Two Bacterial Diterpene Synthases from Allokutzneria albata Produce Bonnadiene, Phomopsene, and Allokutznerene

Mechanistic characterisation of the diterpene synthase for clitopilene and identification of isopentalenene synthase from the fungus Clitopilus passeckerianus

Two terpene synthases from the pleuromutilin producing fungus Clitopilus passeckerianus were functionally characterised. The first enzyme CpTS1 produces the new diterpene clitopilene with a novel 6-6-5-5 tetracyclic skeleton, while the second enzyme CpTS2 makes the new sesquiterpene isopentalenene. The CpTS1 reaction mechanism was studied in depth using experimental and theoretical approaches.

Structural Insights Into the Terpene Cyclization Domains of Two Fungal Sesterterpene Synthases and Enzymatic Engineering for Sesterterpene Diversification

Little is known about the structures and catalytic mechanisms of sesterterpene synthases (StTSs), which greatly hinders the structure-based engineering of StTSs for structural diversity expansion of sesterterpenes. We here report on the crystal structures of the terpene cyclization (TC) domains of two fungal StTSs: sesterfisherol synthase (NfSS) and sesterbrasiliatriene synthase (PbSS). Both TC structures contain benzyltriethylammonium chloride (BTAC), pyrophosphate (PPi), and magnesium ions (Mg2+), clearly defining the catalytic active sites. A combination of theory and experiments including carbocationic intermediates modeling, site-directed mutagenesis, and isotope labeling provided detailed insights into the structural basis for their catalytic mechanisms. Structure-based engineering of NfSS and PbSS resulted in the formation of 20 sesterterpenes including 13 new compounds and four pairs of epimers with different configurations at C18. These results expand the structural diversity of sesterterpenes and provide important insights for future synthetic biology research.

Two Sesterterpene Synthases from Lentzea atacamensis Demonstrate the Role of Conformational Variability in Terpene Biosynthesis

Mining of two multiproduct sesterterpene synthases from Lentzea atacamensis resulted in the identification of the synthases for lentzeadiene (LaLDS) and atacamatriene (LaATS). The main product of LaLDS (lentzeadiene) is a new compound, while one of the side products (lentzeatetraene) is the enantiomer of brassitetraene B and the other side product (sestermobaraene F) is known from a surprisingly distantly related sesterterpene synthase. LaATS produces six new compounds, one of which is the enantiomer of the known sesterterpene Bm1. Notably, for both enzymes the products cannot all be explained from one and the same starting conformation of geranylfarnesyl diphosphate, demonstrating the requirement of conformational flexibility of the substrate in the enzymes' active sites. For lentzeadiene an intriguing thermal [1,5]-sigmatropic rearrangement was discovered, reminiscent of the biosynthesis of vitamin D3. All enzyme reactions and the [1,5]-sigmatropic rearrangement were investigated through isotopic labeling experiments and DFT calculations. The results also emphasize the importance of conformational changes during terpene cyclizations.

Confirmation of the stereochemistry of spiroviolene

We confirm the previously revised stereochemistry of spiroviolene by X-ray crystallographically characterizing a hydrazone derivative of 9-oxospiroviolane, which is synthesized by hydroboration/oxidation of spiroviolene followed by oxidation of the resultant hydroxy group. An unexpected thermal boron migration occurred during the hydroboration process of spiroviolene that resulted in the production of a mixture of 1α-hydroxyspiroviolane, 9α- and 9β-hydroxyspiroviolane after oxidation. The assertion of the cis-orientation of the 19- and 20-methyl groups provided further support for the revised cyclization mechanism of spiroviolene.

Functional and Mechanistic Characterization of the 4,5-diepi-Isoishwarane Synthase from the Liverwort Radula lindenbergiana

The microbial type sesquiterpene synthase RlMTPSL4 from the liverwort Radula lindenbergiana was investigated for its products, showing the formation of several sesquiterpene hydrocarbons. The main product was structurally characterized as the new compound 4,5-diepi-isoishwarane, while the side products included the known hydrocarbons germacrene A, α-selinene, eremophilene and 4,5-diepi-aristolochene. The cyclization mechanism towards 4,5-diepi-isoishwarane catalyzed by RlMTPSL4 was investigated through isotopic labeling experiments, revealing the stereochemical course for the deprotonation step to the neutral intermediate germacrene A, a reprotonation for its further cyclization, and a 1,2-hydride shift along the cascade. The absolute configuration of 4,5-diepi-isoishwarane was determined using a stereoselective deuteration approach, revealing an absolute configuration typically observed for a microbial type sesquiterpene.

