What Is Carotenoid Biosynthesis?

Terpenoids, also called isoprenoids, are organic compounds whose carbon skeleton is derived by linking isoprene (CH₂=C(CH₃)CH=CH₂) units together. Carotenoids, one subtype of terpenoids, are categorized as 30-C, 40-C and so forth, based on their number of skeletal carbon atoms. They can be manufactured in the laboratory by biosynthesis, sometimes called biogenesis, imitating the processes found in nature. Starting with small and simple molecules, such as isopentenyl diphosphate, additions occur stepwise in the presence of catalytic enzymes, until the end-products are reached. Although the reactions are known by their chemical pathway, manufacture may involve the use of microbes.

Carotenoids — including β-carotene, lycopene and the xanthophylls — are yellow-to-red colorants, which occur in carrots, apricots, spinach and other fruits and vegetables. They serve two known essential purposes. Since they absorb light at the blue end of the spectrum, carotenoids extend the range of frequency at which plants can engage in photosynthesis; they also protect the green pigment from oxidative photolytic damage. In addition to their antioxidant properties, some carotenoids possess Vitamin A activity. Foods rich in carotenoids tend to be low in lipids.

Synthesis of carotenoids in nature is accomplished by one of two known processes: one is the mevalonate, the other is the non-mevalonate carotenoid biosynthesis pathway. Both pathways are similar once they reach isopentenyl pyrophosphate (IPP). The next step is conversion into dimethylallyl pyrophosphate (DMPP), then geranyl pyrophosphate (GPP) and, finally, the 15-carbon species, farnesyl pyrophosphate (FPP). This serves as the intermediate in further carotenoid biosynthesis steps. Two of the 15-carbon structures can be joined to form 30-C carotenoids using a catalyst.

If the intent is rather to produce 40-C or 50-C carotenoids, the farnesyl diphosphate receives another IPP to form the 20-carbon atom intermediate, geranylgeranyl diphosphate (GGPP). This is then enzymatically added to itself to produce the 40-C phytoene, which can be rearranged to lycopene. Once lycopene is reached, there are a variety of synthetic pathways to differing end-results. Lycopene can be added to further to produce the 50-C carotenoids. Alternatively, structures can be kept to 40 carbons and be catalytically converted into α-carotene or β-carotene, which initiate the third and fourth route.

Knowledge of the pathways of carotenoid biosynthesis has existed for decades. It was not, however, until the 1990s that gene encoding for the enzymes was sufficiently identified to make industrial manufacture using the methods found in nature practical. Gene cloning has been accomplished for each of the steps of carotenoid biosynthesis, down to the manufacture of the xanthophylls. Molecular biologists believe the carotenoid pathway in plants may be manipulable via gene-transfer technology. This would enable easier and cheaper carotenoid biosynthesis methodologies.