Synthesis, Evaluation, and Mechanism of N,N,N-Trimethyl-D-glucosamine-(1→4)-chitooligosaccharides as Selective Inhibitors of Glycosyl Hydrolase Family 20 β-N-Acetyl-D-hexosaminidases

Efficient and versatile formation of glycosidic bonds via catalytic strain-release glycosylation with glycosyl ortho-2,2-dimethoxycarbonylcyclopropylbenzoate donors

Catalytic glycosylation is a vital transformation in synthetic carbohydrate chemistry due to its ability to expediate the large-scale oligosaccharide synthesis for glycobiology studies with the consumption of minimal amounts of promoters. Herein we introduce a facile and efficient catalytic glycosylation employing glycosyl ortho-2,2-dimethoxycarbonylcyclopropylbenzoates (CCBz) promoted by a readily accessible and non-toxic Sc(III) catalyst system. The glycosylation reaction involves a novel activation mode of glycosyl esters driven by the ring-strain release of an intramolecularly incorporated donor-acceptor cyclopropane (DAC). The versatile glycosyl CCBz donor enables highly efficient construction of O-, S-, and N-glycosidic bonds under mild conditions, as exemplified by the convenient preparation of the synthetically challenging chitooligosaccharide derivatives. Of note, a gram-scale synthesis of tetrasaccharide corresponding to Lipid IV with modifiable handles is achieved using the catalytic strain-release glycosylation. These attractive features promise this donor to be the prototype for developing next generation of catalytic glycosylation.

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.

Structure-based virtual screening for potent inhibitors of GH-20 β-N-acetylglucosaminidase: classical and machine learning scoring functions, and molecular dynamics simulations

GH-20 β-N-acetylglucosaminidases (GlcNAcases) are promising targets in the development of antimicrobial agents against Vibrio infections in humans and aquatic animals. In this study, we set up structure-based virtual screening to identify potential GH-20 GlcNAcase inhibitors from the Reaxys commercial database, using VhGlcNAcase from V. campbellii type strain ATCC® BAA 1116 as the protein target and Redoxal as the reference ligand. Using ChemPLP and RF-Score-VS machine learning scoring functions, eight lead compounds were identified and further evaluated for protein interaction preference and pharmacological properties. Protein-ligand analysis demonstrated that all selected compounds interacted exclusively at subsite - 1 with five hydrophobic residues W487, W505, W546, W582 and V544 at site S1, and with two polar residues, D437 and E438, at site 3. For subsite + 1, the most common residues were R274 and E584 at site 2 and I397 and Q398 at site 4. Based on the data obtained from binding free energy changes (ΔG°_binding), pharmacological property analysis and molecular dynamic simulations, two ChemPLP compounds, 338175 and 1146525, and one RF-Score-VS compound, 337447, were considered as the likely lead compounds. The most promising compound, 1146525, could serve as a scaffold for the future design of novel antimicrobial agents against Vibrio infections.

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.

Kinetic and thermodynamic insights into the inhibitory mechanism of TMG-chitotriomycin on Vibrio campbellii GH20 exo-β-N-acetylglucosaminidase

We investigated the inhibition kinetics of VhGlcNAcase, a GH20 exo-β-N-acetylglucosaminidase (GlcNAcase) from the marine bacterium Vibrio campbellii (formerly V. harveyi) ATCC BAA-1116, using TMG-chitotriomycin, a natural enzyme inhibitor specific for GH20 GlcNAcases from chitin-processing organisms, with p-nitrophenyl N-acetyl-β-d-glucosaminide (pNP-GlcNAc) as the substrate. TMG-chitotriomycin inhibited VhGlcNAcase with an IC₅₀ of 3.0 ± 0.7 μM. Using Dixon plots, the inhibition kinetics indicated that TMG-chitotriomycin is a competitive inhibitor, with an inhibition constant K_i of 2.2 ± 0.3 μM. Isothermal titration calorimetry experiments provided the thermodynamic parameters for the binding of TMG-chitotriomycin to VhGlcNAcase and revealed that binding was driven by both favorable enthalpy and entropy changes (ΔH° = -2.5 ± 0.1 kcal/mol and -TΔS° = -5.8 ± 0.3 kcal/mol), resulting in a free energy change, ΔG°, of -8.2 ± 0.2 kcal/mol. Dissection of the entropic term showed that a favorable solvation entropy change (-TΔS_solv° = -16 ± 2 kcal/mol) is the main contributor to the entropic term.

