Reassessment of chitosanase substrate specificities and classification

Heterogeneously deacetylated chitosans possess an unexpected regular pattern favoring acetylation at every third position

Chitosans are promising biopolymers for diverse applications, with material properties and bioactivities depending i.a. on their pattern of acetylation (PA). Commercial chitosans are typically produced by heterogeneous deacetylation of chitin, but whether this process yields chitosans with a random or block-wise PA has been debated for decades. Using a combination of recently developed in vitro assays and in silico modeling surprisingly revealed that both hypotheses are wrong; instead, we found a more regular PA in heterogeneously deacetylated chitosans, with acetylated units overrepresented at every third position in the polymer chain. Compared to random-PA chitosans produced by homogeneous deacetylation of chitin or chemical N-acetylation of polyglucosamine, this regular PA increases the elicitation activity in plants, and generates different product profiles and distributions after enzymatic and chemical cleavage. A regular PA may be beneficial for some applications but detrimental for others, stressing the relevance of the production process for product development.

Crystal structure of the GH-46 subclass III chitosanase from Bacillus circulans MH-K1 in complex with chitotetraose

BACKGROUND

Chitosanases (EC 3.2.1.132) hydrolyze chitosan which is a polymer of glucosamine (GlcN) linked by β - 1,4 bonds, and show cleavage specificity against partially acetylated chitosan containing N-acetylglucosamine (GlcNAc) residues. Chitosanases' structural underpinnings for cleavage specificity and the conformational switch from open to closed structures are still a mystery.

METHODS

The GH-46 subclass III chitosanase from Bacillus circulans MH-K1 (MH-K1 chitosanase), which also catalyzes the hydrolysis of GlcN-GlcNAc bonds in addition to GlcN-GlcN, has had its chitotetraose [(GlcN)4]-complexed crystal structure solved at 1.35 Å resolution.

RESULTS

The MH-K1 chitosanase's (GlcN)4-bound structure has numerous structural similarities to other GH-46 chitosanases in terms of substrate binding and catalytic processes. However, subsite -1, which is absolutely specific for GlcN, seems to characterize the structure of a subclass III chitosanase due to its distinctive length and angle of a flexible loop. According to a comparison of the (GlcN)4-bound and apo-form structures, the particular binding of a GlcN residue at subsite -2 through Asp77 causes the backbone helix to kink, which causes the upper- and lower-domains to approach closely when binding a substrate.

CONCLUSIONS

Although GH-46 chitosanases vary in the finer details of the subsites defining cleavage specificity, they share similar structural characteristics in substrate-binding, catalytic processes, and potentially in conformational change.

GENERAL SIGNIFICANCE

The precise binding of a GlcN residue to the -2 subsite is essential for the conformational shift that occurs in all GH-46 chitosanases, as shown by the crystal structures of the apo- and substrate-bound forms of MH-K1 chitosanase.

Fast insights into chitosan-cleaving enzymes by simultaneous analysis of polymers and oligomers through size exclusion chromatography

The thorough characterization of chitosan-cleaving enzymes is crucial to unveil structure-function relationships of this promising class of biomolecules for both, enzymatic fingerprinting analyses and to use the enzymes as biotechnological tools to produce tailor-made chitosans for diverse applications. Analyzing polymeric substrates as well as oligomeric products has been established as an effective way to understand the actions of enzymes, but it currently requires separate, rather laborious methods to obtain the full picture. Here, we present ultra high performance size exclusion chromatography coupled to refractive index and mass spectrometry detection (UHPSEC-RI-MS) as a straightforward method for the semi-quantitative analysis of chitosan oligomers of up to ten monomers in length. Additionally, the method allows to determine the average molecular weight of the remaining polymers and its distribution. By sampling live from an ongoing enzymatic reaction, UHPSEC-RI-MS offers the unique opportunity to analyze polymers and oligomers simultaneously-i.e., to monitor the molecular weight reduction of the polymeric substrate over the course of the digestion, while at the same time analyzing the emerging oligomeric products in a semi-quantitative manner. In this way, a single simple analysis yields detailed insights into an enzyme's action on a given substrate.

Identification and evaluation of CAZyme genes, along with functional characterization of a new GH46 chitosanase from Streptomyces sp. KCCM12257

The genomic analysis of Streptomyces sp. KCCM12257 presented 233 CAZyme genes with a predominant glycosyl hydrolase family. This contributes degradation of various polysaccharides including chitin and chitosan, and other promising candidates for the production of different oligosaccharides. We screened the strain providing different polysaccharides as a sole source of carbon and strain KCCM12257, showed higher activity towards colloidal chitosan. Further, we identified and characterized a new chitosanase (MDI5907146) of GH46 family. There was no activity towards chitin, carboxymethylcellulose, or even with chitosan powder. This enzyme acts on colloidal chitosan and hydrolyzes it down into monoacetyl chitobiose, which consists of two glucosamine units with an acetyl group attached to them. The maximum enzyme activity was observed at pH 6.5 and 40 °C using colloidal chitosan as a substrate. The Co2+ metal ions almost double the reaction as compared to other metal ions. The dissociation constant (Km) and of colloidal chitosan (≥90 % and ≥75%DD) were 3.03 mg/ml and 5.01 mg/ml respectively, while maximum velocity (Vmax) values were found to be 36 mg/ml, and 30 μM/μg/min, respectively. Similarly, catalytic efficiency (Kcat/Km) of colloidal chitosan with ≥90 %DD was 1.9 fold higher than colloidal chitosan with ≥75%DD.

Functional Properties of Chitosan Oligomers Obtained by Enzymatic Hydrolysis

The aims of this study were to obtain chitooligosaccharides (COS) from chitosan (CH) with improved functional properties and comparison of the use of two different enzymes: commercial cellulase (CL) and the dedicated enzyme chitosanase (CS). After enzymatic reaction, chitosan oligomers (NFs) were isolated by methanol into two fractions: precipitate (HMF) and supernatant (LMF). The occurrence of a hydrolysis reaction was confirmed by an increased reducing sugar content and viscosity reduction of chitosan oligomers. CPMAS 13C NMR analysis confirmed the dissimilar cleavage mechanism of the enzymes used. LMF and NF fractions were characterised by improved solubility in water (94.56%) compared to the HMF and CH samples (70.64%). Thermogravimetric analysis (TGA) showed that the HMF decomposed in two-stage process while CH, NF, and LMF decomposed in a three-stage process. The greatest mass loss of LMF samples (58.35%) suggests their sensitivity to high-temperature treatments. COS were a mixture of DP (degrees of polymerisation) from 3 to 18 hetero-chitooligomers, with an average Mw of <3 kDa. CL consisted of more low-DP products (DP 3-7) than COS made with CS. LMF characterised by DP~2 showed lower DPPH radical scavenging activity than HMF and NF with DP 3-7. The ability to reduce Escherichia coli increased in the given order: LMF > NF > HMF > CH.

