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At least 19 recordsLinked to original sources
PubMed2006

Xanthophyll accumulation in the human retina during supplementation with lutein or zeaxanthin - the LUXEA (LUtein Xanthophyll Eye Accumulation) study.

The xanthophylls lutein (L) and zeaxanthin (Z) form the macular pigment with the highest density in the macula lutea. We investigated Macular Pigment Optical Density (MPOD) responses to supplementation with identically formulated (Actilease) L or Z (OPTISHARP) or L+Z over 6-12 months using doses of 10 or 20mg/day. MPOD as well as blue light sensitivity in fovea and parafovea were measured monthly by heterochromatic flicker photometry. Average xanthophyll plasma concentrations, analysed monthly by HPLC, increased up to 27-fold. MPOD increased by 15% upon L or L+Z supplementation. Supplementation of Z alone produced similar pigment accumulation in fovea and parafovea, which confounded MPOD measurements. After correction for this, a 14% MPOD increase resulted for Z. Thus, during supplementation with xanthophylls, L is predominantly deposited in the fovea while Z deposition appears to cover a wider retinal area. This may be relevant to health and disease of the retina.

Adolescent↗
PubMed2003

Photophysical properties of xanthophylls in carotenoproteins from human retinas.

The macula of the human retina contains high amounts of the xanthophyll carotenoids lutein and zeaxanthin [a mixture of (3R,3'R)-zeaxanthin and (3R,3'S-meso)-zeaxanthin]. Recently, it was shown that the uptake and the stabilization of zeaxanthin and lutein into the retina are likely to be mediated by specific xanthophyll-binding proteins (XBP). Here, we have used femtosecond pump-probe spectroscopy to study the dynamics of the S1 state of these xanthophylls in xanthophyll-enriched and native XBP. The results from the native XBP and the enriched XBP were then compared with those for carotenoids in organic solvents and in detergent micelles. Steady-state and transient absorption spectra show that the incorporation of xanthophylls into the protein causes a redshift of the spectra, which is stronger for lutein than for zeaxanthin. The transient absorption spectra further indicate that a part of the xanthophylls remains unbound in the xanthophyll-enriched XBP. The transient absorption spectra of the native XBP prove the presence of both xanthophylls in native XBP. Although the S1 lifetime of lutein does not exhibit any changes when measured in solution, micelles or XBP, we have observed the influence of the environment on the S1 lifetime of meso-zeaxanthin, which has a longer (12 ps) lifetime in XBP than in solution (9 ps). The most pronounced effect was found for vibrational relaxation in the S1 state, which is significantly slower for xanthophylls in XBP compared with micelles and solution. This effect is more pronounced for meso-zeaxanthin, suggesting a specific site of binding of this carotenoid to XBP.

Carotenoids↗
PubMed1965

HETEROTROPHIC GROWTH AND PRODUCTION OF XANTHOPHYLLS BY CHLORELLA PYRENOIDOSA.

The growth and level of xanthophylls of several representative species of green algae were investigated as a possible source of pigmentation for the egg yolk and broiler markets. Chlorella pyrenoidosa 7-11-05 was selected for fermentation studies because of its high level of xanthophylls and wide temperature range for growth. The heterotrophic metabolism was preferred because of the ease of adaptability to present fermentation equipment. When used as the sole carbon source, glucose was the only sugar, among many tested, that gave appreciable growth in illuminated shaken flasks. A dry cell weight of 90 g per liter and total xanthophylls of 450 mg per liter were obtained from 190 g per liter of glucose monohydrate in 168-hr illuminated shaken flasks. Higher levels of glucose decreased yields. In combination with glucose, monosaccharides, such as fructose and galactose, were readily assimilated. The 7-11-05 strain was adapted to galactose as the sole carbon source after six vegetative passages. Light of the proper intensity and duration stimulated total xanthophylls approximately 35%. The effect on dry cell weight and total xanthophylls of seven antibiotics added at various levels in shaken flasks was studied. Erythromycin was essentially stable throughout the fermentation and nontoxic up to 25 mug/ml, with only slight toxicity at higher levels. Both erythromycin and ristocetin were effective in controlling a high incidence of bacterial contamination in 30-liter fermentors. With the higher agitation and aeration rates possible in 30-liter fermentors, dry cell weights in excess of 100 g per liter and total xanthophylls of 467 to 512 mg per liter were readily obtained from 230 to 260 g per liter of glucose in 162-hr illuminated batch-type fermentations. Continuous-feed runs yielded a dry cell weight of 302 g per liter and total xanthophylls of 650 mg per liter from 520 g per liter of glucose. The type of Chlorella cell produced was an important consideration with respect to the availability of the xanthophylls in pigmenting egg yolks and broilers.

