Isoprenoids, also named terpenoids, are the chemically and functionally most diversified class of low molecular mass lipids in plants, both primary and secondary metabolites (Iriti and Faoro 2006). They include electron carriers (quinones), membrane constituents (sterols), vitamins (A, D, E and K), plant hormones (side chain of cytokinins, abscisic acid, gibberellins and brassinosteroids), photosynthetic pigments (chlorophyll, phytol and carotenoids) and essential oils (Sacchettini and Poulter 1997).
Acetyl coenzyme A (CoA) represents the precursor for the isprenoid biosynthesis (Fig. 11.5). Firstly, two molecules of acetyl CoA react to give acetoacetyl CoA and then, with a further acetyl CoA to produce p-hydroxy-p-methylglutaryl-coenzime A (HMG-CoA). In plants, the same enzyme, HMG-CoA synthase, catalyses both reactions. The conversion of HMG-CoA into mevalonate, via HMG-CoA reductase, is the rate limiting enzyme of this pathway (Chappell 1995). Mevalonate kinase and mevalonate phosphate kinase phosphorylate, respectively, mevalonate and, then, mevalonate 5-phosphate, yielding mevalonate 5-diphosphate. Further, mevalonate diphosphate decarboxylation, via mevalonate diphosphate decarboxylase, produces isopentenyl diphosphate (IPP), the five-carbon building block for the formation of isoprenoid chains. The enzyme IPP:dimethylallyl-PP isomerase converts IPP into dimethylallyl diphosphate (DMAPP), the acceptor for successive transfers of isopentenyl residues (Bach 1987; Chappell 1995).
Hemiterpenes (C5), such as isoprene, originate from dimethylallyl-PP, upon the release of diphosphate. Differently, dimethylallyl-PP can condense with IPP to form geranyl-PP via geranyl-PP synthase. In the same way, further chain elongation is attained by head to tail condensation of geranyl-PP to IPP, to produce farnesyl-PP via farnesyl-PP synthase. Analogously, geranylgeranyl-PP synthase catalyses the head to tail condensation of farnesyl-PP to IPP thus yielding geranylgeranyl-PP (Poulter and Rilling 1978).
Geranyl-PP is the precursor for the formation of monoterpenes (C10) or essential oils, including highly volatile open chain and cyclic compounds, such as menthol, limonene, geraniol, linalool and pinene. They are active in plant-microbe, plant-pronubi, plant-phytophagous and plant-plant interactions due to their attractiveness and repulsiveness (Croteau 1987).
Farnesyl-PP is the precursor for the synthesis of open chain and cyclic sesquiterpenes (C15), the largest group of isoprenoids, including essential oils and antibiotic compounds (phytoalexins) (Cane 1990). Diterpenes (C20) derive from geranylgeranyl-PP consisting of phytoalexins, plant hormones, the phytol side chain of chlorophylls, tocopherols and phylloquinone (Dogbo and Camara 1987).
Mevalonate phosphate kinase
■ Mevalonate 5-phosphate-
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