Microbial Secondary Metabolite, Phlegmacin B1, as a Novel Inhibitor of Insect Chitinolytic Enzymes

Cryptolepine Derivatives Are Insecticide Leads by Targeting Insect Chitinolytic Enzymes and Bilin-Binding Protein

Multitarget inhibitors (MIs) against insect chitinolytic enzymes are potential pesticide candidates. However, currently known Mis suffers from low biological activity and limited possibilities for structural modification. Here, cryptolepine and its halogenated derivatives were discovered as MIs against all of four insect chitinolytic enzymes from Ostrinia furnacalis (OfChtI, OfChtII, OfChi-h and OfHex1) with Ki values at μM level. Notably, the inhibitory activities of cryptolepine can be increased by 1 order of magnitude through a simple two-site halogenation (CD-4). Molecular docking results indicated that halogen modifications may directly form halogen bonds with polar residues or change the electrostatic potential of the conjugation plane to enhance its π-π stacking interaction with tryptophan residues. Cryptolepine and CD-4 showed insecticidal activities toward both O. furnacalis and Plutella xylostella. Interestingly, CD-4, but not cryptolepine, prevents the formation of the protective color in P. xylostella by competing with biliverdin for binding to the bilin-binding protein. This study not only reports new MIs as insecticide leads, but also provides a new idea for pest control by inhibiting its protective color.

Discovery of Sennidin B as a Potent Multitarget Inhibitor of Insect Chitinolytic Enzymes

Natural products with multitarget inhibitory activity against insect chitinolytic enzymes offer the potential for developing environmentally friendly insecticides. However, most inhibitors show limited efficacy, hindering their practical application. In this study, inspired by the dimeric structure of phlegmacin B1, sennidin B, a rhein analogue, was identified as an effective inhibitor of insect chitinolytic enzymes. Sennidin B exhibited a nanomolar Ki value (80 nM) against OfChi-h from Ostrinia furnacalis, showing at least a 600-fold improvement in potency compared to rhein. Molecular dynamics simulations revealed that sennidin B adopts a folded conformation within the enzyme's active site, enhancing binding affinity through π-π stacking interactions with a key tryptophan residue. Insecticidal assays demonstrated 100% mortality at 5 mM and suppression of larval development at 1 mM. This study positions sennidin B as a lead compound for the development of insecticides targeting the molting process and provides a strategy for the discovery of multitarget inhibitors.

Rational design of azo-aminopyrimidine derivatives as the potent lepidoptera-exclusive chitinase inhibitors

Ostrinia furnacalis (O. furnacalis) is a commonly occurring agricultural pest that can severely impact corn yield and quality. Therefore, establishing and implementing effective control methods against O. furnacalis are of great significance. Chemical insecticides remain the most effective means to mitigate the damage caused by O. furnacalis. With the increasing resistance of O. furnacalis to insecticides, it is imperative to identify and develop compounds with novel mechanisms of action and high safety. The chitinase OfChi-h, identified and characterized in O. furnacalis, has been recognized as a potential insecticide target. In this study, a series of azo-aminopyrimidine analogues were synthesized as OfChi-h inhibitors employing rational molecular optimization. Among them, compounds 9b, 10a and 10g exhibited Ki values of 23.2, 19.4, and 43.2 nM against OfChi-h, respectively. Molecular docking studies were carried out to explore the molecular basis for the high efficacy of these compounds and OfChi-h. In addition, the morphological changes of the cuticle in inhibitor-treated O. furnacalis larvae were assessed using scanning electron microscopy. Furthermore, the target compounds were assayed in leaf dipping and pot experiments, with compound 10a exhibiting greater insecticidal activity against Plutella xylostella (P. xylostella) and O. furnacalis than diflubenzuron and chlorbenzuron. At the same time, the toxicity of these compounds to natural enemies Trichogramma ostriniae and rats was negligible. The present study demonstrates that the azo-aminopyrimidine skeleton can be used as a novel, low-cost scaffold for developing insect chitinolytic enzyme inhibitors, with the potential to be utilized as new environmentally friendly insecticides.

Modification of Azo-Aminopyrimidines as Potent Multitarget Inhibitors of Insect Chitinolytic Enzymes OfChi-h and OfHex1

Multitarget inhibitors exhibit significant advantages in reducing the risk of drug resistance, enhancing therapeutic efficacy, and minimizing dosage, outperforming multicomponent combination drugs. On the basis of glycoside hydrolase family 18 (GH18) chitinases and GH20 β-N-acetylhexosaminidase using the same substrate-assisted catalytic mechanism and similar substrate binding modes, a series of novel azo-aminopyrimidine compounds have been designed and synthesized as multitarget inhibitors targeting chitinolytic enzymes OfChi-h and OfHex1. Compounds AAP4 (OfChi-h, Ki = 29.3 nM; OfHex1, Ki = 4.9 μM) and AAP16 (OfChi-h, Ki = 32.4 nM; OfHex1, Ki = 7.2 μM) were identified to be potent multitarget inhibitors of these enzymes, which were predicted to occupy the -1 subsite and engage in H-binding interactions with catalytic residues. AAP4 also displayed significant insecticidal activity against lepidopteran pests Ostrinia furnacalis through leaf dipping and pot experiments. In addition, the safety of AAP4 to corn and the natural enemy Trichogramma ostriniae was comprehensively evaluated. This present work indicates that azo-aminopyrimidines, as multitarget inhibitors against chitinolytic enzymes, can be further developed as safe and efficient pest control and management agents.

Regioselective Oxidative Phenol Coupling by a Mushroom Unspecific Peroxygenase

Bioactive dimeric (pre-)anthraquinones are ubiquitous in nature and are found in bacteria, fungi, insects, and plants. Their biosynthesis via oxidative phenol coupling (OPC) is catalyzed by cytochrome P450 enzymes, peroxidases, or laccases. While the biocatalysis of OPC in molds (Ascomycota) is well-known, the respective enzymes in mushroom-forming fungi (Basidiomycota) are unknown. Here, we report on the biosynthesis of the atropisomers phlegmacin A1 and B1 of the mushroom Cortinarius odorifer. The biosynthesis of these unsymmetrically 7,10'-homo-coupled dihydroanthracenones was heterologously reconstituted in the mold Aspergillus niger. Methylation of the parental monomer atrochrysone to its 6-O-methyl ether torosachrysone by the O-methyltransferase CoOMT1 precedes the regioselective homocoupling to phlegmacin, catalyzed by the enzyme CoUPO1 annotated as an "unspecific peroxygenase" (UPO). Our results reveal an unprecedented UPO reaction, thereby expanding the biocatalytic portfolio of oxidative phenol coupling beyond the commonly reported enzymes. The results show that Basidiomycota use peroxygenases to selectively couple aryls independently of and convergently to any other group of organisms, emphasizing the central role of OPC in natural processes.

