Cyclization of lycopene

Some bacteria synthesize C50 carotenoids such as decaprenoxanthin (Fig. 22-5), the extra carbon atoms at each end being donated from additional prenyl groups, apparently at the stage of cyclization of lycopene.134 Thus, a carbocation derived by elimination of pyrophosphate from dimethylallyl-PP could replace the H+ shown in the first step of Eq. 22-11. The foregoing descriptions deal with only a few of the many known structural modifications of carotenoids.2 135 136... 

The stereochemistry of the cyclization of lycopene has been discussed by Britton. He concluded that, based on the absolute stereochemistry of trisporic acid and a-carotene, the cyclization involves the initial formation of a boat conformer. However, this deduction assumes that both the a- and j5-ring systems are formed from the same carbonium ion . 

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. 

Carotenoid pathway continues with the cyclization of lycopene, involving the formation of 6C cyclic end groups. Two major enzymes participate at this point lycopene-p-cyclase (LCY-B) and lycopene-8-cyclase (LCY-E). Unusually, tomato contains two lycopene p-cyclases, LCY-B [33] as described above, and also CYC-B, a chromoplast-specific cyclase that would play a major role in carotenoid accumulation in fruits [34]. Lycopene p-cyclase forms initially y-carotene which only accumulates to traces in ripe fruits as a second p-ring is added by the same enzyme to yield p-carotene. 

Del S. chmielewskii, S. chilense, S. pennellii Increases the cyclization of lycopene into 8-carotene (increases 8-carotene at the expense of lycopene). Encodes LCY-E. Oiange-ied colraed finits due to accumulation of 8 -carotene... 

Phytoene, the C40 condensation product, is dehydrogenated to yield lycopene. Cyclization of both ends of lycopene gives... 

Some details of the stereochemistry of carotenoid cyclization have been elucidated. In the C40 series labelling with stable isotopes (deuterium) has been used for the first time in studies of carotenoid biosynthesis. A Flavobacterium species in the presence of nicotine accumulated the acyclic precursor lycopene (175). When the cells were washed free from the inhibitor and suspended in H20 cyclization of the lycopene proceeded, initiated by High-resolution n.m.r. 

There is also reason to suspect that the activity of lycopene may be in part analogous to that of tocotrienols like tocotrienols, the end portions of lycopene have isoprenoid stracture. This is not trae for other carotenoids, which have cyclized tails. The well-known ability of tocotrienols to decrease seram cholesterol is rooted in the ability of these compounds to suppress the expression of HMG-CoA reductase, the rate-limiting enzyme for cholesterol synthesis that statin drags inhibit allosterically by a posttranscriptional mechanism. This effect presumably requires tocotrienols to interact with an as-yet-uncharacterized receptor, a receptor that is postulated to recognize other isoprenoids as well, based on the evidence that tocotrienols are not the only isoprenoids known to down-regulate HMG-CoA reductase. 

Arguments still persist about whether the acyclic intermediate that is cyclized is lycopene or neurosporene, although the mechanism of the cyclization reaction would be the same in both cases. It is more satisfactory to consider that the requirement for cyclization is simply that one half of the carotenoid molecule should have reached the lycopene level of desaturation. 

The direct conversion of lycopene to cyclic carotenes has also been demonstrated with a number of cell-free systems. Extracts from tomato plastids and spinach chloroplasts have been shown to incorporate radioactivity from lycopene into 8-, y-, a-, and jS-carotene (Kushwaha r al., 1%9 1970 Papa-stephanou, 1973). Incorporation of radioactivity from lycopene into -caro-tene and other carotenes has also been reported with bean leaf chloroplasts (Decker and Uehleke, 1%1 Hill et al., 1971) and a cell-free system from P. blakesleeanus (Davies, 1973). The occurrence of a-zeacarotene (7, 8 -dihy-dro- ,il -carotene) and /3-zeacarotene (7, 8 -dihydro-/8,il -carotene) in some organisms suggests that neurosporene can also be cyclized. Whether or not this pathway is of significance in the biosynthesis of y-, 8-, a-, and /3-caro-tenes is not clear, however (Britton, 1976b). Possibly, the enzyme that cy-clizes lycopene also cyclizes neurosporene, but at a much slower rate. Label from neurosporene has also been incorporated into /3-carotene by cell-free... 

