Changing Face: A Key Residue for the Addition of Water by Sclareol Synthase

Redesigning Enzymes for Biocatalysis: Exploiting Structural Understanding for Improved Selectivity

The discovery of new enzymes, alongside the push to make chemical processes more sustainable, has resulted in increased industrial interest in the use of biocatalytic processes to produce high-value and chiral precursor chemicals. Huge strides in protein engineering methodology and in silico tools have facilitated significant progress in the discovery and production of enzymes for biocatalytic processes. However, there are significant gaps in our knowledge of the relationship between enzyme structure and function. This has demonstrated the need for improved computational methods to model mechanisms and understand structure dynamics. Here, we explore efforts to rationally modify enzymes toward changing aspects of their catalyzed chemistry. We highlight examples of enzymes where links between enzyme function and structure have been made, thus enabling rational changes to the enzyme structure to give predictable chemical outcomes. We look at future directions the field could take and the technologies that will enable it.

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.

Tanshinones: Leading the way into Lamiaceae labdane-related diterpenoid biosynthesis

Tanshinones are the bioactive diterpenoid constituents of the traditional Chinese medicinal herb Danshen (Salvia miltiorrhiza), and are examples of the phenolic abietanes widely found within the Lamiaceae plant family. Due to the significant interest in these labdane-related diterpenoid natural products, their biosynthesis has been intensively investigated. In addition to providing the basis for metabolic engineering efforts, this work further yielded pioneering insights into labdane-related diterpenoid biosynthesis in the Lamiaceae more broadly. This includes stereochemical foreshadowing of aromatization, with novel protein domain loss in the relevant diterpene synthase, as well as broader phylogenetic conservation of the relevant enzymes. Beyond such summary of more widespread metabolism, formation of the furan ring that characterizes the tanshinones also has been recently elucidated. Nevertheless, the biocatalysts for the pair of demethylations remain unknown, and the intriguing potential connection of these reactions to the further aromatization observed in the tanshinones are speculated upon here.

Mechanistic analysis for the origin of diverse diterpenes in Tripterygium wilfordii

Tripterygium wilfordii is a valuable medicinal plant rich in biologically active diterpenoids, but there are few studies on the origins of these diterpenoids in its secondary metabolism. Here, we identified three regions containing tandemly duplicated diterpene synthase genes on chromosomes (Chr) 17 and 21 of T. wilfordii and obtained 11 diterpene synthases with different functions. We further revealed that these diterpene synthases underwent duplication and rearrangement at approximately 2.3–23.7 million years ago (MYA) by whole-genome triplication (WGT), transposon mediation, and tandem duplication, followed by functional divergence. We first demonstrated that four key amino acids in the sequences of TwCPS3, TwCPS5, and TwCPS6 were altered during evolution, leading to their functional divergence and the formation of diterpene secondary metabolites. Then, we demonstrated that the functional divergence of three TwKSLs was driven by mutations in two key amino acids. Finally, we discovered the mechanisms of evolution and pseudogenization of miltiradiene synthases in T. wilfordii and elucidated that the new function in TwMS1/2 from the terpene synthase (TPS)-b subfamily was caused by progressive changes in multiple amino acids after the WGT event. Our results provide key evidence for the formation of diverse diterpenoids during the evolution of secondary metabolites in T. wilfordii.

Discovery and Functional Characterization of a Diverse Diterpene Synthase Family in the Medicinal Herb Isodon lophanthoides Var. gerardiana

