George Britton

Structure and properties of carotenoids in relation to function.

The basic principles of structure, stereochemistry, and nomenclature of carotenoids are described and the relationships between structure and the chemical and physical properties on which all the varied biological functions and actions of carotenoids depend are discussed. The conjugated polyene chromophore determines not only the light absorption properties, and hence color, but also the photochemical properties of the molecule and consequent light‐harvesting and photoprotective action. The polyene chain is also the feature mainly responsible for the chemical reactivity of carotenoids toward oxidizing agents and free radicals, and hence for any antioxidant role. In vivo, carotenoids are found in precise locations and orientations in subcellular structures, and their chemical and physical properties are strongly influenced by other molecules in their vicinity, especially proteins and membrane lipids. In turn, the carotenoids influence the properties of these subcellular structures. Structural features such as size, shape, and polarity are essential determinants of the ability of a carotenoid to fit correctly into its molecular environment to allow it to function. A role for carotenoids in modifying structure, properties, and stability of cell mem branes, and thus affecting molecular processes associated with these membranes, may be an important aspect of their possible beneficial effects on human health.—Britton, G. Structure and properties of carotenoids in relation to function. FASEB J. 9, 1551–1558 (1995)

The photochemistry of carotenoids

Preface. Color Plates. Part I: Biosynthetic Pathways and the Distribution of Carotenoids in Photosynthetic Organisms. 1. Carotenoids in Photosynthesis: An Historical Perspective Govindjee. 2. Carotenoid Synthesis and Function in Plants: Insights from Mutant Studies in Arabidopsis thaliana D. DellaPenna. 3. Carotenoids and Carotenogenesis in Anoxygenic Photosynthetic Bacteria S. Takaichi. Part II: Structure of Carotenoid-Chlorophyll Protein Complexes. 4. The Structure and Function of the LH2 Complex from Rhodopseudomonas acidophila Strain 10050, with Special Reference to the Bound Carotenoid R.J. Cogdell, et al. 5. Carotenoids as Components of the Light-harvesting Proteins of Eukaryotic Algae R.G. Hiller. 6. The Structure of Reaction Centers from Purple Bacteria G. Fritzsch, A. Kuglstatter. 7. Carotenoids and the Assembly of Light-Harvesting Complexes H. Paulsen. Part III: Electronic Structure, Stereochemistry, Spectroscopy, Dynamics and Radicals. 8. The Electronic States of Carotenoids R.L. Christensen. 9. Cis-Trans Carotenoids in Photosynthesis: Configurations, Excited-State Properties and Physiological Functions Y. Koyama, R. Fujii. 10. The Electronic Structure, Stereochemistry and Resonance Raman Spectroscopy of Carotenoids B. Robert. 11. Electron Magnetic Resonance of Carotenoids A. Angerhofer. 12. Carotenoid Radicals and the Interaction of Carotenoids with Active Oxygen Species R. Edge, T.G. Truscott. 13. Incorporation of Carotenoids into ReactionCenter and Light-Harvesting Pigment-protein Complexes H.A. Frank. Part IV: Ecophysiology and the Xanthophyll Cycle. 14. Ecophysiology of the Xanthophyll Cycle B. Demmig-Adams, et al. 15. Regulation of the Structure and Function of the Light-Harvesting Complexes of Photosystem II by the Xanthophyll Cycle P. Horton, et al. 16. Biochemistry and Molecular Biology of the Xanthophyll Cycle H.Y. Yamamoto, et al. 17. Relationships Between Antioxidant Metabolism and Carotenoids in the Regulation of Photosynthesis C.H. Foyer, J. Harbinson. Part V: Model Systems. 18. Novel and Biomimetic Functions of Carotenoids in Artificial Photosynthesis T.A. Moore, et al. 19. Physical Properties of Carotenoids in the Solid State H. Hashimoto. 20. Carotenoids in Membranes W.I. Gruszecki. Index.