Animals↗
PubMed2006

Isolation and characterization of a xanthophyll aberrant mutant of the green alga Nannochloropsis oculata.

Novel mutants (xan1 and xan2) of the unicellular green alga Nannochloropsis oculata are impaired in xanthophyll biosynthesis, thereby producing aberrant levels of xanthophylls. High-performance liquid chromatography (HPLC) analysis revealed that the xan1 and xan2 mutants have double the violaxanthin (V) content, but have significantly decreased lutein content in their cells compared to the wild type. Furthermore, these mutants contain two to three times more zeaxanthin than the wild type under low light (LL) growth conditions. However, this xanthophyll aberration in N. oculata did not affect the normal growth and the major cellular chemical composition of the xan1 strain. The xanthophyll pool size of the LL-grown mutant was 1.8-fold greater than that of the wild type. Under high light (HL) growth conditions, V content was substantially decreased in both the mutant and wild types because of the epoxidation state of the xanthophylls. Under LL growth conditions, the deepoxidation states of the xanthophyll pool sizes were 0.1 and 1.2 in the wild type and the mutant, respectively. However, the deepoxidation states of the xanthophyll pool sizes were 0.78 in the wild type and 0.87 in the mutant under HL growth conditions. We observed that the level of one of the commercially important xanthophylls, zeaxanthin, was higher in the mutant than in the wild type under all culture conditions. This mutant is discussed in terms of its commercial value and potential utilization by the algal biotechnology industry for the production of zeaxanthin.

Chlorophyta↗
PubMed2002

Xanthophyll biosynthetic mutants of Arabidopsis thaliana: altered nonphotochemical quenching of chlorophyll fluorescence is due to changes in Photosystem II antenna size and stability.

Xanthophylls (oxygen derivatives of carotenes) are essential components of the plant photosynthetic apparatus. Lutein, the most abundant xanthophyll, is attached primarily to the bulk antenna complex, light-harvesting complex (LHC) II. We have used mutations in Arabidopsis thaliana that selectively eliminate (and substitute) specific xanthophylls in order to study their function(s) in vivo. These include two lutein-deficient mutants, lut1 and lut2, the epoxy xanthophyll-deficient aba1 mutant and the lut2aba1 double mutant. Photosystem stoichiometry, antenna sizes and xanthophyll cycle activity have been related to alterations in nonphotochemical quenching of chlorophyll fluorescence (NPQ). Nondenaturing polyacrylamide gel electrophoresis indicates reduced stability of trimeric LHC II in the absence of lutein (and/or epoxy xanthophylls). Photosystem (antenna) size and stoichiometry is altered in all mutants relative to wild type (WT). Maximal DeltapH-dependent NPQ (qE) is reduced in the following order: WT>aba1>lut1 approximately lut2>lut2aba1, paralleling reduction in Photosystem (PS) II antenna size. Finally, light-activation of NPQ shows that zeaxanthin and antheraxanthin present constitutively in lut mutants are not qE active, and hence, the same can be inferred of the lutein they replace. Thus, a direct involvement of lutein in the mechanism of qE is unlikely. Rather, altered NPQ in xanthophyll biosynthetic mutants is explained by disturbed macro-organization of LHC II and reduced PS II-antenna size in the absence of the optimal, wild-type xanthophyll composition. These data suggest the evolutionary conservation of lutein content in plants was selected for due to its unique ability to optimize antenna structure, stability and macro-organization for efficient regulation of light-harvesting under natural environmental conditions.

