An efficient synthesis of argifin: A natural product chitinase inhibitor with chemotherapeutic potential

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.

Rational Design, Synthesis, and Biological Investigations of N-Methylcarbamoylguanidinyl Azamacrolides as a Novel Chitinase Inhibitor

Chitinase is one of the most important glycoside hydrolyases, widely existing in bacteria, fungi, insects, and plants. It is involved in fungal cell wall remodeling and insect molting. Chitinase inhibitors are an effective means of controlling pathogens and pests. Natural product argifin is a 17-membered pentapeptide that exhibits efficient chitinase inhibitory activity. However, the complexity of the synthetic process results in a lot of restrictions for wide range of applications. In this work, we designed a series of azamacrolide chitinase inhibitors based on the structural features of argifin that have high inhibitory activities against bacterial and insectile chitinase. The most potent chitinase inhibitor compound 19c exhibited IC₅₀ values of 56 nM and 110 nM against OfChi-h and SmChiB, respectively. The molecular docking and molecular dynamics simulations revealed that all inhibitors were bound to the -1 subsite of chitinases via N-methylcarbamoylguanidinyl as well as argifin. Finally, a bioactivity assay against pests was carried out. Compound 18a showed 80% mortality for Mythimna separata at a concentration of 50 mg/L. Besides, insecticides 19b and 19c exhibited high mortality against Plutella xylostella (76 and 73% mortalities at 50 mg/L, respectively).

Design and synthesis of a novel inhibitor of T. Viride chitinase through an in silico target fishing protocol

In the last ten years, we identified and developed a new therapeutic class of antifungal agents, the macrocyclic amidinoureas. These compounds are active against several Candida species, including clinical isolates resistant to currently available antifungal drugs. The mode of action of these molecules is still unknown. In this work, we developed an in-silico target fishing procedure to identify a possible target for this class of compounds based on shape similarity, inverse docking procedure and consensus score rank-by-rank. Chitinase enzyme emerged as possible target. To confirm this hypothesis a novel macrocyclic derivative has been produced, specifically designed to increase the inhibition of the chitinase. Biological evaluation highlights a stronger enzymatic inhibition for the new derivative, while its antifungal activity drops probably because of pharmacokinetic issues. Collectively, our data suggest that chitinase represent at least one of the main target of macrocyclic amidinoureas.

Total synthesis of the putative structure of the proposed Banyasin A

The first total synthesis of four possible isomers of a molecule possessing the configuration proposed for Banyasin A is described. The structure synthesized appears to be different from that of the natural product.

Direct arginine modification in native peptides and application to chemical probe development

An efficient method for the direct labeling of the Arg guanidinium group in native peptides is reported. This straightforward procedure allows modifying the arginine moiety in peptides with various reporter groups, such as fluorophores, biotin, etc., under mild conditions in an operationally simple procedure. The scope of this method tolerates various functionalized amino acids such as His, Ser, Trp, Tyr, Glu, etc., while the only limitations uncovered so far are restricted to cysteine and free amine residues. The utility of this late-stage diversification method was demonstrated in direct labeling of leuprolide, a clinically used drug, for distribution monitoring in Daphnia, and in labeling of microcystin, a cyanobacterial toxin.

Fully deacetylated chitooligosaccharides act as efficient glycoside hydrolase family 18 chitinase inhibitors

Small molecule inhibitors against chitinases have potential applications as pesticides, fungicides, and antiasthmatics. Here, we report that a series of fully deacetylated chitooligosaccharides (GlcN)2-7 can act as inhibitors against the insect chitinase OfChtI, the human chitinase HsCht, and the bacterial chitinases SmChiA and SmChiB with IC50 values at micromolar to millimolar levels. The injection of mixed (GlcN)2-7 into the fifth instar larvae of the insect Ostrinia furnacalis resulted in 85% of the larvae being arrested at the larval stage and death after 10 days, also suggesting that (GlcN)2-7 might inhibit OfChtI in vivo. Crystal structures of the catalytic domain of OfChtI (OfChtI-CAD) complexed with (GlcN)5,6 were obtained at resolutions of 2.0 Å. These structures, together with mutagenesis and thermodynamic analysis, suggested that the inhibition was strongly related to the interaction between the -1 GlcN residue of the inhibitor and the catalytic Glu(148) of the enzyme. Structure-based comparison showed that the fully deacetylated chitooligosaccharides mimic the substrate chitooligosaccharides by binding to the active cleft. This work first reports the inhibitory activity and proposed inhibitory mechanism of fully deacetylated chitooligosaccharides. Because the fully deacetylated chitooligosaccharides can be easily derived from chitin, one of the most abundant materials in nature, this work also provides a platform for developing eco-friendly inhibitors against chitinases.

