Biosynthesis and Function of Long Guluronic Acid-Blocks in Alginate Produced by Azotobacter vinelandii

Microbeads as carriers for Bacillus pumilus: a biofertilizer focus on auxin production

The study aimed to develop a solid biofertilizer using Bacillus pumilus, focusing on auxin production to enhance plant drought tolerance. Methods involved immobilising B. pumilus in alginate-starch beads, focusing on microbial concentration, biopolymer types, and environmental conditions. The optimal formulation showed a diameter of 3.58 mm ± 0.18, a uniform size distribution after 15 h of drying at 30 °C, a stable bacterial concentration (1.99 × 109 CFU g-1 ± 1.03 × 109 over 180 days at room temperature), a high auxin production (748.8 µg g-1 ± 10.3 of IAA in 7 days), and a water retention capacity of 37% ± 4.07. In conclusion, this new formulation of alginate + starch + L-tryptophan + B. pumilus has the potential for use in crops due to its compelling water retention, high viability in storage at room temperature, and high auxin production, which provides commercial advantages.

Alginate: Microbial production, functionalization, and biomedical applications

Alginates are natural polysaccharides widely participating in food, pharmaceutical, and environmental applications due to their excellent gelling capacity. Their excellent biocompatibility and biodegradability further extend their application to biomedical fields. The low consistency in molecular weight and composition of algae-based alginates may limit their performance in advanced biomedical applications. It makes microbial alginate production more attractive due to its potential for customizing alginate molecules with stable characteristics. Production costs remain the primary factor limiting the commercialization of microbial alginates. However, carbon-rich wastes from sugar, dairy, and biodiesel industries may serve as potential substitutes for pure sugars for microbial alginate production to reduce substrate costs. Fermentation parameter control and genetic engineering strategies may further improve the production efficiency and customize the molecular composition of microbial alginates. To meet the specific needs of biomedical applications, alginates may need functionalization, such as functional group modifications and crosslinking treatments, to achieve enhanced mechanical properties and biochemical activities. The development of alginate-based composites incorporated with other polysaccharides, gelatin, and bioactive factors can integrate the advantages of each component to meet multiple requirements in wound healing, drug delivery, and tissue engineering applications. This review provided a comprehensive insight into the sustainable production of high-value microbial alginates. It also discussed recent advances in alginate modification strategies and alginate-based composites for representative biomedical applications.

State of Innovation in Alginate-Based Materials

This review article presents past and current alginate-based materials in each application, showing the widest range of alginate's usage and development in the past and in recent years. The first segment emphasizes the unique characteristics of alginates and their origin. The second segment sets alginates according to their application based on their features and limitations. Alginate is a polysaccharide and generally occurs as water-soluble sodium alginate. It constitutes hydrophilic and anionic polysaccharides originally extracted from natural brown algae and bacteria. Due to its promising properties, such as gelling, moisture retention, and film-forming, it can be used in environmental protection, cosmetics, medicine, tissue engineering, and the food industry. The comparison of publications with alginate-based products in the field of environmental protection, medicine, food, and cosmetics in scientific articles showed that the greatest number was assigned to the environmental field (30,767) and medicine (24,279), whereas fewer publications were available in cosmetic (5692) and food industries (24,334). Data are provided from the Google Scholar database (including abstract, title, and keywords), accessed in May 2023. In this review, various materials based on alginate are described, showing detailed information on modified composites and their possible usage. Alginate's application in water remediation and its significant value are highlighted. In this study, existing knowledge is compared, and this paper concludes with its future prospects.

Mannuronate C-5 epimerases and their use in alginate modification

Alginate is a polysaccharide consisting of β-D-mannuronate (M) and α-L-guluronate (G) produced by brown algae and some bacterial species. Alginate has a wide range of industrial and pharmaceutical applications, owing mainly to its gelling and viscosifying properties. Alginates with high G content are considered more valuable since the G residues can form hydrogels with divalent cations. Alginates are modified by lyases, acetylases, and epimerases. Alginate lyases are produced by alginate-producing organisms and by organisms that use alginate as a carbon source. Acetylation protects alginate from lyases and epimerases. Following biosynthesis, alginate C-5 epimerases convert M to G residues at the polymer level. Alginate epimerases have been found in brown algae and alginate-producing bacteria, predominantly Azotobacter and Pseudomonas species. The best characterised epimerases are the extracellular family of AlgE1-7 from Azotobacter vinelandii (Av). AlgE1-7 all consist of combinations of one or two catalytic A-modules and one to seven regulatory R-modules, but even though they are sequentially and structurally similar, they create different epimerisation patterns. This makes the AlgE enzymes promising for tailoring of alginates to have the desired properties. The present review describes the current state of knowledge regarding alginate-active enzymes with focus on epimerases, characterisation of the epimerase reaction, and how alginate epimerases can be used in alginate production.

