Introduction to Carotenoids and Overview of Their Biosynthesis

The carotenoids are based on or derived from a C40 tetraterpene skeleton, with a long central polyene chromophore that is responsible for their typical yellow-orange colours. Some have been in the natural diet of humans and their ancestors throughout evolution, and special functions have evolved for β-carotene as provitamin A, and for lutein in eye and brain health. Other major dietary carotenoids, such as lycopene, have only been part of the human diet in comparatively recent times, so any biological action is likely to be pharmacological. A brief survey is given of highlights in the development of carotenoid research, including the roles in human health that have been suggested for carotenoids and their breakdown products and metabolites. Biological actions of carotenoids must be consistent with their chemical and physical properties. In the aqueous environment of cells and tissues, these properties may be modified by the medium and the immediate molecular surroundings, and may not be the same as the known properties in true solution in organic solvent. This is also relevant to the biosynthesis of these fat-soluble substances via water-insoluble intermediates. Finally, the overview of carotenoid biosynthesis integrates results from investigations of reaction mechanisms, genes and enzymes. It covers formation of isopentenyl diphosphate, construction of the C40 carotenoid skeleton as phytoene, and subsequent desaturation, isomerisation, cyclisation, hydroxylation and later modifications including cleavage of carotenoids into smaller fragments.