Biochemistry - The Chemical Reactions of Living Cells, Volume 3 - D. Metzler 1980

Light in Biology
Photosynthesis
Photochemical reactions involving carotenoid pigments

It is highly noteworthy that there are no green plants in nature that lack carotenoid pigments [116]. Admittedly, carotenoid-free mutants are used in Photosynthesis research, but they would hardly survive under natural conditions. Carotenoids protect chlorophyll from the destructive action of molecular oxygen generated by light. The Mechanism of this protective action remains unclear.

Carotenoids perform yet another function: they act as accessory light-harvesting pigments. The existence of a third function is indicated by the light-induced reductive deoxygenation of epoxycarotenoids [equation (13-31), stage a] [117, 118].

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Epoxycarotenoids are found exclusively in oxygenic photosynthetic organisms. Their photodeoxygenation and subsequent non-photochemical reoxygenation [equation (13-31), stage b] constitute the violaxanthin cycle. Violaxanthin contains epoxy structures at both ends of the molecule. Photoreduction at one end yields antheraxanthin; if photoreduction involves both ends, zeaxanthin is formed. These three carotenoids are detected in almost all higher green plants and Algae. Very little is known about the enzymology and Biological Significance of the violaxanthin cycle. It may potentially serve a regulatory function. Deepoxidation [equation (13-31), stage a] apparently proceeds with the participation of ascorbic acid and takes place within the thylakoid lumen. This reaction is favored by the strong acidification of the medium that occurs in illuminated METABOLISM/14.html">Chloroplasts. Epoxidation [equation (13-31), stage b] of carotenoids is catalyzed by an "outer" monooxygenase located on the stroma side. As a result, a transmembrane cycle arises, which depends on the translocation of carotenoids across the membrane [118].



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