Isodon lophanthoides var. gerardiana (Lamiaceae), also named xihuangcao, is a traditional Chinese medicinal herb that exhibits a broad range of pharmacological activities. Abietane-type diterpenoids are the characteristic constituents of I. lophanthoides, yet their biosynthesis has not been elucidated. Although the aerial parts are the most commonly used organs of I. lophanthoides, metabolite profiling by gas chromatography-mass spectrometry showed the underground parts also contain large amounts of labdane diterpenoids including abietatriene, miltiradiene and ferruginol, which are distinct from the 13-hydroxy-8(14)-abietene detected in the aerial parts. Comparative transcriptome analysis of root and leaf samples identified a diverse diterpene synthase family including 6 copalyl diphosphate synthase (IlCPS1-6) and 5 kaurene synthase-like (IlKSL1-5). Here we report the functional characterization of six of these enzymes using yeast heterologous expression system. Both IlCPS1 and IlCPS3 synthesized (+)-copalyl diphosphate (CPP), in combination with IlKSL1 resulted in miltiradiene, precursor of abietane-type diterpenoids, while coupling with IlKSL5 led to the formation of hydroxylated diterpene scaffold nezukol. Expression profiling and phylogenetic analysis further support the distinct evolutionary relationship and spatial distribution of IlCPS1 and IlCPS3. IlCPS2 converted GGPP into labda-7,13E-dien-15-ol diphosphate. IlCPS6 was identified as ent-CPS, indicating a role in gibberellin metabolism. We further identified a single residue that determined the water addition of nezukol synthase IlKSL5. Substitution of alanine 513 with isoleucine completely altered the product outcome from hydroxylated nezukol to isopimara-7,15-diene. Together, these findings elucidated the early steps of bioactive abietane-type diterpenoid biosynthesis in I. lophanthoides and the catalytic mechanism of nezukol synthase.

Effect of Salvia officinalis and S. sclarea on rats with a high-fat hypercaloric diet

Phytotherapy for the correction of excess body weight is widely used. However, a comprehensive study of herbal preparations on the organism of model animals has been carried out only for a few plant species. Supplementing the diet of rats with closely related sage species (Salvia officinalis L. and S. sclarea L.) against the background of high-fat hypercaloric diet triggered multidirectional changes in their metabolism. The addition of crushed dry shoots of S. officinalis to the diet of animals led to a sharp increase in their body weight (up to 130.8% of the initial one in 30 days of the experiment). The body weight of the rats treated with S. sclarea for 30 days increased only up to 103.8% of their initial weight and was lower than in the control group. Addition of S. officinalis caused an increase in daily weight gain up to 253.1% of the control group, and S. sclarea – its decrease to 27.8% of the daily weight gain in the control group. In the S. officinalis group, the relative weight of the brain, spleen, and thymus decreased, while in the S. sclarea group, the relative weight of the thymus decreased and that of the colon increased. Under the influence of S. officinalis, the concentration of urea, total bilirubin, and triglycerides in the blood plasma of male rats decreased and the concentration of total protein and the activity of alkaline phosphatase increased. While consuming S. sclarea shoots, there was an increase of alkaline phosphatase activity in the rats’ blood, but atherogenic index (23.1% of the level of the control group) sharply dropped due to an increase in the concentration of high-density lipoprotein cholesterol (286.9% of the control) and a decrease in the concentration of low-density lipoprotein cholesterol (67.7% of control). In rats feeding on S. sclarea shoots, we observed a decrease in the concentration of triglycerides in the blood (39.9% of the control), a decrease in the activity of gamma-glutamyl transferase (62.8%), and an increase in the Ca/P ratio (132.5% of the control group). No significant changes were observed in CBC and WBC differential of male rats when eating S. officinalis and S. sclarea shoots. According to the results of the open field test, the physical and orientational activity of male rats under the influence of S. officinalis significantly decreased by the end of the experiment. Emotional status of rats, on the contrary, decreased when they ate dry crushed shoots of S. sclarea in the composition of the food. Thus, excess body weight of rats in the conditions of hypercaloric diet led to more pronounced deviations from the norm while consuming dry crushed shoots of S. officinalis. The addition of S. sclarea dry crushed shoots to the animals’ diet normalized the body weight in comparison with the control group, reduced the negative manifestations of obesity at the biochemical and organismal levels. In this regard, the substances that contains S. sclarea should be carefully studied for anti-atherosclerotic activity, and tea supplemented with S. sclarea shoots can be recommended as a corrective supplement in the diet of overweight people.

A pair of threonines mark ent-kaurene synthases for phytohormone biosynthesis

All land plants (embryophytes) must contain an ent-kaurene synthase (KS), as the ability to produce this olefin from ent-copalyl diphosphate (ent-CPP) is required for phytohormone biosynthesis. These KSs have frequently given rise to other class I diterpene synthases that catalyze distinct reactions for more specialized plant metabolism. Indeed, the prevalence of such gene duplication and neofunctionalization has obscured phylogenetic assignment of function. Here a pair of threonines is found to be conserved in all land plant KS involved in phytohormone biosynthesis, and their role in enzyme function investigated. Surprisingly, these threonines are not required, nor even particularly important for efficient production of ent-kaurene from ent-CPP. In addition, these threonines do not seem to affect protein structure or stability. Moreover, the absence of codon bias and positioning within an intron do not support a role in transcription or translation either. Despite their lack of apparent function, this pair of threonines are nevertheless completely conserved in all embryophyte KS from phytohormone biosynthesis. Thus, regardless of exact role, this serves as a diagnostic mark for such KS, enabling more confident distinction of these critical enzymes.

