This review focuses on the monoterpene, sesquiterpene, and diterpene synthases of plant origin that use the corresponding C 10 , C 15 , and C 20 prenyl diphosphates as substrates to generate the enormous diversity of carbon skeletons characteristic of the terpenoid family of natural products. A description of the enzymology and mechanism of terpenoid cyclization is followed by a discussion of molecular cloning and heterologous expression of terpenoid synthases. Sequence relatedness and phylogenetic reconstruction, based on 33 members of the Tps gene family, are delineated, and comparison of important structural features of these enzymes is provided. The review concludes with an overview of the organization and regulation of terpenoid metabolism, and of the biotechnological applications of terpenoid synthase genes.The pathways of monoterpene, sesquiterpene, and diterpene biosynthesis are conveniently divided into several stages. The first encompasses the synthesis of isopentenyl diphosphate, isomerization to dimethylallyl diphosphate, prenyltransferase-catalyzed condensation of these two C 5 -units to geranyl diphosphate (GDP), and the subsequent 1Ј-4 additions of isopentenyl diphosphate to generate farnesyl (FDP) and geranylgeranyl (GGDP) diphosphate ( Fig. 1) (1). In the second stage, the prenyl diphosphates undergo a range of cyclizations based on variations on the same mechanistic theme to produce the parent skeletons of each class. Thus, GDP (C 10 ) gives rise to monoterpenes (2), FDP (C 15 ) to sesquiterpenes (3), and GGDP (C 20 ) to diterpenes (4). These transformations catalyzed by the terpenoid synthases (cyclases) may be followed by a variety of redox modifications of the parent skeletal types to produce the many thousands of different terpenoid metabolites of the essential oils, turpentines, and resins of plant origin (5).This review focuses on the synthases that use prenyl diphosphate substrates to generate the enormous diversity of carbon skeletons characteristic of terpenoids. Most of these natural products are cyclic, and many contain multiple ring systems, the basic structures of which are determined by the highly specific terpenoid synthases; examples of synthases that produce acyclic products are also known. The terpenoid synthases may be involved in the regulation of pathway flux because they operate at metabolic branch points and catalyze the first committed steps leading to the various terpene classes (6). The synthases responsible for generating the parent compounds of the various types are quite similar in properties (7), and all operate by electrophilic reaction mechanisms, as do the prenyltransferases (8, 9). Comprehensive treatment of the topic, especially enzymological and mechanistic aspects, has been provided recently (2-4), and the field is periodically surveyed (10, 11). After brief coverage of the three types of terpene synthases from higher plants, with emphasis on common features of structure and function, we focus here on molecular cloning and sequence analysis of these ...

Isopentenyl diphosphate (IPP) is the central intermediate in the biosynthesis of isoprenoids, the most ancient and diverse class of natural products. Two distinct routes of IPP biosynthesis occur in nature: the mevalonate pathway and the recently discovered deoxyxylulose 5-phosphate (DXP) pathway. The evolutionary history of the enzymes involved in both routes and the phylogenetic distribution of their genes across genomes suggest that the mevalonate pathway is germane to archaebacteria, that the DXP pathway is germane to eubacteria, and that eukaryotes have inherited their genes for IPP biosynthesis from prokaryotes. The occurrence of genes specific to the DXP pathway is restricted to plastid-bearing eukaryotes, indicating that these genes were acquired from the cyanobacterial ancestor of plastids. However, the individual phylogenies of these genes, with only one exception, do not provide evidence for a specific affinity between the plant genes and their cyanobacterial homologues. The results suggest that lateral gene transfer between eubacteria subsequent to the origin of plastids has played a major role in the evolution of this pathway.chloroplast ͉ deoxyxylulose 5-phosphate ͉ endosymbiosis ͉ mevalonate ͉ phylogeny

The x-ray crystal structure of dimeric (؉)-bornyl diphosphate synthase, a metal-requiring monoterpene cyclase from Salvia officinalis, is reported at 2.0-Å resolution. Each monomer contains two ␣-helical domains: the C-terminal domain catalyzes the cyclization of geranyl diphosphate, orienting and stabilizing multiple reactive carbocation intermediates; the N-terminal domain has no clearly defined function, although its N terminus caps the active site in the C-terminal domain during catalysis. Structures of complexes with aza analogues of substrate and carbocation intermediates, as well as complexes with pyrophosphate and bornyl diphosphate, provide ''snapshots'' of the terpene cyclization cascade.