Brewing coral terpenes-A yeast based approach to soft coral terpene cyclases

Coral terpenes are important molecules with numerous applications. Here, we describe a robust and simple method to produce coral terpene scaffolds at scale. As an example of the approach, here we discover, express, and characterize further klysimplexin R synthases, expanding the known enzymology of soft coral terpene cyclases. We hope that the underlying method described will enable widespread basic research into the functions of coral terpenes and their biosynthetic genes, as well as the commercial development of biomedically and technologically important molecules.

Catalytic Mechanism and Heterologous Biosynthesis Application of Sesquiterpene Synthases

Sesquiterpenes comprise a diverse group of natural products with a wide range of applications in cosmetics, food, medicine, agriculture, and biofuels. Heterologous biosynthesis is increasingly employed for sesquiterpene production, aiming to overcome the limitations associated with chemical synthesis and natural extraction. Sesquiterpene synthases (STSs) play a crucial role in the heterologous biosynthesis of sesquiterpene. Under the catalysis of STSs, over 300 skeletons are produced through various cyclization processes (C1-C10 closure, C1-C11 closure, C1-C6 closure, and C1-C7 closure), which are responsible for the diversity of sesquiterpenes. According to the cyclization types, we gave an overview of advances in understanding the mechanism of STSs cyclization from the aspects of protein crystal structures and site-directed mutagenesis. We also summarized the applications of engineering STSs in the heterologous biosynthesis of sesquiterpene. Finally, the bottlenecks and potential research directions related to the STSs cyclization mechanism and application of modified STSs were presented.

Mechanistic characterisation of a sesquiterpene synthase for asterisca-1,6-diene from the liverwort Radula lindenbergiana and implications for pentalenene biosynthesis

A sesquiterpene synthase from the liverwort Radula lindenbergiana was characterised and shown to produce the new sesquiterpene hydrocarbon (3R,9R)-asterisca-1,6-diene, besides small amounts of pentalenene. The biosynthesis of asterisca-1,6-diene was studied through isotopic labelling experiments, giving additional insights into the long discussed biosynthesis of pentalenene.

Enzymatic Synthesis of Diterpenoids from iso-GGPP III: A Geranylgeranyl Diphosphate Analog with a Shifted Double Bond

The analog of the diterpene precursor geranylgeranyl diphosphate with a double bond shifted from C14=C15 to C15=C16 (named iso-GGPP III) has been synthesized and enzymatically converted with six bacterial diterpene synthases; this allowed the isolation of nine unnatural diterpenes. For some of the enzyme-substrate combinations, the different reactivity implemented in the substrate analog iso-GGPP III opened reaction pathways that are not observed with natural GGPP, resulting in the formation of diterpenes with novel skeletons. A stereoselective deuteration strategy was used to assign the absolute configurations of the isolated diterpenes.

Isotopic labelings for mechanistic studies

The intricate mechanisms in the biosynthesis of terpenes belong to the most challenging problems in natural product chemistry. Methods to address these problems include the structure-based site-directed mutagenesis of terpene synthases, computational approaches, and isotopic labeling experiments. The latter approach has a long tradition in biosynthesis studies and has recently experienced a revival, after genome sequencing enabled rapid access to biosynthetic genes and enzymes. Today, this allows for a combined approach in which isotopically labeled substrates can be incubated with recombinant terpene synthases. These clearly defined reaction setups can give detailed mechanistic insights into the reactions catalyzed by terpene synthases, and recent developments have substantially deepened our understanding of terpene biosynthesis. This chapter will discuss the state of the art and introduce some of the most important methods that make use of isotopic labelings in mechanistic studies on terpene synthases.

The Diterpenoid Substrate Analogue 19-nor-GGPP Reveals Pronounced Methyl Group Effects in Diterpene Cyclisations

The substrate analogue 19-nor-geranylgeranyl diphosphate (19-nor-GGPP) was synthesised and incubated with 20 diterpene synthases, resulting in the formation of diterpenoids in all cases. A total of 23 different compounds were isolated from these enzyme reactions and structurally characterised, if possible including the experimental determination of absolute configurations through a stereoselective deuteration approach. In several cases the missing 19-Me group in the substrate analogue resulted in opening of completely new reaction paths towards compounds with novel skeletons. DFT calculations were applied to gain a deeper understanding of these observed methyl group effects in diterpene biosynthesis.

