Neurosporene cyclization

The incorporation of " C-labelled neurosporene (138), lycopene, and y-carotene (141) into /3-carotene by cell extracts of Phycomyces blakesleeanus mutants has been demonstrated.Addition of unlabelled lycopene or /3-zeacarotene (140) caused approximately equal reduction of the incorpsration of [ C]neurosporene into /3-carotene, indicating that the alternative routes of Scheme 3 are of equivalent importance. The absolute configuration of C-6 of natural /3,y-carotene (55) is opposite to that of all C40 carotenoids with an e-ring end-group. " Opposite foldings of the aliphatic precursor are therefore required for cyclization to produce the y- and e-end-groups. 

FIGURE 63.1 Starting with mevalonate, carotenoids are biosynthesized by a special branch of the terpenoid pathway. The first C-40 hydrocarbon unit formed is phytoene, a carotenoid with three conjugated double bonds, which then is enzymatically desaturated to successively yield (3-carotene, neurosporene, and lycopene. Other carotenoids such as (3-carotene and oxocarotenoids are produced from lycopene following cyclization and hydroxylation reactions. Thus, lycopene is a central molecule in the biosynthesis pathway of carotenoids. 

Phytoene is desaturated to neurosporene or lycopene by the photosynthetic bacteria. These are then converted to carotenes by desaturation, saturation, cyclization and aromatization, and/or to... 

Composition. Except the polar carotenoid group, the composition of carotenoids in the cells from aerobic culture was 39% bacteriorubi-xanthinal (1 0), 31% zeaxanthin ( ), 19% caloxanthin ( ), 4-% nostoxanthin (5) and 4% cis-zeaxanthin (6). As minor (less than 1%), compounds J[, 2, 2 and Xk were found. Trace amounts of 1J, phytofluene, asymmetrical -carotene, neurosporene, 1 2 and y-carotene were also found. Compounds 11 and 7 were found in the cells from semi-aerobic and DPA-inhibited cultures, respectively. Compounds 8, 1 2 and 1 3 were accumulated in a culture containing nicotine, which inhibits cyclization (FIG. 1). 

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... 

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. 

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