Assembly of Peptidoglycan Fragments-A Synthetic Challenge

Peptidoglycan (PGN) is a major constituent of most bacterial cell walls that is recognized as a primary target of the innate immune system. The availability of pure PGN molecules has become key to different biological studies. This review aims to (1) provide an overview of PGN biosynthesis, focusing on the main biosynthetic intermediates; (2) focus on the challenges for chemical synthesis posed by the unique and complex structure of PGN; and (3) cover the synthetic routes of PGN fragments developed to date. The key difficulties in the synthesis of PGN molecules mainly involve stereoselective glycosylation involving NAG derivatives. The complex synthesis of the carbohydrate backbone commonly involves multistep sequences of chemical reactions to install the lactyl moiety at the O-3 position of NAG derivatives and to control enantioselective glycosylation. Recent advances are presented and synthetic routes are described according to the main strategy used: (i) based on the availability of starting materials such as glucosamine derivatives; (ii) based on a particular orthogonal synthesis; and (iii) based on the use of other natural biopolymers as raw materials.

Temporary ether protecting groups at the anomeric center in complex carbohydrate synthesis

The synthesis of a carbohydrate building block usually starts with introduction of a temporary protecting group at the anomeric center and ends with its selective cleavage for further transformation. Thus, the choice of the anomeric temporary protecting group must be carefully considered because it should retain intact during the whole synthetic manipulation, and it should be chemoselectively removable without affecting other functional groups at a late stage in the synthesis. Etherate groups are the most widely used temporary protecting groups at the anomeric center, generally including allyl ethers, MP (p-methoxyphenyl) ethers, benzyl ethers, PMB (p-methoxybenzyl) eithers, and silyl ethers. This chapter provides a comprehensive review on their formation, cleavage, and applications in the synthesis of complex carbohydrates.

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.

Computational Screening of Potential Inhibitors of β-N-Acetyl-D-Hesosaminidases Using Combined Core-Fragment Growth and Pharmacophore Restraints

As a type of β-N-acetyl-D-hexosaminidase enzyme purified from the Ostriniafurnacalis (Asian corn borer), OfHex1 has been previously reported to participate in chitin degradation, indicating that it may be an ideal target for designing new environmentally friendly pesticides. Besides, a natural product, TMG-chitotriomycin, has been found to be an effective inhibitor of OfHex1, and some studies have shown that the interactions between TMG unit and residues in - 1 subsite of OfHex1 are very conservative and important, inspiring us to design new inhibitors of β-N-acetyl-D-hexosaminidase with a new strategy. In the present study, the virtual screening of TMG unit as the core fragment was conducted using the combined computational methods, such as docking, molecular dynamics, pharmacophore model, and pesticide-likeness rule. Nine compounds with the binding free energy lower than TMG-β-(GlcNAc)₂ were obtained. According to the decomposition energy and the interactions analysis, compounds 2, 3, 6 and 8, forming the hydrogen bonds with Val327 and Trp490, were considered as the possible lead structures for the further study. Our findings indicated that fragment-based lead discovery strategy might provide valuable insights into designing novel potential OfHex1 inhibitors.

