Tetraterpenes C40

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The tetraterpenes are represented by only one group of compounds, the carotenoids, though several hundred natural structural variants are known. These compounds play a role in photosynthesis, but they are also found in non-photosynthetic plant tissues, in fungi and bacteria. Formation of the tetraterpene skeleton, e.g. phytoene, involves tail-to-tail coupling of two molecules of ger-anylgeranyl diphosphate (GGPP) in a sequence essentially analogous to that seen for squalene and triterpenes (Figure 5.67). A cyclopropyl compound, prephytoene diphosphate (compare pres-qualene diphosphate, page 213) is an intermediate in the sequence, and the main difference between the tetraterpene and triterpene pathways is how the resultant allylic cation is discharged. For squa-lene formation, the allylic cation accepts a hydride ion from NADPH, but for phytoene biosynthesis, a proton is lost, generating a double bond in the centre of the molecule, and thus a short conjugated chain is developed. In plants and fungi, this new double bond has the Z (cis) configuration, whilst in bacteria, it is E (trans). This triene system prevents the type of cyclization seen with squalene. Conjugation is extended then by a sequence of desaturation reactions, removing pairs of hydrogens alternately from each side of the triene system, giving eventually lycopene (Figure 5.67), which, in common with the majority of carotenoids, has the all-trans configuration. This means that in plants and fungi, an additional isomerization step is involved to change the configuration of the central double bond.

The extended n-electron system confers colour to the carotenoids, and accordingly they contribute yellow, orange, and red pigmentations to plant tissues. Lycopene is the characteristic carotenoid pigment in ripe tomato fruit (Lycopersicon escu-lente; Solanaceae). The orange colour of carrots (Daucus carota; Umbelliferae/Apiaceae) is caused by P-carotene (Figure 5.68), though this compound is widespread in higher plants. P-Carotene and other natural carotenoids (Figure 5.68) are widely employed as colouring agents for foods, drinks, confectionery, and drugs. P-Carotene displays additional cyclization of the chain ends, which can be rationalized by the carbocation mechanism shown in Figure 5.69. Depending on which proton is lost from the cyclized cation, three different cyclic alkene systems can arise at the end of the chain, described as P-, y-, or e-ring systems. a-Carotene (Figure 5.68) has a P-ring at one end of the chain, and an e-type at the other, and is representative of carotenoids lacking symmetry. y-Carotene (a precursor of P-carotene) and 8-carotene (a precursor of a-carotene) illustrate carotenoids where only one end of the chain has become cyclized. Oxygenated carotenoids (termed xanthophylls) are also widely distributed, and the biosynthetic origins of the oxygenated rings found in some of these, such as zeathanthin, lutein, and violaxanthin (Figure 5.68), all common green leaf carotenoids, are shown in Figure 5.69. The epoxide grouping in violaxanthin allows further chemical modifications, such as ring contraction to a cyclopen-tane, exemplified by capsanthin (Figure 5.68), the brilliant red pigment of peppers (Capsicum annuum; Solanaceae), or formation of an allene as in fucoxanthin, an abundant carotenoid in brown algae (Fucus species; Fucaceae). Astaxan-thin (Figure 5.68) is commonly found in marine animals and is responsible for the pink/red coloration of crustaceans, shellfish, and fish such as salmon. These animals are unable to synthesize carotenoids and astaxanthin is produced by modification of plant carotenoids, e.g. P-carotene, obtained in the diet.

