Studies on acetylation patterns of different chitosan preparations

A bioactivity matrix for antimicrobial activities of chitosans: A review

Chitosans have prominent antimicrobial activities, inhibiting growth of both bacteria and fungi, but molecular structure-function relationships of these activities are still only partially understood. Structurally, chitosans differ in their degree of polymerization (DP), fraction of acetylation (FA), and pattern of acetylation (PA). How these structural parameters are influencing antimicrobial activities is still a matter of debate. A comprehensive screening of all pertinent reviews and original publications dealing with antimicrobial activities of chitosans published in five selected years from 2000 to 2020 was performed. This screening of 2929 publications in total yielded 134 original papers, that contained data suitable for a thorough analysis of the influence of DP and FA on chitosans' antimicrobial activities. Despite many differences between the studies, e.g. in the purity and quality of the chitosans, the microbial species, or the bioassay used, a partial consensus picture emerged. The strongest antimicrobial activity was observed for chitosans with a low to intermediate Mw. Larger polymers had lower activities, and chitosan oligomers were almost inactive. Less clearly, a trend was observed for decreasing activities with increasing FA. Possible reasons for identifying only a partial rather than a comprehensive consensus picture are discussed.

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.

Three intertwining effects guide the mode of action of chitin deacetylase de- and N-acetylation reactions

Chitosans are promising multi-functional biomolecules for various applications whose performance is dependent on three key structural parameters, including the pattern of acetylation (PA). To date, chitin deacetylases (CDAs) are the only tool to control the PA of chitosan polymers via their specific mode of action during de- or N-acetylation. For a start, this review summarizes the current state of research on the classification of carbohydrate esterase 4 enzymes, the features in sequence and structure of CDAs, and the different PAs produced by different CDAs during de- or N-acetylation. In the main part, we introduce three effects that guide the mode of action of these enzymes: the already established subsite capping effect, the subsite occupation effect, and the subsite preference effect. We show how their interplay controls the PA of CDA products and describe their molecular basis. For one thing, this review aims to equip the reader with the knowledge to understand and analyze CDAs - including a guide for in silico and in vitro analyses. But more importantly, we intend to reform and extend the model explaining their mode of action on chitosans to facilitate a deeper understanding of these important enzymes for biology and biotechnology.

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.

Alternative processes to produce chitin, chitosan, and their oligomers

Sustainable recovery of chitin and its derivatives from shellfish waste will be achieved when the industrial production of these polymers is achieved with a high control of their molecular structure, low costs, and acceptable levels of pollution. Therefore, the conventional chemical method for obtaining these biopolymers needs to be replaced or optimized. The goal of the present review is to ascertain what alternative methods are viable for the industrial-scale production of chitin, chitosan, and their oligomers. Therefore, a detailed review of recent literature was undertaken, focusing on the advantages and disadvantages of each method. The analysis of the existing data allows suggesting that combining conventional, biological, and alternative methods is the most efficient strategy to achieve sustainable production, preventing negative impacts and allowing for the recovery of high added-value compounds from shellfish waste. In conclusion, a new process for obtaining chitinous materials is suggested, with the potential of reducing the consumption of reagents, energy, and water by at least 1/10, 1/4, and 1/3 part with respect to the conventional process, respectively.

Effect of Acetylation on the Nanofibril Formation of Chitosan from All-Atom De Novo Self-Assembly Simulations