Biosynthesis of carotenoids.

The carotenoids that are found in the photosynthetic pigment-protein complexes of higher plants, algae and phototrophic bacteria, including cyanobacteria, are C40 tetraterpenes. They are biosynthesised by a specialised branch of the isoprenoid or terpenoid pathway which is also used for the biosynthesis of a wide variety of other important compounds. All isoprenoid compounds are built up from the C5 “isoprene unit” the carbon skeleton of which is clearly seen in the important intermediate isopentenyl diphosphate (IDP). A series of diphosphate intermediates with carbon chains consisting of multiples of five carbon atoms is then built up from this isoprene unit intermediate. In the first step of this process, IDP undergoes a double bond isomerisation to give dimethylallyl diphosphate (DMADP). The prenyl transferase enzymes then build up the isoprenoid chain from these two intermediates. The first condensation, between DMADP and IDP, gives the C10 compound geranyl diphosphate (GDP), which is the precursor of monoterpenes. Further additions of IDP then extend the chain to produce, successively, the Cl5 farnesyl diphosphate (FDP), precursor to the sesquiterpenes, triterpenes and sterols, and the C20 geranylgeranyl diphosphate (GGDP), which gives rise to the carotenoids and to the diterpenes including phytol, which provides the isoprenoid sidechain of the chlorophylls and, in most cases, the bacteriochlorophylls. The general outline of the isoprenoid pathway, illustrating the biosynthesis of carotenoids in relation to that of other isoprenoid compounds, is given in Fig. 4.1

Carotenoids in Photosynthesis

Carotenoids are the secret ingredient in photosynthesis; masked by the green of chlorophyll, they are only revealed in their true glory during senescence, when chlorophyll is degraded to display the glowing colours of autumn. Yet the presence of these orange and yellow pigments is absolutely essential for oxygenic photosynthesis. This Chapter will explain the importance of carotenoids to oxygenic organisms and also their roles in anoxygenic photosynthetic bacteria, where their presence is often more obvious but in other ways may be less crucial.

Glucosinolates and Differential Herbivory in Wild Populations of Brassica oleracea

Glucosinolates are known to elicit responses from Brassica herbivores in laboratory studies. To study their importance in interactions with herbivores in the field, glucosinolate profiles and levels of herbivory were ascertained for wild cabbage plants growing in four neighboring populations in the UK. Glucosinolate profiles differed between plant populations, but not between different habitats within populations. Within habitats, there was no link between individual plant glucosinolate profiles and herbivory by Pieris spp., slugs and snails, flea beetles or aphids. Plants attacked by the micromoth, Selania leplastriana, contained higher levels of 2–hydroxy-3–butenylglucosinolate and 3–indolylmethylglucosinolate than plants within the same population that were not attacked. It is concluded that the differences in glucosinolate profiles between the plant populations are unlikely to be due to differential selection pressures from herbivores feeding on the mature plants over the two years studies.

Enhancement of the ΔpH-dependent dissipation of excitation energy in spinach chloroplasts by light-activation: correlation with the synthesis of zeaxanthin

The extent of energy‐dependent quenching of chlorophyll fluorescence in broken spinach chloroplasts has been quantitatively related to the size of the thylakoid proton gradient as measured by the quenching of 9‐aminoacridine fluorescence by titration at constant irradiance with the uncoupler nigericin or by change in irradiance. It was found that chloroplasts prepared from leaves that had been pre‐illuminated with strong light for 30 min showed energy‐dependent quenching at a lower proton gradient than chloroplasts prepared from dark‐adapted leaves. Measurement of the carotenoid composition of the thylakoids showed that light treatment raised the ratio of zeaxanthin: violaxanthin. The possible dependence of energy‐dependent quenching on xanthophyll composition and the physiological implications of this light‐activation process to the regulation of photosynthetic electron transport are discussed.

