Phytoene structure

IPP react with each other, releasing pyrophosphate to form another allyl pyrophosphate containing 10 carbon atoms. The chain can successively build up by five-carbon units to yield polyisoprenes by head-to-tail condensations alternatively, tail-to-tail condensations of two C15 units can yield squalene, a precursor of sterols. Similar condensation of two C2q units yields phytoene, a precursor of carotenoids. This information is expected to help in the development of genetic methods to control the hydrocarbon structures and yields. 

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... 

Inhibitors of carotenoid synthesis also lead to chlorophyll destruction by destabilizing the photosynthetic apparatus. Total carotenoid content decreased with increased (-)-usnic concentration (Fig. 1.4). Carotenoid biosynthesis can be interrupted by inhibiting the enzyme phytoene desaturase that converts phytoene to carotenes or by inhibiting the enzyme HPPD responsible for plastoquinone (required for phytoene desaturase activity) synthesis.14 Usnic acid possesses some of the structural features of the triketone HPPD inhibitors, such as sulcotrione (Fig. 1.1C).8 (-)-Usnic acid had a strong inhibitory activity on HPPD, with an apparent IC50 of 70 nM, surpassing the activity obtained with the commercial herbicide sulcotrione (Fig. 1.5). 

New Stereochemical Assignments. cis-Isomers. The structure of the poly-cw-carotenoid prolycopene from the Tangerine tomato mutant has been deter-mined by detailed H and "C n.m.r. studies as 7,9,1, 9 -tetra-cis-il/,i /-carotene (14). The phytoene 7,8,ll,12,7, 8, ir,12 -octahydro-(/f,j/ -carotene... 

Most of the compounds cited in this introductory section are produced in metabolic processes where the cyclopropane-containing metabolite appears to be the stable end product or secondary product with as yet unobvious metabolic function. However, this is not the case in at least two types of systems, in which cyclopropyl species are key and necessary intermediate structures in high flux metabolic pathways. The first example is the squalene (76) and phytoene (88) biosynthesis where presqualene pyrophosphate (77) and prephytoene pyrophosphate (89) are obligate cyclopropanoid intermediates in the net head-to-head condensations of two farnesyl pyrophosphate (73) or two geranylgeranyl pyrophosphate (66) molecules respectively. The second example is in plant hormone metabolism where C(3) and C(4) of the amino acid methionine are excised as the simple hormone ethylene via intermediacy of 1-aminocyclopropane-l-carboxylic acid (9). Both examples will be discussed in detail in the Section II. 

A great diversity in molecular structure is observed among herbicides which inhibit carotene biosynthesis as is exemplified by the structures of norflurazon, fluridone and difunone (shown below). Nonetheless, many of these compounds, which comprise a subset of the larger group known as bleaching herbicides, appear to inhibit the same step in the biosynthetic pathway to the carotenoids (1 ). The inhibited step is the desaturation of 15-cis phytoene to 15- cis phytofluene (Figure 1) and the build-up of phytoene in plants and in cell-free systems which have been treated with these herbicides is well documented (2-4). 

If our hypothesis is correct, this hypothetical binding site should also accommodate fluridone, norflurazon and difunone and some possible binding orientations of these molecules are compared with furanone 13 in Figures 7-9. Note that we have attempted to depict the molecules in such a way that key structural features, e.g., the CF3 phenyl and vinylogous amide subunits, occupy the same positions as nearly as possible. Finally, it should be emphasized that considerable further work is required to demonstrate that the furanones actually inhibit phytoene desaturase and to further probe the possibility of a common binding site for the proven inhibitors including those such as fluorochloridone (10) and the m-phenoxybenzamides (4), which do not incorporate the vinylogous amide substructure. 

Presqualene pyrophosphate (32), a compound whose structure has caused considerable controversy in the past, has been isolated from intact rat liver and a yeast microsomal system. Previously, (32) had been detected only in systems which have been starved of NADH and hence the new findings demonstrate that (32) is not an artefact. Despite earlier evidence that lycopersene is a precursor of phytoene, a recent stereochemical analysis of phytoene synthesis makes this appear to be unlikely, and a mechanism has been proposed for the synthesis of cis- and tmnj -phytoene directly from pre-phytoene pyrophosphate (33) (Scheme 7). This mechanism is similar to one proposed for squalene synthesis. ... 

