Cyclic carotenoids

CUNNINGHAM F X Jr, POGSON B, SUN Z, MCDONALD K A, DELLAPENNA D and GANTT E (1996) Functional analysis of the (3 and e lycopene cyclase enzymes of Arabidopsis reveals a mechanism for control of cyclic carotenoid formation . Plant Cell, 8, 1613-26. 

Plant apocarotenoids have a wide variety of structures and functions. As expected, there is a small gene family of CCDs with different cleavage sites and somewhat promiscuous substrate selection. Some CCDs are stereo-specific, for example, 9-cis epoxycarotenoids are the substrates for NCEDs (9-cis expoxy dioxygenases) that produce the precursor of ABA biosynthesis, xanthoxin. Both linear carotenoids (lycopene) and cyclic carotenoids are substrates for cleavage at various double bonds including the central 15-15 and eccentric 5-6, 7-8, 9-10, 9 -10, and 11-12 bonds. Some CCDs cleave both linear and cyclic carotenoids and may cleave the same molecule twice, e.g., both 9-10 and 9 -10 positions. 

Hugueney, P. et ah. Metabolism of cyclic carotenoids a model for the alteration of this biosynthetic pathway in Capsicum annuum chromoplasts. Plant J. 8, 417, 1995. 

A second enzyme, BC02, was identified that cleaves carotenoids asymmetrically at the 9,10-double bond to produce the 10-apocarotenal (C27) and (3-ionone (C13), in a reaction similar to the Arabidopsis CCD7. Examples of BC02 have been cloned from mouse, zebra fish, ferret, and human (Kiefer et al. 2001, von Lintig et al. 2005, Hu et al. 2006). Substrate studies with different BC02s showed that these enzymes prefer acyclic carotenoids such as lycopene over cyclic carotenoids (Kiefer et al. 2001, von Lintig et al. 2005, Hu et al. 2006). These enzymes also seem to be selective for different carotenoid isomers. BC02 from ferret for example cleaves d,v-isomers of lycopene but not all-trans-lycopene (Hu et al. 2006). 

Scheme 7.5 Formation of some aroma compounds after oxidative cleavage of a acyclic carotenoids (e.g., lycopene, phytofluene and phytoene) and b cyclic carotenoids (e.g. a-carotene and / -caro-tene)...

Cyclic Carotenoids.—The previously-reported carotenoids myxobactin (15) and myxobacton (16) are now shown to be glucoside esters rather than inositol esters. In a closely related Myxobacterium, T,2 -dihydro-l -hydroxytorulene glucoside ester (17) and r,2 -dihydro-l -3-dihydroxytorulene glucoside ester (18) were isolated" as well as the analogous rhamnoside esters. 

Bioregulatory agents such as MPTA and DCPTA possess two distinct properties stimulation of the carotenoid biosynthetic pathway and inhibition of the cyclization of the acyclic to the cyclic carotenoids.The stimulation appear to be an indirect effect whereas the inhibitory effect is a direct one. By application of the compounds at lower concentrations at the early stages of its developmental phase, the inhibitory effect can be minmized resulting in the enhancement of desirable color normally associated with the crop. 

The formation of the normally present cyclic carotenoids is Inhibited. The transformation of the acyclic lycopene to the monocyclic gamma-carotene Is partially inhibited and further cyclizatlon to the bicyclic beta-carotene is totally Inhibited. However, when the treatment of the entire tree which Is sprayed with 2,000 ppm MPTA Is conducted preharvest at the fully mature stage of fruit development [2], the carotenoid composition of the endocarp reflects a net synthesis of the cyclic pigments and differs from the pattern seen in the peel as shown in Table II. 

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. 

Inhibition and Regulation. In addition to the work quoted above,CPTA has been used on Turkish lemons and oranges, and found to promote the accumulation of large amounts of lycopene, and to inhibit cyclic carotenoid formation. No specific inhibition was observed when a soluble tomato plastid enzyme system was incubated in the presence of CPTA, diphenylamine, 9-fluorenone, or 2-hydroxybiphenyl. ... 

Hugueney P, Romer S, Kuntz M and Camara B (1992) Characterization and molecular cloning of a flavoprotein catalyzing the synthesis of phytofluene and zeta-carotene in Capsicum chromoplasts. Eur J Biochem 209, 399 07 Hugueney P, Badillo A, Chen HC, Klein A, Hirschberg 1, Camara B and Kuntz M (1995) Metabolism of cyclic carotenoids A model for the alteration of this biosynthetic pathway in Capsicum annum chromoplasts. Plant loumal 8 417-424... 

