Tetraterpeny

Tetraterpeny, also called tetraterpenes, are a class of terpenes characterized by a carbon skeleton of 40 atoms, formed from eight isoprene units. They are a prominent group of terpenoids and include most carotenoids, a family of pigments that impart yellow, orange, or red colors to many plants, algae, and some bacteria.

Chemical structure and classification: Tetraterpenes are typically hydrocarbon or oxygen-containing compounds. The subset known as carotenoids includes both carotenes (hydrocarbons, such as beta-carotene and lycopene) and xanthophylls (oxygenated carotenoids, such as lutein, zeaxanthin, and astaxanthin). They can be acyclic or feature cyclic end groups, often with beta-ionone or similar rings at one or both ends. Conjugated double-bond systems give rise to their vivid colors and to their light-absorbing properties.

Biosynthesis: In photosynthetic organisms, tetraterpenes are synthesized from geranylgeranyl pyrophosphate (GGPP, C20). Two GGPP units condense to form the C40 backbone phytoene, which is then desaturated through multiple steps to lycopene. Cyclization of lycopene and further oxygenation or modification produces the diverse carotenoids. This biosynthetic pathway can proceed via the mevalonate pathway or the MEP/DOXP pathway, depending on the organism.

Occurrence and function: Tetraterpenes are widespread in plants, algae, and some bacteria. They function as accessory pigments in photosynthesis, protecting against photooxidative damage, and as antioxidants. In animals, many tetraterpenes such as beta-carotene serve as provitamin A precursors. They also serve as commercial colorants and nutritional supplements.

Examples and applications: Notable tetraterpenes include beta-carotene, lycopene, lutein, zeaxanthin, canthaxanthin, and astaxanthin. They are used in food coloring, nutraceuticals, and research as model polyenes for studying conjugated systems and photoprotection.