| Description: |
Carotenoids such as beta-carotene, lycopene, lutein and beta-cryptoxanthine are produced in plants and certain bacteria, algae and fungi, where they function as accessory photosynthetic pigments and as scavengers of oxygen radicals for photoprotection. They are also essential dietary nutrients in animals. Carotenoid oxygenases cleave a variety of carotenoids into a range of biologically important products, including apocarotenoids in plants that function as hormones, pigments, flavours, floral scents and defence compounds, and retinoids in animals that function as vitamins, visual pigments and signalling molecules []. Examples of carotenoid oxygenases include:Beta,beta-carotene 15,15'-dioxygenase (BCDO1) from animals, which cleaves beta-carotene symmetrically at the central double bond to yield two molecules of retinal [].Carotenoid-cleaving dioxygenase, mitochondrial (BCDO2) from animals, which cleaves beta-carotene asymmetrically to apo-10'-beta-carotenal and beta-ionone, the latter being converted to retinoic acid. Lycopene is also oxidatively cleaved [, , ].Carotenoid 9,10(9',10')-cleavage dioxygenase (CCD) from plants, which cleaves a variety of carotenoids symmetrically at both the 9-10 and 9'-10' double bonds and catalyzes the formation of 4,9-dimethyldodeca-2,4,6,8,10-pentaene-1,12-dialdehyde from zeaxanthin []. 9-cis-epoxycarotenoid dioxygenase (NCED1/2) from plants, which cleaves 9-cis xanthophylls to xanthoxin, a precursor of the hormone abscisic acid [].Apocarotenoid-15,15'-oxygenase (ACOX) from bacteria and cyanobacteria, which converts beta-apocarotenals rather than beta-carotene into retinal. This protein has a seven-bladed β-propeller structure with four hisitidines that hold the iron active centre [].Retinoid isomerohydrolase (RPE65) from animals, which in its soluble form binds all-trans retinol, and in its membrane-bound form binds all-trans retinyl esters. RPE65 is important for the production of 11-cis retinal during visual pigment regeneration [, , ]. |
