Zeaxanthin 5,6-epoxidation

Jahns Pand MieheP(1996) Kinetic correlation ofrecovery from photoinhibition and zeaxanthin epoxidation. Planta 198 202-210... 

Jahns P (1995) The xanthophyll cycle in intermittent light-grown pea plants Possible functions of chlorophyll a/ -binding proteins. Plant Physiol 108 149-156 Jahns P and Miehe B (1996) Kinetic correlation ofrecovery from photoinhibition and zeaxanthin epoxidation. Planta 198 202-210... 

Siefermann D and Yamamoto HY (1975) Properties ofNADPH and Oxygen-dependent zeaxanthin epoxidation in isolated chloroplasts. A transmembrane model for the xanthophyll cycle. Arch Biochem Biophys 171 70-77... 

It is assumed that in order to have vitamin A activity a molecule must have essentially one-half of its structure similar to that of (i-carotene with an added molecule of water at the end of the lateral polyene chain. Thus, P-carotene is a potent provitamin A to which 100% activity is assigned. An unsubstituted p ring with a Cii polyene chain is the minimum requirement for vitamin A activity. y-Car-otene, a-carotene, P-cryptoxanthin, a-cryptoxanthin, and P-carotene-5,6-epoxide aU have single unsubstimted rings. Recently it has been shown that astaxanthin can be converted to zeaxanthin in trout if the fish has sufficient vitamin A. Vitiated astaxanthin was converted to retinol in strips of duodenum or inverted sacks of trout intestines. Astaxanthin, canthaxanthin, and zeaxanthin can be converted to vitamin A and A2 in guppies. ... 

The 3-hydroxyl P-rings of zeaxanthin are further oxygenated by the introduction of 5,6-epoxy moieties by zeaxanthin epoxidase (ZEP). A mono-epoxidated intermediate, antheraxanthin is produced, followed by the di-epoxy xanthophyU, violaxanthin, as shown in Figure 5.3.3B. 

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). 

Romer, S. et al.. Genetic engineering of a zeaxanthin-rich potato by antisense inactivation and co-suppression of carotenoid epoxidation, Metabol. Eng. 4, 263, 2002. 

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. 

Fig. 2.3. Characteristic chromatogram of paprika paste. Detection at 450 nm. Peak identification 1 = Capsorubin 2 = 5,6-Diepikarpoxanthin 3 = Capsanthin-5,6-epoxide 4 = Capsanthin-3,6-epox-ide 5 = Violaxanthin 6 = Luteoxanthin 2 7 = Luteoxanthin 1 8 = Capsanthin 9 = Antheraxanthin 10 = Mutatoxanthin 11 = Cucurbitaxanthin A 12 = (9/9 Z)-Capsanthins 13 = (13/13 Z)-Capsanthins 14 = Zeaxanthin 15 = Nigroxanthin 16 = (9Z)-Zeaxanthin 17 = (13Z)-Zeaxanthin 18 = Cryptocapsin 19 = a-Cryptoxanthin 20 = /TCryptoxanthin 21 = (Z)-Cryptoxanthin 22 = /1-Carotene 23 = (Z)-jS-Carotene. Reprinted with permission from J. Deli et al. [27].

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]. 

Isolated lettuce chloroplasts could epoxidize zeaxanthin in the presence of reduced pyridine nucleotides and oxygen and the process was stimulated by bovine serum albumin (which protected the epoxidase system from inhibition by fatty acids). Detailed study led to the conclusion that the epoxidase was an external monoxygenase and that the violaxanthin cycle (of which epoxidation of zeaxanthin is a part) was a trans-membrane system wherein de-epoxidation took place on the loculus side and epoxidation on the stroma side of the membrane. This arrangement requires migration of the carotenoids of the violaxanthin cycle across the membrane in a type of shuttle. The possible role of this cycle in some regulatory mechanism of photosynthesis at the membrane level was also discussed. 

Light-harvesting Chi a/b complexes contain carotenoids of the /3-/3 type (/8-carotene, violaxanthin, antheraxanthin, zeaxanthin, neoxanthin) and of the /3-e type (lutein). Some higher plants like lettuce contain an , -carotenoid, lactucaxanthin, both in the major and in minor Chi a/b complexes which may replace either lutein or other xanthophylls (Phillip and Young, 1995). Among /3,/3-carotenoids, violaxanthin, antheraxanthin, and neoxanthin contain epoxides whereas the others do not. In order to... 

Two lut mutants in Arabidopsis that have defects in the -cyclase and in the -ring hydroxylase, accumulate Chi a/b complexes, Chi and total carotenoids to the same levels as wild-type plants (Pogson et al., 1996). Interestingly, zeaxanthin and antheraxanthin are accumulated to much higher levels than in the wildtype in the absence of light stress. They are not epoxidized to violaxanthin, suggesting that they are located in a position where they are not accessible to the epoxidase. This may indicate that they are incorporated into positions normally occupied by lutein. 

Gilmore AM, Mohanty N and Yamamoto HY (1994) Epoxidation of zeaxanthin and antheraxanthin reverses non-photochemical quenching of Photosystem II chlorophyll a fluorescence in the presence of a trans-thylakoid ApH. FEBS Letts 350 271-274... 

Fig. 2. Simple two state allosteric model for qE in which the switch between quenched and unquenched conformation is driven by the effect of protonation and de-epoxidation. In this model LHCll can exist in two states, an unquenched conformation and a quenched conformation. Because the affinity of proton and zeaxanthin binding is greater in the quenched state, the ApH and de-epoxidation state will determine the equilibrium between these states. The existence of co-operativity indicates that these two states consist of a group of LHCll subunits which interact and change conformation in concert. The changed conformation, and intersubunit interaction, may give rise to quenching process itself In this model one or moreofthe different LHCll components may be involved.

---