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

Recent Progress in Enzymatic Preparation of Chitooligosaccharides with a Single Degree of Polymerization and Their Potential Applications in the Food Sector

Chitosan oligosaccharides (COS), derived from chitin, have garnered considerable attention owing to their diverse biological activities and potential applications. Previous investigations into the bioactivity of COS often encountered challenges, primarily stemming from the use of COS mixtures, making it difficult to discern specific effects linked to distinct degrees of polymerization (DP). Recent progress underscores the significant variation in the biological activities of COS corresponding to different DPs, prompting dedicated research towards synthesizing COS with well-defined polymerization. Among the available methods, enzymatic preparation stands out as a viable and environmentally friendly approach for COS synthesis. This article provides a comprehensive overview of emerging strategies for the enzymatic preparation of single COS, encompassing protein engineering, enzymatic membrane bioreactors, and transglycosylation reactions. Furthermore, the bioactivities of single COS, including anti-tumor, antioxidant, antibacterial, anti-inflammatory, and plant defense inducer properties, exhibit close associations with DP values. The potential applications of single COS, such as in functional food, food preservation, and crop planting, are also elucidated.

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.

Engineering of a chitin deacetylase to generate tailor-made chitosan polymers

Chitin deacetylases (CDAs) emerge as a valuable tool to produce chitosans with a nonrandom distribution of N-acetylglucosamine (GlcNAc) and glucosamine (GlcN) units. We hypothesized before that CDAs tend to bind certain sequences within the substrate matching their subsite preferences for either GlcNAc or GlcN units. Thus, they deacetylate or N-acetylate their substrates at nonrandom positions. To understand the molecular basis of these preferences, we analyzed the binding site of a CDA from Pestalotiopsis sp. (PesCDA) using a detailed activity screening of a site-saturation mutagenesis library. In addition, molecular dynamics simulations were conducted to get an in-depth view of crucial interactions along the binding site. Besides elucidating the function of several amino acids, we were able to show that only 3 residues are responsible for the highly specific binding of PesCDA to oligomeric substrates. The preference to bind a GlcNAc unit at subsite -2 and -1 can mainly be attributed to N75 and H199, respectively. Whereas an exchange of N75 at subsite -2 eliminates enzyme activity, H199 can be substituted with tyrosine to increase the GlcN acceptance at subsite -1. This change in substrate preference not only increases enzyme activity on certain substrates and changes composition of oligomeric products but also significantly changes the pattern of acetylation (PA) when N-acetylating polyglucosamine. Consequently, we could clearly show how subsite preferences influence the PA of chitosans produced with CDAs.

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.

Alginate derived functional oligosaccharides: Recent developments, barriers, and future outlooks

Alginate is a biopolymer used extensively in the food, pharmaceutical, and chemical industries. Alginate oligosaccharides (AOS) derived from alginate exhibit superior biological activities and therapeutic potential. Alginate lyases with characteristic substrate specificity can facilitate the production of a broad array of AOS with precise structure and functionality. By adopting innovative analytical tools in conjunction with focused clinical studies, the structure-bioactivity relationship of a number of AOS has been brought to light. This review covers fundamental aspects and recent developments in AOS research. Enzymatic and microbial processes involved in AOS production from brown algae and sequential steps involved in AOS structure elucidation are outlined. Biological mechanisms underlying the health benefits of AOS and their potential industrial and therapeutic applications are elaborated. Withal, various challenges in AOS research are traced out, and future directions, specifically on recombinant systems for AOS preparation, are delineated to further widen the horizon of these exceptional oligosaccharides.

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.

Characteristics and applications of alginate lyases: A review

Brown algae, as the main source of alginate, are a type of marine biomass with a very high output. Alginate, a polysaccharide composed of β-D-mannuronic acid (M) and α-L-guluronic acid (G), has great potential for applications in the food, cosmetic and pharmaceutical industries. Alginate lyases (Alys) can degrade alginate polymers into oligosaccharides or monosaccharides, resulting in a broad application field. Alys can be used for both the production of alginate oligosaccharides and the biorefinery of brown algae. In view of their important functions, an increasing number of Alys have been isolated and characterized. For better application, a comprehensive understanding of Alys is essential. Therefore, in this paper, we summarized recently discovered Alys, discussed their characteristics, and introduced their structural properties, degradation patterns and biological roles in alginate-degrading organisms. In addition, applications of Alys have been illustrated with examples. This paper provides a relatively comprehensive description of Alys, which is significant for Alys exploration and application.

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.

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.

Computational design of Bacillus licheniformis RN-01 levansucrase for control of the chain length of levan-type fructooligosaccharides

Levansucrase (LS) from Gram-positive bacteria generally produces a large quantity of levan polymer, a polyfructose with glucose at the end (GF_n) but a small quantity of levan-type fructooligosaccharides (LFOs). The properties of levan and LFOs depend on their chain lengths, thereby determining their potential applications in food and pharmaceutical industries such as prebiotics and anti-tumor agents. Therefore, an ability to redesign and engineer the active site of levansucrase for synthesis of products with desired degree of polymerization (DP) is very beneficial. We employed computational protein design, docking and molecular dynamics to redesign and engineer the active site of Bacillus licheniformis RN-01 levansucrase for production of LFOs with DP up to five (GF₄), using two approaches: 1) blocking oligosaccharide binding track of GF₃-LS complex with large aromatic residues and 2) eliminating hydrogen bond interactions between terminal glucose of GF₄ and side chains of binding residues of GF₄-LS complex. The designed enzymes and their product patterns from these two approaches were experimentally characterized. The experimental results show that the first approach was successful in creating N251W and N251W/K372Y mutants that synthesized LFOs with DP up to five. This work illustrates how computer-aided approaches can offer novel opportunities to engineer enzymes for desired products.