Tetraterpenoids

Tetraterpenoids are a class of terpenoids with 40 carbon atoms, formed by eight isoprene units, and they constitute the carotenoid family. They are widespread pigments in plants, algae, and some bacteria and fungi, responsible for yellow, orange, and red colors and playing essential roles in photosynthesis and photoprotection.

Structure and classification: Tetraterpenoids include carotenes, which are hydrocarbons such as beta-carotene and lycopene, and xanthophylls, which contain oxygen-containing functional groups, such as lutein and zeaxanthin. Most have a long conjugated polyene chain, often with cyclic end groups. They are lipophilic and localize in plastid membranes and other lipid-rich environments.

Biosynthesis: In plants and many algae, isoprenoid precursors IPP and DMAPP are produced via the MEP pathway in plastids (with some organisms relying on the mevalonate pathway in other compartments). The C20 building block geranylgeranyl pyrophosphate (GGPP) condenses with another GGPP to form C40 phytoene, which is desaturated and isomerized to yield lycopene. Cyclization of lycopene by specific cyclases produces beta- or alpha-carotene, which can be oxygenated to form various xanthophylls. Carotenoids also serve as precursors to retinoids in animals.

Function and significance: Tetraterpenoids function as light-harvesting pigments and photoprotectants, quenching reactive oxygen species in photosynthetic organisms. They contribute nutritional benefits as antioxidants and, for provitamin A carotenoids like beta-carotene, as dietary sources of vitamin A in animals.

Uses and applications: They are widely used as dietary supplements and natural colorants in foods, feeds, cosmetics, and industrial products.