Computational prediction of complex cationic rearrangement outcomes

Recent years have seen revived interest in computer-assisted organic synthesis1,2. The use of reaction- and neural-network algorithms that can plan multistep synthetic pathways have revolutionized this field1,3-7, including examples leading to advanced natural products6,7. Such methods typically operate on full, literature-derived 'substrate(s)-to-product' reaction rules and cannot be easily extended to the analysis of reaction mechanisms. Here we show that computers equipped with a comprehensive knowledge-base of mechanistic steps augmented by physical-organic chemistry rules, as well as quantum mechanical and kinetic calculations, can use a reaction-network approach to analyse the mechanisms of some of the most complex organic transformations: namely, cationic rearrangements. Such rearrangements are a cornerstone of organic chemistry textbooks and entail notable changes in the molecule's carbon skeleton8-12. The algorithm we describe and deploy at https://HopCat.allchemy.net/ generates, within minutes, networks of possible mechanistic steps, traces plausible step sequences and calculates expected product distributions. We validate this algorithm by three sets of experiments whose analysis would probably prove challenging even to highly trained chemists: (1) predicting the outcomes of tail-to-head terpene (THT) cyclizations in which substantially different outcomes are encoded in modular precursors differing in minute structural details; (2) comparing the outcome of THT cyclizations in solution or in a supramolecular capsule; and (3) analysing complex reaction mixtures. Our results support a vision in which computers no longer just manipulate known reaction types1-7 but will help rationalize and discover new, mechanistically complex transformations.

Spiroluchuene A Synthase: A Cyclase from Aspergillus luchuensis Forming a Spirotetracyclic Diterpene

The diterpene synthase AlTS was identified from Aspergillus luchuensis. AlTS catalyses the formation of the diterpene hydrocarbon spiroluchuene A, which exhibits a novel skeleton characterised by a spirocyclic ring system. The cyclisation mechanism towards this compound was elucidated through isotopic labelling experiments in conjunction with DFT calculations and metadynamic simulations. The biosynthetic intermediate luchudiene, besides the derivative spiroluchuene B, was captured from an enzyme variant obtained through site-directed mutagenesis. With its 10-membered ring luchudiene is structurally related to germacrenes and can undergo a Cope rearrangement to luchuelemene.

A Case of Convergent Evolution: The Bacterial Sesquiterpene Synthase for 1-epi-Cubenol from Nonomuraea coxensis

A terpene synthase from Nonomuraea coxensis was identified as (+)-1-epi-cubenol synthase. The enzyme is phylogenetically unrelated to the known enzyme of the same function that is widespread in streptomycetes. Isotopic labelling experiments were performed to unambiguously assign the NMR data and to investigate hydrogen migrations during terpene cyclisations. Epoxidations of (+)-1-epi-cubenol and of the plant derived compounds (-)-cubenol and (-)-1-epi-cubenol confirmed the structure of a natural product isolated from the brown alga Dictyopteris divaricata and allowed to conclude on its absolute configuration. The crystal structures of the epoxides from (+)- and (-)-1-epi-cubenol and the acid catalysed conversion into an isomeric ketone are reported.

Subrutilane-A Hexacyclic Sesterterpene from Streptomyces subrutilus

Mining of a terpene synthase from Streptomyces subrutilus resulted in the identification of the hexacyclic sesterterpene subrutilane, besides eight pentacyclic side products. Subrutilane represents the first case of a saturated sesterterpene hydrocarbon. Its structure, including the absolute configuration, was unambiguously determined through X-ray crystallographic analysis and stereoselective deuteration. The cyclisation mechanism to subrutilane and its side products was investigated in all detail by isotopic labelling experiments and DFT calculations. The subrutilane synthase (SrS) also converted (2Z)-GFPP into one major product. Additional compounds were obtained from the substrate analogues (7R)-6,7-dihydro-GFPP and (2Z,7R)-6,7-dihydro-GFPP with blocked reactivity at the C6-C7 bond. Interestingly, the early steps of the cyclisation cascade with (2Z)-GFPP and the saturated substrate analogues were analogous to those of GFPP, but then deviations from the natural cyclisation mode occur.

Mechanistic Characterisation of Collinodiene Synthase, a Diterpene Synthase from Streptomyces collinus

Two homologs of the diterpene synthase CotB2 from Streptomyces collinus (ScCotB2) and Streptomyces iakyrus (SiCotB2) were investigated for their products by in vitro incubations of the recombinant enzymes with geranylgeranyl pyrophosphate, followed by compound isolation and structure elucidation by NMR. ScCotB2 produced the new compound collinodiene, besides the canonical CotB2 product cyclooctat-9-en-7-ol, dolabella-3,7,18-triene and dolabella-3,7,12-triene, while SiCotB2 gave mainly cyclooctat-9-en-7-ol and only traces of dolabella-3,7,18-triene. The cyclisation mechanism towards the ScCotB2 products and their absolute configurations were investigated through isotopic labelling experiments.