For the dissection one has to keep in mind that the molecule contains positions which permit a coupling in high yield [C(9)-C(10) and especially C(11)-C(12)] whereas others (especially C(7)-C 8) in cyclic carotenoids) give products only in low yields. 

Thorough biochemical analysis of carotenoid biosynthesis, classical genetics, and more recently molecular genetics resulted in the elucidation of the main routes for the synthesis of acyclic and cyclic carotenoids at a molecular level (Sandmann 2001). Little is known, however, about the biosynthesis of carotenoids containing additional modifications of the end groups, the polyene chain, the methyl groups, or molecular rearrangements that contribute to the tremendous structural diversity of carotenoids. At present, hundreds of individual carotenoids have been characterized (Britton et al. 1998), and novel carotenoids continue to be isolated. All carotenoids are derived from the isoprenoid or terpenoid pathway. 

CUNNINGHAM JR., F. X., POGSON, B., SUN, Z., MCDONALD, K. A., DELLAPENNA, D., GANTT, E., Functional analysis of the P and E lycopene cyclase enzjmies of Arahidopsis reveals a mechanism for control of cyclic carotenoid formation. Plant Cell 1996,8,1613-1626. 

The middle part is always a conjugated polyene (symmetrical or unsymmetrical) and can be prepared by a number of established methods which are discussed in detail in Chapter 3 Part I. Some of these middle parts are readily available more common ones, e.g. the Cio-dialdehyde, are manufactured on a ton scale as industrial intermediates for the technical syntheses of (3,(3-carotene (3) and astaxanthin (406). For the synthesis of unsymmetrical carotenoids, the Cio-dialdehyde can be converted into a monoacetal derivative. The free aldehyde moiety is coupled with one end group, and the intermediate product is deprotected and then combined with the second end group. In these reactions, there are some positions which permit a coupling in high yield [C(9)-C(10) and C(11)-C(12)] and others [C(7)-C(8)] which, for cyclic carotenoids, give products in only low yield because of steric hindrance due to the adjacent methyl groups. The choice of the middle part and its synthesis has become a simple matter today. 

The cyclases responsible for formation of cyclic carotenoids are integral membrane proteins, as are the desaturation enzymes described above. The desaturation reactions require oxygen, whereas the cyclase reactions require strictly anaerobic conditions. All -lycopene did not serve as a substrate despite the fact that all the products were exclusively all -isomers. Cyclization was shown to proceed only with the (7Z, 9Z, 7 Z, 9 Z)- and (7Z, 7 Z)-isomers. However, in... 

Epoxide groups, particularly at the 5,6-position of cyclic carotenoids, are fairly common. These are formed stereospecifically, and experiments with have confirmed that the oxygen is derived from molecular oxygen (Yamamoto and Chichester, 1%5). The mechanism of incorporation is unknown, however. 

Biosynthesis. Tail-to-tail condensation of two molecules of geranylgeranylpyrophosphate (see Terpenes) gives phytoene, which undergoes stepwise dehydrogenation to produce the all-trans configuration of the true carotenoids (Fig. 1). Hie ionone rings of cyclic carotenoids arise by addition of a proton at C3, and formation of a bond between C2 and C7. This is followed... 

Lycopene. In addition to the accumulation of lycopene in many fruits and vegetables and the studies above, lycopene is a substrate for cyclization to jS-carotene or other cyclic carotenoids. Evidence was shown in studies involving inhibitors of the cyclization reaction, such as 2-(4-chlorophenylthio)triethylamine hydrochloride (CPTA) and nicotine. These inhibitors caused accumulation of lycopene in cells that normally produce jS-carotene and xanthophylls (42,43). 

Several pathways exist for the biosynthesis of cyclic carotenoids. Starting materials are the different aliphatic carotenes. From lycopene, for instance, the compounds given in Fig. 137 are formed. The biosynthesis of the different types of ionone rings follows the pathway outlined in Fig. 138. A proton is added to C-2 and a bond is formed between C-1 and C-6. Than a hydrogen atom is eliminated from positions 4, 6, or 18, resulting in the formation of the y- or e-ionone rings, respectively. 

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. 

Reactions at the C-1,2 double bond of lycopene 4.123) leads to a series of acyclic carotenoids characteristic of photosynthetic bacteria, or by cyclization to the mono- and bi-cyclic carotenoids typical of plants. Cyclization is believed to be initiated by protonation at C-2 and to proceed as shown in Scheme 4.27 the three ring types, P, e... 

The accumulation of lycopene in place of cyclic carotenoids has also been observed in a number of fruits, including citrus fruits, in roots of higher plants, and in fungi. ... 

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