Tetraterpenes

Tetraterpenes are a class of terpenes that consist of eight isoprene units, totaling forty carbon atoms. In common usage they refer mainly to carotenoids, a family of pigmented tetraterpenes found in plants, algae, bacteria, and fungi. Carotenes are hydrocarbons such as beta-carotene and lycopene; xanthophylls are oxygenated derivatives like lutein and zeaxanthin. Together they provide a broad range of colors from yellow to red and serve as pigments and antioxidants.

Biosynthesis begins with the isoprenoid precursors isopentenyl pyrophosphate (IPP) and dimethylallyl pyrophosphate (DMAPP), produced via the mevalonate pathway in animals and many fungi and some bacteria, or via the MEP pathway in plants and many bacteria. These units are condensed to geranylgeranyl pyrophosphate (GGPP, C20). Two GGPP molecules condense head-to-head to form phytoene (C40), the first committed tetraterpene in the carotenoid pathway. Phytoene then undergoes desaturation to yield lycopene; cyclization and further modifications produce beta-carotene and other carotenoids, which can be oxygenated to form xanthophylls and other derivatives.

Roles and distribution: carotenoids participate in light harvesting and photoprotection in photosynthetic organisms by quenching reactive oxygen species. They also contribute to coloration that can attract pollinators or seed dispersers. In humans, provitamin A carotenoids such as beta-carotene are dietary vitamin A precursors, while others act as antioxidants. Carotenoids are widespread in plants, algae, fungi, and some bacteria, and may be produced by endophytic microbes as well.

Applications: carotenoids are used as natural food colorants (color index E160) and dietary supplements. Industrial production occurs through plant extraction or microbial fermentation, often with metabolic engineering to optimize yields. Carotenoids are traditionally classified into carotenes (hydrocarbons) and xanthophylls (oxygenated derivatives).