Insights into the structure-affinity relationships and solvation effects between OfHex1 and inhibitors using molecular dynamics simulations

OfHex1 is a potential target for the rational design of pesticides. TMG-chitotriomycin is one of the most highly specific known inhibitors of chitinolytic β-GlcNAcases from bacteria, fungi and insects. TMG-chitotriomycin and its analogues show different activities to OfHex1, dependent on the number of GlcNAc units. Subsequently, it is essential to explore how these GlcNAc unit number changes cause alterations in activity. In this study, we examined the free energy patterns and per residue decomposition of binding within the complexes of OfHex1 and a series of inhibitors, utilizing restricted molecular dynamics (MD) and water-mediated MM/GBSA calculations. The results indicated Glu328 could form a stronger polar interaction with OfHex1 inhibitors, while Trp448 and Trp490 had important non-polar contributions. Interestingly, the conformation of Trp448 was different in the open or closed state, when OfHex1 bound different inhibitors. Moreover, the water molecule that mediates the GlcNAc Ⅱ and Trp490 may be critical to stabilizing the hydrophobic interaction. Further study showed that isomerization of TMG-chitotriomycin analogs did not decrease binding affinity, however, there was a highly positive correlation between the calculated binding affinities and the experimental activity data (r² = 0.92) when water molecules were explicitly taken into account. Moreover, the water molecules that mediated GlcNAc II and Trp490 might be critical to the stabilization of the hydrophobic interaction and cause the activity difference between TMG-(GlcNAc)₂ and TMG-(GlcNAc).

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.

Glycoside hydrolase family 18 and 20 enzymes are novel targets of the traditional medicine berberine

Berberine is a traditional medicine that has multiple medicinal and agricultural applications. However, little is known about whether berberine can be a bioactive molecule toward carbohydrate-active enzymes, which play numerous vital roles in the life process. In this study, berberine and its analogs were discovered to be competitive inhibitors of glycoside hydrolase family 20 β-N-acetyl-d-hexosaminidase (GH20 Hex) and GH18 chitinase from both humans and the insect pest Ostrinia furnacalis Berberine and its analog SYSU-1 inhibit insect GH20 Hex from O. furnacalis (OfHex1), with K_i values of 12 and 8.5 μm, respectively. Co-crystallization of berberine and its analog SYSU-1 in complex with OfHex1 revealed that the positively charged conjugate plane of berberine forms π-π stacking interactions with Trp⁴⁹⁰, which are vital to its inhibitory activity. Moreover, the 1,3-dioxole group of berberine binds an unexplored pocket formed by Trp³²², Trp⁴⁸³, and Val⁴⁸⁴, which also contributes to its inhibitory activity. Berberine was also found to be an inhibitor of human GH20 Hex (HsHexB), human GH18 chitinase (HsCht and acidic mammalian chitinase), and insect GH18 chitinase (OfChtI). Besides GH18 and GH20 enzymes, berberine was shown to weakly inhibit human GH84 O-GlcNAcase (HsOGA) and Saccharomyces cerevisiae GH63 α-glucosidase I (ScGluI). By analyzing the published crystal structures, berberine was revealed to bind with its targets in an identical mechanism, namely via π-π stacking and electrostatic interactions with the aromatic and acidic residues in the binding pockets. This paper reports new molecular targets of berberine and may provide a berberine-based scaffold for developing multitarget drugs.

Gold-catalyzed glycosylation in the synthesis of complex carbohydrate-containing natural products

Gold(i)- and gold(iii)-catalyzed glycosylation reactions and their application in the synthesis of natural glycoconjugates are reviewed.

Total synthesis of TMG-chitotriomycin based on an automated electrochemical assembly of a disaccharide building block

The total synthesis of TMG-chitotriomycin using an automated electrochemical synthesizer for the assembly of carbohydrate building blocks is demonstrated. We have successfully prepared a precursor of TMG-chitotriomycin, which is a structurally-pure tetrasaccharide with typical protecting groups, through the methodology of automated electrochemical solution-phase synthesis developed by us. The synthesis of structurally well-defined TMG-chitotriomycin has been accomplished in 10-steps from a disaccharide building block.