Molecular insights into alginate β-lactoglobulin A multivalencies-The foundation for their amorphous aggregates and coacervation

For improved control of biomaterial property design, a better understanding of complex coacervation involving anionic polysaccharides and proteins is needed. Here, we address the initial steps in condensate formation of β-lactoglobulin A (β-LgA) with nine defined alginate oligosaccharides (AOSs) and describe their multivalent interactions in structural detail. Binding of AOSs containing four, five, or six uronic acid residues (UARs), either all mannuronate (M), all guluronate (G), or alternating M and G embodying the block structural components of alginates, was characterized by isothermal titration calorimetry, nuclear magnetic resonance spectroscopy (NMR), and molecular docking. β-LgA was highly multivalent exhibiting binding stoichiometries decreasing from five to two AOSs with increasing degree of polymerization (DP) and similar affinities in the mid micromolar range. The different AOS binding sites on β-LgA were identified by NMR chemical shift perturbation analyses and showed diverse compositions of charged, polar and hydrophobic residues. Distinct sites for the shorter AOSs merged to accommodate longer AOSs. The AOSs bound dynamically to β-LgA, as concluded from saturation transfer difference and ¹ H-ligand-targeted NMR analyses. Molecular docking using Glide within the Schrödinger suite 2016-1 revealed the orientation of AOSs to only vary slightly at the preferred β-LgA binding site resulting in similar XP glide scores. The multivalency coupled with highly dynamic AOS binding with lack of confined conformations in the β-LgA complexes may help explain the first steps toward disordered β-LgA alginate coacervate structures.

Hydrogels based on methylated-alginates as a platform to investigate the effect of material properties on cell activity. The role of material compliance

Alginate-based hydrogels with tunable mechanical properties are developed by chemical methylation of the polysaccharide backbone, which was performed either in homogeneous phase (in solution) or in heterogeneous phase (on hydrogels). Nuclear Magnetic Resonance (NMR) and Size Exclusion Chromatography (SEC-MALS) analyses of methylated alginates allow to identify the presence and location of methyl groups on the polysaccharide, and to investigate the influence of methylation on the stiffness of the polymer chains. The methylated polysaccharides are employed for the manufacturing of calcium-reticulated hydrogels for cell growth in 3D. The rheological characterization shows that the shear modulus of hydrogels is dependent on the amount of cross-linker used. Methylated alginates represent a platform to explore the effect of mechanical properties on cell activity. As an example, the effect of compliance is investigated using hydrogels displaying similar shear modulus. An osteosarcoma cell line (MG-63) was encapsulated in the alginate hydrogels and the effect of material compliance on cell proliferation and localization of YAP/TAZ protein complex is investigated by flow cytometry and immunohistochemistry, respectively. The results point out that an increase of material compliance leads to an increase of the proliferative rate of cells and correlates with the translocation of YAP/TAZ inside the cell nucleus.

Assembly and synthesis of the extracellular matrix in brown algae

In brown algae, the extracellular matrix (ECM) and its constitutive polymers play crucial roles in specialized functions, including algal growth and development. In this review we offer an integrative view of ECM construction in brown algae. We briefly report the chemical composition of its main constituents, and how these are interlinked in a structural model. We examine the ECM assembly at the tissue and cell level, with consideration on its structure in vivo and on the putative subcellular sites for the synthesis of its main constituents. We further discuss the biosynthetic pathways of two major polysaccharides, alginates and sulfated fucans, and the progress made beyond the candidate genes with the biochemical validation of encoded proteins. Key enzymes involved in the elongation of the glycan chains are still unknown and predictions have been made at the gene level. Here, we offer a re-examination of some glycosyltransferases and sulfotransferases from published genomes. Overall, our analysis suggests novel investigations to be performed at both the cellular and biochemical levels. First, to depict the location of polysaccharide structures in tissues. Secondly, to identify putative actors in the ECM synthesis to be functionally studied in the future.

Strain HMM57, Isolated from Sedimentary Layers of Crustose Coralline Algae

Here, we report the genome sequence of Flagellimonas sp. strain HMM57, which was isolated from sedimentary layers of crustose coralline algae in Jeju Island, South Korea. The genome is complete and consists of 4,159,450 bp, with a GC content of 38.5%, 3,616 predicted protein-coding sequences, and 70 RNA genes.