Chitosan-based materials have broad applications, from biotechnology to pharmaceutics. Recent experiments showed that the degree and pattern of acetylation along the chitosan chain modulate its biological and physicochemical properties; however, the molecular mechanism remains unknown. Here, we report, to the best of our knowledge, the first de novo all-atom molecular dynamics (MD) simulations to investigate chitosan's self-assembly process at different degrees and patterns of acetylation. Simulations revealed that 10 mer chitosan chains with 50% acetylation in either block or alternating patterns associate to form ordered nanofibrils comprised of mainly antiparallel chains in agreement with the fiber diffraction data of deacetylated chitosan. Surprisingly, regardless of the acetylation pattern, the same intermolecular hydrogen bonds mediate fibril sheet formation while water-mediated interactions stabilize sheet-sheet stacking. Moreover, acetylated units are involved in forming strong intermolecular hydrogen bonds (NH-O6 and O6H-O7), which offers an explanation for the experimental observation that increased acetylation lowers chitosan's solubility. Taken together, the present study provides atomic-level understanding the role of acetylation plays in modulating chitosan's physiochemical properties, contributing to the rational design of chitosan-based materials with the ability to tune by its degree and pattern of acetylation. Additionally, we disseminate the improved molecular mechanics parameters that can be applied in MD studies to further understand chitosan-based materials.

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.

Chitin and its derivatives: Functional biopolymers for developing bioproducts for sustainable agriculture-A reality?

Chitinous materials (chitin and its derivatives) are obtained from renewable sources, mainly shellfish waste, having a great potential for the development of bioproducts as alternatives to synthetic agrochemicals. Recent studies have provided evidence that the use of these biopolymers can help control postharvest diseases, increase the content of nutrients available to plants, and elicit positive metabolic changes that lead to higher plant resistance against pathogens. However, agrochemicals are still widely and intensively used in agriculture. This perspective addresses the gap in knowledge and innovation to make bioproducts based on chitinous materials more competitive in the market. It also provides the readers with background to understand why these products are scarcely used and the aspects that need to be considered to increase their use. Finally, information on the development and commercialization of agricultural bioproducts containing chitin or its derivatives in the Chilean market is also provided.

Customized chitooligosaccharide production-controlling their length via engineering of rhizobial chitin synthases and the choice of expression system

Chitooligosaccharides (COS) have attracted attention from industry and academia in various fields due to their diverse bioactivities. However, their conventional chemical production is environmentally unfriendly and in addition, defined and pure molecules are both scarce and expensive. A promising alternative is the in vivo synthesis of desired COS in microbial platforms with specific chitin synthases enabling a more sustainable production. Hence, we examined the whole cell factory approach with two well-established microorganisms-Escherichia coli and Corynebacterium glutamicum-to produce defined COS with the chitin synthase NodC from Rhizobium sp. GRH2. Moreover, based on an in silico model of the synthase, two amino acids potentially relevant for COS length were identified and mutated to direct the production. Experimental validation showed the influence of the expression system, the mutations, and their combination on COS length, steering the production from originally pentamers towards tetramers or hexamers, the latter virtually pure. Possible explanations are given by molecular dynamics simulations. These findings pave the way for a better understanding of chitin synthases, thus allowing a more targeted production of defined COS. This will, in turn, at first allow better research of COS' bioactivities, and subsequently enable sustainable large-scale production of oligomers.

Tuning chitosan’s chemical structure for enhanced biological functions

Chitosan, an amino polysaccharide mostly derived from crustaceans, has been recently highlighted for its biological activities that depend on its molecular weight (MW), degree of deacetylation (DD), and acetylation pattern (AP). More importantly, for some advanced biomaterials, the homogeneity of the chitosan structure is an important factor in determining its biological activity. Here we review emerging enzymes and cell factories, respectively, for in vitro and in vivo preparation of chitosan oligosaccharides (COSs), focusing on advances in the analysis of the AP and structural modification of chitosan to tune its functions. By 'mapping' current knowledge on chitosan's in vitro and in vivo activity with its MW and AP, this work could pave the way for future studies in the field.

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.

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.