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: An update for 2011-2012

This review is the seventh update of the original article published in 1999 on the application of MALDI mass spectrometry to the analysis of carbohydrates and glycoconjugates and brings coverage of the literature to the end of 2012. General aspects such as theory of the MALDI process, matrices, derivatization, MALDI imaging, and fragmentation are covered in the first part of the review and applications to various structural types constitute the remainder. The main groups of compound are oligo- and poly-saccharides, glycoproteins, glycolipids, glycosides, and biopharmaceuticals. Much of this material is presented in tabular form. Also discussed are medical and industrial applications of the technique, studies of enzyme reactions, and applications to chemical synthesis. © 2015 Wiley Periodicals, Inc. Mass Spec Rev 36:255-422, 2017.

Structure, Catalysis, and Inhibition of OfChi-h, the Lepidoptera-exclusive Insect Chitinase

Chitinase-h (Chi-h) is of special interest among insect chitinases due to its exclusive distribution in lepidopteran insects and high sequence identity with bacterial and baculovirus homologs. Here OfChi-h, a Chi-h from Ostrinia furnacalis, was investigated. Crystal structures of both OfChi-h and its complex with chitoheptaose ((GlcN)₇) reveal that OfChi-h possesses a long and asymmetric substrate binding cleft, which is a typical characteristics of a processive exo-chitinase. The structural comparison between OfChi-h and its bacterial homolog SmChiA uncovered two phenylalanine-to-tryptophan site variants in OfChi-h at subsites +2 and possibly -7. The F232W/F396W double mutant endowed SmChiA with higher hydrolytic activities toward insoluble substrates, such as insect cuticle, α-chitin, and chitin nanowhisker. An enzymatic assay demonstrated that OfChi-h outperformed OfChtI, an insect endo-chitinase, toward the insoluble substrates, but showed lower activity toward the soluble substrate ethylene glycol chitin. Furthermore, OfChi-h was found to be inhibited by N,N',N″-trimethylglucosamine-N,N',N″,N″'-tetraacetylchitotetraose (TMG-(GlcNAc)₄), a substrate analog which can be degraded into TMG-(GlcNAc)_1-2 Injection of TMG-(GlcNAc)₄ into 5th-instar O. furnacalis larvae led to severe defects in pupation. This work provides insights into a molting-indispensable insect chitinase that is phylogenetically closer to bacterial chitinases than insect chitinases.

Exploring NAG-thiazoline and its derivatives as inhibitors of chitinolytic β-acetylglucosaminidases

NAG-thiazoline (NGT) and its derivatives are well-known inhibitors against most β-acetylglucosaminidases (β-GlcNAcases) except for insect and bacterial chitinolytic β-GlcNAcases, including the molting-indispensable OfHex1 from the insect Ostrinia furnacalis. Here, we report the co-crystal structure of OfHex1 in complex with NGT. This structure reveals a large active pocket in OfHex1 that may account for the poor inhibitory activity of NGT. To test this hypothesis, a bulky substituent was designed and synthesized on the thiazoline ring of NGT. The resulting compound (NMAGT) was determined to be a submicromolar inhibitor of OfHex1 with a Ki value of 0.13 μM, which is 600-fold lower than Ki value of NGT. Molecular dynamics simulation analysis supported the good fit of NMAGT to the active pocket.