Arabidopsis↗
PubMed2006

Xanthophylls in commercial egg yolks: quantification and identification by HPLC and LC-(APCI)MS using a C30 phase.

The xanthophylls lutein and zeaxanthin have attracted a lot of interest since it was presumed that an increased nutritional uptake may prevent adult macula degeneration (AMD). Although egg yolks serve as an important dietary source of lutein and zeaxanthin, data on xanthophyll concentrations in commercial egg yolks are not available. Thus, an high-performance liquid chromatography-diode array detector (HPLC-DAD) method was developed allowing for simultaneous separation of eight xanthophylls used to fortify poultry feed. Peak identification was carried out by liquid chromatography-atmospheric pressure chemical ionization mass spectrometry [LC-(APCI)MS]. Egg yolks of four types of husbandry (seven batches each) were examined. Lutein and zeaxanthin were the predominant xanthophylls in egg yolks produced in accordance with ecological husbandry (class 0) because the concentrations of these xanthophylls ranged from 1274 to 2478 microg/100 g and from 775 to 1288 microg/100 g, respectively. Analysis of variance (ANOVA) proved that both mean lutein and mean zeaxanthin concentrations of eggs from class 0 were statistically discriminable from mean lutein and zeaxanthin concentrations from eggs of all other classes (P < 0.01). Mean concentrations of synthetic xanthophylls in eggs of classes 1 (free range), 2 (barn), and 3 (cage) were as follows: canthaxanthin, 707 +/- 284 microg/100 g; beta-apo-8'-carotenoic acid ethyl ester, 639 +/- 391 microg/100 g; and citranaxanthin, 560 +/- 231 microg/100 g. Experiments with boiled eggs proved that beta-apo-8'-carotenoic acid ethyl ester was the xanthophyll with the highest stability, whereas lutein was degraded to the largest extent (loss of 19%). Detailed knowledge about the xanthophyll amounts in eggs is indispensable to calculate the human uptake.

Atmospheric Pressure↗
PubMed2006

Accumulation of macular xanthophylls in unsaturated membrane domains.

The distribution of macular xanthophylls, lutein and zeaxanthin, between domains formed in membranes made from an equimolar ternary mixture of dioleoylphosphatidylcholine/sphingomyelin/cholesterol, called a raft-forming mixture, was investigated. In these membranes, two domains are formed: the raft domain enriched in saturated lipids and cholesterol (detergent-resistant membranes, DRM), and the bulk domain enriched in unsaturated lipids (detergent-soluble membranes, DSM). These membrane domains have been separated using cold Triton X-100 extraction from membranes containing 1 mol% of either lutein or zeaxanthin. The results indicated that xanthophylls are substantially excluded from DRM and remain concentrated in DSM. Concentrations of xanthophylls in DRM and DSM calculated as the mole ratio of either xanthophyll to phospholipid were 0.005 and 0.03, respectively, and calculated as the mole ratio of either xanthophyll to total lipid (phospholipid + cholesterol) were 0.003 and 0.025, respectively. Thus, xanthophylls are over eight times more concentrated in DSM than in DRM. No significant difference in the distribution of lutein and zeaxanthin was found. It was also demonstrated using saturation-recovery EPR that at 1 mol%, neither lutein nor zeaxanthin affect the formation of membrane domains. The location of xanthophylls in domains formed from unsaturated lipids is ideal if they are to act as a lipid antioxidant, which is the most accepted mechanism through which lutein and zeaxanthin protect the retina from age-related macular diseases.