Rationally engineered chitin deacetylase from Arthrobacter sp. AW19M34-1 with improved catalytic activity toward crystalline chitin

Chitin and its derivatives have anticoagulant, antimicrobial, and antioxidant properties, but the poor solubility of chitin limits its application in different fields. In this study, site-directed mutagenesis was performed to enhance the deacetylation activity of chitin deacetylases CDA from Arthrobacter (ArCE4). The mutant Mut-2-8 with Y172E/E200S/Y201W showed a 2.84- fold and 1.39-fold increase in catalytic efficiency (kcat/Km) for the deacetylation of (GluNAc)₅ and α-chitin, respectively. These results demonstrated that the mutations significantly improved the activation of ArCE4 on crystalline chitin. The molecular docking study confirmed that the enhancement of catalytic efficiency is due to the extra two hydrogen bonds and one acetyl group. In summary, the activity of Mut-2-8 to insoluble chitin was significantly improved by reactional design, which is beneficial to resolve the issues of the expensive cost of the enzymes and low efficiency. Mut-2-8 exhibits potential applications in the chitosan industry.

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.

Enzymatic approaches in the bioprocessing of shellfish wastes

Several tonnes of shellfish wastes are generated globally due to the mass consumption of shellfish meat from crustaceans like prawn, shrimp, lobster, crab, Antarctic krill, etc. These shellfish wastes are a reservoir of valuable by-products like chitin, protein, calcium carbonate, and pigments. In the present scenario, these wastes are treated chemically to recover chitin by the chitin and chitosan industries, using hazardous chemicals like HCl and NaOH. Although this process is efficient in removing proteins and minerals, the unscientific dumping of harmful effluents is hazardous to the ecosystem. Stringent environmental laws and regulations on waste disposal have encouraged researchers to look for alternate strategies to produce near-zero wastes on shellfish degradation. The role of enzymes in degrading shellfish wastes is advantageous yet has not been explored much, although it produces bioactive rich protein hydrolysates with good quality chitin. The main objective of the review is to discuss the potential of various enzymes involved in shellfish degradation and their opportunities and challenges over chemical processes in chitin recovery.

Isolation and characterization of chitosan from Ugandan edible mushrooms, Nile perch scales and banana weevils for biomedical applications

Of recent, immense attention has been given to chitosan in the biomedical field due to its valuable biochemical and physiological properties. Traditionally, the chief source of chitosan is chitin from crab and shrimp shells. Chitin is also an important component of fish scales, insects and fungal cell walls. Thus, the aim of this study was to isolate and characterize chitosan from locally available material for potential use in the biomedical field. Chitosan ash and nitrogen contents ranged from 1.55 to 3.5% and 6.6 to 7.0% respectively. Molecular weight varied from 291 to 348KDa. FTIR spectra revealed high degree of similarity between locally isolated chitosan and commercial chitosan with DD ranging from 77.8 to 79.1%. XRD patterns exhibited peaks at 2θ values of 19.5° for both mushroom and banana weevil chitosan while Nile perch scales chitosan registered 3 peaks at 2θ angles of 12.3°, 20.1° and 21.3° comparable to the established commercial chitosan XRD pattern. Locally isolated chitosan exhibited antimicrobial activity at a very high concentration. Ash content, moisture content, DD, FTIR spectra and XRD patterns revealed that chitosan isolated from locally available materials has physiochemical properties comparable to conventional chitosan and therefore it can be used in the biomedical field.

Systematic Structural Characterization of Chitooligosaccharides Enabled by Automated Glycan Assembly

Chitin, a polymer composed of β(1-4)-linked N-acetyl-glucosamine monomers, and its partially deacetylated analogue chitosan, are abundant biopolymers with outstanding mechanical as well as elastic properties. Their degradation products, chitooligosaccharides (COS), can trigger the innate immune response in humans and plants. Both material and biological properties are dependent on polymer length, acetylation, as well as the pH. Without well-defined samples, a complete molecular description of these factors is still missing. Automated glycan assembly (AGA) enabled rapid access to synthetic well-defined COS. Chitin-cellulose hybrid oligomers were prepared as important tools for a systematic structural analysis. Intramolecular interactions, identified by molecular dynamics simulations and NMR analysis, underscore the importance of the chitosan amino group for the stabilization of specific geometries.