Total Syntheses of Polycyclic Diterpenes Phomopsene, Methyl Phomopsenonate, and iso-Phomopsene via Reorganization of C-C Single Bonds

The first total syntheses of polycyclic diterpenes phomopsene (1), methyl phomopsenonate (2), and iso-phomopsene (3) have been accomplished through the unusual cascade reorganization of C-C single bonds. This approach features: (i) a synergistic Nazarov cyclization/double ring expansions in one-step, developed by authors, to rapid and stereospecific construction of the 5/5/5/5 tetraquinane scaffold bearing contiguous quaternary centers and (ii) a one-pot strategic ring expansion through Beckmann fragmentation/recombination to efficiently assemble the requisite 5/5/6/5 tetracyclic skeleton of the target molecules 1-3. This work enables us to determine that the correct structure of iso-phomopsene is, in fact, the C7 epimer of the originally assigned structure. Finally, the absolute configurations of three target molecules were confirmed through enantioselective synthesis.

Functional characterisation of twelve terpene synthases from actinobacteria

Fifteen type I terpene synthase homologs from diverse actinobacteria that were selected based on a phylogenetic analysis of more than 4000 amino acid sequences were investigated for their products. For four enzymes with functions not previously reported from bacterial terpene synthases the products were isolated and their structures were elucidated by NMR spectroscopy, resulting in the discovery of the first terpene synthases for (+)-δ-cadinol and (+)-α-cadinene, besides the first two bacterial (-)-amorpha-4,11-diene synthases. For other terpene synthases with functions reported from bacteria before the products were identified by GC-MS. The characterised enzymes include a new epi-isozizaene synthase with monoterpene synthase side activity, a 7-epi-α-eudesmol synthase that also produces hedycaryol and germacrene A, and four more sesquiterpene synthases that produce mixtures of hedycaryol and germacrene A. Three phylogenetically related enzymes were in one case not expressed and in two cases inactive, suggesting pseudogenisation in the respective branch of the phylogenetic tree. Furthermore, a diterpene synthase for allokutznerene and a sesterterpene synthase for sesterviolene were identified.

Actinomycetes as Producers of Biologically Active Terpenoids: Current Trends and Patents

Terpenes and their derivatives (terpenoids and meroterpenoids, in particular) constitute the largest class of natural compounds, which have valuable biological activities and are promising therapeutic agents. The present review assesses the biosynthetic capabilities of actinomycetes to produce various terpene derivatives; reports the main methodological approaches to searching for new terpenes and their derivatives; identifies the most active terpene producers among actinomycetes; and describes the chemical diversity and biological properties of the obtained compounds. Among terpene derivatives isolated from actinomycetes, compounds with pronounced antifungal, antiviral, antitumor, anti-inflammatory, and other effects were determined. Actinomycete-produced terpenoids and meroterpenoids with high antimicrobial activity are of interest as a source of novel antibiotics effective against drug-resistant pathogenic bacteria. Most of the discovered terpene derivatives are produced by the genus Streptomyces; however, recent publications have reported terpene biosynthesis by members of the genera Actinomadura, Allokutzneria, Amycolatopsis, Kitasatosporia, Micromonospora, Nocardiopsis, Salinispora, Verrucosispora, etc. It should be noted that the use of genetically modified actinomycetes is an effective tool for studying and regulating terpenes, as well as increasing productivity of terpene biosynthesis in comparison with native producers. The review includes research articles on terpene biosynthesis by Actinomycetes between 2000 and 2022, and a patent analysis in this area shows current trends and actual research directions in this field.

Isolation and structure determination of allopteridic acids A-C and allokutzmicin from an unexplored actinomycete of the genus Allokutzneria

Two classes of new polyketides, allopteridic acids A-C (1-3) and allokutzmicin (4), were isolated from the culture extract of an actinomycete of the genus Allokutzneria. The structures of 1-4 were elucidated through the interpretation of NMR and MS analytical data. Compounds 1-3 possess the same carbon skeleton with pteridic acids but their monocyclic core structures are distinct from the spiro-bicyclic acetal structures of pteridic acids. Compound 4 is a linear polyketide of an unprecedented class, featured by a guanidino-terminus and an epoxide modification. Compounds 1-3 promoted the root elongation of germinated lettuce seeds by ca. 10-40% at 1~10 μM whereas 4 retarded the seed growth. Compound 4 exhibited weak antimicrobial activity against Candida albicans with MIC 25 μg mL^-1.

Synthesis of Spirotricyclic Core of Bonnadiene

We herein describe a new approach for the efficient synthesis of the tricyclic core of diterpene bonnadiene. The synthetically challenging and unusual [6-7-5] spirotricyclic skeleton including the all-carbon quaternary stereocenter, was installed diastereoselectively via a type II [5 + 2] cycloaddition, followed by a unique vinylogous semipinacol rearrangement. The described chemistry demonstrates the feasibility of making the [6-7-5] spirotricyclic skeleton of the final product from the strained bridged [7-8-5] ring system.