Design of Inhibitors Targeting Chitin-Degrading Enzymes by Bioisostere Substitutions and Scaffold Hopping for Selective Control of Ostrinia furnacalis

Chitin-degrading enzymes are critical components in regulating the molting process of the Asian corn borer and serve as potential targets for controlling this destructive pest of maize. Here, we used a scaffold-hopping strategy to design a series of efficient naphthylimide insecticides. Among them, compound 8c exhibited potent inhibition of chitinase from OfChi-h and OfChtI at low nanomolar concentrations (IC50 = 1.51 and 9.21 nM, respectively). Molecular docking simulations suggested that 8c binds to chitinase by mimicking the interaction of chitin oligosaccharide substrates with chitinase. At low ppm concentrations, compound 8c performed comparably to commercial insecticides in controlling the highly destructive plant pest, the Asian corn borer. Tests on a wide range of nontarget organisms indicate that compound 8c has very low toxicity. In addition, the effect of inhibitor treatment on the expression of genes associated with the Asian corn borer chitin-degrading enzymes was further investigated by quantitative real-time polymerase chain reaction. In conclusion, our study highlights the potential of 8c as a novel chitinase-targeting insecticide for effective control of the Asian corn borer, providing a promising solution in the quest for sustainable pest management.

Metabolomics-guided utilization of beneficial microbes for climate-resilient crops

In the rhizosphere, plants and microbes communicate chemically, especially under environmental stress. Over millions of years, plants and their microbiome have coevolved, sharing various chemicals, including signaling molecules. This mutual exchange impacts bacterial communication and influences plant metabolism. Inter-kingdom signal crosstalk affects bacterial colonization and plant fitness. Beneficial microbes and their metabolomes offer eco-friendly ways to enhance plant resilience and agriculture. Plant metabolites are pivotal in this dynamic interaction between host plants and their interacting beneficial microbes. Understanding these associations is key to engineering a robust microbiome for stress mitigation and improved plant growth. This review explores mechanisms behind plant-microbe interactions, the role of beneficial microbes and metabolomics, and the practical applications for addressing climate change's impact on agriculture. Integrating beneficial microbes' activities and metabolomics' application to study metabolome-driven interaction between host plants and their corresponding beneficial microbes holds promise for enhancing crop resilience and productivity.

Novel Butenolide Derivatives as Dual-Chitinase Inhibitors to Arrest the Growth and Development of the Asian Corn Borer

OfChtI and OfChi-h are considered potential targets for the control of Asian corn borer (Ostrinia furnacalis). In this work, the previously reported OfChtI inhibitor 5f was found to show certain inhibitory activity against OfChi-h (Ki = 5.81 μM). Two series of novel butenolide derivatives based on lead compound 5f were designed with the conjugate skeleton, contributing to the π-binding interaction to chitinase, and then synthesized. Compounds 4a-l and 7a-p displayed excellent inhibitory activities against OfChtI and OfChi-h, respectively, at a concentration of 10 μM. Compound 4h was found to be a good dual-Chitinase inhibitor, with Ki values of 1.82 and 2.00 μM against OfChtI and OfChi-h, respectively. The inhibitory mechanism studies by molecular docking suggested that π-π stacking interactions were crucial to the inhibitory activity of novel butenolide derivatives against two different chitinases. A preliminary bioassay indicated that 4h exhibited certain growth inhibition effects against O. furnacalis. Butenolide-like analogues should be further studied as promising novel dual-chitinase inhibitor candidates for the control of O. furnacalis.

Discovery of Rhodanine Inhibitors Targeting Of ChtI Based on the π-Stacking Effect and Aqueous Solubility

The application of some reported inhibitors against the chitinolytic enzyme Of ChtI was limited due to their unsatisfactory insecticidal activities. Hence, we first performed a synergetic design strategy combining the π-stacking effect with aqueous solubility to find novel rhodanine analogues with inhibitory activities against Of ChtI. Novel rhodanine compounds IAa-f and IBa-f have weak aqueous solubility, but they (IAd: Ki = 4.0 μM; IBd: Ki = 2.2 μM) showed better inhibitory activities against Of ChtI and comparable insecticidal efficiency toward Ostrinia furnacalis compared to the high aqueous solubility compounds IIAa-f and IIBa-f (IIAd: Ki = 21.6 μM; IIBd: Ki = 14.3 μM) without a large conjugate plane. Further optimized compounds IIIAa-j with a conjugate plane as well as a higher aqueous solubility exhibited similar good inhibitory activities against Of ChtI (IIIAe: Ki = 2.4 μM) and better insecticidal potency (IIIAe: mortality rate of 63.33%) compared to compounds IAa-f and IBa-f, respectively. Molecular docking studies indicated that the conjugate planarity with the π-stacking effect for rhodanine analogues is responsible for their enzyme inhibitory activity against Of ChtI. This study provides a new strategy for designing insect chitinolytic enzyme inhibitors as insect growth regulators for pest control.

Insect cuticle and insecticide development

Insecticide resistance poses a significant challenge, diminishing the effectiveness of chemical insecticides. To address this global concern, the development of novel and efficient pest management technologies based on chemical insecticides is an ongoing necessity. The insect cuticle, a highly complex and continuously renewing organ, plays a crucial role in this context. On one hand, as the most vital structure, it serves as a suitable target for insecticides. On the other hand, it acts as the outermost barrier, isolating the insect's inner organs from the environment, and thus offering resistance to contact with insecticides, preventing their entry into insect bodies. Our work focuses on key targets concerning cuticle formation and the interaction between the cuticle and contact insecticides. Deeper studying insect cuticles and understanding their structure-function relationship, formation process, and regulatory mechanisms during cuticle development, as well as investigating insecticide resistance related to the barrier properties of insect cuticles, are promising strategies not only for developing novel insecticides but also for discovering general synergists for contact insecticides. With this comprehensive review, we hope to contribute valuable insights into the development of effective pest management solutions and the mitigation of insecticide resistance.

Design, Synthesis, and Biological Investigations of Novel Carbamoylguanidinyl Nitrobenzoxadiazoles against Chitinolytic Enzymes

Chitinase has been identified as an important target for insecticides. In this study, a series of novel chitinase inhibitors was designed and synthesized with nitrobenzoxadiazoles. Compound 8d, which contains the N-methylcarbamoylguanidinyl, exhibited high enzyme inhibitory activity and achieved nanomolar inhibition against OfChtI (IC50 = 12.3 nM). Delightfully, it was also found to possess significant inhibitory activity against OfHex1 (IC50 = 1.76 μM). The computational simulation results indicated that compound 8d interacted with OfChtI and OfHex1 in similar modes through hydrogen bonds and hydrophobic and π-π interactions. Insecticidal activity studies revealed that compound 8d showed high mortality against the Lepidoptera Plutella xylostella (mortality rate = 81%) at 200 mg/L. Toxicity studies indicated that compound 8d exhibited negligible toxicity to the natural enemy Trichogramma ostriniae. These results indicate that compound 8d may be a promising candidate for the development of environmentally friendly chitinase inhibitors. Moreover, this study provides a new angle for the design of innovative inhibitors of chitinolytic enzymes.

