Carotenoids xanthophyll formation

Mein, J.R. Dolnikowski, G.G. Ernst, H. Russell, R.M. Wang, X.D. 2011. Enzymatic formation of apo-carotenoids from the xanthophyll carotenoids lutein, zeaxanthin and P-cyryptoxanthin by ferret carotene-9, lO -monooxygenase. Arch. Biochem. Biophys. 506 109-121. 

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). 

Pathways. Studies of carotenoid transformations that take place when a mutant strain, PGl, of the green alga Scenedesmus obliquus is transferred from dark to light conditions have indicated that the transformations 15-cw-phytoene (180) 15-c/5-phytofluene (181) - 15-cis- -carotene (182) -> trans-C-caro-tene (183) (Scheme 7) take place in the biosynthesis of the normal cyclic carotenoids. The results were also in agreement with the formation of the xanthophylls lutein (16) and zeaxanthin (174) from the corresponding carotenes. 

In order to define the carotenoid structures necessary for LHCII assembly and stabilization, a number of different carotenoids have been used in reconstitution assays with only one carotenoid component present. Not only the xanthophyll cycle carotenoids zeaxanthin and antheraxanthin turned out to promote reconstitution but also heterologous carotenoids as diverse structurally as astaxanthin, okenone, and fucoxanthin. In general, a hydroxyl group in position 3 of at least one of the cyclohexane ring seems to be important for complex formation (D. Phillip, S. Hobe, A. Young, and H. Paulsen, unpublished). Similarly, the major LHCII from... 

Carotenoids represent one of the broadest groups of natural antioxidants (over 600 characterized structurally) with significant biological effects and numerous industrial applications. Lycopene is a typical acyclic carotene that serves as a starting metabolite for formation of carotenoid derivatives via specific routes (p-carotene, torulene, etc.). Xanthophylls include hydroxy-, methoxy- oxo-, epoxy-, carboxy-, and aldehydic groups (torularhodin, zeaxanthin, astaxanthin, etc.), which results in a broad structural variety of carotenoid compounds. 

The biochemistry of the violaxanthin cycle is well-characterised (1) but no clear function of this cycle, in relation to photosynthesis, has been demonstrated. Demmig et al, (2, 3) have suggested that zeaxanthin formation is related to a special function of this carotenoid under photoinhibitory conditions that serves to prevent damage. The xanthophyll cycle thus may play a crucial role in the protection of the photosynthetic apparatus... 

Oxygenated derivatives of carotenoids also are common and widely distributed. These colored substances are known collectively as xanthophylls. The formation of xanthophylls involves an aerobic mechanism. p-Carotene is converted oxidatively to lutein (2), zeaxanthin [(3/ , 37 )-3,p-carotene-3,3 -diol] (15), and, finally, to violaxanthin (3) (Fig. 26.10) (Britton, 1993). Labeling studies have shown that hydroxyla-tions such as those involved in the conversion of zeaxanthin and lutein are formed by the direct replacement of the hydrogen by OH (Britton, 1993). 

Most carotenoids are xanthophylls, not carotenes. These compounds are formed by the introduction of one or more oxygen functions into the molecule. Other modifications of carotenoids involve the formation of allenic and acetylenic groups, additions to the polyene chain, and the loss of carbon atoms. Little is known concerning the biosynthesis of these compounds. 

Cyclization of lycopene proceeds only after an d -trans lycopene is formed by the action of carotene isomerase (CRTISO) in nongreen tissue. In the photosynthetic tissues, this conversion is catalyzed by light and chlorophyll (acting as a sensitizer). Oxygenation of the cyclic carotenoids yields xanthophylls. Introduction of hydroxyl groups at positions 3 and 3 in p-carotene produces zeaxanthin. Zeaxan-thin epoxidase (ZEP) and violaxanthin de-epoxidase (VDE) act in tandem to regulate the formation of violaxanthin. Violaxanthin is next converted to 9-cis-neoxanthin, ABA precursor, by neoxanthin synthase. Lutein is mainly present in photosynthetic tissues, biosynthesized from a-carotene via catalysis by p- and E-hydroxylases. 

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]. 

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