Biochemistry & Molecular Biology of Plants, B. Buchanan, W. Gruissem, R. Jones, Eds. © 2000, American Society of Plant Physiologists CHAPTER 24 Natural Products (Secondary Metabolites) Rodney Croteau Toni M. Kutchan Norman G. Lewis CHAPTER OUTLINE Introduction Introduction Natural products have primary ecological functions. 24.1 Terpenoids 24.2 Synthesis of IPP Plants produce a vast and diverse assortment of organic compounds, 24.3 Prenyltransferase and terpene the great majority of which do not appear to participate directly in synthase reactions growth and development. These substances, traditionally referred to 24.4 Modification of terpenoid as secondary metabolites, often are differentially distributed among skeletons limited taxonomic groups within the plant kingdom. Their functions, 24.5 Toward transgenic terpenoid many of which remain unknown, are being elucidated with increas- production ing frequency. The primary metabolites, in contrast, such as phyto- 24.6 Alkaloids sterols, acyl lipids, nucleotides, amino acids, and organic acids, are 24.7 Alkaloid biosynthesis found in all plants and perform metabolic roles that are essential 24.8 Biotechnological application and usually evident. of alkaloid biosynthesis Although noted for the complexity of their chemical structures research and biosynthetic pathways, natural products have been widely per- 24.9 Phenylpropanoid and ceived as biologically insignificant and have historically received lit- phenylpropanoid-acetate tle attention from most plant biologists. Organic chemists, however, pathway metabolites have long been interested in these novel phytochemicals and have 24.10 Phenylpropanoid and investigated their chemical properties extensively since the 1850s. phenylpropanoid-acetate Studies of natural products stimulated development of the separa- biosynthesis tion techniques, spectroscopic approaches to structure elucidation, and synthetic methodologies that now constitute the foundation of 24.11 Biosynthesis of lignans, lignins, contemporary organic chemistry. Interest in natural products was and suberization not purely academic but rather was prompted by their great utility 24.12 Flavonoids as dyes, polymers, fibers, glues, oils, waxes, flavoring agents, per- 24.13 Coumarins, stilbenes, fumes, and drugs. Recognition of the biological properties of myriad styrylpyrones, and arylpyrones natural products has fueled the current focus of this field, namely, 24.14 Metabolic engineering of the search for new drugs, antibiotics, insecticides, and herbicides. phenylpropanoid production: Importantly, this growing appreciation of the highly diverse biologi- a possible source of enhanced cal effects produced by natural products has prompted a reevalua- fibers, pigments, pharmaceuti- tion of the possible roles these compounds play in plants, especially cals, and flavoring agents in the context of ecological interactions. As illustrated in this chapter, many of these compounds now have been shown to have important 1250 Chapter 24 Natural Products (Secondary Metabolites) adaptive significance in protection against thus a natural product (Fig. 24.1). Even herbivory and microbial infection, as attrac- lignin, the essential structural polymer of tants for pollinators and seed-dispersing ani- wood and second only to cellulose as the mals, and as allelopathic agents (allelochem- most abundant organic substance in plants, icals that influence competition among plant is considered a natural product rather than species). These ecological functions affect a primary metabolite. plant survival profoundly, and we think it In the absence of a valid distinction reasonable to adopt the less pejorative term based on either structure or biochemistry, we “plant natural products” to describe sec- return to a functional definition, with prima- ondary plant metabolites that act primarily ry products participating in nutrition and es- on other species. sential metabolic processes inside the plant, and natural (secondary) products influenc- ing ecological interactions between the plant The boundary between primary and and its environment. In this chapter, we secondary metabolism is blurred. provide an overview of the biosynthesis of the major classes of plant natural products, Based on their biosynthetic origins, plant emphasizing the origins of their structural natural products can be divided into three diversity, as well as their physiological func- major groups: the terpenoids, the alkaloids, tions, human uses, and potential biotechno- and the phenylpropanoids and allied pheno- logical applications. lic compounds. All terpenoids, including both primary metabolites and more than 25,000 secondary compounds, are derived 24.1 Terpenoids from the five-carbon precursor isopentenyl diphosphate (IPP). The 12,000 or so known Terpenoids perhaps are the most structurally alkaloids, which contain one or more nitro- varied class of plant natural products. The gen atoms, are biosynthesized principally name terpenoid, or terpene, derives from the from amino acids. The 8000 or so phenolic fact that the first members of the class were compounds are formed by way of either the shikimic acid pathway or the malonate/ acetate pathway. Primary metabolite Secondary metabolite Primary and secondary metabolites can- not readily be distinguished on the basis of precursor molecules, chemical structures, or biosynthetic origins. For example, both pri- mary and secondary metabolites are found among the diterpenes (C ) and triterpenes 20 COOH COOH (C30). In the diterpene series, both kaurenoic Kaurenoic acid Abietic acid acid and abietic acid are formed by a very similar sequence of related enzymatic reac- tions (Fig. 24.1); the former is an essential in- termediate in the synthesis of gibberellins, N COOH N COOH i.e., growth hormones found in all plants H H (see Chapter 17), whereas the latter is a resin Proline Pipecolic acid component largely restricted to members of the Fabaceae and Pinaceae. Similarly, the es- Figure 24.1 sential amino acid proline is classified as a Kaurenoic acid and proline are primary metabo- lites, whereas the closely related compounds abietic primary metabolite, whereas the C6 analog acid and pipecolic acid are considered secondary pipecolic acid is considered an alkaloid and metabolites. 24.1–Terpenoids 1251 isolated from turpentine (“terpentin” in Ger- known hemiterpene is isoprene itself, a vol- man). All terpenoids are derived by repeti- atile product released from photosynthe- tive fusion of branched five-carbon units tically active tissues. The enzyme isoprene based on isopentane skeleton. These mono- synthase is present in the leaf plastids of nu- mers generally are referred to as isoprene merous C3 plant species, but the metabolic units because thermal decomposition of rationale for the light-dependent production many terpenoid substances yields the alkene of isoprene is unknown (acclimation to high gas isoprene as a product (Fig. 24.2, upper temperatures has been suggested). Estimated panel) and because suitable chemical condi- annual foliar emissions of isoprene are quite tions can induce isoprene to polymerize in substantial (5 × 108 metric tons of carbon), multiples of five carbons, generating numer- and the gas is a principal reactant in the ous terpenoid skeletons. For these reasons, NOx radical–induced formation of tropo- the terpenoids are often called isoprenoids, spheric ozone (see Chapter 22, Fig. 22.37). although researchers have known for well C10 terpenoids, although they consist of over 100 years that isoprene itself is not two isoprene units, are called monoterpenes; the biological precursor of this family of as the first terpenoids isolated from turpen- metabolites. tine in the 1850s, they were considered to be the base unit from which the subsequent nomenclature is derived. The monoterpenes 24.1.1 Terpenoids are classified by the are best known as components of the vol- number of five-carbon units they contain. atile essences of flowers and of the essential oils of herbs and spices, in which they make The five-carbon (isoprene) units that make up as much as 5% of plant dry weight. Mono- up the terpenoids are often joined in a “head- terpenes are isolated by either distillation or to-tail” fashion, but head-to-head fusions are extraction and find considerable industrial also common, and some products are formed use in flavors and perfumes. by head-to-middle fusions (Fig. 24.2, lower The terpenoids that derive from three panel). Accordingly, and because extensive isoprene units contain 15 carbon atoms and structural modifications with carbon–carbon are known as sesquiterpenes (i.e., one and bond rearrangements can occur, tracing the one-half terpenes). Like monoterpenes, original pattern of isoprene units is some- many sesquiterpenes are found in essential times difficult. oils. In addition, numerous sesquiterpenoids The smallest terpenes contain a single act as phytoalexins, antibiotic compounds isoprene unit; as a group, they are named produced by plants in response to microbial hemiterpenes (half-terpenes). The best challenge, and as antifeedants that discour- age opportunistic herbivory. Although the plant hormone abscisic acid is structurally a sesquiterpene, its C15 precursor, xanthoxin, is not synthesized directly from three isoprene Isopentane Isoprene units but rather is produced by asymmetric cleavage of a C40 carotenoid (see Chapter 17). The diterpenes, which contain 20 car- bons (four C5 units), include phytol (the hy- drophobic side chain of chlorophyll), the