P-Carotene biosynthesis

Ye X, Al-Babili S, Kloti A, Zhang J, Lucca P, Beyer P, Potrykus 1 (2000) Engineering provitamin A (P-carotene) biosynthesis pathway into (carotenoid-free) rice endosperm. Science 287 303-305... 

Because rice is part of the staple diet in much of the world, researchers added two P-carotene biosynthesis genes in making a variety of Ory%a saliva rice, producing rice with a greatly enhanced P-carotene content that would help combat Vitamin A deficiency, a worldwide problem. 

Beyer, R, Al-BabUi, S., Ye, X. et al (2002) Golden rice introducing the P-carotene biosynthesis pathway into rice endosperm by genetic engineering to defeat vitamin A deficiency. /. Nutr., 132, 506S-510S. 

In nature, vitamin A aldehyde is produced by the oxidative cleavage of P-carotene by 15,15 - P-carotene dioxygenase. Alternatively, retinal is produced by oxidative cleavage of P-carotene to P-apo-S -carotenal followed by cleavage at the 15,15 -double bond to vitamin A aldehyde (47). Carotenoid biosynthesis and fermentation have been extensively studied both ia academic as well as ia iadustrial laboratories. On the commercial side, the focus of these iavestigations has been to iacrease fermentation titers by both classical and recombinant means. 

Replacement of the hydrogen at the 3 or 3 position of the carotene ring with a hydroxyl is the next step in both branches of the pathway. Hydroxylation of the rings of the carotenes leads to biosynthesis of the xanthophylls, including the well-known lutein and zeaxanthin food pigments. Lutein is formed by hydroxylation of a-carotene zeaxanthin is formed by hydroxylation of P-carotene. 

Besides the engineering of S. cerevisiae for organic acid production, through metabolic engineering it is possible to reconstruct entire pathways. In 1994, Yamano et al. [163] reported the reconstruction of a complete secondary metabolic pathway in S. cerevisiae, resulting in the ability of the yeast to produce p-carotene and lycopene. Carotenoids are a class of pigments used in the food industry and, due to their antioxidant properties, they have wide commercial interest. The biosynthesis of these compounds does naturally not occur in S. cerevisiae and to allow... 

Very few published examples of the genetic manipulation of terpenoid biosynthesis are available, but two interesting successes have been achieved in the area of terpenoid vitamins for example, the ratio of beneficial tocopherol (vitamin E) isomers in oilseeds has been altered by this means and increased P-carotene (a vitamin A precursor) in both rice kernels and rapeseed has been obtained by manipulating the carotenoid pathway [361]. 

A good example of compartmentation, although not yet completely understood, is the apparent segregation of isoprenoid biosynthesis in plants (123). [2- 14C]-Mevalonate was easily incorporated in squalene, phytosterols, P-amyrin and ubiquinone but not in the plastid constituents P-carotene, chlorophyll phytyl group or plastoquinone. The opposite result was obtained with 14C02. 

Data on herbicides are presented and reviewed, which allows the distinction between two different modes of bleaching. The first mode is caused by inhibited carotene biosynthesis exhibited by particular phenylpyridazinones, substituted phenylfuranones or amitrole. Decrease of carotenes leads to subsequent photodestruction of chlorophyll, peroxidation of other membrane components, and decay of electron transport activity. The second mode, represented by p-nitrodl-phenylethers, is associated with peroxidation of membrane-bound polyunsaturated fatty acids concurrently with the breakdown of carotenes, chlorophylls, and decay of photosynthetic electron transport. Short-chain hydrocarbon gases are reliable markers. The action of peroxidizing diphenylethers appears to be related to that of bipyridylium salts, although no light-induced oxygen uptake can be measured. 

Cell-free Systems. More direct evidence can be obtained by investigating cell-free carotene biosynthesis. From Phycomyces blakesleeanus, a fungus in which P-carotene formation is inhibited by norflurazon... 

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