A (-)-kolavenyl diphosphate synthase catalyzes the first step of salvinorin A biosynthesis in Salvia divinorum

The influence of cultivation conditions on the formation of psychoactive salvinorin A, salvinorin B, rosmarinic acid and caffeic acid in Coleus scutellarioides

Coleus scutellarioides (L.) Benh. is a popular species in the world, known for its characteristic magnificent colourful leaves. The study has revealed that the contents of rosmarinic acid and caffeic acid are significantly higher in the plant tissues cultivated in vivo than when under in vitro conditions. The performed qualitative and quantitative analyses confirmed the presence (whose averaged content) of salvinorin A (6.65 µg/1 g of fresh plant) and salvinorin B (50.46 µg/1 g of fresh plant) in tissues of Coleus scutellarioides (L.) Benh. of 'Electric lime' variety. The greatest quantities of these compounds were recorded for plants cultivated in vitro on the MS medium enriched with NAA (naphthyl-1-acetic acid) at a concentration of 0.5 mg∙ dm-3. The research detected differences in the amounts of compounds between plants grown in vivo and those cultivated in vitro. Addition of plant growth regulators into the breeding medium under in vitro conditions was found affecting the amounts of compounds in plant tissues.

Advances in pharmacology, biosynthesis, and metabolic engineering of Scutellaria-specialized metabolites

Scutellaria Linn., which belongs to the family Lamiaceae, is a commonly used medicinal plant for heat clearing and detoxification. In particular, the roots of S. baicalensis and the entire herb of S. barbata have been widely used in traditional medicine for thousands of years. The main active components of Scutellaria, including: baicalein, wogonin, norwogonin, scutellarein, and their glycosides have potential or existing drug usage. However, the wild resources of Scutellaria plants have been overexploited, and degenerated germplasm resources cannot fulfill the requirements of chemical extraction and clinical usage. Metabolic engineering and green production via microorganisms provide alternative strategies for greater efficiency in the production of natural products. Here, we review the progress of: pharmacological investigations, multi-omics, biosynthetic pathways, and metabolic engineering of various Scutellaria species and their active compounds. In addition, based on multi-omics data, we systematically analyze the phylogenetic relationships of Scutellaria and predict candidate transcription factors related to the regulation of active flavonoids. Finally, we propose the prospects of directed evolution of core enzymes and genome-assisted breeding to alleviate the shortage of plant resources of Scutellaria. This review provides important insights into the sustainable utilization and development of Scutellaria resources.

Sequence-Structure Analysis Unlocking the Potential Functional Application of the Local 3D Motifs of Plant-Derived Diterpene Synthases

Plant-derived diterpene synthases (PdiTPSs) play a critical role in the formation of structurally and functionally diverse diterpenoids. However, the specificity or functional-related features of PdiTPSs are not well understood. For a more profound insight, we collected, constructed, and curated 199 functionally characterized PdiTPSs and their corresponding 3D structures. The complex correlations among their sequences, domains, structures, and corresponding products were comprehensively analyzed. Ultimately, our focus narrowed to the geometric arrangement of local structures. We found that local structural alignment can rapidly localize product-specific residues that have been validated by mutagenesis experiments. Based on the 3D motifs derived from the residues around the substrate, we successfully searched diterpene synthases (diTPSs) from the predicted terpene synthases and newly characterized PdiTPSs, suggesting that the identified 3D motifs can serve as distinctive signatures in diTPSs (I and II class). Local structural analysis revealed the PdiTPSs with more conserved amino acid residues show features unique to class I and class II, whereas those with fewer conserved amino acid residues typically exhibit product diversity and specificity. These results provide an attractive method for discovering novel or functionally equivalent enzymes and probing the product specificity in cases where enzyme characterization is limited.

Tandem duplication and sub-functionalization of clerodane diterpene synthase originate the blooming of clerodane diterpenoids in Scutellaria barbata