Production, statistical evaluation and characterization of chitosanase from Fusarium oxysporum D18

Purpose

The present research work focuses on the extraction of chitosanase enzyme from soil fungi. Chitosan hydrolysis by chitosanase is one of the most effective methods to produce chitosan oligosaccharides which are new biomaterials that have many biological activities such as antitumour, antioxidant, antidiabetic and antimicrobial.

Method

A strain producing chitosanase was screened and identified as Fusarium oxysporum D18 with an accession number OL343607. Various physiological parameters (incubation type, carbon source, additive nitrogen source, statistical evaluation, solid state fermentation) were assessed to increase chitosanase production.

Results

Fusarium oxysporum D18 produced a considerable value of chitosanase (1.220 U/ml). After 7 days of incubation, the best carbon source was lactose, and the best nitrogen source was ammonium chloride. Statistical evaluation was carried out by using Plackett–Burman and Box-Behnken designs. The highest chitosanase production (1.994 U/ml) was induced by the medium composition g/l: KH2PO4 (1.5), MgSO4 (0.269), lactose (18), NH4Cl (1.26), pH (6.68), using a 5-day-old inoculum and chitosanase activity was 1.63 folds that of the original medium. The production of chitosanase by Fusarium oxysporum D18 in solid state cultures using different solid substrates was studied and the best solid substrate for higher chitosanase activity (2.246 U/ml) was raw shrimp heads and shells and chitosanase activity was 1.13 folds that of the optimized liquid cultures. An extracellular chitosanase was isolated and partially purified by using 75% saturation of ammonium sulphate. The highest chitosanase activity (3.667 U/ml) with a specific activity of 0.390 U/mg protein was obtained at enzyme protein concentration of 9.391 mg/ml, substrate concentration of 1.2 % (w/v), Vmax of the enzyme of approximately 0.430 U/mg protein, and KM of 0.26 % (w/v), at pH 5.6 and reaction temperature of 50 °C. The activity of the purified and characterized chitosanase increased by 3 times than that the original isolate activity. The enzyme was thermostable and retained about 55% of its original activity after heating at 70 °C for 15 min. The enzyme preparations were activated by Ca2+ ions and inactivated by Zn+2, Cu+2 ions, and EDTA.

Conclusion

An antitumour activity of chitooligosaccharides produced by the chitosanase was applied to the MCF-7 (breast carcinoma cells) and they had a cytotoxicity inhibitory effect against them about IC50 = 448 μg/ml.

Production, Statistical Evaluation and Characterization of Chitosanase from Fusarium oxysporum D18.. [DOI: 10.21203/rs.3.rs-2898996/v1] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

Purpose The present research work focuses on the extraction of chitosanase enzyme from soil fungi. Chitosan hydrolysis by chitosanase is one of the most effective methods to produce chitosan oligosaccharides which are new biomaterials that have many biological activities such as antitumor, antioxidant, antidiabetic and antimicrobial. Method: A strain producing chitosanase was screened and identified as Fusarium oxysporum D18 with an accession number OL343607. Various physiological parameters (incubation type, carbon source, additive nitrogen source, statistical evaluation, solid state fermentation) were assessed to increase chitosanase production. Results: Fusarium oxysporum D18 produced a considerable value of chitosanase, (1.220 U/ml). after 7 days of incubation, the best carbon source was lactose, and the best nitrogen source was ammonium chloride. Statistical evaluation was carried out by using Plackett-Burman and Box-Behnken designs. The highest chitosanase production, (1.994 U/ml) was induced by the medium composition g/L: KH2PO4 (1.5), MgSO4 (0.269), lactose (18), NH4Cl (1.26), pH (6.68), using a 5-day old inoculum and chitosanase activity was 1.63 folds that of the original medium. The production of chitosanase by Fusarium oxysporum D18 in solid state cultures using different solid substrates was studied and the best solid substrate for higher chitosanase activity (2.246 U/ml) was raw shrimp heads and shells and chitosanase activity was 1.13 folds that of the optimized liquid cultures. An extracellular chitosanase was isolated and partially purified by using 75 % saturation of ammonium sulphate. The highest chitosanase activity (3.667 U/ml) was obtained at enzyme protein concentration, (9.391 mg/ml), substrate concentration, (1.20%), Vmax of the enzyme was approximately (4.04 U/ml), km was (0.26%), at pH, (5.6) and reaction temperature, (50°C). The activity of the purified and characterized chitosanase increased by 3 times than that the original isolate activity. The enzyme was thermostable and retained about 55% of its original activity after heating at 70°C for 15 min. The enzyme preparations were activated by Ca2+ ions and inactivated by Zn+2, Cu+2 ions, and EDTA. Conclusion: An antitumor activity of chitooligosaccharides produced by the chitosanase was applied to the MCF-7 (breast carcinoma cells) and they had a cytotoxicity inhibitory effect against them about IC50 = (448 μg/ml).

Biotechnologically produced chitosans with nonrandom acetylation patterns differ from conventional chitosans in properties and activities

Chitosans are versatile biopolymers with multiple biological activities and potential applications. They are linear copolymers of glucosamine and N-acetylglucosamine defined by their degree of polymerisation (DP), fraction of acetylation (F_A), and pattern of acetylation (PA). Technical chitosans produced chemically from chitin possess defined DP and F_A but random PA, while enzymatically produced natural chitosans probably have non-random PA. This natural process has not been replicated using biotechnology because chitin de-N-acetylases do not efficiently deacetylate crystalline chitin. Here, we show that such enzymes can partially N-acetylate fully deacetylated chitosan in the presence of excess acetate, yielding chitosans with F_A up to 0.7 and an enzyme-dependent non-random PA. The biotech chitosans differ from technical chitosans both in terms of physicochemical and nanoscale solution properties and biological activities. As with synthetic block co-polymers, controlling the distribution of building blocks within the biopolymer chain will open a new dimension of chitosan research and exploitation.