Effects of Module Truncation of a New Alginate Lyase VxAly7C from Marine Vibrio xiamenensis QY104 on Biochemical Characteristics and Product Distribution

Alginate lyase has received extensive attention as an important tool for oligosaccharide preparation, pharmaceutical production, and energy biotransformation. Noncatalytic module carbohydrate-binding modules (CBM) have a major impact on the function of alginate lyases. Although the effects of two different families of CBMs on enzyme characteristics have been reported, the effect of two combined CBM32s on enzyme function has not been elucidated. Herein, we cloned and expressed a new multimodular alginate lyase, VxAly7C, from Vibrioxiamenensis QY104, consisting of two CBM32s at N-terminus and a polysaccharide lyase family 7 (PL7) at C-terminus. To explore the function of CBM32s in VxAly7C, full-length (VxAly7C-FL) and two truncated mutants, VxAly7C-TM1 (with the first CBM32 deleted) and VxAly7C-TM2 (with both CBM32s deleted), were characterized. The catalytic efficiency of recombinant VxAly7C-TM2 was 1.82 and 4.25 times higher than that of VxAly7C-TM1 and VxAly7C-FL, respectively, indicating that CBM32s had an antagonistic effect. However, CBM32s improved the temperature stability, the adaptability in an alkaline environment, and the preference for polyG. Moreover, CBM32s contributed to the production of tri- and tetrasaccharides, significantly affecting the end-product distribution. This study advances the understanding of module function and provides a reference for broader enzymatic applications and further enzymatic improvement and assembly.

The Function of CBM32 in Alginate Lyase VxAly7B on the Activity on Both Soluble Sodium Alginate and Alginate Gel

Carbohydrate-binding modules (CBMs), as an important auxiliary module, play a key role in degrading soluble alginate by alginate lyase, but the function on alginate gel has not been elucidated. Recently, we reported alginate lyase VxAly7B containing a CBM32 and a polysaccharide lyase family 7 (PL7). To investigate the specific function of CBM32, we characterized the full-length alginate lyase VxAly7B (VxAly7B-FL) and truncated mutants VxAly7B-CM (PL7) and VxAly7B-CBM (CBM32). Both VxAly7B-FL and native VxAly7B can spontaneously cleavage between CBM32 and PL7. The substrate-binding capacity and activity of VxAly7B-CM to soluble alginate were 0.86- and 1.97-fold those of VxAly7B-FL, respectively. Moreover, CBM32 could accelerate the expansion and cleavage of alginate gel beads, and the degradation rate of VxAly7B-FL to alginate gel beads was threefold that of VxAly7B-CM. Results showed that CBM32 is not conducive to the degradation of soluble alginate by VxAly7B but is helpful for binding and degradation of insoluble alginate gel. This study provides new insights into the function of CBM32 on alginate gel, which may inspire the application strategy of CBMs in insoluble substrates.

Competitive Biosynthesis of Bacterial Alginate Using Azotobacter vinelandii 12 for Tissue Engineering Applications

This study investigated the effect of various cultivation conditions (sucrose/phosphate concentrations, aeration level) on alginate biosynthesis using the bacterial producing strain Azotobacter vinelandii 12 by the full factorial design (FFD) method and physicochemical properties (e.g., rheological properties) of the produced bacterial alginate. We demonstrated experimentally the applicability of bacterial alginate for tissue engineering (the cytotoxicity testing using mesenchymal stem cells (MSCs)). The isolated synthesis of high molecular weight (M_w) capsular alginate with a high level of acetylation (25%) was achieved by FFD method under a low sucrose concentration, an increased phosphate concentration, and a high aeration level. Testing the viscoelastic properties and cytotoxicity showed that bacterial alginate with a maximal M_w (574 kDa) formed the densest hydrogels (which demonstrated relatively low cytotoxicity for MSCs in contrast to bacterial alginate with low M_w). The obtained data have shown promising prospects in controlled biosynthesis of bacterial alginate with different physicochemical characteristics for various biomedical applications including tissue engineering.

Green Coating Polymers in Meat Preservation

Edible coatings, including green polymers are used frequently in the food industry to improve and preserve the quality of foods. Green polymers are defined as biodegradable polymers from biomass resources or synthetic routes and microbial origin that are formed by mono- or multilayer structures. They are used to improve the technological properties without compromising the food quality, even with the purpose of inhibiting lipid oxidation or reducing metmyoglobin formation in fresh meat, thereby contributing to the final sensory attributes of the food and meat products. Green polymers can also serve as nutrient-delivery carriers in meat and meat products. This review focuses on various types of bio-based biodegradable polymers and their preparation techniques and applications in meat preservation as a part of active and smart packaging. It also outlines the impact of biodegradable polymer films or coatings reinforced with fillers, either natural or synthesized, via the green route in enhancing the physicochemical, mechanical, antimicrobial, and antioxidant properties for extending shelf-life. The interaction of the package with meat contact surfaces and the advanced polymer composite sensors for meat toxicity detection are further considered and discussed. In addition, this review addresses the research gaps and challenges of the current packaging systems, including coatings where green polymers are used. Coatings from renewable resources are seen as an emerging technology that is worthy of further investigation toward sustainable packaging of food and meat products.

