Carotenoid compounds biosynthetic pathways

Figure 22-5 Structures and partial biosynthetic pathways for a few of the more than 600 known carotenoid compounds. The origin of some hydrogen atoms from mevalonate is shown, using the numbering for mevalonate. The numbering system for C40 carotenoids is also indicated.

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). 

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. 

Another special class of terpene formed by the isoprenoid biosynthetic pathway are the carotenoids. Carotenoids are important compounds in photosynthesis for harvesting of light energy as well as in the protection of cyanobacteria from excessive amounts of UV radiation. The carotenoids present in cyanobacteria are similar to those found in higher life-forms along with some glycosylated derivatives (see Chapters 1.15-1.18). 

Since the discovery of CPTA, research on onium compounds has generated a substantial amount of information relative to the structure-activity relationships for bioregulation of carotenoid pigments in higher plants and microorganisms. Both the trans-and cis-biosynthetic pathways can be regulated. The carotenoid pattern observed is determined essentially by the nature of the onium compounds employed. 

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. 4.1.2. Structure of commercial herbicides and some herbicidally active compounds that inhibit different enzymes in the biosynthetic pathway leading to the carotenoids.

On the other hand, two different biosynthetic pathways in plants are known for abscisic acid. The first more direct one is following the normal biosynthetic route for sesquiterpenes via mevalonic acid and farnesyl pyrophosphate. The second one imludes the photolytic cleavage of a Cz).o-carotenoid such as violaxanthin to give the C 15-compound xanthoxln which is converted into abscisic acid by oxidation (Eef. 20, 23, 24). 

Several carotenoid metabolites have important functions. The most important of these to consider is vitamin A, which is a metabolite of /8-carotene. This compound plays a key role in vision and in other biological reactions. The role of vitamin A in animals has been reviewed by Pitt (1971). Trisporic acid, also a metabolite of j8-carotene, is important in sexual reproduction in Mucorales, a group of fungi (Bu Lock et al., 1976). Sporopol-lenin, found in the outer layer (exine) of both spores and pollen, is considered a carotenoid polymer (Krinsky, 1971). It has been proposed that abscisic acid, an important plant growth regulator, may be a carotenoid metabolite, but a direct biosynthetic pathway from GGPP seems more probable (Burden and Taylor, 1976). 

Isoprenoid structures for carotenoids, phytol, and other terpenes start biosynthetically from acetyl coenzyme A (89) with successive additions giving mevalonate, isopentyl pyrophosphate, geranyl pyrophosphate, farnesyl pyrophosphate (from which squalene and steroids arise), with further build-up to geranyl geranyl pyrophosphate, ultimately to a- and /3-carotenes, lutein, and violaxanthin and related compounds. Aromatic hydrocarbon nuclei are biosynthesized in many instances by the shikimic acid pathway (90). More complex polycyclic aromatic compounds are synthesized by other pathways in which naphthalene dimerization is an important step (91). 

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