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#beta-carotene

6 public questions tagged with this topic.

Transgenic Plant -2

Beta-carotene is a precursor of:

Beta-carotene is tetraterpenoid C40 carotenoid consisting of two beta-ionone rings linked by eleven conjugated double bonds providing strong antioxidant scavenging singlet oxygen and light filtering. Most importantly it serves as provitamin A carotenoid because symmetrical cleavage at central 15,15' double bond by enzyme beta-carotene 15,15'-monooxygenase BCMO1 in intestinal enterocytes and hepatic stellate cells yields two molecules of retinaldehyde. Retinal can be reduced to retinol bound to retinol binding protein for systemic transport to retina for rhodopsin regeneration mediating scotopic vision, oxidized to retinoic acid activating gene transcription via nuclear receptors RAR and RXR heterodimers regulating embryonic development, epithelial differentiation, immune cell maturation, and spermatogenesis. Humans cannot synthesize carotenoid backbone requiring dietary intake. Vitamin A deficiency leads to xerophthalmia, night blindness, keratomalacia, impaired mucosal immunity increased measles mortality. Beta-carotene bioavailability enhanced by dietary lipids and food processing releasing crystalline aggregates, with conversion factor 12:1 to retinol activity equivalents in mixed diet.

Ref: Lodish Molecular Cell Biology Vitamins; NIH StatPearls Beta-carotene Vitamin A; Olson 1989.

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Transgenic Plant -2

Golden rice is engineered to synthesize:

Golden Rice is paradigm of metabolic engineering for nutritional biofortification to combat vitamin A deficiency blindness and mortality affecting 250 million children in Asia and Africa dependent on polished rice lacking provitamin A. Endosperm of Oryza sativa does not accumulate carotenoids because pathway blocked downstream of geranylgeranyl diphosphate due to absent psy expression and limited desaturase activity. Ingo Potrykus and Peter Beyer introduced minimal carotenogenic pathway: phytoene synthase condensing two GGPP to 15-cis-phytoene, phytoene desaturase performing four desaturations to all-trans lycopene, and lycopene beta-cyclase forming beta-carotene yellow pigment localized in amyloplasts as crystals requiring plastid transit peptide. Second-generation Golden Rice 2 using maize phytoene synthase under glutelin promoter accumulates up to 37 microgram per gram dry weight beta-carotene, turning endosperm golden. Conversion by intestinal BCMO1 cleavage yields retinol delivering estimated 30-50 percent RDA from 130 g rice per day, demonstrating how plant engineering can deliver essential micronutrients lacking in staple without agronomic penalty or yield loss.

Ref: Ye et al. Science 2000 287:303-305 Golden Rice; Paine et al. Nat Biotechnol 2005 GR2; WHO.

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