Chitin and Chitosan Fragments Responsible for Plant Elicitor and Growth Stimulator

Chitin and chitosan are natural polysaccharides with huge application potential in agriculture, such as promoting plant growth, eliciting plant resistance against biotic and abiotic stress, and activating symbiotic signaling between plants and beneficial microorganisms. Chitin and chitosan offer a sustainable alternative for future crop production. The bioactivities of chitin and chitosan closely depend on their structural factors, including molecular size, degree of acetylation, and pattern of acetylation. It is of great significance to identify the key fragments in chitin and chitosan chains that are responsible for these agricultural bioactivities. Herein, we review the recent progress in the structure-function relationship of chitin and chitosan in the field of agriculture application. The preparation of chitin and chitosan fragments and their action mode for plant protection and growth are also discussed.

Study of the Acetylation Pattern of Chitosan by Pure Shift NMR

Chitosan is a biodegradable, antibacterial, and nontoxic biopolymer used in a wide range of applications including biotechnology, pharmacy, and medicine. The physicochemical and biological properties of chitosan have been associated with parameters such as the degree of polymerization (DP) and the fraction of acetylation (F_A). New methods are being developed to yield chitosans of specific acetylation patterns, and, recently, a correlation between biological activity and the distribution of the acetylated units (P_A: pattern of acetylation) has been demonstrated. Although there are numerous well-established methods for the determination of DP and F_A values, this is not the case for P_A. The methods available are either not straightforward or not sensitive enough, limiting their use for routine analysis. In this study, we demonstrate that by applying HOmodecoupled Band-Selective (HOBS) decoupling NMR on signals assigned by multidimensional Pure Shift NMR methods, P_A can be easily and accurately determined on various chitosan samples. This novel methodology-easily implemented for routine analysis-could become a standard for chitosan P_A assessment. In addition, by applying Spectral Aliased Pure Shift HSQC, the analysis was enhanced with the determination of triads.

Chitosan-based particulate systems for drug and vaccine delivery in the treatment and prevention of neglected tropical diseases

Neglected tropical diseases (NTDs) are a diverse group of infections which are difficult to prevent or control, affecting impoverished communities that are unique to tropical or subtropical regions. In spite of the low number of drugs that are currently used for the treatment of these diseases, progress on new drug discovery and development for NTDs is still very limited. Therefore, strategies on the development of new delivery systems for current drugs have been the main focus of formulators to provide improved efficacy and safety. In recent years, particulate delivery systems at micro- and nanosize, including polymeric micro- and nanoparticles, liposomes, solid lipid nanoparticles, metallic nanoparticles, and nanoemulsions, have been widely investigated in the treatment and control of NTDs. Among these polymers used for the preparation of such systems is chitosan, which is a marine biopolymer obtained from the shells of crustaceans. Chitosan has been investigated as a delivery system due to the versatility of its physicochemical properties as well as bioadhesive and penetration-enhancing properties. Furthermore, chitosan can be also used to improve treatment due to its bioactive properties such as antimicrobial, tissue regeneration, etc. In this review, after giving a brief introduction to neglected diseases and particulate systems developed for the treatment and control of NTDs, the chitosan-based systems will be described in more detail and the recent studies on these systems will be reviewed. Graphical abstract.

Selection of Water-Soluble Chitosan by Microwave-Assisted Degradation and pH-Controlled Precipitation