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). 

Following the opposite approach, plants overexpressing lycopene p-cyclase (LCY-B) cDNA resulted in high contents of beta-carotene at the expense of lycopene, which was almost completely cyclized [94]. The plants did not show collateral effects, instead showed an increased productivity. Total carotenoid content was also increased in some progenies and beta-carotene content was twofold that of the beta mutant. 

Shown in Table 1 is the change in major carotenoid components of tomato (1st harvest) as a function of irradiation dose irradiation level of 0.5 and 2 kGy resulted in a rapid accumulation of carotenoids mainly lycopene. In fruits treated with 0.5 kGy the whole carotenoid profile was improved. Whereas conversion of lycopene to p-carotene and lutein via cyclization and hydroxylation reaction respectively was partially distributed in fruit irradiated with a dose of 2 kGy. 

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... 

In the cyclization of phytoene [4.122] a particular stereochemistry is expected for proton and carbon addition to the C-1 double bond. [2- C]Mevalonate labels the ( )-methyl group at C-1 in lycopene [[4.127 = [4.123] = label] and the la-methyl group in zeaxanthin [[4.128 = [4.125 (see Scheme 4.28). Zeaxanthin formed in bacterial cells suspended in deuterium oxide had a 2) -deuteron. It follows that cyclization occurs as shown in Scheme 4.28 [111]. The stereochemistry in the C50 carotenoids, e.g. [4.126] implies a different manner of addition to the C-1 double bond. 

Figure 1 sununarizes the biosynthetic pathway between phytoene and the carotenes. Phytoene is converted to progressively more unsaturated compounds, leading to the formation of lycopene. Lycopene is then cyclized to the monocylic 8- and y-carotenes, which in turn are further cyclized to yield a- and p-carotene, respectively. 

The accumulation of lycopene which is now observed, is again due to 0 which acts in an inverse way in the lycopene cyclization reactions (Fig. 4). When homogenates of chromoplasts are incubated in the presence of radioactive all-trans -carotene, then in the presence of O2, the formation of lycopene is observed as this reaction is dependent on O2 as was found for the formation of -carotene. Under anaerobic conditions (effected here by an enzymatic oxygen trap glucose, glucose oxidase, catalase) no more desaturation is observed, but a rapid cyclization of the accumulated lycopene and formation of a-and B-carotene takes place. 

Cyclization (Fig. 89). Lycopene and all preceding intermediates in the biosynthetic pathway are open chain. It is still unsettled whether the cyclization of the ends of the chain occurs at the level of lycopene or of its precursor neurosporene. We will simply state one of the two possibilities the cyclization occurs with lycopene. 

Synthesis Photosynthctic Bactois.—A characteristic of the carotenoids of purple photosynthetic bacteria is the hydration of the terminal 1 and 1 double bonds. This is frequently followed by methylation of the hydroxyls at C-1 and C-1. Rarely does cyclization occur, but -carotene is formed in Rhodo-microbium vanneilii alongside the main pigment 1-hydroxy-1,2-dihydrolyco-pene (rhodopin). In the presence of 1 mmol 1" nicotine, synthesis of both pigments is inhibited and that of lycopene stimulated by a concomitant amount so nicotine not only inhibits cyclization, which involves the terminal... 

The accumulation of lycopene, at the expense of jS-carotene formation, on treatment of plants with the substituted triethylamine CPTA (Figure 4.9) has been demonstrated in several cases, including citrus fruits and Aphanocapsa cells and cell extracts. In Aphanocapsa, in vivo and in vitro I50 values for lycopene cyclase were 45 and 30 pM, respectively, and a noncompetitive effect on the enzyme was shown. However, in citrus fruits a range of onium compounds related to CPTA regulate the formation of all- carotenoids, in that not only is cyclization blocked, but also the formation of lycopene is stimulated. The latter effect requires protein synthesis. Indeed, the tertiary amine MPTA [2-(4-methylphenoxy)triethyl-amine] causes gene expression and translation of [poly(A) ]RNA on SOS ribosomes. Therefore, CPTA and related compounds may have multiple effects in plants. 

AMO 1618 (Figure 4.9), an inhibitor of squalene cyclization and kaurene synthetase, is also an inhibitor of lycopene cyclization when used at high concentrations with the Aphanocapsa cell-free system. ... 

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