Carotenoids function along with chlorophylls in photosynthesis as accessory light-harvesting

GGPP

GGPP

allylic cation electrophilic addition giving tertiary cation allylic cation electrophilic addition giving tertiary cation

Proton Structure Lycopene

loss of proton with formation of cyclopropane ring loss of diphosphate gives primary cation

prephytoene PP

loss of proton with formation of cyclopropane ring loss of diphosphate gives primary cation

prephytoene PP

apart from the final proton loss, this sequence is exactly analogous to that seen with presqualene PP (Figure 5.54)

W-M 1,3-alkyl shift generates new cyclopropane ring and more favourable tertiary cation a-K-

loss of proton generates alkene bond cleavage produces alkene and favourable allylic cation

loss of proton generates alkene bond cleavage produces alkene and favourable allylic cation

sequence of desaturation reactions; in plants and fungi, the central double bond is also isomerized Z ^ E

lycopene

Figure 5.67

lycopene

Figure 5.67

pigments, effectively extending the range of light absorbed by the photosynthetic apparatus. They also serve as important protectants for plants and algae against photo-oxidative damage, quenching toxic oxygen species. Some herbicides (bleaching herbicides) act by inhibiting carotenoid biosynthesis, and the unprotected plant is subsequently killed by photo-oxidation. Recent research also suggests carotenoids are important antioxidant molecules in humans, quenching singlet oxygen and scavenging peroxyl radicals, thus minimizing cell damage and affording protection against some forms of cancer. The most significant dietary carotenoid in this respect is lycopene, with tomatoes and processed tomato products featuring as the predominant source. The extended conjugated system allows free radical addition reactions and hydrogen abstraction from positions allylic to this chain.

The A group of vitamins* are important metabolites of carotenoids. Vitamin A1 (retinol) (Figure 5.70) effectively has a diterpene structure, but it is derived in mammals by oxidative

Tetraterpene
8-carotene

protonation of double bond gives tertiary cation; then electrophilic addition

O2 NADPH

O2 NADPH

. - NADPH e.g. zeathanthin opening of epoxide ring and loss of proton generates allene

O2 NADPH

e.g. fucoxanthin

O2 NADPH

opening of epoxide ring

y-ring

e.g. fucoxanthin

e.g. violaxanthin opening of epoxide ring

y-ring e-ring e.g. a-carotene

O2 NADPH

e.g. lutein pinacol-like rearrangement generating ketone O

e.g. capsanthin

e.g. capsanthin

Figure 5.69

Figure 5.69

ß-carotene central cleavage a central cleavage generates two molecules of retinal

retinal

NADH

retinol (vitamin Ai)

ß-carotene central cleavage a

oxidative chain shortening

NADH

Diterpene Retinal

desaturation extending conjugation

retinol (vitamin Ai)

desaturation extending conjugation

excentric cleavage b excentric cleavage can generate one molecule of retinal

excentric cleavage b oxidative chain shortening

dehydroretinol (vitamin A2)

dehydroretinol (vitamin A2)

metabolism of a tetraterpenoid, mainly P-carotene, taken in the diet. Cleavage occurs in the mucosal cells of the intestine, and is catalysed by an O2-dependent dioxygenase, probably via an intermediate peroxide. This can theoretically yield two molecules of the intermediate aldehyde, retinal, which is subsequently reduced to the alcohol, retinol (Figure 5.70). Although P-carotene cleaved at the central double bond is capable of giving rise to two molecules of retinol, there is evidence that cleavage can also occur at other double bonds, so-called excentric cleavage (Figure 5.70). Further chain shortening then produces retinal, but only one molecule is produced per molecule of P-carotene. Vitamin A2 (dehydroretinol) (Figure 5.70) is an analogue of retinol containing a cyclohexadiene ring system; the corresponding aldehyde, and retinal, are also included in the A group of vitamins. Retinol and its derivatives are found only in animal products, and these provide some of our dietary needs. Cod-liver

MeO2C

MeO2C

crocetin

Figure 5.71

crocetin

Figure 5.71

oil and halibut-liver oil are rich sources used as dietary supplements. However, carotenoid sources are equally important. These need to have at least one non-hydroxylated ring system of the P-type, e.g. P-carotene, a-carotene, and y-carotene.

Cleavage of carotenoid precursors is likely to explain the formation of bixin and crocetin (Figure 5.71) and, indeed, these are classified as apocarotenoids. Large amounts (up to 10%) of the red pigment bixin are found in the seed

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  • gary
    Is carotene a tetraterpene?
    8 years ago

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