Functions of Intact Carotenoids

The traditional view that carotenoids are a class of plant pigments does not do justice to their versatility. This versatility will become clear from the overview of the biological roles of carotenoids, in animals and microorganisms as well as in plants, that is given in this Chapter. It has become customary and convenient to differentiate biological effects of carotenoids into functions, actions and associations [1]. ‘Functions’ have been defined as effects or properties that are essential for the normal well-being of the organism. Biological responses that follow the administration of carotenoids in the diet or as supplements are considered as ‘actions’. When an effect is seen but a causal relationship to the carotenoid has not been demonstrated, this is described as an ‘association’. The line between these is often not clear, however.

Evaluation of the antioxidant activity and partial characterisation of extracts from browned yam flour diet

Abstract The antioxidant activity of the ethyl acetate extract of browned yam flour, a diet widely consumed in West Africa especially in southern part of Nigeria, was examined in β-carotene linoleate system and egg yolk phosphatidyl choline (EYPC) liposomal membrane exposed to the lipid soluble 2,2-azobis (2-amidinopropane) hydrochloride (AAPH) generated peroxyl radicals. Total antioxidant activity (TAA) of crude ethyl acetate extract was determined using 2,2′ azinobis- (3-ethyl benzothiazoline-6-sulfonic acid) (ABTS) and Horseradish peroxidase as enzyme source. TAA of the extract was estimated to be 7.5 measured as mM of Vitamin C equivalent. The crude extract was fractionated on a sephadex LH-20 column using methanol as eluant at a flow rate of 1.8 ml/min. Fractions 1, 2 and 3 showed similar absorption maxima in the range of 190–230 and 240–269. Fractions 4–7 produced absorption maxima in the range of 270–280. Although, the crude extract showed a higher antioxidant effect than butylated hydroxyanisole (BHA), all the isolated fractions exhibited a lower antioxidant activity than BHA in the β-carotene linoleate model system. Compared to control, fractions 2, 3, 5 and 6 showed good antioxidant effect against the bleaching of β-carotene. Incorporation of the fractions into EYPC liposomes retarded the lipid peroxidation caused by AAPH derived peroxyl radicals to varying extents. Fractions 2, 3, 5 and 6 significantly (P

Effects of high light and desiccation on the operation of the xanthophyll cycle in two marine brown algae

Two brown algae, Pelvetia canaliculata and Laminaria saccharina, from the higher and lower mediolittoral belts respectively, have been tested for their capacity to overcome high-light stress in water and in air (in both fully hydrated and desiccated states). When exposed to supersaturating light irradiance in water, the two species developed non-photochemical quenching of fluorescence (NPQ) which was correlated with an increase in the de-epoxidation ratio (DR) of the xanthophyll cycle carotenoids (violaxanthin, antheraxanthin and zeaxanthin) and was followed by a slower decrease in oxygen evolution. NPQ reached values of up to 9 in P. canaliculata but only 4·5 in L. saccharina, at DRs of 0·65 and 0·5, respectively. In air, the xanthophyll cycle was also operative but the efficiency of de-epoxidation decreased linearly with the degree of hydration of the thallus. Photoprotection capacities in air also appeared higher in P. canaliculata than in L. saccharina, probably due to the higher molar content of the ...

Later Reactions of Carotenoid Biosynthesis

The later reactions of carotenoid biosynthesis are considered in two groups, the basic reactions occurring at the C-1,2 double bond, and the final modifications to the cyclic and acyclic structures thus produced. Recent work on these biosynthetic processes is reviewed, with the emphasis on the mechanism and stereochemistry of the reactions. The concept that carotenoid “half-molecules”, rather than individual compounds are the substrates recognised by the enzymes is used to simplify the overall picture of the biosynthesis of cyclic and acyclic carotenoids, and ideas that enzyme aggregates may be involved are discussed. Attention is drawn to the difficulties of correlating details of the stereochemistry of biosynthesis of different carotenoids in different organisms.