Xanthophylls violaxanthin

Lutein is the predominant xanthophyll in all LHCs and ranges between a low of <40% of total carotenoids in LHClla to >65% in LHCllb. Other carotenoids, particularly the )3-carotene derived xanthophylls, violaxanthin and neoxanthin, vary much more widely in concentration. Neoxanthin is present at similar proportions in LHClla, b and c but is virtually absent from LHClld (Bassi et al, 1993). Interestingly, LHCllb is nearly devoid of/3-carotene but the minor complexes contain 5-10% of their total carotenoids as /3-carotene (Bassi et al, 1993 Ruban et al., 1994). Violaxanthin is also proportionally enriched in the minor LHCs accounting for 30-40% of the total minor LHC carotenoids and in aggregate up to 80%... 

The structural differences between violaxanthin and zeaxanthin may provide an explanation of how the xanthophyll cycle could indirectly control qE. De-epoxidation would change the interaction between LHCII and the xanthophyll. Violaxanthin would stabilize the unquenched conformation and zeaxanthin the quenched conformation (see Fig. 2). But what is the evidence for this kind of indirect control of qE ... 

Besides their function as light collectors and photoprotectors, carotenoids also have important effects as membrane stabilizers in diloroplasts. The xanthophyll violaxanthin and its enzymatic de-epoxidation products antheraxanthin and zea-xanthin partition between the light-harvesting-complexes (LHCs) of PS I and PS II and the lipid phase of the thylakoid membranes, bringing about a decrease in membrane fluidity, an increase in membrane thermostability and a lowered susceptibility to lipid peroxidation [16]. 

The possibility that epoxide carotenoids may function in oxygen evolution and transport has been suggested. In higher plants, three xanthophylls— violaxanthin, zeaxanthin, and antheraxanthin—undergo a series of photoin-duced interconversions (violaxanthin cycle) (Fig. 15), as reviewed by Hager (1975). Evidence as to the significance of this cycle is not conclusive, however. Two Russian workers, Sapozhnikov and Saakov, support the conclusion that the violaxanthin cycle is involved in oxygen transport (for review, see Sapozhnikov, 1973). However, other workers have concluded that ca-... 

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. 

The xanthophyll violaxanthin is not enriched in one of the pigment proteins, it is mainly found in the free pigment fraction (Fig. 1 and Table 1). From this it appears that the apparently not protein-bound major portion of violaxanthin is identical with those 70 to 80% of violaxanthin, which can enzymatically be transformed into zeaxanthin at high light intensities (9). 

The oxidation of carotenes results in the formation of a diverse array of xanthophylls (Fig. 13.7). Zeaxanthin is synthesised from P-carotene by the hydroxylation of C-3 and C-3 of the P-rings via the mono-hydroxylated intermediate P-cryptoxanthin, a process requiring molecular oxygen in a mixed-function oxidase reaction. The gene encoding P-carotene hydroxylase (crtZ) has been cloned from a number of non-photosynthetic prokaryotes (reviewed by Armstrong, 1994) and from Arabidopsis (Sun et al, 1996). Zeaxanthin is converted to violaxanthin by zeaxanthin epoxidase which epoxidises both P-rings of zeaxanthin at the 5,6 positions (Fig. 13.7). The... 

There are basically two types of carotenoids those that contain one or more oxygen atoms are known as xanthophylls those that contain hydrocarbons are known as carotenes. Common oxygen substituents are the hydroxy (as in p-cryptoxanthin), keto (as in canthaxanthin), epoxy (as in violaxanthin), and aldehyde (as in p-citraurin) groups. Both types of carotenoids may be acyclic (no ring, e.g., lycopene), monocyclic (one ring, e.g., y-carotene), or dicyclic (two rings, e.g., a- and p-carotene). In nature, carotenoids exist primarily in the more stable all-trans (or all-E) forms, but small amounts of cis (or Z) isomers do occur. - ... 

Allenic groups — Neoxanthin, a xanthophyll found in many foods, has an allenic group at the C-6,7,8 position where the two double bonds are perpendicular to each other, and the C-7,8 double bond coplanar with the polyene chain contributing effectively to the chromophore since the C-6,7 bond is in a different plane, it makes no contribution. Therefore, neoxanthin, despite its 10 conjugated double bonds, has a UV-Vis spectrum similar to that of a conjugated nonaene such as violaxanthin. 

Fucoxanthin, lutein, neoxanthin, violaxanthin, and zeaxanthin are the most common xanthophylls on our planet. They are found in the photosynthetic machinery of algae (fucoxanthin) and higher plants (Figure 7.1). Interestingly, lutein and zeaxanthin have also been found in the retina of humans and some primates (Khachik et al., 1997 Landrum and Bone, 2001). It is likely that these carotenoids possess some universal photophysical properties essential for both photosynthesis and vision (Britton, 1995). 

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. 

FIGURE 7.4 Absorption (a) and resonance Raman (b) spectra of the four major xanthophylls of I.HCII antenna zeaxanthin (Zea), lutein (Lut), violaxanthin (Vio), and neoxanthin (Neo). 

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. 

In order to obtain nearly absolute purity of the spectra of these xanthophylls, it was necessary to calculate the difference Raman spectra. Therefore, for zeaxanthin, two spectra of samples, one containing violaxanthin and the other enriched in zeaxanthin, were measured at 514.5 nm excitation. After their normalization using chlorophyll a bands at 1354 or 1389 cm-1, a deepoxidized-minus-epoxidized difference spectrum has for the first time been calculated to produce a pure resonance Raman spectrum of zeaxanthin in vivo (Figure 7.10b). A similar procedure was used for the calculation of the pure spectrum for violaxanthin. The only difference is that the 488.0nm excitation wavelength and epoxidized-minus-deepoxidized order of spectra have been applied in the calculation. The spectra produced using this approach have remarkable similarity to the spectra of xanthophyll cycle carotenoids in pure solvents (Ruban et al., 2001). The v, peaks of violaxanthin and zeaxanthin spectra are 7 cm 1 apart and in correspondence to the maxima of this band for isolated zeaxanthin and violaxanthin, respectively. The v3 band for zeaxanthin is positioned at 1003 cm-1, while the one for violaxanthin is upshifted toward 1006 cm-1. 

Leaves were dark-adapted therefore, there is no detectable level of zeaxanthin. Concentrations are nmol pigment (mol chi a — b), the P value from one factor ANOVA is displayed below each column. V-A-Z = xanthophyll pool (violaxanthin, antheraxanthin, zeaxanthin) EPS = epoxidation state. Reprinted with permission from P. J. Ralph et al. [76]. 

The carotenes and carotenoids are very important accessory pigments (Fig. 23-22). The major component in most green plants is (3-carotene. Green sulfur bacteria contain y-carotene in which one end of the molecule has not undergone cyclization and resembles lycopene (Fig. 22-5). Chloroplasts also contain a large variety of oxygenated carotenoids (xanthophylls). Of these, neoxanthin, violaxanthin... 

---