[Study on the discovery of novel chitinase inhibitors based on natural products]

Chitin, the second most abundant polysaccharide in nature, is a constituent of fungal cell walls, the exoskeletons of crustaceans and insects and the microfilarial sheaths of parasitic nematodes. Chitin has, so far, not been found in mammals. Accumulation of chitin by organisms is modulated by chitin synthase-mediated biosynthesis and by chitinase-mediated hydrolytic degradation. Thus, chitinases are expected to be specific targets for antifungal, insecticidal and antiparasitic agents. Paradoxically, while chitin does not exist in mammals, human chitinase family members, such as acidic mammalian chitinase, have recently been described, and offer significant potential for the treatment of asthma and other related diseases in humans. This review covers the development of two chitinase inhibitors of natural origin, Argifin and Argadin, isolated from the cultured broth of microorganisms in our laboratory. In particular, the practical total synthesis of these natural products and discovery methods that generate only highly-active compounds using a kinetic target (chitinase)-guided synthesis approach (termed in situ click chemistry) are described.

Potent family-18 chitinase inhibitors: x-ray structures, affinities, and binding mechanisms

Six novel inhibitors of Vibrio harveyi chitinase A (VhChiA), a family-18 chitinase homolog, were identified by in vitro screening of a library of pharmacologically active compounds. Unlike the previously identified inhibitors that mimicked the reaction intermediates, crystallographic evidence from 14 VhChiA-inhibitor complexes showed that all of the inhibitor molecules occupied the outer part of the substrate-binding cleft at two hydrophobic areas. The interactions at the aglycone location are well defined and tightly associated with Trp-397 and Trp-275, whereas the interactions at the glycone location are patchy, indicating lower affinity and a loose interaction with two consensus residues, Trp-168 and Val-205. When Trp-275 was substituted with glycine (W275G), the binding affinity toward all of the inhibitors dramatically decreased, and in most structures two inhibitor molecules were found to stack against Trp-397 at the aglycone site. Such results indicate that hydrophobic interactions are important for binding of the newly identified inhibitors by the chitinase. X-ray data and isothermal microcalorimetry showed that the inhibitors occupied the active site of VhChiA in three different binding modes, including single-site binding, independent two-site binding, and sequential two-site binding. The inhibitory effect of dequalinium in the low nanomolar range makes this compound an extremely attractive lead compound for plausible development of therapeutics against human diseases involving chitinase-mediated pathologies.

Functional comparison of recombinant acidic mammalian chitinase with enzyme from murine bronchoalveolar lavage

Acidic mammalian chitinase (AMCase) is an enzyme that selectively degrades the biopolymer chitin. Several chitinase enzymes are utilized by mammals to hydrolyze chitin encountered by inhalation and ingestion. AMCase is distinct from other mammalian chitinases as its activity is retained in strongly acidic conditions (pH <2.0). AMCase expression is induced by antigen-induced mouse models of allergic lung inflammation. This protein has also been implicated in the pathogenesis of asthma although its precise role is poorly defined. We describe a novel way to express and purify active murine AMCase. This material retains properties observed in mouse bronchoalveolar lavage (BAL) fluid with regard to pH preference of activity and its inhibition by cyclic peptide inhibitors argifin and argadin. We found that chitinase in BAL from both antigen-challenged and control animals have similar properties in this regard. This strongly supports the notion the same enzyme (AMCase) gives rise to chitinase activity in both challenged and unchallenged animals. We also describe expression of active human AMCase. The methods described in this paper provide a reliable source of recombinant AMCase that can be utilized to expand understanding of AMCase's role in regulating allergic inflammation.

