Violaxanthin 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). 

C 5H2203, Mr 250.34, cryst., mp. 85 - 86 °C, [alp - 56° (CHCI3), a cyclofamesane sesquiterpene formed by photochemical oxidation of violaxanthin. It has been detected in several marine and higher plants. Naturally occurring X. exists as a mixture of -isomers of the 2,3-double bond of the 2,4-pentadienal side chain. It is an inhibitor of plant growth and is involved in the winter rest phase of plants (senescence), in some cases it is also involved in the damage to trees X. is closely related structurally and in its biological activity to abscisic acid and presumably functions as an intermediate in the biosynthesis of it. 

In an indirect manner, the lipoxygenase reaction in higher plants appears to be implicated in the biosynthesis of abscisic acid, a further senescence hormone [23-25], Abscisic acid is formed from the carotenoid violaxanthin that is converted into xanthoxin, which is a plant growth inhibitor and at the same time a precursor of abscisic acid. The formation of xanthoxin requires the co-oxidative activity of a lipoxygenase-linoleic (or a-linolenic) acid system. 

Recent studies with the three ABA-deficient tomato mutants have contributed extensively to the clarification of the biosynthesis of ABA. These studies support the so-called indirect pathway of ABA biosynthesis in which the C40 xanthophyll violaxanthin is the likely precursor of ABA, with xanthoxin as an intermediate [22, 26]. It was concluded that the fic and sit mutations are impaired in the conversion of xanthoxin to ABA, and that the lesion in not is at a step between xanthoxin and violaxanthin. 

Fig. 56.2 Carotenoid biosynthesis pathway in plants. Enzymes (with abbreviations) indicated are isopentenyl pyrophosphate isomerase (IPI), geranylgeranyl pyrophosphate synthase (GGPS), phytoene synthase (PSY), phytoene desaturase (PDS), zeta-carotene desaturase (ZDS), carotenoid isomerase (CRTISO), lycopene beta-cyclase (LCYB), lycopene epsilon-cyclase (LCYE), beta-ring carotene hydroxylase (CHXB), epsilon-ring carotene hydroxylase (CHXE), zeaxanthin epoxidase (ZEP), violaxanthin de-epoxidase (VDE), capsorubin-capsanthin synthase (CCS), neoxanthin synthase (NXS), 9-cis epoxycarotenoid dioxygenase (NCED), and carotenoid cleavage dioxygenase (CCD). (Source [101], drawn using KeGG pathway)...

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]. 

Violaxanthin -> a//-/ran -neoxanthin 9 -c -neoxanthin xanthoxin (C,) are intermediates in the carotenoid pathway leading to the phytohormone abscisic acid (Cjj). Mutants of Nicotiana plumbaginifolia Viviani and Solanum lycopersicum L. sub nom. Lycopersicon esculenlum Mill, were involved in the elucidation of this biosynthesis (Dewick 1999 Oritani and Kiyota 2003 and references therein). 

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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