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

Dissecting and optimizing bioactivities of chitosans by enzymatic modification

Chitosans are versatile biopolymers with antimicrobial and plant-strengthening properties relevant to agriculture. However, a limited understanding of molecular structure-function relationships and cellular modes of action of chitosans hampers the development of effective chitosan-based agro-biologics. We partially hydrolyzed a chitosan polymer (degree of polymerization DP 800, fraction of acetylation FA 0.2) using acetic acid, a GH18 chitinase, or a GH8 chitosanase. All hydrolysates contained mixtures of chitosan oligomers and small polymers, but their composition in terms of DP, FA, and pattern of acetylation (PA) differed greatly. Importantly, chitinase products had mostly deacetylated residues at their ends, flanking mostly deacetylated residues, and vice versa for chitosanase products, while the products of acid hydrolysis had random PA. Acid hydrolysis did not significantly change antifungal and antibacterial activities. In contrast, chitinase hydrolysis slightly increased antibacterial, and chitosanase almost abolished antifungal activity. Elicitor and priming activities in the plant Arabidopsis were unchanged by acid, destroyed by chitinase, and increased by chitosanase hydrolysis. Transcriptomic analysis revealed that the chitosan polymer strongly induced genes involved in photosynthesis, while the chitosanase hydrolysate strongly induced genes involved in disease resistance. Clearly, different bioactivities require different chitosans, and enzymatic modification can fine-tune these activities as required for different agricultural products.

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.

Modulation of a Loop Region in the Substrate Binding Pocket Affects the Degree of Polymerization of Bacillus subtilis Chitosanase Products

The hydrolytic products of chitosanase from Streptomyces avermitilis (SaCsn46A) were found to be aminoglucose and chitobiose, whereas those of chitosanase from Bacillus subtilis (BsCsn46A) were chitobiose and chitotriose. Therefore, the sequence alignment between SaCsn46A and BsCsn46A was conducted, revealing that the structure of BsCsn46A possesses an extra loop region (194N-200T) at the substrate binding pocket. To clarify the impact of this loop on hydrolytic properties, three mutants, SC, TJN, and TJA, were constructed. Eventually, the experimental results indicated that SC changed the ratio of chitobiose to chitotriose hydrolyzed by chitosanase from 1:1 into 2:3, while TJA resulted in a ratio of 15:7. This experiment combined molecular research to unveil a crucial loop within the substrate binding pocket of chitosanase. It also provides an effective strategy for mutagenesis and a foundation for altering hydrolysate composition and further applications in engineering 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.

Addressing the Selectivity of Enzyme Biosensors: Solutions and Perspectives

Enzymatic biosensors enjoy commercial success and are the subject of continued research efforts to widen their range of practical application. For these biosensors to reach their full potential, their selectivity challenges need to be addressed by comprehensive, solid approaches. This review discusses the status of enzymatic biosensors in achieving accurate and selective measurements via direct biocatalytic and inhibition-based detection, with a focus on electrochemical enzyme biosensors. Examples of practical solutions for tackling the activity and selectivity problems and preventing interferences from co-existing electroactive compounds in the samples are provided such as the use of permselective membranes, sentinel sensors and coupled multi-enzyme systems. The effect of activators, inhibitors or enzymatic substrates are also addressed by coupled enzymatic reactions and multi-sensor arrays combined with data interpretation via chemometrics. In addition to these more traditional approaches, the review discusses some ingenious recent approaches, detailing also on possible solutions involving the use of nanomaterials to ensuring the biosensors' selectivity. Overall, the examples presented illustrate the various tools available when developing enzyme biosensors for new applications and stress the necessity to more comprehensively investigate their selectivity and validate the biosensors versus standard analytical methods.

Structure-based rational design of chitosanase CsnMY002 for high yields of chitobiose

Chitosan oligosaccharides (COS) are attractive active molecules for biomedical applications. Currently, the prohibitively high cost of producing fully defined COS hampers extensive studies on their biological activity and restricts their use in various industries. Thus, cost-effective production of pure COS is of major importance. In this report, chitosanase from Bacillus subtilis MY002 (CsnMY002) was prepared for COS production. The structure of apo CsnMY002 displayed an unexpected tunnel-like substrate-binding site and the structure of the CsnMY002_E19A/(GlcN)₆ complex highlighted the "4 + 2″ splitting of hexaglucosamine even though the "3 + 3″ splitting is also observed in the TLC analysis of the enzyme products for hexaglucosamine. Structure based rational design was performed to generate mutants for chitobiose production. The CsnMY002_G21 K mutant produced chitobiose with a relative content > 87 % from chitosan with a low degree of acetylation, and 50.65 mg chitobiose with a purity > 98 % was prepared from 100 mg chitosan. The results provide insight on the catalytic mechanism of chitosanase and underpin future biomedical applications of pure chitobiose.

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.

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.

Efficient Immobilization of Bacterial GH Family 46 Chitosanase by Carbohydrate-Binding Module Fusion for the Controllable Preparation of Chitooligosaccharides

Chitooligosaccharide has been reported to possess diverse bioactivities. The development of novel strategies for obtaining optimum degree of polymerization (DP) chitooligosaccharides has become increasingly important. In this study, two glycoside hydrolase family 46 chitosanases were studied for immobilization on curdlan (insoluble β-1,3-glucan) using a novel carbohydrate binding module (CBM) family 56 domain from a β-1,3-glucanase. The CBM56 domain provided a spontaneous and specific sorption of the fusion proteins onto a curdlan carrier, and two fusion enzymes showed increased enzyme stability in comparison with native enzymes. Furthermore, a continuous packed-bed reactor was constructed with chitosanase immobilized on a curdlan carrier to control the enzymatic hydrolysis of chitosan. Three chitooligosaccharide products with different molecular weights were prepared in optimized reaction conditions. This study provides a novel CBM tag for the stabilization and immobilization of enzymes. The controllable hydrolysis strategy offers potential for the industrial-scale preparation of chitooligosaccharides with different desired DPs.

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.

Significantly increased catalytic activity of Candida antarctica lipase B for the resolution of cis-(±)-dimethyl 1-acetylpiperidine-2,3-dicarboxylate

Structure-based semi-rational engineering approach was applied to alter the binding pocket and substrate channel for enhancing the activity of CALB towards moxifloxacin chiral intermediate.