Tetraterpenes or Carotenoids

Formation of Geranylgeranyl Pyrophosphate Formation of Prephytoene Pyrophosphate Formation of Phytoenes Acyclic Carotenoids Alicyclic Carotenoids Oxygenated Carotenoids Site of Synthesis Chemosystematic Studies Carotenoids in Algae Cartenoids in Fungi Biological Activity 

Carotenoids in Photosynthesis Photoresponses in Plants Carotenoids and Vision Development in animals Pigmentation in Animals Pigmentation of Flowers and Fruits Uses of Carotenoids Metabolites of Carotenoids Plant Growth-Regulating Compounds Carotenoids Metabolites as Fungal Pheromones References 

Carotenoids, which occur in all plants, bacteria, and fungi, are probably the most widely distributed of all natural pigments they are involved in many fundamental processes such as photosynthesis and mammalian vision. These highly unsaturated lipids also serve as vegetative, floral, and fruit pigments in plants and as pigments in the feathers of birds, outer parts of insects, as well as the skins of fish and other animals (Britton, 1976, 1983 Buchecker, 1982 Kayser, 1982). 

Carotenoids are C40 polyolefinic metabolites derived from mevalonic acid. Almost all members of this group are acyclic or have chains that terminate with one or two 6-membered rings. All have ttie potential for a vast number of geometric isomers, but, in practice, almost all have entirely E (trans) configurations many carotenoids are colored because of the extended conjugated systems they contain. The structures for approximately 500 carotenoids are known, but only about one-fourth of these are from plants (Britton, 1983). The leaves of all green plants contain the same major carotenoids 3-carotene (usually 25-30% of the total) (1), lutein (about 45%) [(3/ ,3 / ,6 / )-3, -carotene-3,3 -diol] (2), violaxanthin [(3S,5R,6S,3 S,5% 6 5)-5,6,5, 6 -diepoxy- 

Tetraterpenes or carotenoids are synthesized from mevalo-nate precursors. Those involved in photosynthesis are synthesized in the chloroplast, but the enzymes specific for carotenoid biosynthesis are encoded in the nucleus, synthesized in cytoplasmic ribosomes, and transported into the chloroplast (Britton, 1993). Chloroplasts at different stages in development seem to differ in their ability to synthesize carotenoids autonomously from CO2 or by importation of isopentenyl pyrophosphate (isopentenyl diphosphate) (Britton, 1993). 

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More than 600 different carotenoids from natural sources have been isolated and characterized. Physical properties and natural functions and actions of carotenoids are determined by their chemical properties, and these properties are defined by their molecular structures. Carotenoids consist of 40 carbon atoms (tetraterpenes) with conjugated double bonds. They consist of eight isoprenoid units j oined in such a manner that the arrangement of isoprenoid units is reversed at the center of the molecule so that the two central methyl groups are in a 1,6-position and the remaining nonterminal methyl groups are in a 1,5-position relationship. They can be acyclic or cyclic (mono- or bi-, alicyclic or aryl). Whereas green leaves contain unesterified hydroxy carotenoids, most carotenoids in ripe fruit are esterified with fatty acids. However, those of a few... 

Among carotenoids, fat-soluble plant pigments, generally classified as tetraterpenes as well as among other isoprenoids and polyenes, isolated from plants or animals, many substances are found for the synthesis of which the Wittig reaction is of paramount importance. Quite often, these compounds consist of two symmetrically linked molecular halves and it might be sufficient to prepare one molecular half only which can subsequently be dimerized . 

Terpenoids are derived from the cytosolic mevalonate pathway or from the plastidial 2-C-methyl-D-erythritol-4-phosphate (MEP) pathway (see also Terpenoid Biosynthesis). Both pathways lead to the formation of the C5 units isopentenyl diphosphate and its allylic isomer dimethylallyl diphosphate, which are the basic terpenoid biosynthesis building blocks (Fig. 1). Although increasing evidence suggests that exchange of intermediates occurs between these compartments, the cytoplasmic mevalonate pathway is generally considered to supply the precursors for the production of sesquiterpenes and triterpenes (including sterols) and to provide precursors for protein prenylation and for ubiquinone and heme-A production in mitochondria. In the plastids, the MEP pathway supplies the precursors for the production of isoprene, monoterpenes, diterpenes (e.g., GAs), and tetraterpenes (e.g., carotenoids). 