Antioxidants↗
PubMed2002

Dynamics of chromophore binding to Lhc proteins in vivo and in vitro during operation of the xanthophyll cycle.

Three plant xanthophylls are components of the xanthophyll cycle in which, upon exposure of leaves to high light, the enzyme violaxanthin de-epoxidase (VDE) transforms violaxanthin into zeaxanthin via the intermediate antheraxanthin. Previous work () showed that xanthophylls are bound to Lhc proteins and that substitution of violaxanthin with zeaxanthin induces conformational changes and fluorescence quenching by thermal dissipation. We have analyzed the efficiency of different Lhc proteins to exchange violaxanthin with zeaxanthin both in vivo and in vitro. Light stress of Zea mays leaves activates VDE, and the newly formed zeaxanthin is found primarily in CP26 and CP24, whereas other Lhc proteins show a lower exchange capacity. The de-epoxidation system has been reconstituted in vitro by using recombinant Lhc proteins, recombinant VDE, and monogalactosyl diacylglycerol (MGDG) to determine the intrinsic capacity for violaxanthin-to-zeaxanthin exchange of individual Lhc gene products. Again, CP26 was the most efficient in xanthophyll exchange. Biochemical and spectroscopic analysis of individual Lhc proteins after de-epoxidation in vitro showed that xanthophyll exchange occurs at the L2-binding site. Xanthophyll exchange depends on low pH, implying that access to the binding site is controlled by a conformational change via lumenal pH. These findings suggest that the xanthophyll cycle participates in a signal transduction system acting in the modulation of light harvesting versus thermal dissipation in the antenna system of higher plants.

Chlorophyll↗
PubMed1999

Photoadaptation of two members of the Chlorophyta (Scenedesmus and Chlorella) in laboratory and outdoor cultures: changes in chlorophyll fluorescence quenching and the xanthophyll cycle.

The role of the xanthophyll cycle in the adaptation of two chlorococcal algae Scenedesmus quadricauda and Chlorella sorokiniana to high irradiance was studied under laboratory and outdoor conditions. We wished to elucidate whether the xanthophyll cycle plays a key role in dissipating the excesses of absorbed light, as in higher plants, and to characterise the relationship between chlorophyll fluorescence parameters and the content of xanthophyll-cycle pigments. The xanthophyll cycle was found to be operative in both species; however, its contribution to overall non-photochemical quenching (NPQ) could only be distinguished in Scenedesmus (15-20% of total NPQ). The Scenedesmus cultures showed a larger pool of xanthophyll-cycle pigments than Chlorella, and lower sensitivity to photoinhibition as judged from the reduction of maximum quantum yield of photosystem II. In general, both algae had a larger xanthophyll-cycle pool when grown outdoors than in laboratory cultures. Comparing the two species, Scenedesmus exhibited a higher capacity to adapt to high irradiance, due to an effective quenching mechanism and high photosynthetic capacity; in contrast, Chlorella represents a species with a larger antennae system, less-efficient quenching and lower photosynthetic performance. Non-photochemical quenching (NPQ) induced through the xanthophyll cycle can, to a limited extent, represent a regulatory factor in diluted algal cultures grown in outdoor solar photobioreactors, as well as in natural algal phytoplankton populations exposed transiently to high irradiance. However, it does not play an appreciable role in dense, well-mixed microalgal suspensions.

Adaptation, Physiological↗
PubMed1997

Xanthophylls in light-harvesting complex II of higher plants: light harvesting and triplet quenching.

