Carotenoids are terpenoids classified as orange, red or yellow fat- soluble pigments. These are responsible for the colouring of plants, such as tomatoes. Carotenoids are also essential in photosynthesis. This pathway involves many sub-pathways, such as retinol metabolism, lutein biosynthesis, the xanthophyll cycle, and abscisic acid biosynthesis. As with other terpenoid pathways, carotenoid biosynthesis begins with geranylgeranyl-PP. This takes place in the chloroplast, and the pathway continues here for almost all the subsequent reactions. Geranylgeranyl-PP is then converted to prephytoene diphosphate by using the enzyme phytoene synthase. This enzyme continues to catalyze compounds in the next reaction, using prephytoene diphosphate to produce phytoene. Phytoene is then converted to phytofluene, using the enzyme 15-cis-phytoene desaturase. Phytofluene then uses this enzyme again to create 9-cis,9'-cis-7,7',8,8'-tetrahydro-psi,psi-carotene. From here, neurosporene is created with the help of zeta-carotene desaturase. This enzyme works again to convert neurosporene to lycopene. Lycopene uses lycopene beta cyclase to become y-Carotene, which uses the same enzyme to create b-carotene. Continuing in the chloroplast, b-carotene teams up with two enzymes, beta-carotene 3-hydroxylase 1 and protein lutein deficient 5 to produce b-cryptoxanthin. These two enzymes then convert b-cryptoxanthin to zeaxanthin. This marks the beginning of the xanthophyll cycle, which end in the creation of violaxanthin, an oxygen-containing xanthophyll. From here, abscisic acid biosynthesis begins, in the cytoplasm, and ends in the endoplasmic reticulum lumen.
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