Biosynthesis Pathway for Vitamin A



                                   geranylgeranyl diphosphate (GGDP)

                                   GGDP occurs naturally in rice endosperm.

                                   Two molecules of GGDP are used in this process.


phytoene synthase. A gene taken from daffodil is responsible for producing the enzyme phytoene synthase.  The gene is controlled by a rice promoter and is only expressed in the endosperm (Toenniessen 2000).  Phytoene synthase catalyzes the reaction that converts two GGDP molecules to phytoene, the first step in provitamin-A synthesis.








Phytoene desaturase, an enzyme originating from the bacterium Erwinia uredovora, catalyzes two steps in the pathway.  A total of four double bonds are introduced to convert phytoene to lycopene (Ye et al. 2000; McLaren and Frigg 2001).







                                                                                  Lycopene b-cyclase, also originating

 from daffodil, catalyses the fourth step.  This step results in the cyclization of lycopene, producing provitamin-A (Ye et al. 2000; McLaren and Frigg 2001).



                                                 provitamin A






                                                   provitamin A



Once ingested, our body cleaves provitamin A at its central carbon-carbon bond resulting in vitamin A (retinol) (Carey 1987).





                                                            Vitamin A (retinol)





Jason Sutton in the Department of Bioagricultural Sciences and Pest Management at Colorado State University contributed the content for this page.