2.5 Tetraterpenoids and Carotenoids
A plethora of natures yellow, orange, red and purple colours are mostly by virtue of the presence of carotenoids. The essentially consist of an important group of C40 tetraterpenoids. Invariably, there are two specific regions in a living plant wherein the biogenesis of carotenoids usually occur, namely: chloroplasts and chromatophores of bacteria and fungi.
There are two characteristic features that have been observed in such types of naturally occurring compounds, namely:
(a) Additional isopentenyl moieties (H3C—CH==CH—CH2CH3) could be embeded onto the tetraterpenoid backbone to result into the formation of either C45 or C50 carotenoids, as seen in certain microbes.
Example Homocarotenoids, and
(b) Oxidation of C40 carotenoids aften yields such carotenoids that do possess less than 40 carbon atoms,
Example Apocarotenoids
So far nearly 600 carotenoids have been duly isolated and identified from naturally occurring sources, such as: plants, bacteria, fungi and marine organisms. The ones obtained from the marine sources are found most abundantly and usually contain acetylenic moieties (HC≡CH).
Characteristic Features of Carotenoids Some characteristic features of carotenoids are
enumerated below:
(i) Most widely known carotenoids are either simple unsaturated hydrocarbons having the basic lycopene structure or their corresponding oxygenated analogues, usually termed as Xanthophylls,
(ii) Eight isoprene units are found to be joined head to tail in lycopene to give it a conjugated system that eventually is responsible for attributing the chromophoric character to the molecule i.e.; producing colour, and
(iii) Cyclization of lycopene at both terminals of the molecule yields a bicyclic hydrocarbon commonly known as β-carotenes, which occur most abundantly in the higher plants.
Interestingly, both in plants and micro-organisms the carotenoids have been observed to serve three major roles, namely: first, as photosynthetic pigments; secondly, as photoprotective agents; and thirdly, as membrane stabilization substances. In contrast, carotenoids in animals serve as a precursor of vitamin A and other retenoids. Besides, they also act not only as cancer preventive agents but also as photoprotective agents. Perhaps the protective characteristic features of carotenoids, in general, may be due to the easy accessibility to various singly oxygen atoms and ample free radicals, collectively checking the oxidation damage to cells and catering as antioxidants.
With the advent of various innovative aspects of biotechnology a quantum jump in the availability of carotenoid production is very much on the cards.
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* Thornton, M.D., ‘Phytochemistry’, 12, 391, 1973.
** Reimhoed, H, and K.P. Sieber, ‘Z. Naturfoesch., 36C, 466, 1981.