Mechanistic Characterisation and Engineering of Sesterviolene Synthase from Streptomyces violens

The sesterviolene synthase from Streptomyces violens was identified and represents the second known sesterterpene synthase from bacteria. Isotopic labelling experiments in conjunction with DFT calculations were performed that provided detailed insight into its complex cyclisation mechanism. Enzyme engineering through site-directed mutagenesis gave access to a high-yielding enzyme variant that provided six additional minor products and the main product in sufficient quantities to study its chemistry.

Diterpene Biosynthesis from Geranylgeranyl Diphosphate Analogues with Changed Reactivities Expands Skeletal Diversity

Two analogues of the diterpene precursor geranylgeranyl diphosphate with shifted double bonds, named iso-GGPP I and iso-GGPP II, were enzymatically converted with twelve diterpene synthases from bacteria, fungi and protists. The changed reactivity in the substrate analogues resulted in the formation of 28 new diterpenes, many of which exhibit novel skeletons.

Research Progress on Fungal Sesterterpenoids Biosynthesis

Sesterterpenes are 25-carbon terpenoids formed by the cyclization of dimethyl allyl diphosphate (DMAPP) and isopentenyl diphosphate (IPP) as structural units by sesterterpenes synthases. Some (not all) sesterterpenoids are modified by cytochrome P450s (CYP450s), resulting in more intricate structures. These compounds have significant physiological activities and pharmacological effects in anti-inflammatory, antibacterial, antitumour, and hypolipidemic communities. Despite being a rare class of terpenoids, sesterterpenoids derived from fungi show a wide range of structural variations. The discovered fungal sesterterpenoid synthases are composed of C-terminal prenyltransferase (PT) and N-terminal terpene synthase (TS) domains, which were given the name PTTSs. PTTSs have the capacities to catalyze chain lengthening and cyclization concurrently. This review summarizes all 52 fungal PTTSs synthases and their biosynthetic pathways involving 100 sesterterpenoids since the discovery of the first PTTSs synthase from fungi in 2013.

Discovery of non-squalene triterpenes

All known triterpenes are generated by triterpene synthases (TrTSs) from squalene or oxidosqualene¹. This approach is fundamentally different from the biosynthesis of short-chain (C₁₀–C₂₅) terpenes that are formed from polyisoprenyl diphosphates^2–4. In this study, two fungal chimeric class I TrTSs, Talaromyces verruculosus talaropentaene synthase (TvTS) and Macrophomina phaseolina macrophomene synthase (MpMS), were characterized. Both enzymes use dimethylallyl diphosphate and isopentenyl diphosphate or hexaprenyl diphosphate as substrates, representing the first examples, to our knowledge, of non-squalene-dependent triterpene biosynthesis. The cyclization mechanisms of TvTS and MpMS and the absolute configurations of their products were investigated in isotopic labelling experiments. Structural analyses of the terpene cyclase domain of TvTS and full-length MpMS provide detailed insights into their catalytic mechanisms. An AlphaFold2-based screening platform was developed to mine a third TrTS, Colletotrichum gloeosporioides colleterpenol synthase (CgCS). Our findings identify a new enzymatic mechanism for the biosynthesis of triterpenes and enhance understanding of terpene biosynthesis in nature.

Chimeric triterpene synthases are identified that catalyse non-squalene-dependent triterpene biosynthesis.

Mechanism of the Bifunctional Multiple Product Sesterterpene Synthase AcAS from Aspergillus calidoustus

The multiproduct chimeric sesterterpene synthase AcAS from Aspergillus calidoustus yielded spirocyclic calidoustene, which exhibits a novel skeleton, besides five known sesterterpenes. The complex cyclisation mechanism to all six compounds was investigated by isotopic labelling experiments in combination with DFT calculations. Chemically synthesised 8-hydroxyfarnesyl diphosphate was converted with isopentenyl diphosphate and AcAS into four oxygenated sesterterpenoids that structurally resemble cytochrome P450 oxidation products of the sesterterpene hydrocarbons. Protein engineering of AcAS broadened the substrate scope and gave significantly improved enzyme yields.

Enzymatic Synthesis of Variediene Analogs

Five analogs of dimethylallyl diphosphate (DMAPP) with additional or shifted Me groups were converted with isopentenyl diphosphate (IPP) and the fungal variediene synthase from Aspergillus brasiliensis (AbVS). These enzymatic reactions resulted in the formation of several new terpene analogs that were isolated and structurally characterised by NMR spectroscopy. Several DMAPP analogs showed a changed reactivity giving access to compounds with unusual skeletons. Their formation is mechanistically rationalised and the absolute configurations of all obtained compounds were determined through a stereoselective deuteration strategy, revealing absolute configurations that are analogous to that of the natural enzyme product variediene.

