Carotenoids are synthesized in nature by plants and many microorganisms. Animals can metabolize carotenoids in a characteristic manner, but they are not able to synthesize carotenoids.
Carotenoids, being terpenoids, are synthesized from the basic C5-terpenoid precursor, isopentenyl diphosphate (IPP) (XVII) (Fig.1). This compound is converted to geranylgeranyl diphosphate (C20) (XVIII). The dimerization of XVIII leads to phytoene (XIX) and the stepwise dehydrogenation via phytofluene (XX), ζ-carotene (XXI), and neurosporene (XXII) gives lycopene (I). Subsequent cyclizations, dehydrogenations, oxidations etc., lead to the individual naturally occurring carotenoids. Little is known about the biochemistry of the many interesting final structural modifications that give rise to the hundreds of diverse natural carotenoids.
There are now exciting prospects for rapid progress through the application of molecular genetics techniques in combination with other biochemical and chemical approaches. The benefits of these findings are not purely academic. The industrial production of natural carotenoids through microbial biotechnology is already established and expanding, mainly through the exploitation of some microalgae (particularly Dunaliella) which can synthesize large amount of carotenoids.