A crystal structure-guided rational design switching non-carbohydrate inhibitors' specificity between two β-GlcNAcase homologs

Selective inhibition of function-specific β-GlcNAcase has great potential in terms of drug design and biological research. The symmetrical bis-naphthalimide M-31850 was previously obtained by screening for specificity against human glycoconjugate-lytic β-GlcNAcase. Using protein-ligand co-crystallization and molecular docking, we designed an unsymmetrical dyad of naphthalimide and thiadiazole, Q2, that changes naphthalimide specificity from against a human glycoconjugate-lytic β-GlcNAcase to against insect and bacterial chitinolytic β-GlcNAcases. The crystallographic and in silico studies reveal that the naphthalimide ring can be utilized to bind different parts of these enzyme homologs, providing a new starting point to design specific inhibitors. Moreover, Q2-induced closure of the substrate binding pocket is the structural basis for its 13-fold increment in inhibitory potency. Q2 is the first non-carbohydrate inhibitor against chitinolytic β-GlcNAcases. This study provides a useful example of structure-based rationally designed inhibitors as potential pharmaceuticals or pesticides.

Crystallographic structure of ChitA, a glycoside hydrolase family 19, plant class IV chitinase from Zea mays

Maize ChitA chitinase is composed of a small, hevein-like domain attached to a carboxy-terminal chitinase domain. During fungal ear rot, the hevein-like domain is cleaved by secreted fungal proteases to produce truncated forms of ChitA. Here, we report a structural and biochemical characterization of truncated ChitA (ChitA ΔN), which lacks the hevein-like domain. ChitA ΔN and a mutant form (ChitA ΔN-EQ) were expressed and purified; enzyme assays showed that ChitA ΔN activity was comparable to the full-length enzyme. Mutation of Glu62 to Gln (ChitA ΔN-EQ) abolished chitinase activity without disrupting substrate binding, demonstrating that Glu62 is directly involved in catalysis. A crystal structure of ChitA ΔN-EQ provided strong support for key roles for Glu62, Arg177, and Glu165 in hydrolysis, and for Ser103 and Tyr106 in substrate binding. These findings demonstrate that the hevein-like domain is not needed for enzyme activity. Moreover, comparison of the crystal structure of this plant class IV chitinase with structures from larger class I and II enzymes suggest that class IV chitinases have evolved to accommodate shorter substrates.

TMG-chitotriomycin as a probe for the prediction of substrate specificity of β-N-acetylhexosaminidases

TMG-chitotriomycin (1) produced by the actinomycete Streptomyces annulatus NBRC13369 was examined as a probe for the prediction of substrate specificity of β-N-acetylhexosaminidases (HexNAcases). According to the results of inhibition assays, 14 GH20 HexNAcases from various organisms were divided into 1-sensitive and 1-insensitive enzymes. Three representatives of each group were investigated for their substrate specificity. The 1-sensitive HexNAcases hydrolyzed N-acetylchitooligosaccharides but not N-glycan-type oligosaccharides, whereas the 1-insensitive enzymes hydrolyzed N-glycan-type oligosaccharides but not N-acetylchitooligosaccharides, indicating that TMG-chitotriomycin can be used as a molecular probe to distinguish between chitin-degrading HexNAcases and glycoconjugate-processing HexNAcases.

Structural insights into cellulolytic and chitinolytic enzymes revealing crucial residues of insect β-N-acetyl-D-hexosaminidase

The chemical similarity of cellulose and chitin supports the idea that their corresponding hydrolytic enzymes would bind β-1,4-linked glucose residues in a similar manner. A structural and mutational analysis was performed for the plant cellulolytic enzyme BGlu1 from Oryza sativa and the insect chitinolytic enzyme OfHex1 from Ostrinia furnacalis. Although BGlu1 shows little amino-acid sequence or topological similarity with OfHex1, three residues (Trp⁴⁹⁰, Glu³²⁸, Val³²⁷ in OfHex1, and Trp³⁵⁸, Tyr¹³¹ and Ile¹⁷⁹ in BGlu1) were identified as being conserved in the +1 sugar binding site. OfHex1 Glu³²⁸ together with Trp⁴⁹⁰ was confirmed to be necessary for substrate binding. The mutant E328A exhibited a 8-fold increment in K_m for (GlcNAc)₂ and a 42-fold increment in K_i for TMG-chitotriomycin. A crystal structure of E328A in complex with TMG-chitotriomycin was resolved at 2.5 Å, revealing the obvious conformational changes of the catalytic residues (Glu³⁶⁸ and Asp³⁶⁷) and the absence of the hydrogen bond between E328A and the C3-OH of the +1 sugar. V327G exhibited the same activity as the wild-type, but acquired the ability to efficiently hydrolyse β-1,2-linked GlcNAc in contrast to the wild-type. Thus, Glu³²⁸ and Val³²⁷ were identified as important for substrate-binding and as glycosidic-bond determinants. A structure-based sequence alignment confirmed the spatial conservation of these three residues in most plant cellulolytic, insect and bacterial chitinolytic enzymes.