Diterpenes Specially Produced by Fungi: Structures, Biological Activities, and Biosynthesis (2010–2020)

Fungi have traditionally been a very rewarding source of biologically active natural products, while diterpenoids from fungi, such as the cyathane-type diterpenoids from Cyathus and Hericium sp., the fusicoccane-type diterpenoids from Fusicoccum and Alternaria sp., the guanacastane-type diterpenoids from Coprinus and Cercospora sp., and the harziene-type diterpenoids from Trichoderma sp., often represent unique carbon skeletons as well as diverse biological functions. The abundances of novel skeletons, biological activities, and biosynthetic pathways present new opportunities for drug discovery, genome mining, and enzymology. In addition, diterpenoids peculiar to fungi also reveal the possibility of differing biological evolution, although they have similar biosynthetic pathways. In this review, we provide an overview about the structures, biological activities, evolution, organic synthesis, and biosynthesis of diterpenoids that have been specially produced by fungi from 2010 to 2020. We hope this review provides timely illumination and beneficial guidance for future research works of scholars who are interested in this area.

Isotopic Labeling Experiments Solve the Hedycaryol Problem

Hedycaryol is a widespread sesquiterpene alcohol and important biosynthetic intermediate toward eudesmols and guaiols. A full NMR assignment for this compound has been hampered because of the unique molecular mechanics of its conformers in complex mixtures. This problem was solved through the enzymatic synthesis of isotopically labeled materials using a mutated plant and a bacterial enzyme for access to both enantiomers of hedycaryol, which also allowed us to follow the stereochemical course of its Cope rearrangement.

The enzyme mechanism of patchoulol synthase

Different mechanisms for the cyclisation of farnesyl pyrophosphate to patchoulol by the patchoulol synthase are discussed in the literature. They are based on isotopic labelling experiments, but the results from these experiments are contradictory. The present work reports on a reinvestigation of patchoulol biosynthesis by isotopic labelling experiments and computational chemistry. The results are in favour of a pathway through the neutral intermediates germacrene A and α-bulnesene that are both reactivated by protonation for further cyclisation steps, while previously discussed intra- and intermolecular hydrogen transfers are not supported. Furthermore, the isolation of the new natural product (2S,3S,7S,10R)-guaia-1,11-dien-10-ol from patchouli oil is reported.

Isotopic labelling experiments and enzymatic preparation of iso-casbenes with casbene synthase from Ricinus communis

Casbene synthase was used to convert GGPP isomers into iso-casbenes. The enzyme mechanism and absolute configurations were investigated through stereoselective deuteration. 13C-labellings gave insights into the mass spectrometric fragmentation.

Mechanistic Investigations on Microbial Type I Terpene Synthases through Site-Directed Mutagenesis

During the past three decades many terpene synthases have been characterised from all kingdoms of life. Enzymes of type I, from bacteria, fungi and protists, commonly exhibit several highly conserved motifs and single residues, and the available crystal structures show a shared α-helical fold, while the overall sequence identity is generally low. Several enzymes have been studied by site-directed mutagenesis, giving valuable insights into terpene synthase catalysis and the intriguing mechanisms of terpene synthases. Some mutants are also preparatively useful and give higher yields than the wild type or a different product that is otherwise difficult to access. The accumulated knowledge obtained from these studies is presented and discussed in this review.

1 Introduction

2 Residues for Substrate Binding and Catalysis

3 Residues with Structural Function

4 Residues Contouring the Active Site Cavity

5 Other Residues

6 Conclusions

Targeting active site residues and structural anchoring positions in terpene synthases

The sesterterpene synthase SmTS1 from Streptomyces mobaraensis contains several unusual residues in positions that are otherwise highly conserved. Site-directed mutagenesis experiments for these residues are reported that showed different effects, resulting in some cases in an improved catalytic activity, but in other cases in a loss of enzyme function. For other enzyme variants a functional switch was observed, turning SmTS1 from a sesterterpene into a diterpene synthase. This article gives rational explanations for these findings that may generally allow for protein engineering of other terpene synthases to improve their catalytic efficiency or to change their functions.

Functional Switch and Ethyl Group Formation in the Bacterial Polytrichastrene Synthase from Chryseobacterium polytrichastri

A reinvestigation of the linalool synthase from Chryseobacterium polytrichastri uncovered its diterpene synthase activity, yielding polytrichastrene A and polytrichastrol A with new skeletons, besides known wanju-2,5-diene and thunbergol. The enzyme mechanism was investigated by isotopic labeling experiments and DFT calculations to explain an unusual ethyl group formation. Rationally designed exchanges of active site residues showed major functional switches, resulting for I66F in the production of five more new compounds, including polytrichastrene B and polytrichastrol B, while A87T, A192V and the double exchange A87T, A192V gave a product shift towards wanju-2,5-diene.

