Phytoene from geranylgeranyl diphosphate

Figure 73. The carotenoid biosynthetic pathway. Enzymes are named according to the designation of their genes Ccs, capsanthin-capsorubin synthase CrtL-b, lycopene-b-cyclase CrtL-e, lycopene-e-cyclase CrtR-b, b-ring hydroxylase, CrtR-e, e-ring hydroxylase DMADP, dimethylallyl diphosphate GGDP, geranylgeranyl diphosphate Ggps, geranylgeranyl-diphosphate synthase IDP, isopentenyl diphosphate Ipi, IDP isomerase Pds, phytoene desaturase Psy, phytoene synthase Vde, violaxanthin de-epoxidase Zds, z-carotene desaturase Zep, zeaxanthin epoxidase. (From van den Berg and others 2000.)...

Fig. 4 Reconstruction of the carotenoids biosynthetic pathway in S. cerevisiae. Heterologous activities t-HMGl 3 hydroxy-3-methylglutaryl-coenzymeA, ctrE geranylgeranyldiphosphate synthase (from Xanthophyllomyces dendrorhous), Ctrl phytoene desaturase, ctrYB bifunctional enzyme phytoene synthase and lycopene cyclase. Endogenous yeast activities ISIS I geranylgeranyldiphosphate synthase. Metabolites IPP isopentenyl diphosphate, GPP geranyl diphosphate, FPP famesyl-diphosphate, GGPP geranylgeranyl diphosphate...

Whereas the isoprene units of the higher plant carotenoids that are biosynthesized within the chloroplasts are formed via the 1-deoxyxylulose pathway, the fungal carotenoids are biosynthesized from acetate via mevalonic add. The C40 carbon skeleton of the carotenoids is formed by the head-to-head coupling of two C20 geranylgeranyl diphosphate units by phytoene synthase. 

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). 

Biosynthesis The biosynthesis of the C. proceeds in chloroplasts or chromoplasts in analogy to other iso-prenoids by way of 3-isopentenyl diphosphate ( active isoprene ). The colorless 15-cis-phytoene (C40) is formed from 2 molecules of geranylgeranyl diphosphate, further transformations furnish, wiaall-trans- -carotene (colored), all-trans- ycopenc. Subsequent cy-clization reactions allow the formation of other carotinoids. Xanthophvlls are formed from C. by incorporation of oxygen... 

Terpenes, like myrcene, are formed according to the terpene rule by a head-to-tail coupling of two isoprene units. This is for example contrasted by the tail-to-tail coupling of two geranylgeranyl diphosphate molecules to phytoene, which collides with this rule the same is true forthe mechanistically related biosynthesis of chrysanthemic acid from two molecules of dimethylaUyl diphosphate. 

The simplest prototype of carotenes, called I/-carotenes, is the polyunsaturated acyclic hydrocarbon (15Z)-7,8,11,12,7, 8, llM2 -octahydro- r, Ir-carotene, known as phytoene, which is synthesised from two molecules of geranylgeranyl diphosphate. Isomerisation of phytoene yields the trans-isomer phytofluene (7,8,1 l,12,7, 8 -hexahydro- Ir, Ir-carotene). Oxidation of phytofluene gradually gives -carotene (7,8,7, 8 -tetrahydro-vIf,vIf-carotene), neurosporene (7,8-dihydro- f, Ir-carotene) and lycopene(vIf,vIr-carotene), which is the final product of the biosynthesis (9-180) and the main pigment of tomatoes (30-200 mg/kg), watermelons (33 121 mg/kg) and rose hips (101-834 mg/kg... 

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