Insights into promiscuous chitosanases: the known and the unknown

Chitosanase, a glycoside hydrolase (GH), catalyzes the cleavage of β-1,4-glycosidic bonds in polysaccharides and is widely distributed in nature. Many organisms produce chitosanases, and numerous chitosanases in the GH families have been intensely studied. The reported chitosanases mainly cleaved the inter-glucosamine glycosidic bonds, while substrate specificity is not strictly unique due to the existence of bifunctional or multifunctional activity profiles. The promiscuity of chitosanases is essential for the different pathways of biomass polysaccharide conversion and understanding of the chitosanase evolutionary process. However, the reviews for this aspect are completely unknown. This review provides an overview of the promiscuous activities, also considering the substrate and product specificity of chitosanases observed to date. These contribute to important implications for the future discovery and research of promiscuous chitosanases and applications related to biomass conversion. KEY POINTS: • The promiscuity of chitosanases is reviewed for the first time. • The current review provides insights into the substrate specificity of chitosanases. • The mode-product relationship and prospect of promiscuous chitosanases are highlighted.

New Insights into Bifunctional Chitosanases with Hydrolysis Activity toward Chito- and Cello-Substrates

In the present study, we carried out a comprehensive investigation of glycoside hydrolase (GH) 46 model-chitosanases based on cleavage specificity classification to understand their unknown bifunctional activity. We for the first time show that GH46 chitosanase CsnMHK1 from Bacillus circulans MH-K1, which was previously thought to be strictly exclusive to chitosan, can hydrolyze both chito- and cello-substrates. We determined the digestion direction of bifunctional chitosanase CsnMHK1 from class III and compared it with class II chitosanase belonging to GH8, providing insight into unique substrate specificities and a new perspective on its reclassification. The results lead us to challenge the current understanding of chitosanase substrate specificity based on GH taxonomy classification and suggest that the prevalence from the common bifunctional activity may have occurred. Altogether, these data contribute to the understanding of chitosanase recognition and hydrolysis toward chito- and cello-substrates, which is valuable for future studies on chitosanases.

Heterologous Expression of GH5 Chitosanase in Pichia pastoris and Antioxidant Biological Activity of Its Chitooligosacchride Hydrolysate

Chitosanase was widely used in the production of bioactive chitooligosacchride (CHOS) due to their safety, controllability, environmental protection, and biodegradability. Studies showed that the bioactivity of CHOS is closely related to its degree of polymerization. Therefore, the production of ideal polymerized CHOS becomes our primary goal. In this study, the glycosyl hydrolase (GH) family 5 chitosanase was successfully expressed heterologously in Pichia pastoris. After 96h of high-density fermentation, the chitosanase activity reached 90.62 U·mL^-1, the protein content reached 9.76mg·mL^-1. When 2% chitosan was hydrolyzed by crude enzyme (20U/mL), the hydrolysis rate reached 91.2% after 8h, producing a mixture of CHOS with 2-4 desirable degrees of polymerization (DP). Then, the antioxidant activity of CHOS mixture was investigated, and the results showed that the antioxidant effect was concentration-dependent and had great application potential in the field of nutrition.

A Novel Chitosanase from Penicillium oxalicum M2 for Chitooligosaccharide Production: Purification, Identification and Characterization

This study discovered a novel chitosanase from Penicillium oxalicum M2 based on a new screening strategy. An extracellular chitosanase was isolated and purified from the fermentation broth of Penicillium oxalicum M2. A 19.34-fold purification was achieved on a cation exchange column. Using sodium dodecylsulfate-polyacrylamide gel electrophoresis (SDS-PAGE) analysis, chitosanase was determined at approximately 42 kDa without any subunits. The sequence of peptide in the protein was identified as SALNKNYITNFSTLR by MALTI-TOF/TOF MS. The maximum catalytic activity of the purified enzyme was 60.45 U/mg at the optimum pH and temperature of 5.5 and 60 °C. The enzyme activity held stability in the range of 35-50 °C and pH 3-4.5. Ca²⁺, Mn²⁺, non-ionic surfactants (Tween 20/40/60/80 and Trition X-100) and some common reducing agents (DTT and β-ME) could significantly activate chitosanase. The purified enzyme showed rigorous specificity to chitosan as a substrate. The hydrolysate in the final stage of hydrolysis consisted of chitooligosaccharides with a degree of polymerization ranging from 2 to 5 and without glucosamine or acetylglucosamine. The monomeric enzyme obtained by one-step purification reveal applications potential in sugar industry, and expanded our understanding of the GH75 family chitosanases simultaneously.

Deciphering the ChitoCode: fungal chitins and chitosans as functional biopolymers

Chitins and chitosans are among the most widespread and versatile functional biopolymers, with interesting biological activities and superior material properties. While chitins are evolutionary ancient and present in many eukaryotes except for higher plants and mammals, the natural distribution of chitosans, i.e. extensively deacetylated derivatives of chitin, is more limited. Unequivocal evidence for its presence is only available for fungi where chitosans are produced from chitin by the action of chitin deacetylases. However, neither the structural details such as fraction and pattern of acetylation nor the physiological roles of natural chitosans are known at present. We hypothesise that the chitin deacetylases are generating chitins and chitosans with specific acetylation patterns and that these provide information for the interaction with specific chitin- and chitosan-binding proteins. These may be structural proteins involved in the assembly of the complex chitin- and chitosan-containing matrices such as fungal cell walls and insect cuticles, chitin- and chitosan-modifying and -degrading enzymes such as chitin deacetylases, chitinases, and chitosanases, but also chitin- and chitosan-recognising receptors of the innate immune systems of plants, animals, and humans. The acetylation pattern, thus, may constitute a kind of 'ChitoCode', and we are convinced that new in silico, in vitro, and in situ analytical tools as well as new synthetic methods of enzyme biotechnology and organic synthesis are currently offering an unprecedented opportunity to decipher this code. We anticipate a deeper understanding of the biology of chitin- and chitosan-containing matrices, including their synthesis, assembly, mineralisation, degradation, and perception. This in turn will improve chitin and chitosan biotechnology and the development of reliable chitin- and chitosan-based products and applications, e.g. in medicine and agriculture, food and feed sciences, as well as cosmetics and material sciences.