DFT Study on the Biosynthesis of Verrucosane Diterpenoids and Mangicol Sesterterpenoids: Involvement of Secondary-Carbocation-Free Reaction Cascades

Some experimental observations indicate that a sequential formation of secondary (2°) carbocations might be involved in some biosynthetic pathways, including those of verrucosane-type diterpenoids and mangicol-type sesterterpenoids, but it remains controversial whether or not such 2° cations are viable intermediates. Here, we performed comprehensive density functional theory calculations of these biosynthetic pathways. The results do not support previously proposed pathways/mechanisms: in particular, we find that none of the putative 2° carbocation intermediates is involved in either of the biosynthetic pathways. In verrucosane biosynthesis, the proposed 2° carbocations (II and IV) in the early stage are bypassed by the formation of the adjacent 3° carbocations and by unusual skeletal rearrangement reactions, and in the later stage, the putative 2° carbocation intermediates (VI, VII, and VIII) are not present as the proposed forms but as nonclassical structures between homoallyl and cyclopropylcarbinyl cations. In the mangicol biosynthesis, one of the two proposed 2° carbocations (X) is bypassed by a C-C bond-breaking reaction to generate a 3° carbocation with a C=C bond, while the other (XI) is bypassed by a strong hyperconjugative interaction leading to a nonclassical carbocation. We propose new biosynthetic pathways/mechanisms for the verrucosane-type diterpenoids and mangicol-type sesterterpenoids. These pathways are in good agreement with the findings of previous biosynthetic studies, including isotope-labeling experiments and byproducts analysis, and moreover can account for the biosynthesis of related terpenes.

Systematic mining of fungal chimeric terpene synthases using an efficient precursor-providing yeast chassis

Chimeric terpene synthases, termed PTTSs, are a unique family of enzymes occurring only in fungi. Characterizing PTTSs is challenging due to the complex reactions they catalyze and the structural complexity of their products. Here, by devising an efficient precursor-providing yeast chassis and incorporating a high-throughput automated platform, we identified 34 active PTTSs, which was considerably more than the number of known functional PTTSs. This effective and rapid pipeline can be employed for the characterization of other PTTSs or related terpenoid biosynthetic enzymes. By systematically analyzing the presence/absence of PTTS genes together with phylogenetic analysis, the ancestral PTTS gene was inferred to have undergone duplication and functional divergence, which led to the development of two distinct cyclization mechanisms.

Chimeric terpene synthases, which consist of C-terminal prenyltransferase (PT) and N-terminal class I terpene synthase (TS) domains (termed PTTSs here), is unique to fungi and produces structurally diverse di- and sesterterpenes. Prior to this study, 20 PTTSs had been functionally characterized. Our understanding of the origin and functional evolution of PTTS genes is limited. Our systematic search of sequenced fungal genomes among diverse taxa revealed that PTTS genes were restricted to Dikarya. Phylogenetic findings indicated different potential models of the origin and evolution of PTTS genes. One was that PTTS genes originated in the common Dikarya ancestor and then underwent frequent gene loss among various subsequent lineages. To understand their functional evolution, we selected 74 PTTS genes for biochemical characterization in an efficient precursor-providing yeast system employing chassis-based, robot-assisted, high-throughput automatic assembly. We found 34 PTTS genes that encoded active enzymes and collectively produced 24 di- and sesterterpenes. About half of these di- and sesterterpenes were also the products of the 20 known PTTSs, indicating functional conservation, whereas the PTTS products included the previously unknown sesterterpenes, sesterevisene (1), and sesterorbiculene (2), suggesting that a diversity of PTTS products awaits discovery. Separating functional PTTSs into two monophyletic groups implied that an early gene duplication event occurred during the evolution of the PTTS family followed by functional divergence with the characteristics of distinct cyclization mechanisms.

