Catalytic role of carbonyl oxygens and water in selinadiene synthase

Simulation-Guided Engineering Enables a Functional Switch in Selinadiene Synthase toward Hydroxylation

Engineering sesquiterpene synthases to form predefined alternative products is a major challenge due to their diversity in cyclization mechanisms and our limited understanding of how amino acid changes affect the steering of these mechanisms. Here, we use a combination of atomistic simulation and site-directed mutagenesis to engineer a selina-4(15),7(11)-diene synthase (SdS) such that its final reactive carbocation is quenched by trapped active site water, resulting in the formation of a complex hydroxylated sesquiterpene (selin-7(11)-en-4-ol). Initially, the SdS G305E variant produced 20% selin-7(11)-en-4-ol. As suggested by modeling of the enzyme-carbocation complex, selin-7(11)-en-4-ol production could be further improved by varying the pH, resulting in selin-7(11)-en-4-ol becoming the major product (48%) at pH 6.0. We incorporated the SdS G305E variant along with genes from the mevalonate pathway into bacterial BL21(DE3) cells and demonstrated the production of selin-7(11)-en-4-ol at a scale of 10 mg/L in batch fermentation. These results highlight opportunities for the simulation-guided engineering of terpene synthases to produce predefined complex hydroxylated sesquiterpenes.

Stability trends in carbocation intermediates stemming from germacrene A and hedycaryol

In the current work, we analyzed the origin of difference in stabilities among the germacrene A and hedycaryol-derived carbocations. This study focused on twelve hydrocarbons derived from germacrene A and twelve from hedycaryol, which can be divided into three groups: four molecules containing 6-6 bicyclic rings, four 5-7 bicyclic compounds with the carbocation being on the seven-membered ring and the remaining four 5-7 bicyclic compounds with the carbocation on the five-membered ring. The variations in energy within the groups of carbocations (i.e., 6-6 and two kinds of 5-7 bicyclic carbocations) can be ascribed to intramolecular repulsion interactions, as seen from non-covalent interactions plots. Despite the structural similarities between germacrene A and hedycaryol cations, they possess a somewhat different stability trend. These differences are attributed to C+···OH intramolecular interactions present in some hedycaryol cations, which are absent in the carbocations derived from germecrene A.

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.

Production of non-natural terpenoids through chemoenzymatic synthesis using substrate analogs

Chemoenzymatic synthesis of non-natural terpenes using the promiscuous activity of terpene synthases allows for the expansion of the chemical space of terpenoids with potentially new bioactivities. In this report, we describe protocols for the preparation of a novel aphid attractant, (S)-14,15-dimethylgermacrene D, by exploiting the promiscuity of (S)-germacrene D synthase from Solidago canadensis and using an engineered biocatalytic route to convert prenols to terpenoids. The method uses a combination of five enzymes to carry out the preparation of terpenoid semiochemicals in two steps: (1) diphosphorylation of five or six carbon precursors (prenol, isoprenol and methyl-isoprenol) catalyzed by Plasmodium falciparum choline kinase and Methanocaldococcus jannaschii isopentenyl phosphate kinase to form DMADP, IDP and methyl-IDP, and (2) chain elongation and cyclization catalyzed by Geobacillus stearothermophilus (2E,6E)-farnesyl diphosphate synthase and S. canadensis (S)-germacrene D synthase to produce (S)-germacrene D and (S)-14,15-dimethylgermacrene D. Using this method, new non-natural terpenoids are readily accessible and the approach can be adopted to produce different terpene analogs and terpenoid derivatives with potential novel applications.

Characterization and Engineering of Two Highly Paralogous Sesquiterpene Synthases Reveal a Regioselective Reprotonation Switch

Sesquiterpene synthases (STPSs) catalyze carbocation-driven cyclization reactions that can generate structurally diverse hydrocarbons. The deprotonation-reprotonation process is widely used in STPSs to promote structural diversity, largely attributable to the distinct regio/stereoselective reprotonations. However, the molecular basis for reprotonation regioselectivity remains largely understudied. Herein, we analyzed two highly paralogous STPSs, Artabotrys hexapetalus (-)-cyperene synthase (AhCS) and ishwarane synthase (AhIS), which catalyze reactions that are distinct from the regioselective protonation of germacrene A (GA), resulting in distinct skeletons of 5/5/6 tricyclic (-)-cyperene and 6/6/5/3 tetracyclic ishwarane, respectively. Isotopic labeling experiments demonstrated that these protonations occur at C3 and C6 of GA in AhCS and AhIS, respectively. The cryo-electron microscopy-derived AhCS complex structure provided the structural basis for identifying different key active site residues that may govern their functional disparity. The structure-guided mutagenesis of these residues resulted in successful functional interconversion between AhCS and AhIS, thus targeting the three active site residues [L311-S419-C458]/[M311-V419-A458] that may act as a C3/C6 reprotonation switch for GA. These findings facilitate the rational design or directed evolution of STPSs with structurally diverse skeletons.