Redesigning the Molecular Choreography to Prevent Hydroxylation in Germacradien-11-ol Synthase Catalysis

Natural sesquiterpene synthases have evolved to make complex terpenoids by quenching reactive carbocations either by proton transfer or by hydroxylation (water capture), depending on their active site. Germacradien-11-ol synthase (Gd11olS) from Streptomyces coelicolor catalyzes the cyclization of farnesyl diphosphate (FDP) into the hydroxylated sesquiterpene germacradien-11-ol. Here, we combine experiment and simulation to guide the redesign of its active site pocket to avoid hydroxylation of the product. Molecular dynamics simulations indicate two regions between which water molecules can flow that are responsible for hydroxylation. Point mutations of selected residues result in variants that predominantly form a complex nonhydroxylated product, which we identify as isolepidozene. Our results indicate how these mutations subtly change the molecular choreography in the Gd11olS active site and thereby pave the way for the engineering of terpene synthases to make complex terpenoid products.

Switching on a Nontraditional Enzymatic Base - Deprotonation by Serine in the ent-Kaurene Synthase from Bradyrhizobium japonicum

Terpene synthases often catalyze complex carbocation cascade reactions. It has been previously shown that single residue switches involving replacement of a key aliphatic residue with serine or threonine can "short-circuit" such reactions, presumed to act indirectly via dipole stabilization of intermediate carbocations. Here a similar switch was found in the structurally characterized ent-kaurene synthase from Bradyrhizobium japonicum. Application of a recently developed computational approach to terpene synthases, TerDockin, surprisingly indicates direct action of the introduced serine hydroxyl as a catalytic base. Notably, this model suggests alternative interpretation of previous results, and potential routes towards reengineering terpene synthase activity more generally.

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.

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.

Functional Diversity of Diterpene Synthases in the Biofuel Crop Switchgrass

Diterpenoids constitute a diverse class of metabolites with critical functions in plant development, defense, and ecological adaptation. Major monocot crops, such as maize (Zea mays) and rice (Oryza sativa), deploy diverse blends of specialized diterpenoids as core components of biotic and abiotic stress resilience. Here, we describe the genome-wide identification and functional characterization of stress-related diterpene synthases (diTPSs) in the dedicated bioenergy crop switchgrass (Panicum virgatum). Mining of the allotetraploid switchgrass genome identified an expansive diTPS family of 31 members, and biochemical analysis of 11 diTPSs revealed a modular metabolic network producing a diverse array of diterpenoid metabolites. In addition to ent-copalyl diphosphate (CPP) and ent-kaurene synthases predictably involved in gibberellin biosynthesis, we identified syn-CPP and ent-labda-13-en-8-ol diphosphate (LPP) synthases as well as two diTPSs forming (+)-labda-8,13E-dienyl diphosphate (8,13-CPP) and ent-neo-cis-trans-clerodienyl diphosphate (CT-CLPP) scaffolds not observed previously in plants. Structure-guided mutagenesis of the (+)-8,13-CPP and ent-neo-CT-CLPP synthases revealed residue substitutions in the active sites that altered product outcome, representing potential neofunctionalization events that occurred during diversification of the switchgrass diTPS family. The conversion of ent-CPP, ent-LPP, syn-CPP, and ent-neo-CT-CLPP by promiscuous diTPSs further yielded distinct labdane-type diterpene olefins and alcohols. Of these metabolites, the formation of 9β-hydroxy-syn-pimar-15-ene and the expression of the corresponding genes were induced in roots and leaves in response to oxidative stress and ultraviolet irradiation, indicating their possible roles in abiotic stress adaptation. Together, these findings expand the known chemical space of diterpenoid metabolism in monocot crops toward systematically investigating and ultimately improving stress resilience traits in crop species.