Elucidating the degradation pattern of a new cold-tolerant pectate lyase used for efficient preparation of pectin oligosaccharides

The cold-active pectate lyases have drawn increasing attention in food and biotechnological applications due to their ability to retain high catalytic efficiency under lower temperatures, which could be helpful for energy saving, cost reduction and flavor preservation. Herein, a new cold-tolerant pectate lyase (ErPelPL1) gene from Echinicola rosea was cloned and heterologously expressed in Escherichia coli. Interestingly, ErPelPL1 retained high catalytic activity even at a low temperature (4 °C). ErPelPL1 exhibited optimal activity at 35 ℃, pH 8.0 with 1 mM of Ca2+. It showed high specific activity towards polygalacturonic acid (34.7 U/mg) and sodium polygalacturonate (59.3 U/mg). The combined thin-layer chromatography (TLC), fast protein liquid chromatography (FPLC) and electrospray ionization mass spectrometry (ESI-MS) results indicated that ErPelPL1 endolytically degraded pectic substances into the oligosaccharides with degrees of depolymerization (Dps) of 1-6. In conclusion, this study mainly conducted biochemical characterization and product analysis of a cold-tolerant pectate lyase. Therefore, it provides a promising enzyme candidate for food and biotechnological applications.

Engineering of a chitosanase fused to a carbohydrate-binding module for continuous production of desirable chitooligosaccharides

Chitooligosaccharides (CHOS) with multiple biological activities are usually produced through enzymatic hydrolysis of chitosan or chitin. However, purification and recycling of the enzyme have largely limited the advancement of CHOS bioproduction. Here, we engineered a novel enzyme by fusing the native chitosanase Csn75 with a carbohydrate-binding module (CBM) that can specifically bind to curdlan. The recombinase Csn75-CBM was successfully expressed by Pichia pastoris and allowed one-step purification and immobilization in the chitosanase immobilized curdlan packed-bed reactor (CICPR), where a maximum adsorption capacity of 39.59 mg enzyme/g curdlan was achieved. CHOS with degrees of polymerization of 2-5 (a hydrolysis yield of 97.75%), 3-6 (75.45%), and 3-7 (73.2%) were continuously produced by adjusting the ratio of enzyme and chitosan or the flow rate of chitosan. Moreover, the CICPR exhibited good stability and reusability after several cycles. The recombinase Csn75-CBM has greatly improved the efficiency of the bioproduction of CHOS.

The succinoglycan riclin restores beta cell function through the regulation of macrophages on Th1 and Th2 differentiation in type 1 diabetic mice

Bacterial succinoglycan is found suitable as a viscosifying and emulsifying agent in the food industry. Riclin is a de-succinyl succinoglycan from an Agrobacterium isolate. Our previous study has revealed that riclin exerts special anti-inflammatory effects in vitro and in vivo. This study aims to determine the effects of riclin on preventing against immunological injury of beta cells in a type 1 diabetic model. We found that orally riclin effectively restores beta-cell function and improves the complications of streptozotocin (STZ)-induced diabetes. Riclin also reduces STZ-induced liver and kidney damage, and balances the inappropriate ratio of T helper type 1 cell (Th1)/type 2 cell (Th2) in the spleen and pancreatic draining lymph nodes of the STZ-induced diabetic mice. In a co-culture system with the islet β cell MIN6 and macrophage RAW 264.7, riclin reduces the levels of IFN-γ and IL-1β, protecting against STZ-caused MIN6 cell injury. We identified that riclin specifically binds to the membrane of macrophages and regulates the ratio of IL-10 and IL-12, thereby inhibiting the macrophage-mediated polarization of Th1 cells and promoting the differentiation of Th2 cells, which depends on the dendritic cell-specific intercellular adhesion molecule-3-grabbing non-integrin (DC-SIGN) receptor. Moreover, orally riclin significantly decreases the incidence of STZ-induced hyperglycemia (7.1% in riclin vs. 92.9% in STZ), and prevents autoimmune diabetes in non-obese diabetic (NOD) mice, with 87.5% of mice free of diabetes compared to 46.6% of the control mice. These results suggest that riclin has potential to be a functional food to prevent and improve autoimmune diabetes and related diseases.

In silico and in vitro analysis of an Aspergillus niger chitin deacetylase to decipher its subsite sugar preferences

Chitin deacetylases (CDAs) are found in many different organisms ranging from marine bacteria to fungi and insects. These enzymes catalyze the removal of acetyl groups from chitinous substrates generating various chitosans, linear copolymers consisting of N-acetylglucosamine (GlcNAc) and glucosamine. CDAs influence the degree of acetylation of chitosans as well as their pattern of acetylation, a parameter that was recently shown to influence the physicochemical properties and biological activities of chitosans. The binding site of CDAs typically consists of around four subsites, each accommodating a single sugar unit of the substrate. It has been hypothesized that the subsite preferences for GlcNAc or glucosamine units play a crucial role in the acetylation pattern they generate, but so far, this characteristic was largely ignored and still lacks structural data on the involved residues. Here, we determined the crystal structure of an Aspergillus niger CDA. Then, we used molecular dynamics simulations, backed up with a variety of in vitro activity assays using different well-defined polymeric and oligomeric substrates, to study this CDA in detail. We found that Aspergillus niger CDA strongly prefers a GlcNAc sugar unit at its −1 subsite and shows a weak GlcNAc preference at the other noncatalytic subsites, which was apparent both when deacetylating and N-acetylating oligomeric substrates. Overall, our results show that the combination of in vitro and in silico methods used here enables the detailed analysis of CDAs, including their subsite preferences, which could influence their substrate targets and the characteristics of chitosans produced by these species.

Pectinolytic lyases: a comprehensive review of sources, category, property, structure, and catalytic mechanism of pectate lyases and pectin lyases

Pectate lyases and pectin lyases have essential roles in various biotechnological applications, such as textile industry, paper making, pectic wastewater pretreatment, juice clarification and oil extraction. They can effectively cleave the α-1,4-glycosidic bond of pectin molecules back bone by β-elimination reaction to produce pectin oligosaccharides. In this way, it will not generate highly toxic methanol and has the advantages of good enzymatic selectivity, less by-products, mild reaction conditions and high efficiency. However, numerous researches have been done for several decades; there are still no comprehensive reviews to summarize the recent advances of pectate lyases and pectin lyases. This review tries to fill this gap by providing all relevant information, including the substrate, origin, biochemical properties, sequence analysis, mode of action, the three-dimensional structure and catalytic mechanism.