Scalable syntheses of both enantiomers of DNJNAc and DGJNAc from glucuronolactone: the effect of N-alkylation on hexosaminidase inhibition

The efficient scalable syntheses of 2-acetamido-1,2-dideoxy-D-galacto-nojirimycin (DGJNAc) and 2-acetamido-1,2-dideoxy-D-gluco-nojirimycin (DNJNAc) from D-glucuronolactone, as well as of their enantiomers from L-glucuronolactone, are reported. The evaluation of both enantiomers of DNJNAc and DGJNAc, along with their N-alkyl derivatives, as glycosidase inhibitors showed that DGJNAc and its N-alkyl derivatives were all inhibitors of α-GalNAcase but that none of the epimeric DNJNAc derivatives inhibited this enzyme. In contrast, both DGJNAc and DNJNAc, as well as their alkyl derivatives, were potent inhibitors of β-GlcNAcases and β-GalNAcases. Neither of the L-enantiomers showed any significant inhibition of any of the enzymes tested. Correlation of the in vitro inhibition with the cellular data, by using a free oligosaccharide analysis of the lysosomal enzyme inhibition, revealed the following structure-property relationship: hydrophobic side-chains preferentially promoted the intracellular access of iminosugars to those inhibitors with more-hydrophilic side-chain characteristics.

Active-pocket size differentiating insectile from bacterial chitinolytic β-N-acetyl-D-hexosaminidases

Chitinolytic β-N-acetyl-D-hexosaminidase is a branch of the GH20 (glycoside hydrolase family 20) β-N-acetyl-D-hexosaminidases that is only distributed in insects and micro-organisms, and is therefore a potential target for the action of insecticides. PUGNAc [O-(2-acetamido-2-deoxy-D-glucopyransylidene)-amino-N-phenylcarbamate] was initially identified as an inhibitor against GH20 β-N-acetyl-D-hexosaminidases. So far no crystal structure of PUGNAc in complex with any GH20 β-N-acetyl-D-hexosaminidase has been reported. We show in the present study that the sensitivities of chitinolytic β-N-acetyl-D-hexosaminidases towards PUGNAc can vary by 100-fold, with the order being OfHex1 (Ostrinia furnacalis β-N-acetyl-D-hexosaminidase)<SmCHB (Serratia marcescens chitobiase)<SpHex (Streptomyces plicatus β-N-acetyl-D-hexosaminidase). To explain this difference, the crystal structures of wild-type OfHex1 as well as mutant OfHex1(V327G) in complex with PUGNAc were determined at 2.0 Å (1 Å=0.1 nm) and 2.3 Å resolutions and aligned with the complex structures of SpHex and SmCHB. The results showed that the sensitivities of these enzymes to PUGNAc were determined by the active pocket size, with OfHex1 having the largest but narrowest entrance, whereas SpHex has the smallest entrance, suitable for holding the inhibitor, and SmCHB has the widest entrance. By widening the size of the active pocket entrance of OfHex1 through replacing the active site Val327 with a glycine residue, the sensitivity of OfHex1 to PUGNAc became similar to that of SmCHB. The structural differences among chitinolytic β-N-acetyl-D-hexosaminidases leading to different sensitivities to PUGNAc may be useful for developing species-specific pesticides and bactericides.