title: Topology and photoprotective role of carotenoids in photosystem II of chloroplast: a hypothesis.
abstract: 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.
authors.affiliation: School of Life Sciences, Sambalpur University, Jyoti Vihar 768019, Orissa, India.
authors.forename: Lalitendu
authors.initials: L
authors.lastname: Nayak
authors.forename: Mukesh K
authors.initials: MK
authors.lastname: Raval
authors.forename: Basanti
authors.initials: B
authors.lastname: Biswal
authors.forename: Udaya C
authors.initials: UC
authors.lastname: Biswal
mesh_terms: D002338:Carotenoids / Q000737:chemistry / Q000378:metabolism*; D002736:Chloroplasts / Q000378:metabolism*; D045342:Light-Harvesting Protein Complexes; D008968:Molecular Conformation; D045322:Photosynthetic Reaction Center Complex Proteins / Q000737:chemistry* / Q000378:metabolism*; D045332:Photosystem II Protein Complex; D010944:Plants / Q000378:metabolism; D019207:beta Carotene / Q000737:chemistry / Q000378:metabolism
chemical_list: D045342:Light-Harvesting Protein Complexes; D045322:Photosynthetic Reaction Center Complex Proteins; D045332:Photosystem II Protein Complex; D019207:beta Carotene; D002338:Carotenoids