Scutellaria barbata is a traditional Chinese herb medicine and a major source of bioactive clerodane diterpenoids. However, barely clerodanes have been isolated from the closely related S. baicalensis. Here we assembled a chromosome-level genome of S. barbata and identified three class II clerodane diterpene synthases (SbarKPS1, SbarKPS2 and SbaiKPS1) from these two organisms. Using in vitro and in vivo assays, SbarKPS1 was characterized as a monofunctional (-)-kolavenyl diphosphate synthases ((-)-KPS), while SbarKPS2 and SbaiKPS1 produced major neo-cleroda-4(18),13E-dienyl diphosphate with small amount of (-)-KPP. SbarKPS1 and SbarKPS2 shared a high protein sequence identity and formed a tandem gene pair, indicating tandem duplication and sub-functionalization probably led to the evolution of monofunctional (-)-KPS in S. barbata. Additionally, SbarKPS1 and SbarKPS2 were primarily expressed in the leaves and flowers of S. barbata, which was consistent with the distribution of major clerodane diterpenoids scutebarbatine A and B. In contrast, SbaiKPS1 was barely expressed in any tissue of S. baicalensis. We further explored the downstream class I diTPS by functional characterizing of SbarKSL3 and SbarKSL4. Unfortunately, no dephosphorylated product was detected in the coupled assays with SbarKSL3/KSL4 and four class II diTPSs (SbarKPS1, SbarKPS2, SbarCPS2 and SbarCPS4) when a phosphatase inhibitor cocktail was included. Co-expression of SbarKSL3/KSL4 with class II diTPSs in yeast cells did not increase the yield of the corresponding dephosphorylated products, either. Together, these findings elucidated the involvement of two class II diTPSs in clerodane biosynthesis in S. barbata, while the class I diTPS is likely not responsible for the subsequent dephosphorylation step.

The genomes of medicinal skullcaps reveal the polyphyletic origins of clerodane diterpene biosynthesis in the family Lamiaceae

The presence of anticancer clerodane diterpenoids is a chemotaxonomic marker for the traditional Chinese medicinal plant Scutellaria barbata, although the molecular mechanisms behind clerodane biosynthesis are unknown. Here, we report a high-quality assembly of the 414.98 Mb genome of S. barbata into 13 pseudochromosomes. Using phylogenomic and biochemical data, we mapped the plastidial metabolism of kaurene (gibberellins), abietane, and clerodane diterpenes in three species of the family Lamiaceae (Scutellaria barbata, Scutellaria baicalensis, and Salvia splendens), facilitating the identification of genes involved in the biosynthesis of the clerodanes, kolavenol, and isokolavenol. We show that clerodane biosynthesis evolved through recruitment and neofunctionalization of genes from gibberellin and abietane metabolism. Despite the assumed monophyletic origin of clerodane biosynthesis, which is widespread in species of the Lamiaceae, our data show distinct evolutionary lineages and suggest polyphyletic origins of clerodane biosynthesis in the family Lamiaceae. Our study not only provides significant insights into the evolution of clerodane biosynthetic pathways in the mint family, Lamiaceae, but also will facilitate the production of anticancer clerodanes through future metabolic engineering efforts.

The sage genome provides insight into the evolutionary dynamics of diterpene biosynthesis gene cluster in plants

The widely cultivated medicinal and ornamental plant sage (Salvia officinalis L.) is an evergreen shrub of the Lamiaceae family, native to the Mediterranean. We assembled a high-quality sage genome of 480 Mb on seven chromosomes, and identified a biosynthetic gene cluster (BGC) encoding two pairs of diterpene synthases (diTPSs) that, together with the cytochromes P450 (CYPs) genes located inside and outside the cluster, form two expression cascades responsible for the shoot and root diterpenoids, respectively, thus extending BGC functionality from co-regulation to orchestrating metabolite production in different organs. Phylogenomic analysis indicates that the Salvia clades diverged in the early Miocene. In East Asia, most Salvia species are herbaceous and accumulate diterpenoids in storage roots. Notably, in Chinese sage S. miltiorrhiza, the diterpene BGC has contracted and the shoot cascade has been lost. Our data provide genomic insights of micro-evolution of growth type-associated patterning of specialized metabolite production in plants.

Biosynthesis and synthetic biology of psychoactive natural products

Psychoactive natural products play an integral role in the modern world. The tremendous structural complexity displayed by such molecules confers diverse biological activities of significant medicinal value and sociocultural impact. Accordingly, in the last two centuries, immense effort has been devoted towards establishing how plants, animals, and fungi synthesize complex natural products from simple metabolic precursors. The recent explosion of genomics data and molecular biology tools has enabled the identification of genes encoding proteins that catalyze individual biosynthetic steps. Once fully elucidated, the "biosynthetic pathways" are often comparable to organic syntheses in elegance and yield. Additionally, the discovery of biosynthetic enzymes provides powerful catalysts which may be repurposed for synthetic biology applications, or implemented with chemoenzymatic synthetic approaches. In this review, we discuss the progress that has been made toward biosynthetic pathway elucidation amongst four classes of psychoactive natural products: hallucinogens, stimulants, cannabinoids, and opioids. Compounds of diverse biosynthetic origin - terpene, amino acid, polyketide - are identified, and notable mechanisms of key scaffold transforming steps are highlighted. We also provide a description of subsequent applications of the biosynthetic machinery, with an emphasis placed on the synthetic biology and metabolic engineering strategies enabling heterologous production.