A spectral and functional assignment of the xanthophylls in monomeric and trimeric light-harvesting complex II of green plants has been obtained using HPLC analysis of the pigment composition, laser-flash induced triplet-minus-singlet, fluorescence excitation, and absorption spectra. It is shown that violaxanthin is not present in monomeric preparations, that it has most likely a red-most absorption maximum at 510 nm in the trimeric complex, and that it is involved in both light-harvesting and Chl-triplet quenching. Two xanthophylls (per monomer) have an absorption maximum at 494 nm. These play a major role in both singlet and triplet transfer. These two are most probably the two xanthophylls resolved in the crystal structure, tentatively assigned to lutein, that are close to several chlorophyll molecules [Kühlbrandt, W., Wang, N. D., & Fujiyoshi, Y. (1994) Nature 367, 614-621]. A last xanthophyll contribution, with an absorption maximum at 486 nm, does not seem to play a significant role in light-harvesting or in Chl-triplet quenching. On the basis of the assumption that the two structurally resolved xanthophylls are lutein, this 486 nm absorbing xanthophyll should be neoxanthin. The measurements demonstrate that violaxanthin is connected to at least one chlorophyll a with an absorption maximum near 670 nm, whereas the xanthophylls absorbing at 494 nm are connected to at least one chlorophyll a with a peak near 675 nm.

Chromatography, High Pressure Liquid↗
PubMed1990

Characterization of the Xanthophyll Cycle and Other Photosynthetic Pigment Changes Induced by Iron Deficiency in Sugar Beet (Beta vulgaris L.).

In this work we characterize the changes induced by iron deficiency in the pigment composition of sugar beet (Beta vulgaris L.) leaves. When sugar beet plants were grown hydroponically under limited iron supply, neoxanthin and beta-carotene decreased concomitantly with chlorophyll a, whereas lutein and the carotenoids within the xanthophyll cycle were less affected. Iron deficiency caused major increases in the lutein/chlorophyll a and xanthophyll cycle pigments/chlorophyll a molar ratios. Xanthophyll cycle carotenoids in Fe-deficient plants underwent epoxidations and de-epoxidations in response to ambient light conditions. In dark adapted Fe-deficient plants most of the xanthophyll cycle pigment pool was in the epoxidated form violaxanthin. We show, both by HPLC and by in vivo 505 nanometers absorbance changes, that in Fe deficient plants and in response to light, the de-epoxidated forms antheraxanthin and zeaxanthin were rapidly formed at the expense of violaxanthin. Several hours after returning to dark, the xanthophyll cycle was shifted again toward violaxanthin. The ratio of variable to maximum chlorophyll fluorescence from intact leaves was decreased by iron deficiency. However, in iron deficient leaves this ratio was little affected by light conditions which displace the xanthophyll cycle toward epoxidation or de-epoxidation. This suggests that the functioning of the xanthophyll cycle is not necessarily linked to protection against excess light input.

Journal Article↗
PubMed1996

Comparative time-resolved photosystem II chlorophyll a fluorescence analyses reveal distinctive differences between photoinhibitory reaction center damage and xanthophyll cycle-dependent energy dissipation.

The photosystem II (PSII) reaction center in higher plants is susceptible to photoinhibitory molecular damage of its component pigments and proteins upon prolonged exposure to excess light in air. Higher plants have a limited capacity to avoid such damage through dissipation, as heat, of excess absorbed light energy in the PSII light-harvesting antenna. The most important photoprotective heat dissipation mechanism, induced under excess light conditions, includes a concerted effect of the trans-thylakoid pH gradient (delta pH) and the carotenoid pigment interconversions of the xanthophyll cycle. Coincidentally, both the photoprotective mechanism and photoinhibitory PSII damage decrease the PSII chlorophyll a (Chl a) fluorescence yield. In this paper we present a comparative fluorescence lifetime analysis of the xanthophyll cycle- and photoinhibition-dependent changes in PSII Chl a fluorescence. We analyze multifrequency phase and modulation data using both multicomponent exponential and bimodal Lorentzian fluorescence lifetime distribution models; further, the lifetime data were obtained in parallel with the steady-state fluorescence intensity. The photoinhibition was characterized by a progressive decrease in the center of the main fluorescence lifetime distribution from approximately 2 ns to approximately 0.5 ns after 90 min of high light exposure. The damaging effects were consistent with an increased nonradiative decay path for the charge-separated state of the PSII reaction center. In contrast, the delta pH and xanthophyll cycle had concerted minor and major effects, respectively, on the PSII fluorescence lifetimes and intensity (Gilmore et al., 1996, Photosynth. Res., in press). The minor change decreased both the width and lifetime center of the longest lifetime distribution; we suggest that this change is associated with the delta pH-induced activation step, needed for binding of the deepoxidized xanthophyll cycle pigments. The major change increased the fractional intensity of a short lifetime distribution at the expense of a longer lifetime distribution; we suggest that this change is related to the concentration-dependent binding of the deepoxidized xanthophylls in the PSII inner antenna. Further, both the photoinhibition and xanthophyll cycle mechanisms had different effects on the relationship between the fluorescence lifetimes and intensity. The observed differences between the xanthophyll cycle and photoinhibition mechanisms confirm and extend our current basic model of PSII exciton dynamics, structure and function.