Improve thermostability of Bacillus sp. TS chitosanase through structure-based alignment

Chitosanases can catalyze the release of chitooligosaccharides which have a number of medical applications. Therefore, Chitosanases are good candidates for large-scale enzymatic synthesis due to their favorable thermostability properties and high catalytic efficiency. To further improve the thermostability of a chitosanase from Bacillus sp. TS, which has a half-life of 5.32 min, we mutated specific serine residues that we identified as potentially relevant through structure comparison with thermophilic CelA from Clostridium thermocellum. Out of a total of 15 mutants, three, namely S265G, S276A, and S347G, show higher thermostability. Their half-lives at 60 °C were calculated as 34.57 min, 36.79 min and 7.2 min. The K_m values of S265G, S276A and S347G mutants show substrate binding ability comparable to that of the wild-type enzyme, while the S265G mutant displays a significant decrease of enzymatic activities. Additionally, we studied the synergistic effects of combined mutations, observing that all double mutants and the triple mutant are more stable than the wild-type enzyme and single mutants. Finally, we investigated the mechanisms which might give a reasonable explanation for the improved thermostability via comparative analysis of the resulting 3D structures.

Expression and Biochemical Characterization of a Novel Marine Chitosanase from Streptomyces niveus Suitable for Preparation of Chitobiose

It is known that bioactivities of chitooligosaccharide (COS) are closely related to the degree of polymerization (DP); therefore, it is essential to prepare COS with controllable DP, such as chitobiose showing high antioxidant and antihyperlipidemia activities. In this study, BLAST, sequence alignment and phylogenetic analysis of characterized glycoside hydrolase (GH) 46 endo-chitosanases revealed that a chitosanase Sn1-CSN from Streptomyces niveus was different from others. Sn1-CSN was overexpressed in E. coli, purified and characterized in detail. It showed the highest activity at pH 6.0 and exhibited superior stability between pH 4.0 and pH 11.0. Sn1-CSN displayed the highest activity at 50 °C and was fairly stable at ≤45 °C. Its apparent kinetic parameters against chitosan (DDA: degree of deacetylation, >94%) were determined, with Km and kcat values of 1.8 mg/mL and 88.3 s-1, respectively. Cu2+ enhanced the activity of Sn1-CSN by 54.2%, whereas Fe3+ inhibited activity by 15.1%. Hydrolysis products of chitosan (DDA > 94%) by Sn1-CSN were mainly composed of chitobiose (87.3%), whereas partially acetylated chitosan with DDA 69% was mainly converted into partially acetylated COS with DP 2-13. This endo-chitosanase has great potential to be used for the preparation of chitobiose and partially acetylated COS with different DPs.

Differential expression of bio-active metabolites produced by chitosan polymers-based Bacillus amyloliquefaciens fermentation

Bacillus amyloliquefaciens strain PPL shows a potential for the control of phytopathogenic fungi. In the present study, upon growing the strain PPL on various forms of chitosan (0.5 % powder, 0.1 % soluble, and 0.15 % colloidal) as the carbon source, the antifungal activity on tomato Fusarium wilt correlated with the activity of chitosanase and β-1,3-glucanase. The colloidal substrate-based strain PPL fermentation displayed the highest degree of spore germination inhibition (79.5 %) and biocontrol efficiency (76.0 %) in tomato by increased biofilm formation. The colloidal culture upregulated the expression of chitosanase gene (5.9-fold), and the powder attributed to the expression of cyclic lipopeptides-genes (2.5-5.7 fold). Moreover, the three chitosan cultures induced the morphological changes of Fusarium oxysporum. These results suggest that the choice of growth substrate synergistically affects the production of secondary metabolites by PPL strain, and consequently its antifungal activity.

Enzymatic Synthesis and Characterization of Different Families of Chitooligosaccharides and Their Bioactive Properties

Chitooligosaccharides (COS) are homo- or hetero-oligomers of D-glucosamine (GlcN) and N-acetyl-D-glucosamine (GlcNAc) that can be obtained by chitosan or chitin hydrolysis. Their enzymatic production is preferred over other methodologies (physical, chemical, etc.) due to the mild conditions required, the fewer amounts of waste and its efficiency to control product composition. By properly selecting the enzyme (chitinase, chitosanase or nonspecific enzymes) and the substrate properties (degree of deacetylation, molecular weight, etc.), it is possible to direct the synthesis towards any of the three COS types: fully acetylated (faCOS), partially acetylated (paCOS) and fully deacetylated (fdCOS). In this article, we review the main strategies to steer the COS production towards a specific group. The chemical characterization of COS by advanced techniques, e.g., high-performance anion-exchange chromatography with pulsed amperometric detection (HPAEC-PAD) and MALDI-TOF mass spectrometry, is critical for structure–function studies. The scaling of processes to synthesize specific COS mixtures is difficult due to the low solubility of chitin/chitosan, the heterogeneity of the reaction mixtures, and high amounts of salts. Enzyme immobilization can help to minimize such hurdles. The main bioactive properties of COS are herein reviewed. Finally, the anti-inflammatory activity of three COS mixtures was assayed in murine macrophages after stimulation with lipopolysaccharides.

A bottom-up approach towards a bacterial consortium for the biotechnological conversion of chitin to L-lysine

Chitin is an abundant waste product from shrimp and mushroom industries and as such, an appropriate secondary feedstock for biotechnological processes. However, chitin is a crystalline substrate embedded in complex biological matrices, and, therefore, difficult to utilize, requiring an equally complex chitinolytic machinery. Following a bottom-up approach, we here describe the step-wise development of a mutualistic, non-competitive consortium in which a lysine-auxotrophic Escherichia coli substrate converter cleaves the chitin monomer N-acetylglucosamine (GlcNAc) into glucosamine (GlcN) and acetate, but uses only acetate while leaving GlcN for growth of the lysine-secreting Corynebacterium glutamicum producer strain. We first engineered the substrate converter strain for growth on acetate but not GlcN, and the producer strain for growth on GlcN but not acetate. Growth of the two strains in co-culture in the presence of a mixture of GlcN and acetate was stabilized through lysine cross-feeding. Addition of recombinant chitinase to cleave chitin into GlcNAc₂, chitin deacetylase to convert GlcNAc₂ into GlcN₂ and acetate, and glucosaminidase to cleave GlcN₂ into GlcN supported growth of the two strains in co-culture in the presence of colloidal chitin as sole carbon source. Substrate converter strains secreting a chitinase or a β-1,4-glucosaminidase degraded chitin to GlcNAc₂ or GlcN₂ to GlcN, respectively, but required glucose for growth. In contrast, by cleaving GlcNAc into GlcN and acetate, a chitin deacetylase-expressing substrate converter enabled growth of the producer strain in co-culture with GlcNAc as sole carbon source, providing proof-of-principle for a fully integrated co-culture for the biotechnological utilization of chitin.