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.

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.

Impacts of QM region sizes and conformation numbers on modelling enzyme reactions: a case study of polyethylene terephthalate hydrolase

A quantum mechanics/molecular mechanics (QM/MM) approach is a broadly used tool in computational enzymology. Treating the QM region with a high-level DFT method is one of the important branches. Here, taking leaf-branch compost cutinase-catalyzed polyethylene terephthalate depolymerization as an example, the convergence behavior of energy barriers as well as key structural and charge features with respect to the size of the QM region (up to 1000 atoms) is systematically investigated. BP86/6-31G(d)//CHARMM and M06-2X/6-311G(d,p)//CHARMM level of theories were applied for geometry optimizations and single-point energy calculations, respectively. Six independent enzyme conformations for all the four catalytic steps (steps (i)-(iv)) were considered. Most of the twenty-four cases show that at least 500 QM atoms are needed while only two rare cases show that ∼100 QM atoms are sufficient for convergence when only a single conformation was considered. This explains why most previous studies showed that 500 or more QM atoms are required while a few others showed that ∼100 QM atoms are sufficient for DFT/MM calculations. More importantly, average energy barriers and key structural/charge features from six conformations show an accelerated convergence than that in a single conformation. For instance, to reach energy barrier convergence (within 2.0 kcal mol-1) for step (ii), only ∼100 QM atoms are required if six conformations are considered while 500 or more QM atoms are needed with a single conformation. The convergence is accelerated to be more rapid if hundreds and thousands of conformations were considered, which aligns with previous findings that only several dozens of QM atoms are required for convergence with semi-empirical QM/MM MD simulations.

Active Site Loop Engineering Abolishes Water Capture in Hydroxylating Sesquiterpene Synthases

Terpene synthases (TS) catalyze complex reactions to produce a diverse array of terpene skeletons from linear isoprenyl diphosphates. Patchoulol synthase (PTS) from Pogostemon cablin converts farnesyl diphosphate into patchoulol. Using simulation-guided engineering, we obtained PTS variants that eliminate water capture. Further, we demonstrate that modifying the structurally conserved Hα-1 loop also reduces hydroxylation in PTS, as well as in germacradiene-11-ol synthase (Gd11olS), leading to cyclic neutral intermediates as products, including α-bulnesene (PTS) and isolepidozene (Gd11olS). Hα-1 loop modification could be a general strategy for engineering sesquiterpene synthases to produce complex cyclic hydrocarbons without the need for structure determination or modeling.

Decoding Catalysis by Terpene Synthases

The review by Christianson, published in 2017 on the twentieth anniversary of the emergence of the field, summarizes the foundational discoveries and key advances in terpene synthase/cyclase (TS) biocatalysis (Christianson, D. W. Chem Rev2017, 117 (17), 11570-11648. DOI: 10.1021/acs.chemrev.7b00287). Here, we review the TS literature published since then, bringing the field up to date and looking forward to what could be the near future of TS rational design. Many revealing discoveries have been made in recent years, building on the knowledge and fundamental principles uncovered during those initial two decades of study. We use these to explore TS reaction chemistry and see how a combined experimental and computational approach helps to decipher the complexities of TS catalysis. Revealed are a suite of catalytic motifs which control product outcome in TSs, some obvious, some more subtle. We examine each in detail, using the most recent papers and insights to illustrate how exactly this fascinating class of enzymes takes a single acyclic substrate and turns it into the many thousands of complex terpenoids found in Nature. We then explore some of the recent strategies for TS engineering, including machine learning and other data-driven approaches. From this, rational and predictive engineering of TSs, "designer terpene synthases", will begin to emerge as a realistic goal.