Bacterial terpenome

Covering: up to mid-2020 Terpenoids, also called isoprenoids, are the largest and most structurally diverse family of natural products. Found in all domains of life, there are over 80 000 known compounds. The majority of characterized terpenoids, which include some of the most well known, pharmaceutically relevant, and commercially valuable natural products, are produced by plants and fungi. Comparatively, terpenoids of bacterial origin are rare. This is counter-intuitive to the fact that recent microbial genomics revealed that almost all bacteria have the biosynthetic potential to create the C5 building blocks necessary for terpenoid biosynthesis. In this review, we catalogue terpenoids produced by bacteria. We collected 1062 natural products, consisting of both primary and secondary metabolites, and classified them into two major families and 55 distinct subfamilies. To highlight the structural and chemical space of bacterial terpenoids, we discuss their structures, biosynthesis, and biological activities. Although the bacterial terpenome is relatively small, it presents a fascinating dichotomy for future research. Similarities between bacterial and non-bacterial terpenoids and their biosynthetic pathways provides alternative model systems for detailed characterization while the abundance of novel skeletons, biosynthetic pathways, and bioactivies presents new opportunities for drug discovery, genome mining, and enzymology.

Rerouting and Improving Dauc-8-en-11-ol Synthase from Streptomyces venezuelae to a High Yielding Biocatalyst

The dauc‐8‐en‐11‐ol synthase from Streptomyces venezuelae was investigated for its catalytic activity towards alternative terpene precursors, specifically designed to enable new cyclisation pathways. Exchange of aromatic amino acid residues at the enzyme surface by site‐directed mutagenesis led to a 4‐fold increase of the yield in preparative scale incubations, which likely results from an increased enzyme stability instead of improved enzyme kinetics.

Revision of the Cyclisation Mechanism for the Diterpene Spiroviolene and Investigations of Its Mass Spectrometric Fragmentation

The diterpene spiroviolene, its diterpene synthase from Streptomyces violens and the experimentally determined terpene cyclisation mechanism were reported in 2017. Recently, the structure of spiroviolene was revised based on a total synthesis, with consequences for the cyclisation mechanism. Herein, a reinvestigation of the terpene cyclisation to spiroviolene and the mass spectrometric fragmentation mechanism investigated by 13 C-labelling experiments are presented.

Mechanistic divergence between (4S,7R)-germacra-(1(10)E,5E)-dien-11-ol synthases from Dictyostelium purpureum and Streptomyces coelicolor

The main product of DpTPS9 from the social amoeba Dictyostelium purpureum was identified as (4S,7R)-germacra-(1(10)E,5E)-dien-11-ol that is also known as an intermediate of bacterial geosmin synthase, but the experimentally verified cyclisation mechanisms differ. Together with the low sequence identity this points to convergent evolution. The functionality of selected residues in DpTPS9 was investigated via site-directed mutagenesis experiments.

The Sesquiterpene Synthase PtTPS5 Produces (1S,5S,7R,10R)-Guaia-4(15)-en-11-ol and (1S,7R,10R)-Guaia-4-en-11-ol in Oomycete-Infected Poplar Roots

Pathogen infection often leads to the enhanced formation of specialized plant metabolites that act as defensive barriers against microbial attackers. In this study, we investigated the formation of potential defense compounds in roots of the Western balsam poplar (Populus trichocarpa) upon infection with the generalist root pathogen Phytophthora cactorum (Oomycetes). P. cactorum infection led to an induced accumulation of terpenes, aromatic compounds, and fatty acids in poplar roots. Transcriptome analysis of uninfected and P. cactorum-infected roots revealed a terpene synthase gene PtTPS5 that was significantly induced upon pathogen infection. PtTPS5 had been previously reported as a sesquiterpene synthase producing two unidentified sesquiterpene alcohols as major products and hedycaryol as a minor product. Using heterologous expression in Escherichia coli, enzyme assays with deuterium-labeled substrates, and NMR analysis of reaction products, we could identify the major PtTPS5 products as (1S,5S,7R,10R)-guaia-4(15)-en-11-ol and (1S,7R,10R)-guaia-4-en-11-ol, with the former being a novel compound. The transcript accumulation of PtTPS5 in uninfected and P. cactorum-infected poplar roots matched the accumulation of (1S,5S,7R,10R)-guaia-4(15)-en-11-ol, (1S,7R,10R)-guaia-4-en-11-ol, and hedycaryol in this tissue, suggesting that PtTPS5 likely contributes to the pathogen-induced formation of these compounds in planta.

Isoishwarane synthase from Streptomyces lincolnensis

The product of a terpene synthase from Streptomyces lincolnensis has been identified as the new natural product isoishwarane. The enzyme mechanism was studied by isotopic labelling experiments and site-directed mutagenesis.

Germacrene A-A Central Intermediate in Sesquiterpene Biosynthesis

This review summarises known sesquiterpenes whose biosyntheses proceed through the intermediate germacrene A. First, the occurrence and biosynthesis of germacrene A in Nature and its peculiar chemistry will be highlighted, followed by a discussion of 6-6 and 5-7 bicyclic compounds and their more complex derivatives. For each compound the absolute configuration, if it is known, and the reasoning for its assignment is presented.