Several phytoene desaturases are known that differ in the number of desaturation steps and in their structures. Among them, the bacteriaPfungal type is encoded by crti, and the cyanobacterial/algal/plant type is encoded by crtP or pds. Phytoene desaturase converts phytoene to -carotene with phytofluene as an intermediate. The reaction is stimulated by NAD, NADP, and oxygen. 

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).

Owing to the similarity of desaturation reactions catalyzed by PDS or ZDS, differentiation in the plant is not easy to detect. Most of the herbicidal inhibitors probably inhibit both, although to a different extent [6j. If strong inhibition of PDS has taken place with accumulation of phytoene, then the compound s ability to inhibit ZDS cannot be seen. Figure 4.1.2 shows that the commercial products primarily inhibit PDS [6, 8, 27-29]. Cell-free studies exemplified by norflurazon and fluridone have shown them to act as reversible noncompetitive inhibitors of PDS [27]. Other PDS active structures are shown below in Table 4.1.2 and in Section 4.1.4.10. 

Phytoene Desaturase Inhibitors j 197 Table 4.1.2 Structural evolution of phenoxypyridine ethers since 1994. 

Table 35.13 Chemically different classes of phytoene desaturase inhibitors (80-86) (see Fig. 35.20 for the basic structure).

As a methodology to search for a new lead, we focused on the chemical approach using A,S-heterocycles. This approach led to the evolution of 5-methylene-thiazolidines as the lead to bleaching herbicides. Further structural modifications based on the parent compound resulted in the creation of a new family of bleaching herbicides, 2-(W-difluoroacelylimino)-S-methyl-3-(3-trifiuoromethylphenyl)-l,3-thiazoline, (S-3085) widi potent preemergence herbicidal activity and selectivity on cotton crops. On die basis of biochemical studies, 3-phenylthiazolines have been characterized as a new inhibitor of phytoene desaturase. Facile syndieses of 1,3-thiazolines and the manufacturing process toward S-3085 will be reported in future publications. 

Fig. 11. The pathway of conversion of phytoene to lycopene. In this pathway trans structures are shown for all compounds. In plants the first two compounds in this pathway (phytoene and phytofluene) have a central 15,15 -cis double bond. An isomerization reaction converts cis-phytofluene to trans-phytolluene in plants.

The structure of the C-5 monomethyl ether of azafrin (6) methyl ester was determined by partial analysis of the NMR spectrum (170), and a new isomer of phytoene (21) has been assigned the Z,E,Z or Z,E,E configuration from NMR evidence (18). Further application of NMR spectroscopy for the structural elucidation of carotenoids will be facilitated by the assigned spectra of authentic carotenoids gradually available. 

Carotenoid desaturation proceeds in stages, starting with phytoene, a colorless C40 terpenoid, which is formed by the condensation of two molecules of ger-anylgeranyl pjrrophosphate through the action of the enzyme, ph3rtoene synthase (PSY). The general scheme for phytoene desaturation, first proposed by Porter and Lincoln in 1950 before the structures were known for the individual carotenoids, is very close to the scheme depicted in Figure 1 (7,8). The Porter... 

In contrast to the head-to-tail connected polyisoprenoids, squalene (C30) and phytoene (C40) arise from a head-to-head condensation of two molecules FPP or GGPP, respectively. The squalene synthase requires divalent metal ions Mn " as well as an NADPH cofactor. Superimposition of the crystal structure of human squalene synthase with avian FPP synthase reveals a close structural homology, although the sequence identity in the superimposed parts is low [164]. Due to their distinct functionality, the highly a-helical squalene synthases (Fig. 87.11) have been designated an own type of head-to-head prenyltransferases termed E-class. The highly conserved DDXXD motifs in a-prenyltransferases are altered to DTLED and DYLED motifs in the human squalene synthase. 

Harada J, Nagashima KVP, Takaichi S, Misawa N, Matsuura K, Shimada K (2001) Phytoene desaturase, CrtI, of the purple photosynthetic bacterium, Rubrivivax gelatinosus, produces both neurosporene and lycopene. Plant Cell Physiol 42 1112-1118 Francis GW, Liaaen-Jensen S (1970) Bacterial carotenoids XXXIII. Carotenoids of thiorhodaceae 9. The structures of the carotenoids of the rhodopinal series. Acta Chem Scand 24 2705-2712... 

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