Carotenoid biosynthesis cyclization reactions

Biosynthesis and Metabolism.—Pathways and Reactions. Two reviews of carotenoid biosynthesis discuss, respectively, the early steps and the later reactions." The former paper deals with the mechanism of formation of phytoene and the series of desaturation reactions by which phytoene is converted into lycopene, and also describes in detail the biosynthesis of bacterial C30 carotenoids. The second paper" presents details of the mechanism and stereochemistry of cyclization and the other reactions that involve the carotenoid C-1 —C-2 double bond and the later modifications, especially the introduction of oxygen functions. 

Inhibitors have been used to investigate the biosynthesis of 1,2-dihydroneuro-sporene [l,2,7,8-tetrahydro-i/, i/f-carotene (143)] and related 1,2-dihydro-carotenoids in Rhodopseudomonas viridis, and possible alternative sequences are presented.The C-1,2 hydrogenation reaction is inhibited by CPTA [2-(4-chlorophenylthio)triethylammonium chloride], a compound known to inhibit cyclization and C-1,2-hydration in other systems, thus indicating possible similarity of the reactions involved. 

FIGURE 63.1 Starting with mevalonate, carotenoids are biosynthesized by a special branch of the terpenoid pathway. The first C-40 hydrocarbon unit formed is phytoene, a carotenoid with three conjugated double bonds, which then is enzymatically desaturated to successively yield (3-carotene, neurosporene, and lycopene. Other carotenoids such as (3-carotene and oxocarotenoids are produced from lycopene following cyclization and hydroxylation reactions. Thus, lycopene is a central molecule in the biosynthesis pathway of carotenoids. 

The biosynthesis of carotenoids in plants has been reviewed extensively in recent years and is only briefly described here (Britton, 1988 Bartley and Scolnik, 1994 Sandmann, 1994). The committed step to carotenoid synthesis is the formation of the first compound phytoene by the head-to-head condensation of two molecules of GGDP by phytoene synthase. Phytoene is subjected to a series of four sequential desaturation reactions, by two separate enzymes to yield lycopene, which has eleven conjugated double bonds. Lycopene is then cyclized to /3-carotene by two /3-cyclizations or to a-carotene... 

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