Photoprotective function

Despite their absence in phycobilisomes, carotenoids, especially the so-called secondary carotenoids such as echinenone, were presumed to play a role in cyanobacterial photoprotection. Indeed, classic biochemical approaches have led to several reports of cyanobacterial carotenoid-proteins and evidence for their photoprotective function (Kerfeld et al. 2003, Kerfeld 2004b). One of these, the water soluble orange carotenoid protein (OCP), has been structurally characterized and has recently emerged as a key player in cyanobacterial photoprotection. 

Kodis, G., C. Herrero, R. Palacios, E. Marino-Ochoa, S. Gould, L. de la Garza, R. van Grondelle, D. Gust, T. A. Moore, A. L. Moore, and J. T. M. Kennis. 2004. Light harvesting and photoprotective functions of carotenoids in compact artificial photosynthetic antenna designs. J. Phys. Chem. B 108 414-425. 

Antioxidant and Photoprotection Functions and Reactions Involving Singlet Oxygen and Reactive Oxygen Species... 

The photoprotection hypothesis for anthocyanins is attractive because it can explain why red-leafed plants occur across such diverse environments. In the tropics, for example, anthocyanins in flushing red leaves may provide a critical photoprotective role to the nascent chloroplasts until adequate levels of xanthophylls have been synthesized. In the understorey, anthocyanins might protect shade-acclimated plants from the effects of sunflecks, which can be 2000-fold brighter than the usual light level, or canopy gaps caused by tree blowdown. A photoprotective function of anthocyanins would also benefit leaves in... 

The photoprotection hypothesis is not without its critics, and it certainly does not account for every occurrence of anthocyanins, such as in the leaf veins, in stems and petioles, and in underground organs such as roots and tubers. Nonetheless, there is now a compelling body of empirical data consistent with a photoprotective function in the photosynthetic cells of leaves. Photoprotection might well be the primary role of anthocyanin in the vegetative shoots of many species. 

In the case of the carotenoid-containing LH2 complex, the triplet states of BChl a and carotenoid (spheroidene) were generated immediately after excitation, but the triplet-state BChl a was quenched efficiently by the carotenoid so that no BChl a cation-radical was generated. Thus, the photoprotective function of the carotenoid in this antenna complex has been proven. 

Some adult invertebrates in tropical areas have relatively high MAA concentrations in external surfaces. Holothuroids preferentially accumulate MAAs in the epidermis, giant clams have highest concentrations of MAAs in the outermost layers of siphonal mantle tissue (more than four times the concentrations in subsurface mantle layers), sea hares have high MAA levels in skin, and ascidian tunics have higher MAA concentrations in the surface cells than in basal dermal lay-ers 102,152 154 jn coraiSj e upper exposed surface of colonies can have five-fold higher UV absorption than the less irradiated vertical faces.155 These topical distribution patterns reinforce the premise that MAAs have a photoprotective function. 

These results demonstrate forcefully that a photoprotective strategy predicated upon extremely rapid chlorophyll or porphyrin to carotene triplet energy transfer can succeed. We note here that the yield of the energy transfer process in the absence of oxygen is not the figure of merit for photoprotective function. It is 100% for both 24 and 26. 

The reader is referred to Siefermann-Harms (1987a, b) for earlier discussions on the light harvesting and photoprotective functions of carotenoids. 

In recent years the universal presence of various 15-cw carotenoids in the reaction centres of phototrophic organisms has been established and associated with photoprotective functions in PS2. It has been demonstrated that the 15-c/s configuration has a unique property of efficient isomerisation to the all-trans configuration upon triplet excitation [43],... 

Lichtenthaler HK, Schindler C. Studies on the photoprotective function of zeaxanthin at high-light conditions. In Murata N, editor. Research in Photosynthesis, Vol. IV. Dordrecht Kluwer Academic Publishers, 1992 517-520. 

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