Antifeedant, cytotoxic, and anti-inflammatory neo-clerodane diterpenoids in the peltate glandular trichomes and fresh leaves of Ajuga forrestii

The Lamiaceae plant Ajuga forrestii Diels is a traditional Chinese herbal medicine with abundant glandular trichomes (GTs), but their chemistry and biological functions remain uninvestigated. Here, a panel of six highly functionalized neo-clerodane diterpenoids was localized to the peltate GTs of A. forrestii using laser microdissection coupled with HPLC analysis, indicating that the GTs of A. forrestii are an excellent material for the elucidation of the yet unclear biosynthetic pathway of natural neo-clerodane diterpenoids. In addition, four undescribed neo-clerodane diterpenoids with an acyclic C-9 side chain including two pairs of 1:1 mixture of inseparable diastereomers, ajuforrestins D-G, were isolated from the fresh leaves of A. forrestii together with six known compounds. The structures of the undescribed compounds were elucidated by spectroscopic (including 1D and 2D NMR and HR-ESI-MS) analyses. Biological assays indicated that the major GT compound ajugacumbin B and undescribed ajuforrestins D/E showed antifeedant activity against Helicoverpa armigera, suggesting that neo-clerodanes in A. forrestii should be involved in plant defence against insects. Moreover, the abietane diterpenoid ajuforrestin B exhibited significant anti-inflammatory activity on the secretion of interleukin-2 (IL-2) and cytotoxicity against three cancer cell lines, NCI-H1975, HepG2 and MCF-7, suggesting that ajuforrestin B could positively contribute to the therapeutic effects of this traditional Chinese medicine.

Recent progress and new perspectives for diterpenoid biosynthesis in medicinal plants

Diterpenoids, including more than 18,000 compounds, represent an important class of metabolites that encompass both phytohormones and some industrially relevant compounds. These molecules with complex, diverse structures and physiological activities, have high value in the pharmaceutical industry. Most medicinal diterpenoids are extracted from plants. Major advances in understanding the biosynthetic pathways of these active compounds are providing unprecedented opportunities for the industrial production of diterpenoids by metabolic engineering and synthetic biology. Here, we summarize recent developments in the field of diterpenoid biosynthesis from medicinal herbs. An overview of the pathways and known biosynthetic enzymes is presented. In particular, we look at the main findings from the past decade and review recent progress in the biosynthesis of different groups of ringed compounds. We also discuss diterpenoid production using synthetic biology and metabolic engineering strategies, and draw on new technologies and discoveries to bring together many components into a useful framework for diterpenoid production.

DARK Classics in Chemical Neuroscience: Salvinorin A

Salvinorin A is the main bioactive compound in Salvia divinorum, an endemic plant with ancestral use by the inhabitants of the Mazateca mountain range (Sierra Mazateca) in Oaxaca, México. The main use of la pastora, as locally known, is in spiritual rites due to its extraordinary hallucinogenic effects. Being the first known nonalkaloidal opioid-mediated psychotropic molecule, salvinorin A set new research areas in neuroscience. The absence of a protonated amine group, common to all previously known opioids, results in a fast metabolism with the concomitant fast elimination and swift loss of activity. The worldwide spread and psychotropic effects of salvinorin A account for its misuse and classification as a drug of abuse. Consequently, salvinorin A and Salvia divinorum are now banned in many countries. Several synthetic efforts have been focused on the improvement of physicochemical and biological properties of salvinorin A: from total synthesis to hundreds of analogues. In this Review, we discuss the impact of salvinorin A in chemistry and neuroscience covering the historical relevance, isolation from natural sources, synthetic efforts, and pharmacological and safety profiles. Altogether, the chemistry behind and the taboo that encloses salvinorin A makes it one of the most exquisite naturally occurring drugs.

