Violaxanthin structures

Physiologically, violaxanthin is an important component of the xanthophyU cycle a high light stress-induced de-epoxidation of the violaxanthin pool to the more photoprotective zeaxanthin is mediated by violaxanthin de-epoxidase (VDE). Violaxanthin and neoxanthin, an enzymatically (NXS)-produced structural isomer, are the precursors for the abscisic acid (ABA) biosynthetic pathway (Figure 5.3.1, Pathway 4 and Figure 5.3.2). In non-photosynthetic tissues, namely ripe bell peppers, antheraxanthin and violaxanthin are precursors to the red pigments, capsanthin and capsorubin, respectively (Figure 5.3.3B). 

The structure of the major trimeric LHCII complex has been recently obtained at 2.72 A (Figure 7.3) (Liu et al., 2004). It was revealed that each 25kDa protein monomer contains three transmembrane and three amphiphilic a-helixes. In addition, each monomer binds 14 chlorophyll (8 Chi a and 6 Chi b) and 4 xanthophyll molecules 1 neoxanthin, 2 luteins, and 1 violaxanthin. The first three xanthophylls are situated close to the integral helixes and are tightly bound to some amino acids by hydrogen bonds to hydroxyl oxygen atoms and van der Waals interactions to chlorophylls, and hydrophobic amino acids such as tryptophan and phenylalanine. 

Neoxanthin and the two lutein molecules have close associations with three transmembrane helixes, A, B, and C, forming three chlorophyll-xanthophyll-protein domains (Figure 7.5). Considering the structure of LHCII complex in terms of domains is useful for understanding how the antenna system works, and the functions of the different xanthophylls. Biochemical evidence suggests that these xanthophylls have a much stronger affinity of binding to LHCII in comparison to violaxanthin... 

FIGURE 7.5 Structural domains of LHCII xanthophylls. Aromatic amino acids tyrosine in the neoxanthin domain and tryptophan and phenylalanine in the violaxanthin domain are labeled as Y, W, and F, respectively. 

According to literature data, the total pigment content of ripe paprika consists of about 50 to 60 organic compounds, which are stable but different in their structures. Seven of them comprise 90 to 95% of the total pigment content. These are capsanthin, capsorubin, P-carotene, cryptoxanthin, lutein, violaxanthin and zeaxanthin. They mostly consist of 40 carbons and are linear compounds with many conjugated double bonds, and with rings at the ends of the chain. (See Figure 9.6-7)... 

Violaxanthin also functions as a precursor to the plant hormone abscisic acid. Compare the structure of the latter (Fig. 22-4) with those of carotenoids. Oxidative cleavage of violaxanthin or related epoxy-carotenoids initiates the pathway of synthesis of this hormone.142 143... 

Similarly, Karrer and oo-workers discovered that the carotenoid flower pigment trollixanthin (XV),103 as veil aa the related substances anthoroxsnthin, M, i4 violaxanthin, 1 and epoxylutein, 10-(1 all contain epoxide units. The. subject of naturally-occurring carotenoid epoxides ha been reviewed recently.1. 8 8 and attention called to the possible need for revision in certain of the structural assignments made by Karrer and hie associates.885... 

Structures of major carotenoids /3-carotene, lutein, violaxanthin, and neoxanthin. 

Isoprenoid structures for carotenoids, phytol, and other terpenes start biosynthetically from acetyl coenzyme A (89) with successive additions giving mevalonate, isopentyl pyrophosphate, geranyl pyrophosphate, farnesyl pyrophosphate (from which squalene and steroids arise), with further build-up to geranyl geranyl pyrophosphate, ultimately to a- and /3-carotenes, lutein, and violaxanthin and related compounds. Aromatic hydrocarbon nuclei are biosynthesized in many instances by the shikimic acid pathway (90). More complex polycyclic aromatic compounds are synthesized by other pathways in which naphthalene dimerization is an important step (91). 

Figure 9.38 Chemical structures of select carotenoids can be divided into two groups the carotenes (e.g., /3-carotene) which are hydrocarbons and the xanthophylls (e.g., violaxanthin, fucoxanthin) which are molecules that contain at least one oxygen atom.

The majority of carotenoids tested in this study increased the rhodamine accumulation of the Colo 320 MDR/MRP human colon cancer cells by the inhibition of the MDR1-mediated efflux pump activity. The cell size and the intracellular or sub cellular structures of carotenoid-treated cells were not modified during the short period of the flow cytometric experiments. The mean fluorescence and the shift of the fluorescence peak increased to various extents in the presence of carotenoids. The most active compounds were antheraxanthin, violeoxanthin, apple peel phytox-anthin, lutein and violaxanthin, while the luteoxanthin, neoxanthin and /f-cryploxanlhin were only moderate in their inhibition of the efflux pump (Tables 7, 8). 

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