Recent development of two chitinase inhibitors, Argifin and Argadin, produced by soil microorganisms

Chitin, the second most abundant polysaccharide in nature, occurs in fungi, some algae and many invertebrates, including insects. Thus, chitin synthesis and degradation could represent specific targets for fungicides and insecticides. Chitinases hydrolyze chitin into oligomers of N-acetyl-D-glucosamine at key points in the life cycles of organisms, consequently, chitinase inhibitors have become subject of increasing interest. This review covers the development of two chitinase inhibitors of natural origin, Argifin and Argadin, isolated from the cultured broth of microorganisms in our laboratory. In particular, the practical total synthesis of these natural products, the synthesis of lead compounds via computer-aided rational molecular design, and discovery methods that generate only highly-active compounds using a kinetic target(chitinase)-guided synthesis approach (termed in situ click chemistry) are described.

Computer-aided rational molecular design of argifin-derivatives with increased inhibitory activity against chitinase B from Serratia marcescens

Argifin, a novel pentapeptide chitinase inhibitor isolated from Gliocladium fungal culture, is a promising candidate for the development of new fungicides, insecticides, and anti-asthma medications. In this study, we undertook rational molecular design of argifin-derivatives and tested them against chitinase B from Serratia marcescens (SmChiB). The work involved molecular dynamics simulation with explicit water molecules, the molecular docking calculation, and free-energy analysis using the molecular mechanics Poisson-Boltzmann surface area method. The custom-designed derivatives were synthesized via effective solid phase synthesis, developed recently in our laboratory, and their inhibitory activities were measured against SmChiB. Finally, we identified and obtained a derivative which exhibited 28-fold more inhibition than argifin itself, a compound in which the d-Ala(5) of argifin was replaced with d-Leu and the 4-benzylpiperdine was attached to l-Asp(4).

Solid-phase synthesis of cyclic peptide chitinase inhibitors: SAR of the argifin scaffold

A new, highly efficient, all-solid-phase synthesis of argifin, a natural product cyclic pentapeptide chitinase inhibitor, is reported. The synthesis features attachment of an orthogonally protected Asp residue to the solid support and assembly of the linear peptide chain by Fmoc SPPS prior to cyclisation and side-chain manipulation on-resin. Introduction of the key N-methyl carbamoyl-substituted Arg side chain is achieved via derivatisation of a selectively protected Orn residue, prior to cleavage from the resin and side-chain deprotection. A severe aspartimide side-reaction observed upon final deprotection is circumvented by the use of a novel aqueous acidolysis procedure. The flexibility of the synthesis is demonstrated by the preparation of a series of argifin analogues designed from the X-ray structure of the natural product in complex with a representative family 18 chitinase.

Structure-based dissection of the natural product cyclopentapeptide chitinase inhibitor argifin

Chitinase inhibitors have chemotherapeutic potential as fungicides, pesticides, and antiasthmatics. Argifin, a natural product cyclopentapeptide, competitively inhibits family 18 chitinases in the nanomolar to micromolar range and shows extensive substrate mimicry. In an attempt to map the active fragments of this large natural product, the cyclopentapeptide was progressively dissected down to four linear peptides and dimethylguanylurea, synthesized using a combination of solution and solid phase peptide synthesis. The peptide fragments inhibit chitinase B1 from Aspergillus fumigatus (AfChiB1), the human chitotriosidase, and chitinase activity in lung homogenates from a murine model of chronic asthma, with potencies ranging from high nanomolar to high micromolar inhibition. X-ray crystallographic analysis of the chitinase-inhibitor complexes revealed that the conformations of the linear peptides were remarkably similar to that of the natural product. Strikingly, the dimethylguanylurea fragment, representing only a quarter of the natural product mass, was found to harbor all significant interactions with the protein and binds with unusually high efficiency. The data provide useful information that could lead to the generation of drug-like, natural product-based chitinase inhibitors.