[29] Biosynthesis of carotenoids: An overview

Publisher Summary The three basic carotenoid structures lycopene, α -carotene, and β -carotene were elucidated in the early1930s. All known naturally occurring carotenoids, about 400, are variations on these structures. Carotenoids are tetraterpenoids—that is, they are composed of eight branched C 5 isoprenoid units made up into two C 20 units formed by tail to head condensation joined head to head at the center of the molecule. This satisfies Ruzicka's generalization that all terpenoids can be considered to be derived by cyclization or other rearrangements from an aliphatic precursor made up of isoprene units. The chapter discusses the formation of phytoene, which is the first C 40 precursor. The first step involves the isomerization of isopentenyl pyrophosphate (IPP) to dimethylallyl pyrophosphate (DMAPP) and the sequential addition of three further IPP molecules to DMAPP. These reactions, catalyzed by prenyl transferases, yield the C 20 compound geranygeranyl pyrophosphate (GGPP). This pathway is shared with the sterol pathway up to the C l5 stage, when farnesyl pyrophosphate (FPP) branches off to form squalene, the C 30 acyclic precursor of sterols. In carotenogeneic systems, two GGPP molecules condense head to head to form, via prephytoene pyrophosphate, phytoene. The stereochemistry of the reaction is described in the chapter for the formation of both trans -phytoene and cis -phytoene.