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.

Arabidopsis Mutants Deleted in the Light-Harvesting Protein Lhcb4 Have a Disrupted Photosystem II Macrostructure and Are Defective in Photoprotection

“…The molecular mechanism of enhanced resistance to photooxidation by Zea is not yet clear. The hypotheses include (1) draining excitation energy from the PS core by xanthophylls bound at the interface with Lhcbs; (2) quenching of 3 Chl* energy from RC, possibly by the Dexter exchange mechanism (Nayak et al, 2002) by Zea located in site L2 on specific Lhcs; and (3) preferential scavenging of 1 O 2 by Zea into site V1 of LHCII (Johnson et al, 2007). Although all of these mechanisms are plausible for explaining the higher photosensitivity of koLhcb4, their relative contribution needs further investigation.…”

Control of Photosynthetic and High-Light-Responsive Genes by the Histidine Kinase DspA: Negative and Positive Regulation and Interactions between Signal Transduction Pathways

“…Both microalgae and vascular plants have evolved mechanisms for photoacclimation that enable them to tolerate the absorption of excess excitation energy (10,24,25,31,32). Acclimation mechanisms include, but are not limited to, changes in the composition of light-harvesting and/or reaction center pigment-protein complexes (4,6,7,38), dissipation of excess absorbed excitation energy as heat, and synthesis of enzymes with antioxidant function, such as superoxide dismutase (26,35), catalase (27,29,42), and peroxidases (11,16,40).…”

Bulk-heterojunction Based on Blending of Red and Blue Luminescent Silicon Nanocrystals and P3HT Polymer

“…An excitation transfer energy from a conjugated polymer to a nanocrystal results in a red-shift of nanocrystals/polymer blends as a function of the temperature. In the Dexter mechanism, the spectral overlap is independent of the oscillator strength of the transitions and is efficient at very small distances (< 10 Å) [17]. In the temperature range 4-300 K the red-shift is of about 40 nm in the case of red luminescent Si-ncs and the ~70 nm-shift for the blue luminescent ones.…”