Chlorophyll↗
PubMed2007

The selective retention of lutein, meso-zeaxanthin and zeaxanthin in the retina of chicks fed a xanthophyll-free diet.

Lutein and zeaxanthin are pigmented oxygenated carotenoids, or xanthophylls, derived from plants and concentrated in the retina of primates and birds. We investigated the transport, distribution and depletion of lutein and zeaxanthin in the plasma and tissues of newly hatched chicks fed xanthophyll-free diets. One-day-old Leghorn chicks were randomly divided into two groups. A control group was fed a diet containing lutein and zeaxanthin (5.2 and 1.7 mg/kg diet, respectively) for 28 days. An experimental group was fed a diet containing no lutein and zeaxanthin for 28 days. Plasma and tissues were analyzed for lutein and zeaxanthin at 28 days (control) and on days 1, 14 and 28 (experimental). At hatching, lutein and zeaxanthin were the predominant carotenoids present in the blood and tissues. As indicated by their similar mass contents, there was complete transfer of these carotenoids from egg yolk to chick. Lutein and zeaxanthin concentrations in the plasma and tissues of chicks fed the xanthophyll-free diet decreased rapidly to almost zero (with a depletion time of seven days [t(1/2)]). In contrast, the retina retained its initial concentrations of lutein and zeaxanthin similar to the control group. meso-Zeaxanthin and cis-zeaxanthin were identified only in the retina. The retina concentrated zeaxanthin over lutein. Lutein and zeaxanthin were selectively retained in the retinas of chicks fed a xanthophyll-free diet. In contrast, the plasma and other tissues lost up to 90% of their original content of xanthophylls. These data emphasize the relative stability of lutein and zeaxanthin in the cone-rich retina where they are present as esters in oil droplets. The tissue depletion suggests the need for a regular dietary intake of lutein and zeaxanthin because of rapid depletion in the body. It is clear that these xanthophylls may have an essential role in the cone-rich retina of the chick as evidenced by their selective retention.

Animals↗
PubMed2001

Photochemical behavior of xanthophylls in the recombinant photosystem II antenna complex, CP26.

The steady state absorption and fluorescence spectroscopic properties of the xanthophylls, violaxanthin, zeaxanthin, and lutein, and the efficiencies of singlet energy transfer from the individual xanthophylls to chlorophyll have been investigated in recombinant CP26 protein overexpressed in Escherichia coli and then refolded in vitro with purified pigments. Also, the effect of the different xanthophylls on the extents of static and dynamic quenching of chlorophyll fluorescence has been investigated. Absorption, fluorescence, and fluorescence excitation demonstrate that the efficiency of light harvesting from the xanthophylls to chlorophyll a is relatively high and insensitive to the particular xanthophyll that is present. A small effect of the different xanthophylls is observed on the extent of quenching of Chl fluorescence. The data provide the precise wavelengths of the absorption and fluorescence features of the bound pigments in the highly congested spectral profiles from these light-harvesting complexes. This information is important in assessing the mechanisms by which higher plants dissipate excess energy in light-harvesting proteins.

