NO - Besoin d'assistance

Composition of tomatoes and tomato products in antioxidants (WG1) page 15
__________________________________________________________________________________________
4. Breeding and biotechnology methods for high lycopene content
The biochemical pathway involved in lycopene production has been largely
elucidated. Until recently, the mevalonate (MVA) (Fig 1) pathway was believed to be the only
route for the provision of the key isoprenoid intermediate, IPP (Britton 1990).
Figure 1. Main stages of carotenoid synthesis in plants
Mevalonate
Isopentenyl diphosphate (IPP)
Geranylgeranydiphosphate (GGDP)
Phytoene synthase Phytoene
Phytoene desaturase Lycopene
Lycopene cyclase β- carotene
Recently a non-MVA pathway has been identified (Schwender et al.1996). This is
localised in the plastid in contrast to the MVA pathway which is localised in the cytoplasm of
the cell. In this pathway, glyceraldehyde 3-phosphate (GAD) and pyruvate are the first
precursors. Pyruvate is first decarboxylated and then condensated on the carbonyl of GAP
yielding 1-deoxy-d-xylulose 5-phosphate and finally IPP. This novel mevalonate-independent
biosynthesis pathway was first demonstrated in bacteria and then was shown to be involved in
the carotenoid synthesis in higher plants.
Then condensation of IPP molecules results in Geranylgeranyl diphosphate (GGDP)
formation. The first C40 carotenoid phytoene is formed from two molecules of GGDP. The
non-coloured phytoene undergoes a series of sequential desaturation reactions to produce
phytofluene, ζ-carotene, neurosporene and finally the coloured lycopene. It was suggested
that during these reactions cis-trans double bond isomerization occured. Finally, cyclization