15,15 -Z-phytoene

Phytoene (Fig. 22-5) is apparently formed from geranylgeranyl-PP via prephytoene-Pf whose structure is entirely analogous to that of presqualene-pp 44,117 However, no reduction by NADH is required (Eq. 22-8). It is known that the 5-pro-R hydrogen atoms of mevalonate are retained in the phytoene as indicated by a shaded box in Eq. 22-8. Elimination of the other (pro-S) hydrogen yields 15,15 -Z phytoene (a s-phytoene), while elimination of the pro-R hydrogen yields all-E (trans) phytoene. Higher plants and fungi form mostly a s-phytoene, but some bacteria produce the all-E isomer.118... 

Phytoene can be converted to the carotenes by pathways indicated in part in Fig. 22-5 and Eq. 22-10. One of the first products is lycopene, the red pigment of tomatoes and watermelons, which is an all-trans compound. If 15-Z phytoene is formed, it must, at some point, be isomerized to an all-E isomer, and four additional double bonds must be introduced. The isomerization may be nonenzymatic. The double bonds are created by an oxygen-dependent desaturation, which occurs through the trans loss of hydrogen atoms. 

Fig. 5.3 Carotenoid biosynthesis in maize endosperm. Compounds IPP, isopentenyl pyrophosphate FPP, famesyl pyrophosphate GGPP, geranylgeranyl pyrophosphate DMAPP, dimethallyl pyrophosphate. Carotenoid biosynthetic pathway enzymes PSY, phytoene synthase PDS, phytoene desaturase ZDS, zetacarotene desaturase ISO, carotene isomerase LCY-B, lycopene beta cyclase LCY-E, lycopene epsilon cyclase HYD-B, beta-carotene hydroxylase HYD-E, alpha-carotene hydroxylase Isonrenoid biosynthetic pathway enzymes IPPI (IPP isomerase) GGPPS (GGPP synthase). Structures are not representative of the geometrical isomer substrates (e.g. Z-phytoene is a bent structure).

Fig. 26.3. Proposed mechanism for the conversion of prephytoene pyrophosphate to lycopersene, Z-phytoene, and -phytoene (Poulter, 1990 adapted and used with permission of the copyright owner, the American Chemical Society, Washington, DC).

It does not appear that lycopersene (13), a tetraterpene homologous to squalene, is an intermediate in tetraterpene biosynthesis (Bramley, 1985). This compound does, however, occur in a number of organisms including carrot roots. In most plants that have been studied, 15-Z-phytoene (7,8,1 l,12,7, 8Mr,12 -octahydro-i i,il -carotene) (11) is the predominant form, although in some bacteria the 15-E form (12) is most common (Spurgeon and Porter, 1983). 

When (3/ ,S,5/ )-[2- C,5- Hi]mevalonate was incubated with slices of tomato, the phytoene isolated showed a ratio of 8 8 (Fig. 26.4). These results indicate that there was no loss of the 5R hydrogen of mevalonate (based on Fig. 26.4, formation of Z-phytoene occurred). However, in experiments with [2- " C,5- H2]mevalonate, the ratio was closer to 14 8, instead of the 16 8 ratio expected, corresponding to loss of two hydrogens. Thus, in the formation of Z-phytoene (11), both of the pro-iR) hydrogens of mevalonate are retained, whereas the two pro-S hydrogens are lost (Fig. 26.3 and 26.4) (Spurgeon and Porter, 1983). 

Once formed, 15,15 -Z-phytoene (11) is desaturated to produce lycopene (5) via stereospecific removal of pairs of... 

In tomatoes and other higher plants, formation of lycopene (5) from Z-phytoene (11) occurs via -carotene (16). Z-phytoene (11) is converted to Z-phytofluene (14), E-phytofluene (15), -carotene (16), neurosporene (17), and, finally, lycopene (5) by extracts of tomato fruit plastids (Fig. 26.5) (Porter et al., 1984 Spurgeon and Porter, 1983). 

Since phytoene is the 15-Z isomer, whereas the fully desaturated carotenoids in plants are all-E, an isomerization of the 15-15 bond must occur in the sequence. It is thought to take place at the phytofluene stage in tomato plastids" and Capsicum fruits, although it occurs at the -carotene level in a strain of Scenedesmus, The direct, in vitro conversion of 15-Z phytoene into unsaturated carotenes has been demonstrated with tomato fruit," Capsicum, and daffodil chromoplasts. ... 

Figure 4.6. Separation by HPLC of plant carotenoids extracted from untreated (A) and diflufenican-treated (B, C) carrot cells. Traces A and B were monitored at 450 nm trace C was monitored at 285 nm. 1, All- neoxanthin 2, violaxanthin 3, antheraxanthin 4, lutein 5, a-carotene 6, )3-carotene 7, phytofluenes 8, 15-Z phytoene. Data kindly provided by Dr. K. Pallett.

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