In the field of gene therapy, chitosan (CS) gained interest for its promise as a non-viral DNA vector. However, commercial sources of CS lack precise characterization and do not generally reach sufficient solubility in aqueous media for in vitro and in vivo evaluation. As low molecular weight CS showed improved solubility, we investigated the process of CS depolymerization by acidic hydrolysis, using either long time heating at 80 °C or short time microwave-enhanced heating. The resulting depolymerized chitosan (dCS) were analyzed by gel permeation chromatography (GPC) and 1H nuclear magnetic resonance (NMR) to determine their average molecular weight (Mn, Mp and Mw), polydispersity index (PD) and degree of deacetylation (DD). We emphasized the production of water-soluble CS (solubility > 5 mg/mL), obtained in reproducible yield and characteristics, and suitable for downstream functionalization. Optimal microwave-assisted conditions provided dCS with a molecular weight (MW) = 12.6 ± 0.6 kDa, PD = 1.41 ± 0.05 and DD = 85%. While almost never discussed in the literature, we observed the partial post-production aggregation of dCS when exposed to phase changes (from liquid to solid). Repeated cycles of freezing/thawing allowed the selection of dCS fractions which were exempt of crystalline particles formation upon solubilization from frozen samples.

How the Lack of Chitosan Characterization Precludes Implementation of the Safe-by-Design Concept

Efficacy and safety of nanomedicines based on polymeric (bio)materials will benefit from a rational implementation of a Safe-by-Design (SbD) approach throughout their development. In order to achieve this goal, however, a standardization of preparation and characterization methods and their accurate reporting is needed. Focusing on the example of chitosan, a biopolymer derived from chitin and frequently used in drug and vaccine delivery vector preparation, this review discusses the challenges still to be met and overcome prior to a successful implementation of the SbD approach to the preparation of chitosan-based protein drug delivery systems.

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.

Enzymatic Modification of Native Chitin and Conversion to Specialty Chemical Products

: Chitin is one of the most abundant biomolecules on earth, occurring in crustacean shells and cell walls of fungi. While the polysaccharide is threatening to pollute coastal ecosystems in the form of accumulating shell-waste, it has the potential to be converted into highly profitable derivatives with applications in medicine, biotechnology, and wastewater treatment, among others. Traditionally this is still mostly done by the employment of aggressive chemicals, yielding low quality while producing toxic by-products. In the last decades, the enzymatic conversion of chitin has been on the rise, albeit still not on the same level of cost-effectiveness compared to the traditional methods due to its multi-step character. Another severe drawback of the biotechnological approach is the highly ordered structure of chitin, which renders it nigh impossible for most glycosidic hydrolases to act upon. So far, only the Auxiliary Activity 10 family (AA10), including lytic polysaccharide monooxygenases (LPMOs), is known to hydrolyse native recalcitrant chitin, which spares the expensive first step of chemical or mechanical pre-treatment to enlarge the substrate surface. The main advantages of enzymatic conversion of chitin over conventional chemical methods are the biocompability and, more strikingly, the higher product specificity, product quality, and yield of the process. Products with a higher Mw due to no unspecific depolymerisation besides an exactly defined degree and pattern of acetylation can be yielded. This provides a new toolset of thousands of new chitin and chitosan derivatives, as the physio-chemical properties can be modified according to the desired application. This review aims to provide an overview of the biotechnological tools currently at hand, as well as challenges and crucial steps to achieve the long-term goal of enzymatic conversion of native chitin into specialty chemical products.

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.

Melanin deposition in two Cryptococcus species depends on cell-wall composition and flexibility

Cryptococcus neoformans and Cryptococcus gattii are two species complexes in the large fungal genus Cryptococcus and are responsible for potentially lethal disseminated infections. These two complexes share several phenotypic traits, such as production of the protective compound melanin. In C. neoformans, the pigment associates with key cellular constituents that are essential for melanin deposition within the cell wall. Consequently, melanization is modulated by changes in cell-wall composition or ultrastructure. However, whether similar factors influence melanization in C. gattii is unknown. Herein, we used transmission EM, biochemical assays, and solid-state NMR spectroscopy of representative isolates and "leaky melanin" mutant strains from each species complex to examine the compositional and structural factors governing cell-wall pigment deposition in C. neoformans and C. gattii. The principal findings were the following. 1) C. gattii R265 had an exceptionally high chitosan content compared with C. neoformans H99; a rich chitosan composition promoted homogeneous melanin distribution throughout the cell wall but did not increase the propensity of pigment deposition. 2) Strains from both species manifesting the leaky melanin phenotype had reduced chitosan content, which was compensated for by the production of lipids and other nonpolysaccharide constituents that depended on the species or mutation. 3) Changes in the relative rigidity of cell-wall chitin were associated with aberrant pigment retention, implicating cell-wall flexibility as an independent variable in cryptococcal melanin assembly. Overall, our results indicate that cell-wall composition and molecular architecture are critical factors for the anchoring and arrangement of melanin pigments in both C. neoformans and C. gattii species complexes.