Identification of Triazolo-quinazolinone Derivatives as Novel and Potent Chitinase OfChi-h Inhibitors Based on Structure-Based Virtual Screening

Insect chitinase, OfChi-h, from Ostrinia furnacalis, is considered as a promising target for the development of green pesticides. On the basis of the crystal structure of OfChi-h, we successfully obtained a triazolo-quinazolinone scaffold as the novel class of OfChi-h inhibitor via a structure-based virtual screening approach. Rational compound screening enabled us to acquire a potent OfChi-h inhibitor TQ19 with a Ki value of 0.33 μM. Furthermore, the in vivo biological activity of target compounds was assayed. The results showed that compounds TQ8 and TQ19 could dramatically inhibit the growth and development of Ostrinia nubilalis larvae, and most of the compounds showed higher insecticidal activity than hexaflumuron. This present work reveals that triazolo-quinazolinone derivatives can serve as novel candidates for insect growth regulators.

Structural diversification of natural substrates modified by the O-methyltransferase AurJ from Fusarium Graminearum

Aromatic polyketide and phenylpropanoid derivatives are a large class of natural products produced by bacteria, fungi, and plants. The O-methylation is a unique decoration that can increase structural diversity of aromatic compounds and improve their pharmacological properties, but the substrate specificity of O-methyltransferase hinders the discovery of more natural products with O-methylation through biosynthesis. Here, we reported that the O-methyltransferase AurJ from plant pathogenic fungus Fusarium graminearum could methylate a broad range of natural substrates of monocyclic, bicyclic, and tricyclic aromatic precursors, exhibiting excellent substrate tolerance. This finding will partly change our stereotype about the specificity of traditional methyltransferases, and urge us to mine more O-methyltransferases with good substrate tolerance and discover more methylated natural products for drug discovery and development through directed evolution and combinatorial biosynthesis.

Recent advances in glycoside hydrolase family 20 and 84 inhibitors: Structures, inhibitory mechanisms and biological activities

Glycoside hydrolase family 20 (GH20) β-N-acetyl-d-hexosaminidase (Hex) catalyzes the cleavage of glycosidic linkages in glycans, glycolipids and glycoproteins, and is involved in glycoprotein modification, metabolism of glycoconjugate and the degradation of chitin in fungal cell walls and arthropod exoskeletons. GH84 O-β-N-acetyl-d-glucosaminidase (OGA), which is mechanistically similar related to GH20, participates in the O-GlcNAcylation modification, hydrolyzing the O-GlcNAc moiety from protein acceptors. Hex and OGA are of interest due to their potential for the treatment of disorder diseases and plant protection. Hex inhibitors act as molecular chaperones to treat lysosomal storage disease and as growth regulators to arrest insect molting. Inhibition of OGA is a promising therapeutic approach to treat tau pathology in neurodegenerative diseases such as Alzheimer's disease. However, since Hex and OGA exhibit similar active sites, there are challenges in designing highly selective inhibitors. The elucidation of the structural basis of the catalytic mechanism and substrate binding mode of Hex and OGA has provided core information for virtual screening and rational design of inhibitors. A large number of high-potency and selective inhibitors have been developed in the last five years. In this review, we focus on the recent advances in the structural modification, inhibitory activity, binding mechanisms and biological evaluation of Hex and OGA inhibitors, which will facilitate the development of new drugs and agrochemicals.

Discovery of Potent N-Methylcarbamoylguanidino Insect Growth Regulators Targeting OfChtI and OfChi-h

Insect growth regulators (IGRs) are important insecticides that reduce the harm caused by insects to crops by controlling pest population growth. Chitinases are closely associated with insect growth and are among the most important glycoside hydrolases. Thus, Chitinase is an attractive target for the development of novel insecticides. In this study, we designed and synthesized a series of novel and highly potent insecticides targeting OfChtI and OfChi-h in insects. Enzymatic activity tests showed that most compounds exhibited a potent inhibitory activity against OfCh-h. Binding mode analysis revealed that the target compounds bound to the -1 active subsite of Chitinase through the key pharmacophore N-methylcarbamoylguanidino. Compounds 6e, 6g, 6j, and 6o significantly affected the growth and development of Plutella xylostella at 200 mg/L. Our study provides novel insights for the development of potent insecticide-targeted Chitinase combinations based on receptors and ligands.

Application of Natural Bioresources to Sustainable Agriculture: A C-Glycoside Insecticide Based on N-Acetyl-glucosamine for Regulating Insect Molting of Ostrinia furnacalis

In order to increase the application of natural bioresources in drug discovery and development, a study on N-acetyl-glucosamine (GlcNAc) derivatives of chitin as green pesticides was necessary. In this study, we designed and synthesized a series of novel C-glycoside naphthalimides using GlcNAc as a starting material. Compound 10l showed high inhibitory activity against OfHex1 (IC₅₀ = 1.77 μM), with a nearly 30-fold increase in activity over our previously reported C-glycoside CAUZL-A (IC₅₀ = 47.47 μM). By observing the morphology of the Ostrinia furnacalis, we found that the synthesized compounds significantly inhibited the molting process. In addition, we further explored the morphological changes of the inhibitor-treated O. furnacalis cuticle using scanning electron microscopy. This is the first study to validate the insecticidal mechanism of OfHex1 inhibitors at the microscale level. Several compounds also exhibited excellent larvicidal activity against Plutella xylostella. Moreover, the toxicity measurements and predictions indicated that the C-glycoside naphthalimides have little effect on the natural enemy Trichogramma ostriniae and rats. Together, our results highlight an approach for the design of green pesticides, taking advantage of natural bioresources to control pests in agriculture.

Group h Chitinase: A Molecular Target for the Development of Lepidopteran-Specific Insecticides

Sustainable agriculture requires insecticides that are selective between insects and mammals and even between harmful and beneficial insects. Lepidoptera includes the largest number of insect pests that threaten crops, and Hymenoptera contains the natural enemies for these pests. Discovery of lepidopteran-specific molecular targets is one route to develop such selective pesticides. Group h chitinase (Chi-h) is an ideal target for lepidopteran-specific insecticides because it is only distributed in Lepidoptera and is critical to their molting processes. This minireview focuses on the latest progress in developing Chi-h as a lepidopteran-specific insecticide target. We describe the biological function, crystal structure, and small-molecule inhibitors of the enzyme. Notably, two unique pockets were discovered in the crystal structure of Chi-h for the binding of the selective inhibitors, phlegmacin B₁ and lynamicin B. Moreover, lynamicin B was found to exhibit significant insecticidal activity toward lepidopteran pests but is harmless toward their natural enemies. These findings are advancing the development of selective insecticides to meet the needs of sustainable agriculture.