New Class of Chitosanase from Bacillus amyloliquefaciens for the Generation of Chitooligosaccharides

Chitooligosaccharides (COS) generated from either chitin (chitin oligosaccharides) or chitosan (chitosan oligosaccharides) have a wide range of applications in agriculture, medicine, and other fields. Here, we report the characterization of a chitosanase from Bacillus amyloliquefaciens (BamCsn) and the importance of a tryptophan (Trp), W204, for BamCsn activity. BamCsn hydrolyzed the chitosan polymer by an endo mode. It also hydrolyzed chitin oligosaccharides and interestingly exhibited transglycosylation activity on chitotetraose and chitopentaose. Mutation of W204, a nonconserved amino acid in chitosanases, to W204A abolished the hydrolytic activity of BamCsn, with a change in the structure that resulted in a decreased affinity for the substrate and impaired the catalytic ability. Phylogenetic analysis revealed that BamCsn could belong to a new class of chitosanases that showed unique properties like transglycosylation, cleavage of chitin oligosaccharides, and the presence of W204 residues, which is important for activity. Chitosanases belonging to the BamCsn class showed a high potential to generate COS from chitinous substrates.

Structure-function analysis of Gynuella sunshinyii chitosanase uncovers the mechanism of substrate binding in GH family 46 members

Chitooligosaccharides (COS) is a kind of functional carbohydrates with great application potential as its various biological functions in food, cosmetics, and pharmaceutical fields. Exploring the relationship between structure and function of chitosanase is essential for the controllable preparation of chitooligosaccharides with the specific degree of polymerization (DP). GsCsn46A is a cold-adapted glycosyl hydrolase (GH) family 46 chitosanase with application potential for the controllable preparation of chitooligosaccharides. Here, we present two complex structures with substrate chitopentaose and chitotetraose of GsCsn46A, respectively. The overall structure of GsCsn46A contains nine α-helices and two β-strands that folds into two globular domains with the substrate between them. The unique binding positions of both chitopentaose and chitotetraose revealed two novel sugar residues in the negatively-numbered subsites of GH family 46 chitosanases. The structure-function analysis of GsCsn46A uncovers the substrate binding and catalysis mechanism of GH family 46 chitosanases. Structural basis mutagenesis in GsCsn46A indicated that altering interactions near +3 subsite would help produce hydrolysis products with higher DP. Specifically, the mutant N21W of GsCsn46A nearly eliminated the ability of hydrolyzing chitotetraose after long-time degradation.

Preparation of Defined Chitosan Oligosaccharides Using Chitin Deacetylases

During the past decade, detailed studies using well-defined 'second generation' chitosans have amply proved that both their material properties and their biological activities are dependent on their molecular structure, in particular on their degree of polymerisation (DP) and their fraction of acetylation (F_A). Recent evidence suggests that the pattern of acetylation (PA), i.e., the sequence of acetylated and non-acetylated residues along the linear polymer, is equally important, but chitosan polymers with defined, non-random PA are not yet available. One way in which the PA will influence the bioactivities of chitosan polymers is their enzymatic degradation by sequence-dependent chitosan hydrolases present in the target tissues. The PA of the polymer substrates in conjunction with the subsite preferences of the hydrolases determine the type of oligomeric products and the kinetics of their production and further degradation. Thus, the bioactivities of chitosan polymers will at least in part be carried by the chitosan oligomers produced from them, possibly through their interaction with pattern recognition receptors in target cells. In contrast to polymers, partially acetylated chitosan oligosaccharides (paCOS) can be fully characterised concerning their DP, F_A, and PA, and chitin deacetylases (CDAs) with different and known regio-selectivities are currently emerging as efficient tools to produce fully defined paCOS in quantities sufficient to probe their bioactivities. In this review, we describe the current state of the art on how CDAs can be used in forward and reverse mode to produce all of the possible paCOS dimers, trimers, and tetramers, most of the pentamers and many of the hexamers. In addition, we describe the biotechnological production of the required fully acetylated and fully deacetylated oligomer substrates, as well as the purification and characterisation of the paCOS products.

Marine-polysaccharide degrading enzymes: Status and prospects

Marine-polysaccharide degrading enzymes have recently been studied extensively. They are particularly interesting as they catalyze the cleavage of glycosidic bonds in polysaccharide macromolecules and produce oligosaccharides with low degrees of polymerization. Numerous findings have demonstrated that marine polysaccharides and their biotransformed products possess beneficial properties including antitumor, antiviral, anticoagulant, and anti-inflammatory activities, and they have great value in healthcare, cosmetics, the food industry, and agriculture. Exploitation of enzymes that can degrade marine polysaccharides is in the ascendant, and is important for high-value use of marine biomass resources. In this review, we describe research and prospects regarding the classification, biochemical properties, and catalytic mechanisms of the main types of marine-polysaccharide degrading enzymes, focusing on chitinase, chitosanase, alginate lyase, agarase, and carrageenase, and their product oligosaccharides. The state-of-the-art discussion of marine-polysaccharide degrading enzymes and their properties offers information that might enable more efficient production of marine oligosaccharides. We also highlight current problems in the field of marine-polysaccharide degrading enzymes and trends in their development. Understanding the properties, catalytic mechanisms, and modification of known enzymes will aid the identification of novel enzymes to degrade marine polysaccharides and facilitation of their use in various biotechnological processes.

The Significance of Bacillus spp. in Disease Suppression and Growth Promotion of Field and Vegetable Crops

Bacillus spp. produce a variety of compounds involved in the biocontrol of plant pathogens and promotion of plant growth, which makes them potential candidates for most agricultural and biotechnological applications. Bacilli exhibit antagonistic activity by excreting extracellular metabolites such as antibiotics, cell wall hydrolases, and siderophores. Additionally, Bacillus spp. improve plant response to pathogen attack by triggering induced systemic resistance (ISR). Besides being the most promising biocontrol agents, Bacillus spp. promote plant growth via nitrogen fixation, phosphate solubilization, and phytohormone production. Antagonistic and plant growth-promoting strains of Bacillus spp. might be useful in formulating new preparations. Numerous studies of a wide range of plant species revealed a steady increase in the number of Bacillus spp. identified as potential biocontrol agents and plant growth promoters. Among different mechanisms of action, it remains unclear which individual or combined traits could be used as predictors in the selection of the best strains for crop productivity improvement. Due to numerous factors that influence the successful application of Bacillus spp., it is necessary to understand how different strains function in biological control and plant growth promotion, and distinctly define the factors that contribute to their more efficient use in the field.