Chitosan hydrogel micro-bio-devices with complex capillary patterns via reactive-diffusive self-assembly

We present chitosan hydrogel microfluidic devices with self-assembled complex microcapillary patterns, conveniently formed by a diffusion-reaction process. These patterns in chitosan hydrogels are formed by a single-step procedure involving diffusion of a gelation agent into the polymer solution inside a microfluidic channel. By changing the channel geometry, it is demonstrated how to control capillary length, trajectory and branching. Diffusion of nanoparticles (NPs) in the capillary network is used as a model to effectively mimic the transport of nano-objects in vascularized tissues. Gold NPs diffusion is measured locally in the hydrogel chips, and during their two-step transport through the capillaries to the gel matrix and eventually to embedded cell clusters in the gel. In addition, the quantitative analyses reported in this study provide novel opportunities for theoretical investigation of capillary formation and propagation during diffusive gelation of biopolymers. STATEMENT OF SIGNIFICANCE: Hydrogel micropatterning is a challenging task, which is of interest in several biomedical applications. Creating the patterns through self assembly is highly beneficial, because of the accessible and practical preparation procedure. In this study, we introduced complex self-assembled capillary patterns in chitosan hydrogels using a microfluidic approach. To demonstrate the potential application of these capillary patterns, a vascularized hydrogel with microwells occupied by cells was produced, and the diffusion of gold nanoparticles travelling in the capillaries and diffusing in the gel were evaluated. This model mimics a simplified biological tissue, where nanomedicine has to travel through the vasculature, extravasate into and diffuse through the extracellular matrix and eventually reach targeted cells.

Sustainable ecofriendly phytoextract mediated one pot green recovery of chitosan

Chitin and chitosan are biopolymers that have diverse applications in medicine, agriculture, food, cosmetics, pharmaceuticals, wastewater treatment and textiles. With bio-origins, they easily blend with biological systems and show exemplified compatibility which is mandatory when it comes to biomedical research. Chitin and chitosan are ecofriendly, however the processes that are used to recover them aren’t ecofriendly. The focus of this work is to attempt an ecofriendly, sustainable phytomediated one pot recovery of chitosan from commercial chitin and from crab and shrimp shells and squid pen solid wastes. Graviola extracts have been employed, given the fact file that their active ingredients acetogenins actively interact with chitin in insects (resulting in its application as an insecticide). With that as the core idea, the graviola extracts were chosen for orchestrating chitin recovery and a possible chitin to chitosan transformation. The results confirm that graviola extracts did succeed in recovery of chitosan nanofibers from commercial chitin flakes and recovery of chitosan particles directly from solid marine wastes of crab, shrimp and squids. This is a first time report of a phyto-extract mediated novel chitosan synthesis method.

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.

Influence of Preparation Methods of Chitooligosaccharides on Their Physicochemical Properties and Their Anti-Inflammatory Effects in Mice and in RAW264.7 Macrophages

The methods to obtain chitooligosaccharides are tightly related to the physicochemical properties of the end products. Knowledge of these physicochemical characteristics is crucial to describing the biological functions of chitooligosaccharides. Chitooligosaccharides were prepared either in a single-step enzymatic hydrolysis using chitosanase, or in a two-step chemical-enzymatic hydrolysis. The hydrolyzed products obtained in the single-step preparation were composed mainly of 42% fully deacetylated oligomers plus 54% monoacetylated oligomers, and they attenuated the inflammation in lipopolysaccharide-induced mice and in RAW264.7 macrophages. However, chitooligosaccharides from the two-step preparation were composed of 50% fully deacetylated oligomers plus 27% monoacetylated oligomers and, conversely, they promoted the inflammatory response in both in vivo and in vitro models. Similar proportions of monoacetylated and deacetylated oligomers is necessary for the mixtures of chitooligosaccharides to achieve anti-inflammatory effects, and it directly depends on the preparation method to which chitosan was submitted.