Discovery of aromatic 2-(3-(methylcarbamoyl) guanidino)-N-aylacetamides as highly potent chitinase inhibitors

Chitinases are important glycoside hydrolases that are closely related to bacterial pathogenesis, fungal cell wall remodelling, and insect moulting. Consequently, chitinases have become attractive targets for therapeutic drugs and pesticides. In this study, we designed and synthesised a series of novel chitinase inhibitors based on the N-methylcarbamoylguanidinyl group of the natural product argifin. The most active compound 8h showed strong inhibitory activity against the group I chitinases HsChit1, SmChiB, and OfChi-h, with IC₅₀ values of 0.19 µM, 4.2 nM, and 25 nM, respectively. Binding mode studies revealed that the compound 8h formed π-π stacking/hydrophobic interactions at +1 or +2 subsite of chitinases. In addition, a key hydrogen bond net was formed between the pharmacophore N-methylcarbamoylguanidinyl and key residues at the -1 subsite. Together, the findings of this study provide novel insights into the development of potent small-molecule chitinase inhibitors using a combination of planar structures and N-methylcarbamoylguanidinyl.

Rational Design of N-Methylcarbamoylguanidinyl Derivatives as Highly Potent Dual-Target Chitin Hydrolase Inhibitors for Retarding Growth of Pest Insects

Chitin degradation is a vital process for the growth of insects. Chitin hydrolase OfChtI and β-N-acetylhexosaminidase OfHex1 are two key enzymes involved in hydrolyzing the chitin of insects' cuticles. Thus, they are considered promising targets for preventing and controlling agricultural pests. In this study, we designed and synthesized a series of compounds bearing N-methylcarbamoylguanidinyl and N-methoxycarbonylguanidinyl as dual-target inhibitors of OfChtI and OfHex1. The most potent dual-target inhibitor, compound 10d, exhibited half-maximal inhibitory concentration (IC₅₀) values of 27.1 and 249.1 nM against OfChtI and OfHex1, respectively. Furthermore, the insecticidal activity studies showed that compounds 10a-c, 10k, and 10l bear significant effects on the growth and development of Plutella xylostella. This work provides a promising method for the development of novel chitin hydrolase inhibitors as potential pest control and management agents.

Structure-Based Virtual Screening of Natural Products and Optimization for the Design and Synthesis of Novel CeCht1 Inhibitors as Nematicide Candidates

Nematode chitinases are critical components of the nematode life cycle, and CeCht1 is a potential target for developing novel nematicides. Herein, lunidonine, a natural quinoline alkaloid, was first discovered to have inhibitory activity against CeCht1, which was acquired from a library of over 16,000 natural products using a structure-based virtual screening methodology. A pocket-based lead optimization strategy was employed based on the predicted binding mode of lunidonine. Subsequently, a series of benzo[d][1,3]dioxole-5-carboxylate derivatives were designed and synthesized, and their inhibitory activities against CeCht1 as well as in vitro nematicidal activities against Caenorhabditis elegans were assessed. The analysis of structure-activity relationship and inhibitory mechanisms provided insights into their interactions with the CeCht1 active site, which could facilitate future research in improving the potency of the inhibitory activity. Especially, compound a12 interacted well with CeCht1 and exhibited excellent in vitro nematicidal activity against C. elegans with a LC₅₀ value of 41.54 mg/L, suggesting that it could be a promising candidate for a novel chemical nematicide targeting CeCht1. The known binding modes and structural features of these inhibitors will contribute to the design of stronger CeCht1-based nematicides to control nematodes in agriculture.

Discovery of Azo-Aminopyrimidines as Novel and Potent Chitinase OfChi-h Inhibitors via Structure-Based Virtual Screening and Rational Lead Optimization

Chitinase OfChi-h, from the destructive agricultural pest Ostrinia furnacalis, is considered as a promising target for green pest control and management. In this study, structure-based virtual screening and rational molecular optimization led to the synthesis of a series of azo-aminopyrimidine derivatives as a novel class of OfChi-h inhibitors. Among them, the most potent compound 8f, with a benzyl on the amino group at the 4-position of pyrimidine, exhibited a K_i value of 64.7 nM against OfChi-h. In addition, molecular docking studies were carried out to investigate the basis for the potency of the aminopyrimidines against OfChi-h. Furthermore, the insecticidal activity of the target compounds against Plutella xylostella and Ostrinia nubilalis was assessed, and the potent OfChi-h inhibitors 8f and 8i showed higher insecticidal activity than the control pesticide hexaflumuron. The present work revealed that the azo-aminopyrimidine skeletons characterized by concise chemical structure and high efficiency could be further developed as potential pesticides for the control of lepidopteran pests.

Rational Design and Identification of Novel Piperine Derivatives as Multichitinase Inhibitors

Asian corn borer (Ostrinia furnacalis) is one of the most destructive pests in agriculture. Three chitinases OfChtI, OfChtII, and OfChi-h are regarded as potential targets for discovering novel agrochemicals to control O. furnacalis. In this study, piperine (K_i = 43.78∼83.03 μM) was first shown to exhibit inhibitory activities against all three chitinases. Subsequently, 19 novel piperine derivatives were rationally designed based on the conserved aromatic residues of three chitinases and then synthesized. Among them, Compound 5k (K_i = 11.78∼22.82 μM) was identified as the most effective multichitinase inhibitor and indeed displayed higher insecticidal activity against O. furnacalis than dual- or single-chitinase inhibitors. Molecular mechanism studies clarified that Compound 5k interacted with two conserved TRP and TYR of three chitinases in identical modes through hydrogen bonds, hydrophobic, and π-π interactions. Moreover, the microinjection experiment indicated that Compound 5k exhibited substantial sublethal effects against O. furnacalis by regulating its growth and development. This study provides evidence of multichitinase inhibitors to be applied in the control of O. furnacalis.

Design, synthesis, biologically evaluation and molecular docking of C-glycosidic oximino carbamates as novel OfHex1 inhibitors

The inhibition of function-specific β-N-acetyl-D-hexosaminidases, such as OfHex1 from the Asian corn borer (Ostrinia furnacalis), is a promising strategy for the development of green pesticides. Among reported OfHex1 inhibitors, glycosyl inhibitors show especially high inhibitory activity. In this study, a series of novel C-glycosidic oximino carbamate derivatives were designed using the OfHex1 crystal structure and synthesized. Among the C-Glycoside derivatives studied, compound 7k exhibited the best inhibitory activity against OfHex1 (IC₅₀ = 47.47 μM). Compound 7k also exhibited excellent larvicidal activity against Plutella xylostella. The potential inhibitory mechanism of 7k was studied using molecular docking. Notably, compound 7k is the first reported C-glycoside inhibitor of OfHex1. These results provide direction for the rational design of novel OfHex1 inhibitors.