Divergent Evolution of the Diterpene Biosynthesis Pathway in Tea Plants (Camellia sinensis) Caused by Single Amino Acid Variation of ent-Kaurene Synthase

Most plant terpenoids are classified as secondary metabolites. A small portion of them are products of primary metabolism biosynthesized by relatively conserved pathways. Gibberellins (GAs), which are essential for plant growth and development, are diterpenoid phytohormones. (E,E,E)-Geranylgeranyl diphosphate (GGPP) is the precursor for both GAs and other diterpenoids of secondary metabolism. ent-Kaurene biosynthesis from GGPP is a key step of GA formation, which is catalyzed by two sequential and dedicated diterpene synthases (diTPSs): ent-copalyl diphosphate synthase (CPS) and ent-kaurene synthase (KS) of the terpene synthase gene family. Sharing a common evolutionary origin, CPS and KS belong to different TPS subfamilies. Tea plant (Camellia sinensis), the subject of this study, is a leaf-based economic crop. Budbreak mainly manipulated by GAs is a primary factor for targeted tea breeding. The key genes for gibberellin biosynthesis are known; however, they have not yet been characterized in tea plants. Here, we identified and functionally characterized three diterpene biosynthesis-related genes, including one CPS and two highly similar KSs in tea plants. These genes were initially identified through transcriptome sequencing. The functional characterization determined by enzymatic activity assay indicated that CsCPS could catalyze GGPP to form ent-copalyl diphosphate (ent-CPP), which was further used as the substrate by CsKS1 to produce ent-kaurene or by CsKS2 to produce 16α-hydroxy-ent-kaurane with ent-kaurene as a minor product, respectively. We demonstrated that the divergent evolution of diterpene biosynthesis in tea plants resulted from gene duplication of KSs, followed by functional divergence caused by single amino acid variation. This study would provide an insight into the diterpenoid metabolism and GA biosynthesis in tea plants to further understand leaf bud development or insect resistance and to provide a genetic basis for tea plant breeding.

Generation of a chromosome-scale genome assembly of the insect-repellent terpenoid-producing Lamiaceae species, Callicarpa americana

BACKGROUND

Plants exhibit wide chemical diversity due to the production of specialized metabolites that function as pollinator attractants, defensive compounds, and signaling molecules. Lamiaceae (mints) are known for their chemodiversity and have been cultivated for use as culinary herbs, as well as sources of insect repellents, health-promoting compounds, and fragrance.

FINDINGS

We report the chromosome-scale genome assembly of Callicarpa americana L. (American beautyberry), a species within the early-diverging Callicarpoideae clade of Lamiaceae, known for its metallic purple fruits and use as an insect repellent due to its production of terpenoids. Using long-read sequencing and Hi-C scaffolding, we generated a 506.1-Mb assembly spanning 17 pseudomolecules with N50 contig and N50 scaffold sizes of 7.5 and 29.0 Mb, respectively. In all, 32,164 genes were annotated, including 53 candidate terpene synthases and 47 putative clusters of specialized metabolite biosynthetic pathways. Our analyses revealed 3 putative whole-genome duplication events, which, together with local tandem duplications, contributed to gene family expansion of terpene synthases. Kolavenyl diphosphate is a gateway to many of the bioactive terpenoids in C. americana; experimental validation confirmed that CamTPS2 encodes kolavenyl diphosphate synthase. Syntenic analyses with Tectona grandis L. f. (teak), a member of the Tectonoideae clade of Lamiaceae known for exceptionally strong wood resistant to insects, revealed 963 collinear blocks and 21,297 C. americana syntelogs.

CONCLUSIONS

Access to the C. americana genome provides a road map for rapid discovery of genes encoding plant-derived agrichemicals and a key resource for understanding the evolution of chemical diversity in Lamiaceae.

More is better: the diversity of terpene metabolism in plants

All plants synthesize a diverse array of terpenoid metabolites. Some are common to all, but many are synthesized only in specific taxa and presumably evolved as adaptations to specific ecological conditions. While the basic terpenoid biosynthetic pathways are common in all plants, recent discoveries have revealed many variations in the way plants synthesized specific terpenes. A major theme is the much greater number of substrates that can be used by enzymes belonging to the terpene synthase (TPS) family. Other recent discoveries include non-TPS enzymes that catalyze the formation of terpenes, and novel transport mechanisms.