Assessment of the Quorum Sensing Inhibition Activity of a Non-Toxic Chitosan in an N-Acyl Homoserine Lactone (AHL)-Based Escherichia coli Biosensor

New approaches to deal with drug-resistant pathogenic bacteria are urgent. We studied the antibacterial effect of chitosans against an Escherichia coli quorum sensing biosensor reporter strain and selected a non-toxic chitosan to evaluate its quorum sensing (QS) inhibition activity and its effect on bacterial aggregation. To this end, chitosans of varying degree of acetylation (DA) (12 to 69%) and molecular weight (Mw) (29 to 288 kDa) were studied. Only chitosans of low DA (~12%) inhibited bacterial growth, regardless of their Mw. A chitosan with medium degree of polymerization (named MDP) DA30, with experimental DA 42% and Mw 115 kDa was selected for further QS inhibition and scanning electron microscopy (SEM) imaging studies. MDP DA30 chitosan exhibited QS inhibition activity in an inverse dose-dependent manner (≤12.5 µg/mL). SEM images revealed that this chitosan, when added at low concentration (≤30.6 µg/mL), induced substantial bacterial aggregation, whereas at high concentration (234.3 µg/mL), it did not. Aggregation explains the QS inhibition activity as the consequence of retardation of the diffusion of N-acylated homoserine lactones (AHLs).

Chitin Deacetylases: Structures, Specificities, and Biotech Applications

Depolymerization and de-N-acetylation of chitin by chitinases and deacetylases generates a series of derivatives including chitosans and chitooligosaccharides (COS), which are involved in molecular recognition events such as modulation of cell signaling and morphogenesis, immune responses, and host-pathogen interactions. Chitosans and COS are also attractive scaffolds for the development of bionanomaterials for drug/gene delivery and tissue engineering applications. Most of the biological activities associated with COS seem to be largely dependent not only on the degree of polymerization but also on the acetylation pattern, which defines the charge density and distribution of GlcNAc and GlcNH₂ moieties in chitosans and COS. Chitin de-N-acetylases (CDAs) catalyze the hydrolysis of the acetamido group in GlcNAc residues of chitin, chitosan, and COS. The deacetylation patterns are diverse, some CDAs being specific for single positions, others showing multiple attack, processivity or random actions. This review summarizes the current knowledge on substrate specificity of bacterial and fungal CDAs, focusing on the structural and molecular aspects of their modes of action. Understanding the structural determinants of specificity will not only contribute to unravelling structure-function relationships, but also to use and engineer CDAs as biocatalysts for the production of tailor-made chitosans and COS for a growing number of applications.

Reassessment of chitosanase substrate specificities and classification

Chitosanases can be used to produce partially acetylated chitosan oligosaccharides (paCOS) for different applications, provided they are thoroughly characterized. However, recent studies indicate that the established classification system for chitosanases is too simplistic. Here, we apply a highly sensitive method for quantitatively sequencing paCOS to reassess the substrate specificities of the best-characterized class I–III chitosanases. The enzymes’ abilities to cleave bonds at GlcNAc residues positioned at subsite (−1) or (+1), on which the classification system is based, vary especially when the substrates have different fractions of acetylation (F_A). Conflicts with the recent classification are observed at higher F_A, which were not investigated in prior specificity determinations. Initial analyses of pectin-degrading enzymes reveal that classifications of other polysaccharide-degrading enzymes should also be critically reassessed. Based on our results, we tentatively suggest a chitosanase classification system which is based on specificities and preferences of subsites (−2) to (+2).