Unsymmetrically Regioselective Homodimerization Depends on the Subcellular Colocalization of Laccase/Fasciclin Protein in the Biosynthesis of Phlegmacins

Phlegmacins are homodimeric dihydroanthracenone natural products featuring two torosachrysone monomers unsymmetrically conjugated by 7,10'-coupling. Herein, we report the identification and characterization of the biosynthetic gene cluster of phlegmacins in ascomycete Talaromyces sp. F08Z-0631. On the basis of the heterologous reconstitution of the phlegmacin pathway in Aspergillus oryzae, we demonstrated an unprecedented laccase-involved unsymmetrically regioselective oxidative coupling reaction. The association of laccase PhlC and the fasciclin partner protein PhlB was verified to be indispensable for the coupling activity. Intriguingly, both proteins can be transferred and located independently at the mitochondrial membrane. Notably, only their subcellular colocalization led to the occurrence of oxidative dimerization. These observations add new insights into the poorly understood catalytic mechanisms of various laccases involved in the biosynthesis of secondary metabolites, particularly those functioning with variable partners.

The Role of Chitooligosaccharidolytic β-N-Acetylglucosamindase in the Molting and Wing Development of the Silkworm Bombyx mori

The insect glycoside hydrolase family 20 β-N-acetylhexosaminidases (HEXs) are key enzymes involved in chitin degradation. In this study, nine HEX genes in Bombyx mori were identified by genome-wide analysis. Bioinformatic analysis based on the transcriptome database indicated that each gene had a distinct expression pattern. qRT-PCR was performed to detect the expression pattern of the chitooligosaccharidolytic β-N-acetylglucosaminidase (BmChiNAG). BmChiNAG was highly expressed in chitin-rich tissues, such as the epidermis. In the wing disc and epidermis, BmChiNAG has the highest expression level during the wandering stage. CRISPR/Cas9-mediated BmChiNAG deletion was used to study the function. In the BmChiNAG-knockout line, 39.2% of female heterozygotes had small and curly wings. The ultrastructure of a cross-section showed that the lack of BmChiNAG affected the stratification of the wing membrane and the formation of the correct wing vein structure. The molting process of the homozygotes was severely hindered during the larva to pupa transition. Epidermal sections showed that the endocuticle of the pupa was not degraded in the mutant. These results indicate that BmChiNAG is involved in chitin catabolism and plays an important role in the molting and wing development of the silkworm, which highlights the potential of BmChiNAG as a pest control target.

Talaromyces-Insect Relationships

Facing the urgent need to reduce the input of agrochemicals, in recent years, the ecological relationships between plants and their associated microorganisms have been increasingly considered as an essential tool for improving crop production. New findings and data have been accumulated showing that the application of fungi can go beyond the specific role that has been traditionally assigned to the species, employed in integrated pest management as entomopathogens or mycoparasites, and that strains combining both aptitudes can be identified and possibly used as multipurpose biocontrol agents. Mainly considered for their antagonistic relationships with plant pathogenic fungi, species in the genus Talaromyces have been more and more widely reported as insect associates in investigations carried out in various agricultural and non-agricultural contexts. Out of a total of over 170 species currently accepted in this genus, so far, 27 have been found to have an association with insects from 9 orders, with an evident increasing trend. The nature of their mutualistic and antagonistic relationships with insects, and their ability to synthesize bioactive compounds possibly involved in the expression of the latter kind of interactions, are analyzed in this paper with reference to the ecological impact and applicative perspectives in crop protection.

Lynamicin B is a Potential Pesticide by Acting as a Lepidoptera-Exclusive Chitinase Inhibitor

Insect group h chitinase is a promising target for designing non-target safe pesticides in that it is exclusively distributed in lepidopteran insects, over 80% of which are agricultural pests. In this work, lynamicin B was discovered to be an inhibitor of OfChi-h, the group h chitinase from the lepidopteran pest Ostrinia furnacalis. Lynamicin B was revealed to competitively inhibit OfChi-h with a K_i value of 8.76 μM and does not significantly inhibit other chitinases. The co-crystal structure of lynamicin B and OfChi-h revealed that the dichloroindolyl group of lynamicin B occupies an unexplored pocket below subsites +1 and +2 of the substrate-binding cleft, which is vital for its selectivity. Feeding experiments demonstrated that lynamicin B exhibited high insecticidal activities against other lepidopteran pests Mythimna separata and Spodoptera frugiperda besides O. furnacalis. Moreover, lynamicin B did not affect Trichogramma ostriniae, a natural enemy of O. furnacalis. This study provides a natural-derived potent pesticide for the control of lepidopteran pests, leaving its natural enemy unaffected.

Discovery of Biphenyl-Sulfonamides as Novel β-N-Acetyl-d-Hexosaminidase Inhibitors via Structure-Based Virtual Screening

Novel insecticidal targets are always in demand due to the development of resistance. OfHex1, a β-N-acetyl-d-hexosaminidase identified in Ostrinia furnacalis (Asian corn borer), is involved in insect chitin catabolism and has proven an ideal target for insecticide development. In this study, structure-based virtual screening, structure simplification, and biological evaluation are used to show that compounds with a biphenyl-sulfonamide skeleton have great potential as OfHex1 inhibitors. Specifically, compounds 10k, 10u, and 10v have K_i values of 4.30, 3.72, and 4.56 μM, respectively, and thus, they are more potent than some reported nonglycosyl-based inhibitors such as phlegmacin B₁ (K_i = 26 μM), berberine (K_i = 12 μM), 2 (K_i = 11.2 μM), and 3 (K_i = 28.9 μM). Furthermore, inhibitory kinetic assessments reveal that the target compounds are competitive inhibitors with respect substrate, and based on toxicity predictions, most of them have potent drug properties. The obtained results indicate that the biphenyl-sulfonamide skeleton characterized by simple chemical structure, synthetic tractability, potent activity, and low toxicity has potential for further development in pest management targeting OfHex1.

Discovery of Natural Products as Multitarget Inhibitors of Insect Chitinolytic Enzymes through High-Throughput Screening

Small-molecule inhibitors of insect chitinolytic enzymes are potential insecticides. However, the reported inhibitors that target one enzyme usually exhibit unsatisfactory bioactivity. On the basis of the multitarget strategy, we performed a high-throughput screening of a natural product library to find insecticide leads against four chitinolytic enzymes from the Asian corn borer Ostrinia furnacalis (OfChtI, OfChtII, OfChi-h, and OfHex1). Several phytochemicals were discovered to be multitarget inhibitors of these enzymes and were predicted to occupy the -1 substrate-binding subsite and engage in polar interactions with catalytically important residues. Shikonin and wogonin, which had good inhibitory activities toward all four enzymes, also exhibited significant insecticidal activities against lepidopteran agricultural pests. This study provides the first example of using a multitarget high-throughput screening strategy to exploit natural products as insecticide leads against chitin biodegradation during insect molting.

