Tetraterpenes, pigments

Farnesyl pyrophosphate can react with another molecule of isopentenyl pyrophosphate to form geranylgeranyl pyrophosphate, a 20-carbon compound. Two geranylger-anyl pyrophosphates can join to form phytoene, a 40-carbon compound. Phytoene is the precursor of the carotenoid (tetraterpene) pigments in plants. 

Section 26 16 Carotenoids are tetraterpenes They have 40 carbons and numerous dou ble bonds Many of the double bonds are conjugated causing carotenes to absorb visible light and be brightly colored They are often plant pigments... 

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 . 

The terpenoids, which are composed of the five-carbon isoprenoids, constitute the largest family of natural products with over 22,000 individual compounds in this class having been described. The terpenoids (hemiterpenes, monoterpenes, sesquiterpenes, diterpenes, triterpenes, tetraterpenes, polyterpenes, and the like) play diverse functional roles in plants as hormones, photosynthetic pigments, electron carriers, mediators of polysaccharide assembly, structural components of membranes, and defense compounds. Many compounds used by man including resins, latex, waxes, and oils contain plant terpenoids. 

The occurrence of terpenes is ubiquitous. Natural terpenes are found in plants and animals in minute amounts. Especially in higher plants, terpenes characterize the type of plant (chemotaxonomy) mono- and sesquiterpenes in essential oils, sesqui-, di-, triterpenes in balsams and resins, tetraterpenes in pigments and polyterpenes in latexes.Therefore, terpenes are often emitted from natural products such as citrus fruits or trees, e.g. conifers. 

Vitamin A, or retinol, occurs only in the animal world, where the best sources are cod-liver oil and other fish-liver oils, animal liver, and dairy products. Vitamin A in the form of a precursor, or provitamin, is found in the plant world in a group of tetraterpene (C4o) pigments called carotenes. The most common of these is j8-carotene, abundant in carrots, but also found in some other vegetables, particularly yellow and green ones. )S-Carotene has no vitamin A activity however, after ingestion, it is cleaved at the central carbon-carbon double bond to give retinol (vitamin A) ... 

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. 

C. represent a major class of natural pigments, occurring widely in plants and animals. In animals they occur mainly in surface tissues such as skin, shells, scales, feathers and beaks, but also in birds egg yolks and as visual pigments. C. in animals are all of plant origin, as animals are incapable of their de novo synthesis. However, animals do convert plant C into other forms, e.g. carotenes are oxidized to vitamin A. For C. biosynthesis, see Tetraterpenes. 

Most of the natural carotenoid pigments are tetraterpenes, being formed from two C20 geranylgeranyl pyrophosphate units through phytoene 4.122). The mechanism of phytoene formation has much in common with squalene biosynthesis. In particular, head-on fusion of the two C20 units occurs via prephytoene pyrophosphate 4.120), analogous to presqualene pyrophosphate 4.48) formation from two... 

Terpenoids, which include in particular tetraterpenoid and some other pigments derived from tetraterpenes, known as carotenoids, monoterpenic pigments iridoids and certain other coloured compounds. 

Carotenes Plant pigments based on tetraterpene structure that are nonsaponiliable, and impart yellow coloration to the endosperm. Carotenoids are divided into carotenes or hydrocarbon tetraterpenes (p-carotene, lycopene) and xanthophyUs or hydroxylated carotenes (lutein, zeaxanthin). P-Carotenes are precursors of retinol or vitamin A. 

Xanthophyll Plant pigments, such as lutein and zeaxanthin, based on a tetraterpene that contains at least one or more hydroxyl groups borne on the cyclic ends of the... 

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