The term carotenoids designates a group of structurally related colorants that are mainly found in plants. At present, more than 600 carotenoids have been identified. Their basic structure is a symmetrical tetraterpene skeleton, formed by head-to-tail condensation of two 20-carbon units. (Formula 9.1). Based on their composition, carotenoids are subdivided into two groups carotenes, which contain only carbon and hydrogen atoms, e.g., a-, P-, and y-carotenes and lycopene (Formulae 9.2-9.4) and xanthophylls — oxocarotenoids — which contain at least one oxygen function, such as hydroxy, keto, or epoxy groups (Formulae 9.5-9.9). 

Carotenoids are tetraterpenes derived from a symmetrical C40 skeleton. Carotenoids can be classified into two great groups carotenes, which are strictly hydrocarbons, and xanthophylls, derived from the former that contain oxygenated functions. Stracturally, the carotenoids may be acyclic (e.g., lycopene) or contain a ring of five or six carbons at one or both ends of the molecule (e.g., /3-carotene). Figure 10.1 shows the stmcture and the system of numbering using lycopene and /3-carotene as models of acyclic and bicyclic carotenoids respectively. Stractures of some representative carotenes and xanthophylls, commonly found in fruits are also illustrated. 

About 200 naturally abundant tetraterpenes are known to date and referred to as carotenoids because all of them represent structural variants or degradation derivatives of p-carotene from the carrot Daucus carota (Umbelliferae) with 11 to 12 conjugated CC double bonds. The generally accepted parent name is "carotene" two Greek letters (p, y, s, 9, k, % and /) define all seven of the known end groups. 

Carotenoids a large class of yellow and red pigments, which are highly unsaturated aliphatic and ali-cyclic hydrocarbons and their oxidation products. C. are biosynthesized from isoprene units (CsHj) and therefore have the methyl branches typical of isopre-noid compounds. Most C. have 40 C atoms and are thus tetraterpenes (consisting of 8 isoprene units). A few C. contain 45 and 50 C atoms, particularly in nonphotosynthetic bacteria. C. with fewer than 40 C atoms are called nor-, seco- or apocarotenoids. 

Mono-, sesqui-, di-, and sesterpenes contain the isoprene units linked in a head-to-tail fashion. The triterpenes and carotenoids (tetraterpenes) contain two C15 and C20 units, respectively, linked head to head. Many terpenes are hydrocarbons, but oxygen-containing compounds such as alcohols, aldehydes, or ketones are also found to fall in the class of terpenes. These derivatives are more frequently named as terpenoids. 

Tetraterpenes comprise only one group of compounds, the carotenoids. They are synthesized from two molecules GGPP by tail-to-tail addition. Double bonds are inserted to yield an extended conjugated system with dXX-trans configuration that is responsible for the yellow, orange and red color of the carotenoids. Either one or both ends of the tetraterpene chain are cyclized to a six-membered ring. Carotenoids with hydroxy or epoxy functions are classified as xanthophylls (Dewick, 2002). 

Let us consider further the consequences of head-tail additions. If an additional molecule of IPP is added head-to-tail to farnesyl pyrophosphate geranylgeranyl pyrophosphate, a diterpene, is obtained. The series of events outlined above can now be repeated at a higher level of complexity geranylgeranyl pyrophosphate can either be converted to other diter-penes or two molecules of geranylgeranyl pyrophosphate can be joined tail-to-tail to give 40 C bodies. In this way tetraterpenes, i.e. carotenoids, are obtained. Further head-to-tail additions of IPP lead, finally, to the polyterpenes rubber, gutta-percha, and balata. 

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