Terpenoid pathways carotenoid synthesis

Thorough biochemical analysis of carotenoid biosynthesis, classical genetics, and more recently molecular genetics resulted in the elucidation of the main routes for the synthesis of acyclic and cyclic carotenoids at a molecular level (Sandmann 2001). Little is known, however, about the biosynthesis of carotenoids containing additional modifications of the end groups, the polyene chain, the methyl groups, or molecular rearrangements that contribute to the tremendous structural diversity of carotenoids. At present, hundreds of individual carotenoids have been characterized (Britton et al. 1998), and novel carotenoids continue to be isolated. All carotenoids are derived from the isoprenoid or terpenoid pathway. 

All carotenoids are derived from the isoprenoid or terpenoid pathway. From prenyl diphosphates of different chain lengths, specific routes branch off into various terpenoid end products. The prenyl diphosphates are formed by different prenyl transferases after isomerization of IPP to DMAPP by successive T-4 condensations with IPP molecules. Condensation of one molecule of dimethylallyl diphosphate (DMADP) and three molecules of isopentyl diphosphate (IDP) produces the diter-pene geranylgeranyl diphosphate (GGDP) that forms one-half of all C40 carotenoids. The head-to-head condensation of two GGDP molecules results in the first colorless carotenoid, phytoene. Phytoene synthesis is the first committed step in C40 carotenoid biosynthesis (Britton et al. 1998, Sandmann 2001). 

Since GAs as diterpenes share many intermediates in the biosynthetic steps leading to other terpenoids, eg, cytokinins, ABA, sterols, and carotenoids, inhibitors of the mevalonate (MVA) pathway of terpene synthesis also inhibit GA synthesis (57). Biosynthesis of GAs progresses in three stages, ie, formation of / Akaurene from MVA, oxidation of /-kaurene to GA 2" hyde, and further oxidation of the GA22-aldehyde to form the different GAs more than 70 different GAs have been identified. 

Carotenoids of higher plants, algae, and fungi are C40 tetraterpenes biosynthesized by the well-known isoprenoid pathway [1, 5, 6, 8, 17, 18]. The early steps, involving the formation of the C5 isoprenoid units and the subsequent synthesis of prenyldiphosphate intermediates, are common to all classes of terpenoids. 

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