Diterpene synthases facilitating production of the kaurane skeleton of eriocalyxin B in the medicinal plant Isodon eriocalyx

The Isodon plants (Lamiaceae) have been used in traditional Chinese medicine to alleviate sufferings from inflammations and cancers. This feature has been attributed to the presence of pharmacologically active ent-kaurane diterpenoids such as eriocalyxin B and oridonin. The Isodon eriocalyx (Dunn) Kudô species native to southwest China can accumulate a particularly high content of ent-kaurane diterpenoids (∼1.5% w/w of dried leaves). We previously identified diterpene synthases IeCPS1 and IeCPS2 as ent-copalyl diphosphate synthases (ent-CPS) potentially involved in Isodon ent-kaurane diterpenoids biosynthesis. In this study, analysis of RNA-seq transcriptome of the I. eriocalyx plant revealed three other diterpene synthase genes (IeCPS3, IeKS1, and IeKSL1). Their functional characterization through coupled in vitro enzyme assays has confirmed that IeCPS3 is an ent-CPS specifically producing ent-copalyl diphosphate (ent-CPP). IeKS1 accepted ent-CPP to produce exclusively ent-kaurene and may thus be defined as an ent-kaurene synthase (ent-KS). When IeKSL1 was combined with IeCPS2 or IeCPS3, no product was detected. Based on tissue-specific expression and metabolic localization studies, the IeCPS3 and IeKS1 transcripts were significantly accumulated in leaves where the ent-kaurane diterpenoid eriocalyxin B dominates, whereas weak expression of both were observed in germinating seeds in which gibberellin biosynthetic pathway is normally active. Our findings suggest that both IeCPS3 and IeKS1 possess dual roles in general (gibberellins) and specialized diterpenoid metabolism, such as that of the Isodon ent-kaurane diterpenoids.

A database-driven approach identifies additional diterpene synthase activities in the mint family (Lamiaceae)

Members of the mint family (Lamiaceae) accumulate a wide variety of industrially and medicinally relevant diterpenes. We recently sequenced leaf transcriptomes from 48 phylogenetically diverse Lamiaceae species. Here, we summarize the available chemotaxonomic and enzyme activity data for diterpene synthases (diTPSs) in the Lamiaceae and leverage the new transcriptomes to explore the diTPS sequence and functional space. Candidate genes were selected with an intent to evenly sample the sequence homology space and to focus on species in which diTPS transcripts were found, yet from which no diterpene structures have been previously reported. We functionally characterized nine class II diTPSs and 10 class I diTPSs from 11 distinct plant species and found five class II activities, including two novel activities, as well as a spectrum of class I activities. Among the class II diTPSs, we identified a neo-cleroda-4(18),13E-dienyl diphosphate synthase from Ajuga reptans, catalyzing the likely first step in the biosynthesis of a variety of insect-antifeedant compounds. Among the class I diTPSs was a palustradiene synthase from Origanum majorana, leading to the discovery of specialized diterpenes in that species. Our results provide insights into the diversification of diterpene biosynthesis in the mint family and establish a comprehensive foundation for continued investigation of diterpene biosynthesis in the Lamiaceae.

Diterpenoids of terrestrial origin

This review covers the isolation and chemistry of diterpenoids from terrestrial sources from 2017.

Terpene Synthases as Metabolic Gatekeepers in the Evolution of Plant Terpenoid Chemical Diversity

Terpenoids comprise tens of thousands of small molecule natural products that are widely distributed across all domains of life. Plants produce by far the largest array of terpenoids with various roles in development and chemical ecology. Driven by selective pressure to adapt to their specific ecological niche, individual species form only a fraction of the myriad plant terpenoids, typically representing unique metabolite blends. Terpene synthase (TPS) enzymes are the gatekeepers in generating terpenoid diversity by catalyzing complex carbocation-driven cyclization, rearrangement, and elimination reactions that enable the transformation of a few acyclic prenyl diphosphate substrates into a vast chemical library of hydrocarbon and, for a few enzymes, oxygenated terpene scaffolds. The seven currently defined clades (a-h) forming the plant TPS family evolved from ancestral triterpene synthase- and prenyl transferase-type enzymes through repeated events of gene duplication and subsequent loss, gain, or fusion of protein domains and further functional diversification. Lineage-specific expansion of these TPS clades led to variable family sizes that may range from a single TPS gene to families of more than 100 members that may further function as part of modular metabolic networks to maximize the number of possible products. Accompanying gene family expansion, the TPS family shows a profound functional plasticity, where minor active site alterations can dramatically impact product outcome, thus enabling the emergence of new functions with minimal investment in evolving new enzymes. This article reviews current knowledge on the functional diversity and molecular evolution of the plant TPS family that underlies the chemical diversity of bioactive terpenoids across the plant kingdom.