Robust enzymatic-mass spectrometric fingerprinting analysis of the fraction of acetylation of chitosans

We developed a rapid and precise method to determine the fraction of acetylation (F_A) of unknown chitosan samples using a combination of enzymatic sample hydrolysis, isotopic labeling, and HILIC-ESI-MS analysis. Chitosans are β-(1,4)-linked, partially N-acetylated and linear polyglucosamines representing an interesting group of functional biopolymers with a broad range of applications. For a better understanding of their structure-function relationships, it is key to have sensitive, accurate structural analysis tools available to determine parameters like the degree of polymerization (DP), fraction of acetylation (F_A), or pattern of acetylation (P_A). Here, we describe an improved enzymatic/mass spectrometric method for F_A analysis of chitosan polymers. In contrast to the original chitinosanase-based mass spectrometric fingerprinting analysis of F_A, the new method is independent of the P_A and the intermolecular variation in F_A (Đ_FA) of the chitosan sample. This allows accurate analysis of heterogeneously de-N-acetylated samples representing the majority of commercially available chitosans.

High-Throughput Screening Using UHPLC-MS To Characterize the Subsite Specificities of Chitosanases or Chitinases

Partially acetylated chitosan oligosaccharides (paCOS), consisting of β-1,4-linked N-acetyl-d-glucosamine and d-glucosamine units, possess diverse bioactivities that can be used for applications in, e.g., biomedicine, agriculture, and pharmaceutics. Establishing structure-function relationships and revealing modes of action requires the availability of structurally defined paCOS that can best be produced using chitin- and chitosan-modifying enzymes, such as chitinases and chitosanases, with known and defined subsite specificities. To enlarge the spectrum of such enzymes and, consequently, defined paCOS available, we have developed a two-step, microtiter plate-based high-throughput screening assay that allows quantification of the activity and subsite specificities of chitosan hydrolases. In a first step, the activities of the enzymes are quantified using a reducing end assay, and enzymes with sufficient activity are then screened for their subsite specificities using mass spectrometric analysis of their products when acting on well-defined chitosan polymers as substrates. The rapid UHPLC-ELSD-ESI-MS² method does not require labeling steps or addition of standards, and the principal component analysis of the fragment ion intensities of just two isomeric oligomer groups, GlcNAc₁GlcN₃ and GlcNAc₂GlcN₂, sufficed to identify, in a directed evolution, the site-saturation mutagenesis library of Bacillus sp. MN chitosanase consisting of 167 muteins, enzymes that significantly differed in their subsite specificities from the wildtype enzyme. Detailed analyses of a few selected muteins proved that the screening method is efficient and accurate in predicting altered subsite specificities.

The Pattern of Acetylation Defines the Priming Activity of Chitosan Tetramers

The biological activity of chitosans depends on their degree of polymerization (DP) and degree of acetylation (DA). However, information could also be carried by the pattern of acetylation (PA): the sequence of β-1,4-linked glucosamine (deacetylated/D) and N-acetylglucosamine (acetylated/A) units. To address this hypothesis, we prepared partially acetylated chitosan oligosaccharides from a chitosan polymer (DA = 35%, DP_w = 905) using recombinant chitosan hydrolases with distinct substrate and cleavage specificities. The mixtures were separated into fractions DP4-DP12, which were tested for elicitor and priming activities in rice cells. We confirmed that both activities were influenced by DP, but also observed apparent DA-dependent priming activity, with the ADDD+DADD fraction proving remarkably effective. We then compared all four monoacetylated tetramers prepared using different chitin deacetylases and observed significant differences in priming activity. This demonstrates for the first time that PA influences the biological activity of chitosans, which can now be recognized as bona fide information-carrying molecules.

Cloning and Characterization of a Cold-adapted Chitosanase from Marine Bacterium Bacillus sp. BY01

Chitosanase plays an important role in the production of chitooligosaccharides (CHOS), which possess various biological activities. Herein, a glycoside hydrolase (GH) family 46 chitosanase-encoding gene, csnB, was cloned from marine bacterium Bacillus sp. BY01 and heterologously expressed in Escherichia coli. The recombinant chitosanase, CsnB, was optimally active at 35 °C and pH 5.0. It was also revealed to be a cold-adapted enzyme, maintaining 39.5% and 40.4% of its maximum activity at 0 and 10 °C, respectively. Meanwhile, CsnB showed wide pH-stability within the range of pH 3.0 to 7.0. Then, an improved reaction condition was built to enhance its thermostability with a final glycerol volume concentration of 20%. Moreover, CsnB was determined to be an endo-type chitosanase, yielding chitosan disaccharides and trisaccharides as the main products. Overall, CsnB provides a new choice for enzymatic CHOS production.

Enzymatic Modifications of Chitin, Chitosan, and Chitooligosaccharides

Chitin and its N-deacetylated derivative chitosan are two biological polymers that have found numerous applications in recent years, but their further deployment suffers from limitations in obtaining a defined structure of the polymers using traditional conversion methods. The disadvantages of the currently used industrial methods of chitosan manufacturing and the increasing demand for a broad range of novel chitosan oligosaccharides (COS) with a fully defined architecture increase interest in chitin and chitosan-modifying enzymes. Enzymes such as chitinases, chitosanases, chitin deacetylases, and recently discovered lytic polysaccharide monooxygenases had attracted considerable interest in recent years. These proteins are already useful tools toward the biotechnological transformation of chitin into chitosan and chitooligosaccharides, especially when a controlled non-degradative and well-defined process is required. This review describes traditional and novel enzymatic methods of modification of chitin and its derivatives. Recent advances in chitin processing, discovery of increasing number of new, well-characterized enzymes and development of genetic engineering methods result in rapid expansion of the field. Enzymatic modification of chitin and chitosan may soon become competitive to conventional conversion methods.