Novel Inhibitors of an Insect Pest Chitinase: Design and Optimization of 9-O-Aromatic and Heterocyclic Esters of Berberine

OfChi-h, a lepidopteran-exclusive glycoside hydrolase family 18 (GH18) chitinase from the agricultural insect pest Ostrinia furnacalis, is a promising molecular target candidate for pest control and management. Berberine (BER), a traditional Chinese medicine, binds to a wide variety of glycosyl hydrolases via an identical mechanism, showing potential as a pesticide lead compound. In this work, we found that BER was a moderate inhibitor of OfChi-h with a Ki of 16.1 μM. To improve its efficacy, a series of BER derivatives featuring an ester bond linked to an aromatic or heterocyclic aromatic ring at the 9-position were designed and evaluated as effective OfChi-h inhibitors. The most potent compound, compound 19e with a nicotinate group, exhibited a Ki of 0.093 μM. Molecular docking analysis suggested that the common binding mode of BER derivatives featured a network of π-π stacking and electrostatic interactions and that the group at the 9-position enhanced the van der Waals and hydrogen bonding interactions. Administration of the BER derivative 19c to 4th-instar O. furnacalis larvae in an artificial diet led to their impaired growth and metamorphosis. This work provides a new starting point for the modification of BER for use in pest control.

A Piperine-Based Scaffold as a Novel Starting Point to Develop Inhibitors against the Potent Molecular Target OfChtI

The insect chitinase OfChtI from the agricultural pest Ostrinia furnacalis (Asian corn borer) is a promising target for green insecticide design. OfChtI is a critical chitinolytic enzyme for the cuticular chitin degradation at the stage of molting. In this study, piperine, a natural amide compound isolated from black pepper, Piper nigrum L., was discovered for the first time to have inhibitory activity toward OfChtI. The compound-enzyme interaction was presumed to take place between the piperine benzo[d][1,3] dioxole skeleton and subsite -1 of the substrate-binding pocket of OfChtI. Hence, on the basis of the deduced inhibitory mechanism and crystal structure of the substrate-binding cavity of OfChtI, compounds 5a-f were designed and synthesized by introducing a butenolide scaffold into the lead compound piperine. The enzymatic activity assay indicated that compounds 5a-f (K_i = 1.03-2.04 μM) exhibited approximately 40-80-fold higher inhibitory activity than the lead compound piperine (I) (K_i = 81.45 μM) toward OfChtI. The inhibitory mechanism of the piperonyl butenolide compounds was elucidated by molecular dynamics, which demonstrated that the introduced butenolide skeleton improved the binding affinity to OfChtI. Moreover, the in vivo activity assay indicated that these compounds also displayed moderate insecticidal activity toward O. furnacalis. This work introduces the natural product piperine as a starting point for the development of novel insecticides targeting OfChtI.

Crystal structure-guided design of berberine-based novel chitinase inhibitors

Glycoside hydrolase family 18 (GH18) chitinases play an important role in various organisms ranging from bacteria to mammals. Chitinase inhibitors have potential applications as pesticides, fungicides, and anti-asthmatics. Berberine, a plant-derived isoquinoline alkaloid, was previously reported to inhibit against various GH18 chitinases with only moderate K_i values ranging between 20 and 70 μM. In this report, we present for the first time the berberine-complexed crystal structure of SmChiB, a model GH18 chitinase from the bacterium Serratia marcescens. Based on the berberine-binding mode, a hydrophobic cavity-based optimisation strategy was developed to increase their inhibitory activity. A series of berberine derivatives were designed and synthesised, and their inhibitory activities against GH18 chitinases were evaluated. The compound 4c showed 80-fold-elevated inhibitory activity against SmChiB and the human chitinase hAMCase with K_i values at the sub-micromolar level. The mechanism of improved inhibitory activities was proposed. This work provides a new strategy for developing novel chitinase inhibitors.

X-ray Structure and Molecular Docking Guided Discovery of Novel Chitinase Inhibitors with a Scaffold of Dipyridopyrimidine-3-carboxamide

Chitinases are the glycosyl hydrolase for catalyzing the degradation of chitin and play an indispensable role in bacterial pathogenesis, fungal cell wall remodeling, and insect molting. Thus, chitinases are attractive targets for therapeutic drugs and pesticides. Here, we present a strategy of developing a novel chemotype of chitinase inhibitors by the construction of planar heterocycles that can stack with conserved aromatic residues. The rational design, guided by crystallographic analysis and docking results, leads to a series of dipyridopyrimidine-3-carboxamide derivatives as chitinase inhibitors. Among them, compound 6t showed the most potent activity against bacterial chitinase SmChiB and insect chitinase OfChi-h, with a K_i value of 0.14 and 0.0056 μM, respectively. The strong stacking interaction of compound 6p with Trp99 and Trp220 found in the SmChiB-6p co-crystal structure verifies the feasibility of our design. Our results provide novel insights into developing potent chitinase inhibitors for pathogen and pest control.

New lead discovery of insect growth regulators based on the scaffold hopping strategy

Insect growth regulators (IGRs), which can interrupt or inhibit pest life cycles, are low-toxicity pesticides widely used in integrated pest management (IPM). Ecdysone analogues and chitinase inhibitors are familiar IGRs that have attracted considerable attention because of their unique modes of action and low toxicity to non-target organisms. To find new and highly effective candidate IGRs with novel mechanisms, D-08 (N-(4-(tert-butyl)phenyl)-2-phenyl-2,4,5,6,7,8-hexahydrocyclohepta[c]pyrazole-5-carboxamide) was chosen as a lead compound, and a series of novel heptacyclic pyrazolamide derivatives were designed and synthesized using the scaffold hopping strategy. The bioassay showed that III-27 (N-(2-methylphenethyl)-1-phenyl-1,4,5,6,7,8-hexahydrocyclohepta[c]pyrazole-5-carboxamide) had excellent activity against Plutella xylostella. Protein verification and molecular docking indicated that III-27 could act on both the ecdysone receptor (EcR) and Ostrinia furnacalis chitinase (Of ChtI) and is a promising new lead IGRs. The interaction mechanism of III-27 with EcR and Of ChtI was then studied by molecular docking. These results provide important guidance for the study of new dual-target IGRs.

Virtual screening, synthesis, and bioactivity evaluation for the discovery of β-N-acetyl-D-hexosaminidase inhibitors

BACKGROUND

Molting is an essential insect developmental process, in which a variety of enzymes are involved. The inhibition of these enzymes effect normal insect growth and development and may even cause death. OfHex1, one of the β-N-acetyl-D-hexosaminidases, is a key enzyme involved in the molting process of the Asian corn borer (Ostrinia furnacalis), and is deemed a potential insecticidal target.

RESULTS

Based on the crystal structure of OfHex1, virtual screening was carried out to obtain a novel class of OfHex1 inhibitors, of which, 28 compounds were subjected to bioactivity evaluation. The compound 3, N-(3-cyano-4,5,6,7-tetrahydrobenzo[b]thiophen-2-yl)benzamide, showed good inhibition against OfHex1 with a K_i value of 11.2 μM. Structure optimization and molecular docking were applied for the structure-activity relationship analysis. The results also showed that the cyano group of this compound was essential for the maintenance of its inhibitory activity against OfHex1. Additionally, the interaction between this compound and Trp490, Glu328, Tyr475 and Trp524 were important for inhibitory activity.

