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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
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
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)