Isoprenoids make up a group of hydrocarbons found in unicellular and multicellular organisms (Eubacteria, Archaebacteria, protozoa, fungi, algae, insects, animals and plants) that are diverse in chemistry, structure and function.1-3 Over 30,000 natural isoprenoid compounds have been characterized and still more are being discovered and identified. The precursors of isoprenoids are isopentenyl diphosphate (IPP) and dimethylallyl diphosphate (DMAPP), isomers which constitute the building blocks for the formation of this large family of naturally occurring compounds.1,4 Isoprenoid compounds that are generated from the C5 monomers IPP and DMAPP include C10 monoterpenes, C15 sesquiterpenes, C20 diterpenes, C30 triterpenes and C40 tetraterpenes.1,4

Many diverse isoprenoid compounds occur naturally in the plant kingdom (Figure 8.1). Plant-derived flavors and scents are due to the presence of these compounds, e.g., menthol, limonene, camphor, geraniol. Plant resins contain terpenes, from which rosin and turpentine are obtained. Other plant-derived isoprenoids include terpenes (linalool, linalyl acetate, terpinen-4-ol, citronellol and α-pinene),

8.1 What Are Isoprenoids? ............................................................................... 123 8.2 Where Are Isoprenoids Found? .................................................................. 123 8.3 How Are Isoprenoids Synthesized? ............................................................ 125 8.4 Why Are Isoprenoids Important? ............................................................... 126 8.5 How Does Plant Isoprene Emission Affect the Environment? .................. 126 8.6 Which Isoprenoids in Foods Keep Us Healthy? ......................................... 127 8.7 Applications by the Inhibition of Enzymes in Isoprenoid Biosynthesis .... 128 8.8 Can We Genetically Engineer Plants and Microbes for the Production

of Desired Isoprenoids? .............................................................................. 128 8.9 Can Plants Be Genetically Altered to Produce Isoprenoids that

Promote Plant Defense against Herbivores? ............................................... 129 8.10 What Other Isoprenoids Are Important in Industry? ................................. 130 8.11 Conclusions ................................................................................................. 130

which are utilized as cutaneously applied essential oils.5 Natural rubber, guayule and gutta-percha are polyterpenes. The plant hormones abscisic acid and gibberellic acid, are also isoprenoid derivatives. Sterols that are biologically important in the formation of membrane structures and hormones are C30 isoprenoid compounds.4 Other examples include C40 carotenoids, chlorophylls and ubiquinones, which are composed of isoprenoid chains (Figure 8.1).