Biosynthesis of b-carotene derivatives and the creation of vitamin A

The plastid is involved in the synthesis of many vitamins and compounds in the plant. This section of plant biology advice takes a look at the derivatives that are created from b-carotene (for information on how b-carotene is created in the plants see the previous section).

The creation of beta-carotene and zeaxanthin

As mentioned previously once lycopene has been produced it can take two different routes, the b,b branch is responsible for beta-carotene and its derivatives, and involves the addition of two beta rings to the cyclic end groups of lycopene.

Lycopene b-cyclases have been identified in both the Arabidopsis and tomato plants. In tomato the gene has been shown to have a role in fruit ripening and the pigmentation of fruit. It is also thought that the pepper enzyme capsanthin-capsorubin synthase is very closly related to these lycopene b-cyclases.

Plant Xanthiophylls such as zeaxanthin have hydroxyl moieties, these are located on the third carbon of the cyclic b-ionine end group. There are many b-ring hydroxylases that are involved in the addition of these end groups from carotene to produce zeaxanthin in higher plants.

Production and use of Vitamin A (Retinaldehyde)

Vitamin A is essential in the human diet, and lack of it may lead to blindness and other health consequences. Because of this much work has been carried out on vitamin A research. This has led to plant breeding that has led to increased b-carotene levels in sweet potatoes and in Rice.

Vitamin A itself is a C20 product of the carotenoid pathway. It has been found that any carotenoid that has an unmodified b-ionine ring can have provitamin A activity, and is therefore able to produce retinaldehyde. Vitamin A is produced is used in the body as both retinoic acids and retinals.

a-carotene and leutin biosynthesis

The production of leutin and a-carotene involves the addition of a beta and an epsilon ring to the cyclic end of lycopene. this is carried out by b-cyclase and e-cyclase enzymes.

Other plant carotenoid products

In addition to vitamin A, Leutin and b-carotene production, other important compounds are produced via the plastid carotenoid pathway; these include abscisic acid (a plant hormone) that is created via the formation of xanthoxin; and carotenoid products created by cleavage by carotenoid cleavage dioxygenases, in addition to their roles in plants these have many uses in industry such as food colouring and use in perfumes.

The next section takes a look at the biosynthesis of tocochromanol (vitamin E) in plants.

DellaPenna and Pogson (2006). Vitamin Synthesis in Plants: Tocopherols and Carotenoids. Annual revue plant biology. 57: 711 to 738
Pogson et al (1996). Arabidopsis carotenoid mutants demonstrate lutein is not essential for photosynthesis in higher plants. Plant Cell 8:1627–39
Ronen et al (2000). An alternative pathway to beta-carotene formation in plant chromoplasts discovered by map-based cloning of Beta and old-gold color mutations in tomato. Proc. Nat. Acad. Sci. USA 97:11102–7
von Lintig et al (2005). Towards a better understanding of carotenoid metabolism in animals. Biochim. Biophys. Acta 1740:122–31

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