QM/MM Modeling Aided Enzyme Engineering in Natural Products Biosynthesis

Natural products and their derivatives are widely used across various industries, particularly pharmaceuticals. Modern engineered biosynthesis provides an alternative way of producing and meeting the growing need for diverse natural products. Natural enzymes, on the other hand, often exhibit unsatisfactory catalytic characteristics and necessitate further enzyme engineering modifications. QM/MM, as a powerful and extensively used computational tool in the field of enzyme catalysis, has been increasingly applied in rational enzyme engineering over the past decade. In this review, we summarize recent advances in QM/MM computational investigation on enzyme catalysis and enzyme engineering for natural product biosynthesis. The challenges and perspectives for future QM/MM applications aided enzyme engineering in natural product biosynthesis will also be discussed.

Molecular insights into the catalytic promiscuity of a bacterial diterpene synthase

Diterpene synthase VenA is responsible for assembling venezuelaene A with a unique 5-5-6-7 tetracyclic skeleton from geranylgeranyl pyrophosphate. VenA also demonstrates substrate promiscuity by accepting geranyl pyrophosphate and farnesyl pyrophosphate as alternative substrates. Herein, we report the crystal structures of VenA in both apo form and holo form in complex with a trinuclear magnesium cluster and pyrophosphate group. Functional and structural investigations on the atypical 115DSFVSD120 motif of VenA, versus the canonical Asp-rich motif of DDXX(X)D/E, reveal that the absent second Asp of canonical motif is functionally replaced by Ser116 and Gln83, together with bioinformatics analysis identifying a hidden subclass of type I microbial terpene synthases. Further structural analysis, multiscale computational simulations, and structure-directed mutagenesis provide significant mechanistic insights into the substrate selectivity and catalytic promiscuity of VenA. Finally, VenA is semi-rationally engineered into a sesterterpene synthase to recognize the larger substrate geranylfarnesyl pyrophosphate.

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.

Labelling studies in the biosynthesis of polyketides and non-ribosomal peptides

Covering: 2015 to 2022In this review, we discuss the recent advances in the use of isotopically labelled compounds to investigate the biosynthesis of polyketides, non-ribosomally synthesised peptides, and their hybrids. Also, we highlight the use of isotopes in the elucidation of their structures and investigation of enzyme mechanisms. The biosynthetic pathways of selected examples are presented in detail to reveal the principles of the discussed labelling experiments. The presented examples demonstrate that the application of isotopically labelled compounds is still the state of the art and can provide valuable information for the biosynthesis of natural products.

Germacrene B - a central intermediate in sesquiterpene biosynthesis

Germacranes are important intermediates in the biosynthesis of eudesmane and guaiane sesquiterpenes. After their initial formation from farnesyl diphosphate, these neutral intermediates can become reprotonated for a second cyclisation to reach the bicyclic eudesmane and guaiane skeletons. This review summarises the accumulated knowledge on eudesmane and guaiane sesquiterpene hydrocarbons and alcohols that potentially arise from the achiral sesquiterpene hydrocarbon germacrene B. Not only compounds isolated from natural sources, but also synthetic compounds are dicussed, with the aim to give a rationale for the structural assignment for each compound. A total number of 64 compounds is presented, with 131 cited references.

Discovery, Structure, and Mechanism of a Class II Sesquiterpene Cyclase

Terpene cyclases (TCs), extraordinary enzymes that create the structural diversity seen in terpene natural products, are traditionally divided into two classes, class I and class II. Although the structural and mechanistic features of class I TCs are well-known, the corresponding details in class II counterparts have not been fully characterized. Here, we report the genome mining discovery and structural characterization of two class II sesquiterpene cyclases (STCs) from Streptomyces. These drimenyl diphosphate synthases (DMSs) are the first STCs shown to possess β,γ-didomain architecture. High-resolution X-ray crystal structures of DMS from Streptomyces showdoensis (SsDMS) in complex with both a farnesyl diphosphate and Mg²⁺ unveiled an induced-fit mechanism, with an unprecedented Mg²⁺ binding mode, finally solving one of the lingering questions in class II TC enzymology. This study supports continued genome mining for novel bacterial TCs and provides new mechanistic insights into canonical class II TCs that will lead to advances in TC engineering and synthetic biology.

Mutation of the eunicellane synthase Bnd4 alters its product profile and expands its prenylation ability

Bnd4 catalyzes the first committed step in the biosynthesis of the bacterial diterpenoid benditerpenoic acid and was the first eunicellane synthase identified from nature. We investigated the catalytic roles of the aromatic residues in the active site of Bnd4 through a series of mutation studies. These experiments revealed that large hydrophobic or aromatic side chains are required at F162 and Y197 for eunicellane formation and that selected mutations at W316 converted Bnd4 into a cembrane synthase. In addition, the Bnd4Y197A variant expanded the native prenylation ability of Bnd4 from accepting C5 and C10 prenyl donors to C15. This study supports the mechanism of eunicellane formation by Bnd4 and encourages further engineering of terpene synthases into practical and efficient prenyltransferases.

