC II to the quenching complex is surmised. It is suggested that the carotenoids in RC II and in different LHCs form an integrated photoprotective unit.