Non-volatile natural products in plant glandular trichomes: chemistry, biological activities and biosynthesis

The investigation methods, chemistry, bioactivities, and biosynthesis of non-volatile natural products involving 489 compounds in plant glandular trichomes are reviewed.

Transcriptomic Insight into Terpenoid Biosynthesis and Functional Characterization of Three Diterpene Synthases in Scutellaria barbata

Scutellaria barbata (Lamiaceae) is an important medicinal herb widely used in China, Korea, India, and other Asian countries. Neo-clerodane diterpenoids are the largest known group of Scutellaria diterpenoids and show promising cytotoxic activity against several cancer cell lines. Here, Illumina-based deep transcriptome analysis of flowers, the aerial parts (leaf and stem), and roots of S. barbata was used to explore terpenoid-related genes. In total, 121,958,564 clean RNA-sequence reads were assembled into 88,980 transcripts, with an average length of 1370 nt and N50 length of 2144 nt, indicating high assembly quality. We identified nearly all known terpenoid-related genes (33 genes) involved in biosynthesis of the terpenoid backbone and 14 terpene synthase genes which generate skeletons for different terpenoids. Three full length diterpene synthase genes were functionally identified using an in vitro assay. SbTPS8 and SbTPS9 were identified as normal-CPP and ent-CPP synthase, respectively. SbTPS12 reacts with SbTPS8 to produce miltiradiene. Furthermore, SbTPS12 was proven to be a less promiscuous class I diterpene synthase. These results give a comprehensive understanding of the terpenoid biosynthesis in S. barbata and provide useful information for enhancing the production of bioactive neo-clerodane diterpenoids through genetic engineering.

Biosynthesis of bioactive diterpenoids in the medicinal plant Vitex agnus-castus

Vitex agnus-castus L. (Lamiaceae) is a medicinal plant historically used throughout the Mediterranean region to treat menstrual cycle disorders, and is still used today as a clinically effective treatment for premenstrual syndrome. The pharmaceutical activity of the plant extract is linked to its ability to lower prolactin levels. This feature has been attributed to the presence of dopaminergic diterpenoids that can bind to dopamine receptors in the pituitary gland. Phytochemical analyses of V. agnus-castus show that it contains an enormous array of structurally related diterpenoids and, as such, holds potential as a rich source of new dopaminergic drugs. The present work investigated the localisation and biosynthesis of diterpenoids in V. agnus-castus. With the assistance of matrix-assisted laser desorption ionisation-mass spectrometry imaging (MALDI-MSI), diterpenoids were localised to trichomes on the surface of fruit and leaves. Analysis of a trichome-specific transcriptome database, coupled with expression studies, identified seven candidate genes involved in diterpenoid biosynthesis: three class II diterpene synthases (diTPSs); three class I diTPSs; and a cytochrome P450 (CYP). Combinatorial assays of the diTPSs resulted in the formation of a range of different diterpenes that can account for several of the backbones of bioactive diterpenoids observed in V. agnus-castus. The identified CYP, VacCYP76BK1, was found to catalyse 16-hydroxylation of the diol-diterpene, peregrinol, to labd-13Z-ene-9,15,16-triol when expressed in Saccharomyces cerevisiae. Notably, this product is a potential intermediate in the biosynthetic pathway towards bioactive furan- and lactone-containing diterpenoids that are present in this species.

Structural and Chemical Biology of Terpenoid Cyclases

The year 2017 marks the twentieth anniversary of terpenoid cyclase structural biology: a trio of terpenoid cyclase structures reported together in 1997 were the first to set the foundation for understanding the enzymes largely responsible for the exquisite chemodiversity of more than 80000 terpenoid natural products. Terpenoid cyclases catalyze the most complex chemical reactions in biology, in that more than half of the substrate carbon atoms undergo changes in bonding and hybridization during a single enzyme-catalyzed cyclization reaction. The past two decades have witnessed structural, functional, and computational studies illuminating the modes of substrate activation that initiate the cyclization cascade, the management and manipulation of high-energy carbocation intermediates that propagate the cyclization cascade, and the chemical strategies that terminate the cyclization cascade. The role of the terpenoid cyclase as a template for catalysis is paramount to its function, and protein engineering can be used to reprogram the cyclization cascade to generate alternative and commercially important products. Here, I review key advances in terpenoid cyclase structural and chemical biology, focusing mainly on terpenoid cyclases and related prenyltransferases for which X-ray crystal structures have informed and advanced our understanding of enzyme structure and function.