Chlorophyll↗
PubMed2004

The xanthophyll cycle in green algae (chlorophyta): its role in the photosynthetic apparatus.

Light-dependent conversion of violaxanthin to zeaxanthin, the so-called xanthophyll cycle, was shown to serve as a major, short-term light acclimation mechanism in higher plants. The role of xanthophylls in thermal dissipation of surplus excitation energy was deduced from the linear relationship between zeaxanthin formation and the magnitude of non-photochemical quenching. Unlike in higher plants, the role of the xanthophyll cycle in green algae (Chlorophyta) is ambiguous, since its contribution to energy dissipation can significantly vary among species. Here, we have studied the role of the xanthophyll cycle in the adaptation of several species of green algae (Chlorella, Scenedesmus, Haematococcus, Chlorococcum, Spongiochloris) to high irradiance. The xanthophyll cycle has been found functional in all tested organisms; however its contribution to non-photochemical quenching is not as significant as in higher plants. This conclusion is supported by three facts: (i) in green algae the content of zeaxanthin normalized per chlorophyll was significantly lower than that reported from higher plants, (ii) antheraxanthin + zeaxanthin content displayed different diel kinetics from NPQ and (iii) in green algae there was no such linear relationship between NPQ and Ax + Zx, as found in higher plants. We assume that microalgae rely on other dissipation mechanism(s), which operate along with xanthophyll cycle-dependent quenching.

Acclimatization↗
PubMed2001

The major antenna complex of photosystem II has a xanthophyll binding site not involved in light harvesting.

We have characterized a xanthophyll binding site, called V1, in the major light harvesting complex of photosystem II, distinct from the three tightly binding sites previously described as L1, L2, and N1. Xanthophyll binding to the V1 site can be preserved upon solubilization of the chloroplast membranes with the mild detergent dodecyl-alpha-d-maltoside, while an IEF purification step completely removes the ligand. Surprisingly, spectroscopic analysis showed that when bound in this site, xanthophylls are unable to transfer absorbed light energy to chlorophyll a. Pigments bound to sites L1, L2, and N1, in contrast, readily transfer energy to chlorophyll a. This result suggests that this binding site is not directly involved in light harvesting function. When violaxanthin, which in normal conditions is the main carotenoid in this site, is depleted by the de-epoxidation in strong light, the site binds other xanthophyll species, including newly synthesized zeaxanthin, which does not induce detectable changes in the properties of the complex. It is proposed that this xanthophyll binding site represents a reservoir of readily available violaxanthin for the operation of the xanthophyll cycle in excess light conditions.

Binding Sites↗
PubMed2003

Xanthophyll binding sites of the CP29 (Lhcb4) subunit of higher plant photosystem II investigated by domain swapping and mutation analysis.

The binding sites for xanthophylls in the CP29 antenna protein of higher plant Photosystem II have been investigated using recombinant proteins refolded in vitro. Despite the presence of three xanthophyll species CP29 binds two carotenoids per polypeptide. The localization of neoxanthin was studied producing a chimeric protein constructed by swapping the C-helix domain from CP29 to LHCII. The resulting holoprotein did not bind neoxanthin, confirming that the N1 site is not present in CP29. Neoxanthin in CP29 was, instead, bound to the L2 site, which is thus shown to have a wider specificity with respect to the homologous site L2 in LHCII. Lutein was found in the L1 site of CP29. For each site the selectivity for individual xanthophyll species was studied as well as its role in protein stabilization, energy transfer, and photoprotection. Putative xanthophyll binding sequences, identified by primary structure analysis as a stretch of hydrophobic residues including an acidic term, were analyzed by site-directed mutagenesis or, in one case, by deleting the entire sequence. The mutant proteins were unaffected in their xanthophyll composition, thus suggesting that the target motifs had little influence in determining xanthophyll binding, whereas hydrophobic sequences in the membrane-spanning helices are important.

Amino Acid Sequence↗