Antheraxanthin biosynthesis

Carotene is the major dietary precursor of vitamin A and therefore represents a fundamental component in our diet. The later steps of carotenoid biosynthesis in plants involve the formation of xanthophylls, which are oxygenated derivatives. Among these, capsanthin results from the activity of a bifunctional enzyme, the capsanthin-capsorabin synthase (CCS), that catalyses the conversion of the ubiquitous antheraxanthin and violaxanthin, into capsanthin and capsorubin (Fig. 11.3). 

The aba mutant of Arabidopsis is impaired in epoxy carotenoid biosynthesis and thus, makes zeaxanthin but not violaxanthin, antheraxanthin, and neoxanthin (Rock and Zeevaart, 1991 Rock et al, 1992). The pigment stoichiometries indicate a 1 1 replacement of neoxanthin and violaxanthin by zeaxanthin. The lutein content decreases compared to wildtype Arabidopsis, indicating a reduced Chi a/b antenna. Also comparing the composition of the antenna changes, there is less ofthe major LHCII and more of the minor Chi a/b complexes (Hurry et al, 1997). 

Camara, B. and R. Moneger, Carotenoid biosynthesis. In vitro conversion of antheraxanthin to capsanthin by a chromoplast enriched fraction of Capsicum fruits, Biochem. Biophys. Res. Commun., 99, 1117-1122 (1981). 

There is no biochemical evidence for the mechanism of formation of al-lenic and acetylenic bonds. Some speculative proposals have been made, however (Britton, 1976a). Schemes have also been proposed for the synthesis of capxanthin from antheraxanthin and for the biosynthesis of retro- and secocarotenoids (Britton, 1976a). 

Epoxidation of zeaxanthin by zeaxanthin epoxidase (ZE) would result in the production of violaxanthin via antheraxanthin. From that substrate, the enzyme neoxanthin synthase (NXS) would yield neoxanthin opening the cyclohexenyl 5-6 epoxide ring in violaxanthin [38]. Neoxanthin would be the last product of carotenoid biosynthesis in green parts of the plant, and it would derive in the abscisic acid (ABA) synthesis pathway. The accumulation of neoxanthin and violaxanthin in flowers results in wildtype yellow petals. A defective mutation in the gene encoding CRTR-B2 prevents formation of these xanthophylls, resulting in the white-flower phenotype [18]. 

The 5,6-epoxycarotenoids such as violaxanthin, neoxanthin, and antheraxanthin are also widely distributed. Zeaxanthin is epoxidized to antheraxanthin, which in turn is epoxidized to form violaxanthin in the thylakoid membrane. These three xanthophylls constitute the violaxanthin cycle, found in the chloroplast. Although aspects of its enzymology have been elucidated (see Section 4.4.2.4), the physiological role of the cycle is a matter of debate. " The formation of violaxanthin in the chloroplast envelope is thought to be separate from the violaxanthin cycle. The formation of some xanthophylls in vitro has been demonstrated with a few higher plant systems (see Ref. 66). Details of the biosynthesis of the other xanthophylls found in plants can be found in several reviews." " ... 

Very few enzymatic studies on xanthophyll biosynthesis in plants have been reported. The conversion of zeaxanthin into antheraxanthin and violaxanthin by lettuce chloroplasts requires molecular oxygen and NAD PH, with pH optima at 7.8 and 7.4 in the light and dark, respectively. The hydroxylation of carotene to )8-cryptoxanthin (3-hydroxy-)3-carotene) also requires NADPH and is probably catalyzed by a mixed-function oxygenase, as the hydroxy group originates from 02- The 5,6-and 5, 6 -epoxy groups are probably formed by a similar mechanism. 

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