Chitosanases are classified according to their specificity in cleaving bonds at GlcNAc residues but the current system may be too simplistic. Here, the authors use quantitative mass spectrometry to revisit chitosanase specificity and propose additional determinants for their classification.

Chitosan Analysis by Enzymatic/Mass Spectrometric Fingerprinting and in Silico Predictive Modeling

Chitosans, β-1,4-linked partially N-acetylated linear polyglucosamines, are very versatile and promising functional biopolymers. Understanding their structure-function relationships requires sensitive and accurate structural analyses to determine parameters like degree of polymerization (DP), fraction of acetylation (F_A), or pattern of acetylation (P_A). NMR, the gold standard for F_A analysis, requires large amounts of sample. Here, we describe an enzymatic/mass spectrometric fingerprinting method to analyze the F_A of chitosan polymers. The method combines the use of chitinosanase, a sequence-specific hydrolase that cleaves chitosan polymers into oligomeric fingerprints, ultrahigh-performance liquid chromatography-electrospray ionization-mass spectrometry (UHPLC-ESI-MS), and partial least-squares regression (PLSR). We also developed a technique to simulate enzymatic fingerprints in silico that were used to build the PLS models for F_A determination. Overall, we found our method to be as accurate as NMR while at the same time requiring only microgram amounts of sample. Thus, the method represents a powerful technique for chitosan analysis.

Dynamic Structuration of Physical Chitosan Hydrogels

We studied the microstructure of physical chitosan hydrogels formed by the neutralization of chitosan aqueous solutions highlighting the structural gradients within thick gels (up to a thickness of 16 mm). We explored a high polymer concentrations range (C_p ≥ 1.0% w/w) with different molar masses of chitosan and different concentrations of the coagulation agent. The effect of these processing parameters on the morphology was evaluated mainly through small-angle light scattering (SALS) measurements and confocal laser scanning microscopy (CLSM) observations. As a result, we reported that the microstructure is continuously evolving from the surface to the bulk, with mainly two structural transitions zones separating three types of hydrogels. The first zone (zone I) is located close to the surface of the hydrogel and constitutes a hard (entangled) layer formed under fast neutralization conditions. It is followed by a second zone (zone II) with a larger thickness (∼3-4 mm), where in some cases large pores or capillaries (diameter ∼10 μm) oriented parallel to the direction of the gel front are present. Deeper in the hydrogel (zone III), a finer oriented microstructure, with characteristic sizes lower than 2-3 μm, gradually replace the capillary morphology. However, this last bulk morphology cannot be regarded as structurally uniform because the size of small micrometer-range-oriented pores continuously increases as the distance to the surface of the hydrogel increases. These results could be rationalized through the effect of coagulation kinetics impacting the morphology obtained during neutralization.

An accurate coarse-grained model for chitosan polysaccharides in aqueous solution

Computational models can provide detailed information about molecular conformations and interactions in solution, which is currently inaccessible by other means in many cases. Here we describe an efficient and precise coarse-grained model for long polysaccharides in aqueous solution at different physico-chemical conditions such as pH and ionic strength. The Model is carefully constructed based on all-atom simulations of small saccharides and metadynamics sampling of the dihedral angles in the glycosidic links, which represent the most flexible degrees of freedom of the polysaccharides. The model is validated against experimental data for Chitosan molecules in solution with various degree of deacetylation, and is shown to closely reproduce the available experimental data. For long polymers, subtle differences of the free energy maps of the glycosidic links are found to significantly affect the measurable polymer properties. Therefore, for titratable monomers the free energy maps of the corresponding links are updated according to the current charge of the monomers. We then characterize the microscopic and mesoscopic structural properties of large chitosan polysaccharides in solution for a wide range of solvent pH and ionic strength, and investigate the effect of polymer length and degree and pattern of deacetylation on the polymer properties.