CONCLUSION

The advantages of the derivatives of 2-amino-4,5,6,7-tetrahydrobenzo[b]thiophene-3-carbonitrile, which have simple chemical structures and are easily synthesized, suggests them to be developed further as potential OfHex1 inhibitors for pest control. © 2020 Society of Chemical Industry.

Synthesis of ureido thioglycosides as novel insect β‑N‑acetylhexosaminidase OfHex1 inhibitors

The insect β-N-acetylhexosaminidase OfHex1 from Ostrinia furnacalis (one of the most destructive agricultural pests) has been considered as a promising pesticide target. In this study, a series of novel and readily available ureido thioglycosides were designed and synthesized based on the catalytic mechanism and the co-crystal structures of OfHex1 with substrates. After evaluation via enzyme inhibition experiments, thioglycosides 11c and 15k were found to have inhibitory activities against OfHex1 with the K_i values of 25.6 µM and 53.8 µM, respectively. In addition, all these ureido thioglycosides exhibited high selectivity toward OfHex1 over hOGA and HsHexB (K_i > 100 μM). Furthermore, to investigate the inhibitory mechanism, the possible binding modes of 11c and 15k with OfHex1 were deduced based on molecular docking analysis. This work may provide useful structural starting points for further rational design of potent inhibitors of OfHex1.

Design, Synthesis, and Biological Activity of Novel Heptacyclic Pyrazolamide Derivatives: A New Candidate of Dual-Target Insect Growth Regulators

Insect growth regulators (IGRs) can cause abnormal growth and development in insects, resulting in incomplete metamorphosis or even death of the larvae. Ecdysone receptor (EcR) and chitinase in insects play indispensable roles in the molting process. Ecdysone analogues and chitinase inhibitors are considered as potential IGRs. In order to find new and highly effective IGR candidates, based on the structure-activity relationship and molecular docking results of the active compound 6i (3-(tert-butyl)-N-(4-(tert-butyl)phenyl)-1-phenyl-1H-pyrazole-5-carboxamide) discovered in our previous work, we changed the t-butyl group on the pyrazole ring into heptacycle to enhance the hydrophobicity. Consequently, a series of novel heptacyclic pyrazolamide derivatives were designed and synthesized. The bioassay results demonstrated that some compounds showed obvious insecticidal activity. Especially, D-27 (N-(4-(tert-butyl)phenyl)-2-phenyl-2,4,5,6,7,8-hexahydrocyclohepta[c]pyrazole-5-carboxamide) showed good activities against Plutella xylostella (LC₅₀, 51.50 mg·L^-1) and Mythimna separata (100% mortality at 2.5 mg·L^-1). Furthermore, protein validation indicated that D-27 acts not only on the EcR but also on chitinase Of ChtI. Molecular docking and molecular dynamics simulation explained the vital factors in the interaction between D-27 and receptors. D-27 may be a new lead candidate with a dual target in which Of ChtI shall be the main one. This work created a new starting point for discovering a novel type of IGRs.

Development of Novel Pesticides Targeting Insect Chitinases: A Minireview and Perspective

Chitinase (EC 3.2.1.14) is an enzyme to breakdown β-1,4-glycosidic bonds in chitin and chitooligosaccharides. The loss of chitinase enzymatic activity in insects results in severe exoskeleton defects and lethality at all developmental stages, indicating that insect chitinases can be promising pesticide targets. However, there are no pesticides known to target chitinases. This perspective will focus on the latest research progress of insect chitinases, paying special attention to crystal structures and chemical biology advances in the field. The physiological importance and unique structural features of insect chitinases may ensure the development of new pesticides through a novel acting mode.

Functional expression of the Spodoptera exigua chitinase to examine the virtually screened inhibitor candidates

Chitinase is responsible for insect chitin hydrolyzation, which is a key process in insect molting and pupation. However, little is known about the chitinase of Spodoptera exigua (SeChi). In this study, based on the SeChi gene (ADI24346) identified in our laboratory, we constructed the recombinant baculovirus P-Chi for the expression of recombinant SeChi (rSeChi) in Hi5 cells. The rSeChi was purified by chelate affinity chromatography, and the purified protein showed activity comparable with that of a commercial SgChi, suggesting that we harvested active SeChi for the first time. The purified protein was subsequently tested for enzymatic properties and revealed to exhibit its highest activity at pH 8 and 40 C. Using homology modeling and molecular docking techniques, the three-dimensional model of SeChi was constructed and screened for inhibitors. In two rounds of screening, twenty compounds were selected. With the purified rSeChi, we tested each of the twenty compounds for inhibitor activity against rSeChi, and seven compounds showed obvious activity. This study provided new information for the chitinase of beet armyworm and for chitinase inhibitor development.

Progress and prospects of arthropod chitin pathways and structures as targets for pest management

Chitin is a structural component of the arthropod cuticular exoskeleton and the peritrophic matrix of the gut, which play crucial roles in growth and development. In the past few decades, our understanding of the composition, biosynthesis, assembly, degradation, and regulation of chitinous structures has increased. Many chemicals have been developed that target chitin biosynthesis (benzoyphenyl ureas, etoxazole), chitin degradation (allosamidin, psammaplin), and chitin regulation (benzoyl hydrazines), thus resulting in molting deformities and lethality. In addition, proteins that disrupt chitin structures, such as lectins, proteases, and chitinases have been utilized to halt feeding and induce mortality. Chitin-degrading enzymes, such as chitinases are also useful for improving the efficacy of bio-insecticides. Transgenic plants, baculoviruses, fungi, and bacteria have been engineered to express chitinases from a variety of organisms for control of arthropod pests. In addition, RNA interference targeting genes involved in chitin pathways and structures are now being investigated for the development of environmentally friendly pest management strategies. This review describes the chemicals and proteins used to target chitin structures and enzymes for arthropod pest management, as well as pest management strategies based upon these compounds, such as plant-incorporated-protectants and recombinant entomopathogens. Recent advances in RNA interference-based pest management, and how this technology can be used to target chitin pathways and structures are also discussed.