Rational protein design of Bacillus sp. MN chitosanase for altered substrate binding and production of specific chitosan oligomers

BACKGROUND

Partially acetylated chito-oligosaccharides (paCOS) have a variety of potential applications in different fields, but to harness their benefits, pure paCOS or well-defined paCOS mixtures are essential. For example, if one could produce fully acetylated (A4) and fully deacetylated (D4) tetramers in abundance, all possible variants of tetrameric paCOS could be generated reliably from them. A promising approach for generating defined paCOS is by enzymatic depolymerization of chitosan polymers using chitosanases, since these enzymes' subsite specificities directly influence the composition of the paCOS produced; however, enzymatic production of e.g. D4 is challenging because the substrate is generally hydrolyzed further by most chitosanases. To overcome this, chitosanases could potentially be engineered so that upon hydrolyzing chitosan, they are unable to hydrolyze certain substrates, leaving well-defined oligomers intact in the hydrolysate.

RESULTS

For this purpose, we performed rational protein engineering on the extensively studied GH 8 chitosanase CSN from Bacillus sp. MN. By specifically targeting residues with a predicted function in substrate binding, we created new muteins incapable of efficiently hydrolyzing the fully deacetylated tetramer D4, and we were able to demonstrate efficient large-scale production of D4 with an altered version of CSN. Furthermore, we were able to uncover differences in the substrate positioning and subsite specificities of the muteins, which result in altered paCOS mixtures produced from partially acetylated chitosan polymers, with possibly altered bioactivities.

CONCLUSION

The value of protein engineering as a tool for the more efficient production of pure oligomers and potentially bioactive paCOS mixtures was demonstrated by the results and the suitability of specific muteins for the large-scale production of strictly defined, pure paCOS in a batch process was shown using the example of D4.

Structural and biochemical insight into mode of action and subsite specificity of a chitosan degrading enzyme from Bacillus spec. MN

Chitosans, partially de-N-acetylated derivatives of chitin, are multifunctional biopolymers. In nature, biological activities of partially acetylated chitosan polymers are mediated in part by their oligomeric breakdown products, which are generated in situ by the action of chitosanolytic enzymes. Understanding chitosanolytic enzymes, therefore, can lead to the production of chitosan oligomers with fully defined structures that may confer specific bioactivities. To address whether defined oligomer products can be produced via chitosanolytic enzymes, we here characterized a GH8 family chitosanase from Bacillus spec. MN, determining its mode of action and product profiles. We found that the enzyme has higher activity towards polymers with lower degree of acetylation. Oligomeric products were dominated by GlcN₃, GlcN₃GlcNAc₁, and GlcN₄GlcNAc₁. The product distribution from oligomers were GlcN₃ > GlcN₂. Modeling and simulations show that the binding site comprises subsites ranging from (-3) to (+3), and a putative (+4) subsite, with defined preferences for GlcN or GlcNAc at each subsite. Flexible loops at the binding site facilitate enzyme-substrate interactions and form a cleft at the active site which can open and close. The detailed insight gained here will help to engineer enzyme variants to produce tailored chitosan oligomers with defined structures that can then be used to probe their specific biological activities.

Purification and Characterization of A New Cold-Adapted and Thermo-Tolerant Chitosanase from Marine Bacterium Pseudoalteromonas sp. SY39

Chitosanases play an important role in chitosan degradation, forming enzymatic degradation products with several biological activities. Although many chitosanases have been discovered and studied, the enzymes with special characteristics are still rather rare. In this study, a new chitosanase, CsnM, with an apparent molecular weight of 28 kDa was purified from the marine bacterium Pseudoalteromonas sp. SY39. CsnM is a cold-adapted enzyme, which shows highest activity at 40 °C and exhibits 30.6% and 49.4% of its maximal activity at 10 and 15 °C, respectively. CsnM is also a thermo-tolerant enzyme that recovers 95.2%, 89.1% and 88.1% of its initial activity after boiling for 5, 10 and 20 min, respectively. Additionally, CsnM is an endo-type chitosanase that yields chitodisaccharide as the main product (69.9% of the total product). It’s cold-adaptation, thermo-tolerance and high chitodisaccharide yield make CsnM a superior candidate for biotechnological application to produce chitooligosaccharides.

Endochitinase 1 (Tv-ECH1) from Trichoderma virens has high subsite specificities for acetylated units when acting on chitosans

Chitosans with defined characteristics have been shown to possess reproducible bioactivities for numerous applications. A promising approach for producing chitosans with defined degrees of polymerization (DP), degrees of acetylation (DA), and patterns of acetylation (PA) involves using chitin-modifying enzymes. One such enzyme, the chitinase Tv-ECH1 belonging to the glycoside hydrolase (GH) family 18, seems to have an important role in the biocontrol properties of the fungus Trichoderma virens, suggesting its potential in generating novel chitosans for plant health applications. In this study, the Tv-ECH1 enzyme was overexpressed in the methylotrophic yeast Pichia pastoris, yielding large amounts (up to 2mgmL^-1) of purified recombinant enzyme of high activity, high purity, and high stability, making the system promising for industrial production of Tv-ECH1. The purified Tv-ECH1 chitinase displayed a wide optimal pH range from 4.5 to 6 and an optimal temperature of 37°C. Detailed subsite specificity analyses revealed high preference for acetylated residues at all four subsites analyzed (-2, -1, +1, +2), making Tv-ECH1 a promising candidate for the biotechnological production of specific chitosan oligomers and for the characterization of chitosan polymers via enzymatic fingerprinting.

Protein-engineering of chitosanase from Bacillus sp. MN to alter its substrate specificity

Partially acetylated chitosan oligosaccharides (paCOS) have various potential applications in agriculture, biomedicine, and pharmaceutics due to their suitable bioactivities. One method to produce paCOS is partial chemical hydrolysis of chitosan polymers, but that leads to poorly defined mixtures of oligosaccharides. However, the effective production of defined paCOS is crucial for fundamental research and for developing applications. A more promising approach is enzymatic depolymerization of chitosan using chitinases or chitosanases, as the substrate specificity of the enzyme determines the composition of the oligomeric products. Protein-engineering of these enzymes to alter their substrate specificity can overcome the limitations associated with naturally occurring enzymes and expand the spectrum of specific paCOS that can be produced. Here, engineering the substrate specificity of Bacillus sp. MN chitosanase is described for the first time. Two muteins with active site substitutions can accept N-acetyl-D-glucosamine units at their subsite (-2), which is impossible for the wildtype enzyme.