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.

How a 10-epi-Cubebol Synthase Avoids Premature Reaction Quenching to Form a Tricyclic Product at High Purity

Terpenes are the largest class of natural products and are attractive targets in the fuel, fragrance, pharmaceutical, and flavor industries. Harvesting terpenes from natural sources is environmentally intensive and often gives low yields and purities, requiring further downstream processing. Engineered terpene synthases (TSs) offer a solution to these problems, but the low sequence identity and high promiscuity among TSs are major challenges for targeted engineering. Rational design of TSs requires identification of key structural and chemical motifs that steer product outcomes. Producing the sesquiterpenoid 10-epi-cubebol from farnesyl pyrophosphate (FPP) requires many steps and some of Nature’s most difficult chemistry. 10-epi-Cubebol synthase from Sorangium cellulosum (ScCubS) guides a highly reactive carbocationic substrate through this pathway, preventing early quenching and ensuring correct stereochemistry at every stage. The cyclizations carried out by ScCubS potentially represent significant evolutionary expansions in the chemical space accessible by TSs. Here, we present the high-resolution crystal structure of ScCubS in complex with both a trinuclear magnesium cluster and pyrophosphate. Computational modeling, experiment, and bioinformatic analysis identified residues important in steering the reaction chemistry. We show that S206 is crucial in 10-epi-cubebol synthesis by enlisting the nearby F211 to shape the active site contour and prevent the formation of early escape cadalane products. We also show that N327 and F104 control the distribution between several early-stage cations and whether the final product is derived from the germacrane, cadalane, or cubebane hydrocarbon scaffold. Using these insights, we reengineered ScCubS so that its main product was germacradien-4-ol, which derives from the germacrane, rather than the cubebane, scaffold. Our work emphasizes that mechanistic understanding of cation stabilization in TSs can be used to guide catalytic outcomes.

Investigation of Acid–Base Catalysis in Halimadienyl Diphosphate Synthase Involved in Mycobacterium tuberculosis Virulence

The devastating human pathogenMycobacterium tuberculosis (Mtb) is able to parasitize phagosomal compartments within alveolar macrophage cells due, in part, to the activity of its cell-surface lipids. Prominent among these is 1-tuberculosinyl-adenosine (1-TbAd), a derivative of the diterpenoid tuberculosinyl (halima-5,13-dienyl) diphosphate produced by the class II diterpene cyclase encoded by Rv3377c, termed here MtHPS. Given the demonstrated ability of 1-TbAd to act as a virulence factor for Mtb and the necessity for Rv3377c for its production, there is significant interest in MtHPS activity. Class II diterpene cyclases catalyze a general acid–base-mediated carbocation cascade reaction initiated by protonation of the terminal alkene in the general diterpenoid precursor (E,E,E)-geranylgeranyl diphosphate and terminated by deprotonation of the final cyclized (and sometimes also rearranged) intermediate. Here, structure-guided mutagenesis was applied to characterize the various residues contributing to activation of the enzymatic acid, as well as identify the enzymatic base in MtHPS. Particularly given the ability of conservative substitution for the enzymatic base (Y479F) to generate an alternative product (labda-7,13-dienyl diphosphate) via deprotonation of an earlier unrearranged intermediate, further mutational analysis was carried out to introduce potential alternative catalytic bases. The results were combined with mechanistic molecular modeling to elucidate how these mutations affect the catalytic activity of this important enzyme. This not only provided detailed structure–function insight into MtHPS but also further emphasized the inert nature of the active site of MtHPS and class II diterpene cyclases more generally.

Hedycaryol – Central Intermediates in Sesquiterpene Biosynthesis, Part II

The known sesquiterpenes that arise biosynthetically from hedycaryol are summarised. Reasonings for the assignments of their absolute configurations are discussed. The analysis provided here suggests that reprotonations at the C1=C10 double bond of hedycaryol are directed toward C1 and generally lead to 6–6 bicyclic compounds, while reprotonations at the C4=C5 double bond occur at C4 and result in 5–7 bicyclic compounds. Read more in the Review by H. Xu and J. S. Dickschat (DOI: 10.1002/chem.202200405).

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.