Synthesis, Optimization, and Evaluation of Glycosylated Naphthalimide Derivatives as Efficient and Selective Insect β- N-Acetylhexosaminidase OfHex1 Inhibitors

Insect chitinolytic β- N-acetylhexosaminidase OfHex1, from the agricultural pest Ostrinia furnacalis (Guenée), is considered as a potential target for green pesticide design. In this study, rational molecular design and optimization led to the synthesis of compounds 15r ( K_i = 5.3 μM) and 15y ( K_i = 2.7 μM) that had superior activity against OfHex1 than previously reported lead compounds. Both compounds 15r and 15y had high selectivity toward OfHex1 over human β- N-acetylhexosaminidase B (HsHexB) and human O-GlcNAcase (hOGA). In addition, to investigate the basis for the potency of glycosylated naphthalimides against OfHex1, molecular docking and molecular dynamics simulations were performed to study possible binding modes. Furthermore, the in vivo biological activity of target compounds with efficient OfHex1 inhibitory potency was assayed against Myzus persicae, Plutella xylostella, and O. furnacalis. This present work indicates that glycosylated naphthalimides can be further developed as potential pest control and management agents targeting OfHex1.

Computational Studies on the Potency and Selectivity of PUGNAc Derivatives Against GH3, GH20, and GH84 β-N-acetyl-D-hexosaminidases

β-N-acetyl-D-hexosaminidases have attracted significant attention due to their crucial role in diverse physiological functions including antibacterial synergists, pathogen defense, virus infection, lysosomal storage, and protein glycosylation. In particular, the GH3 β-N-acetyl-D-hexosaminidase of V. cholerae (VcNagZ), human GH20 β-N-acetyl-D-hexosaminidase B (HsHexB), and human GH84 β-N-acetyl-D-hexosaminidase (hOGA) are three important representative glycosidases. These have been found to be implicated in β-lactam resistance (VcNagZ), lysosomal storage disorders (HsHexB) and Alzheimer's disease (hOGA). Considering the profound effects of these three enzymes, many small molecule inhibitors with good potency and selectivity have been reported to regulate the corresponding physiological functions. In this paper, the best-known inhibitors PUGNAc and two of its derivatives (N-valeryl-PUGNAc and EtBuPUG) were selected as model compounds and docked into the active pockets of VcNagZ, HsHexB, and hOGA, respectively. Subsequently, molecular dynamics simulations of the nine systems were performed to systematically compare their binding modes from active pocket architecture and individual interactions. Furthermore, the binding free energy and free energy decomposition are calculated using the MM/GBSA methods to predict the binding affinities of enzyme-inhibitor systems and to quantitatively analyze the contribution of each residue. The results show that PUGNAc is deeply-buried in the active pockets of all three enzymes, which indicates its potency (but not selectivity) against VcNagZ, HsHexB, and hOGA. However, EtBuPUG, bearing branched 2-isobutamido, adopted strained conformations and was only located in the active pocket of VcNagZ. It has completely moved out of the pocket of HsHexB and lacks interactions with HsHexB. This indicates why the selectivity of EtBuPUG to VcNagZ/HsHexB is the largest, reaching 968-fold. In addition, the contributions of the catalytic residue Asp253 (VcNagZ), Asp254 (VcNagZ), Asp175 (hOGA), and Asp354 (HsHexB) are important to distinguish the activity and selectivity of these inhibitors. The results of this study provide a helpful structural guideline to promote the development of novel and selective inhibitors against specific β-N-acetyl-D-hexosaminidases.

Pocket-based Lead Optimization Strategy for the Design and Synthesis of Chitinase Inhibitors

Insect chitinases play an indispensable role in shedding old cuticle during molting. Targeting chitinase inhibition is a promising pest control strategy. Of ChtI, a chitinase from the destructive insect pest Ostrinia furnacalis (Asian corn borer), has been suggested as a potential target for designing green pesticides. A 4,5,6,7-tetrahydrobenzo[ b]thiophene-3-carboxylate scaffold was previously obtained, and further derivatization generated the lead compound 1 as Of ChtI inhibitor. Here, based on the predicted binding mode of compound 1, the pocket-based lead optimization strategy was applied. A series of analogues was synthesized, and their inhibitory activities against Of ChtI were evaluated. Compound 8 with 6- tert-pentyl showed preferential inhibitory activity with a K_i value of 0.71 μM. Their structure-activity relationships suggested that the compound with larger steric hindrance at the 6-nonpolar group was essential for inhibitory activity due to its stronger interactions with surrounding amino acids. This work provides a strategy for designing potential chitinase inhibitors.

Chitinous Structures as Potential Targets for Insect Pest Control

Chitinous structures are physiologically fundamental in insects. They form the insect exoskeleton, play important roles in physiological systems and provide physical, chemical and biological protections in insects. As critically important structures in insects, chitinous structures are attractive target sites for the development of new insect-pest-control strategies. Chitinous structures in insects are complex and their formation and maintenance are dynamically regulated with the growth and development of insects. In the past few decades, studies on insect chitinous structures have shed lights on the physiological functions, compositions, structural formation, and regulation of the chitinous structures. Current understanding of the chitinous structures has indicated opportunities for exploring new target sites for insect control. Mechanisms to disrupt chitinous structures in insects have been studied and strategies for the potential development of new means of insect control by targeting chitinous structures have been proposed and are practically to be explored.

Glycosyl triazoles as novel insect β-N-acetylhexosaminidase OfHex1 inhibitors: Design, synthesis, molecular docking and MD simulations

The insect enzyme GH20 β-N-acetyl-d-hexosaminidase OfHex1 represents an important chitinolytic enzyme found in the agricultural pest Ostrinia furnacalis (Guenée) and inhibition of this enzyme has been considered a promising strategy for the development of eco-friendly pesticides. In this article, based on the structure of the catalytic domains of OfHex1, a series of novel glycosyl triazoles were designed and synthesized via Cu-catalyzed azide-alkyne [3+2] cycloaddition reaction. To investigate the potency and selectivity of these glycosyl triazoles, the inhibition activities towards OfHex1 and HsHexB (human β-N-acetylhexosaminidase B) were studied. Particularly compound 17c (OfHex1, K_i = 28.68 μM; HsHexB, K_i > 100 μM) exhibited a suitable activity and selectivity against OfHex1. Furthermore, the possible inhibitory mechanisms of 17c with OfHex1 were studied using molecular docking and MD simulations. The structure-activity relationship results as well as the formed binding patterns may provide promising insights into the further development of novel OfHex1 inhibitors.

Structure-Based Virtual Screening, Compound Synthesis, and Bioassay for the Design of Chitinase Inhibitors

Chitinases play a vital part in the molting phase of insect pests. Inhibiting their activities by the use of drug-like small chemical molecules is thought to be an efficient strategy in pesticide design and development. On the basis of the crystal structure of OfChtI, a chitinase indispensable for the molting of the insect pest Ostrinia furnacalis (Asian corn borer), here we report a chemical fragment and five variant compounds as inhibitors of OfChtI obtained from a library of over 200 000 chemicals by a structure-based-virtual-screening approach. The compounds were synthesized with high atom economy and tested for their OfChtI-inhibitory activities in a bioassay. Compound 3 showed preferential inhibitory activity with a K_i value of 1.5 μΜ against OfChtI. Analysis of the structure-activity relationships of the compounds provided insight into their interactions with the enzyme active site, which may inform future work in improving the potencies of their inhibitory activities.