Alginate degrading enzymes: an updated comprehensive review of the structure, catalytic mechanism, modification method and applications of alginate lyases

Alginate, a kind of linear acidic polysaccharide, consists of α-L-guluronate (G) and β-D-mannuronate (M). Both alginate and its degradation products (alginate oligosaccharides) possess abundant biological activities such as antioxidant activity, antitumor activity, and antimicrobial activity. Therefore, alginate and alginate oligosaccharides have great value in food, pharmaceutical, and agricultural fields. Alginate lyase can degrade alginate into alginate oligosaccharides via the β-elimination reaction. It plays an important role in marine carbon recycling and the deep utilization of brown algae. Elucidating the structural features of alginate lyase can improve our knowledge of its catalytic mechanisms. With the development of structural analysis techniques, increasing numbers of alginate lyases have been characterized at the structural level. Hence, it is essential and helpful to summarize and discuss the up-to-date findings. In this review, we have summarized progress on the structural features and the catalytic mechanisms of alginate lyases. Furthermore, the molecular modification strategies and the applications of alginate lyases have also been discussed. This comprehensive information should be helpful to expand the applications of alginate lyases.

Insights into the roles of charged residues in substrate binding and mode of action of mannuronan C-5 epimerase AlgE4

Mannuronan C-5 epimerases catalyse the epimerization of monomer residues in the polysaccharide alginate, changing the physical properties of the biopolymer. The enzymes are utilized to tailor alginate to numerous biological functions by alginate-producing organisms. The underlying molecular mechanisms that control the processive movement of the epimerase along the substrate chain is still elusive. To study this, we have used an interdisciplinary approach combining molecular dynamics simulations with experimental methods from mutant studies of AlgE4, where initial epimerase activity and product formation were addressed with NMR spectroscopy, and characteristics of enzyme-substrate interactions were obtained with isothermal titration calorimetry and optical tweezers. Positive charges lining the substrate-binding groove of AlgE4 appear to control the initial binding of poly-mannuronate, and binding also seems to be mediated by both electrostatic and hydrophobic interactions. After the catalytic reaction, negatively charged enzyme residues might facilitate dissociation of alginate from the positive residues, working like electrostatic switches, allowing the substrate to translocate in the binding groove. Molecular simulations show translocation increments of two monosaccharide units before the next productive binding event resulting in MG-block formation, with the epimerase moving with its N-terminus towards the reducing end of the alginate chain. Our results indicate that the charge pair R343-D345 might be directly involved in conformational changes of a loop that can be important for binding and dissociation. The computational and experimental approaches used in this study complement each other, allowing for a better understanding of individual residues' roles in binding and movement along the alginate chains.

Molecular weight and guluronic/mannuronic ratio of alginate produced by Azotobacter vinelandii at two bioreactor scales under diazotrophic conditions

Alginates can be used to elaborate hydrogels, and their properties depend on the molecular weight (MW) and the guluronic (G) and mannuronic (M) composition. In this study, the MW and G/M ratio were evaluated in cultures of Azotobacter vinelandii to 3 and 30 L scales at different oxygen transfer rates (OTRs) under diazotrophic conditions. An increase in the maximum OTR (OTR_max) improved the alginate production, reaching 3.3 ± 0.2 g L^-1. In the cultures conducted to an OTR of 10.4 mmol L^-1 h^-1 (500 rpm), the G/M increased during the cell growth phase and decreased during the stationary phase; whereas, in the cultures at 19.2 mmol L^-1 h^-1 was constant throughout the cultivation. A higher alginate MW (520 ± 43 kDa) and G/M ratio (0.86 ± 0.01) were obtained in the cultures conducted at 10.4 mmol L^-1 h^-1. The OTR as a criterion to scale up alginate production allowed to replicate the concentration and the alginate production rate; however, it was not possible reproduce the MW and G/M ratio. Under a similar specific oxygen uptake rate (qO₂) (approximately 65 mmol g^-1 h^-1) the alginate MW was similar (approximately 365 kDa) in both scales. The evidences revealed that the qO₂ can be a parameter adequate to produce alginate MW similar in two bioreactor scales. Overall, the results have shown that the alginate composition could be affected by cellular respiration, and from a technological perspective the evidences contribute to the design process based on oxygen consumption to produce alginates defined.

Properties and potential applications of mannuronan C5-epimerase: A biotechnological tool for modifying alginate

Given the excellent characteristics of alginate, it is an industrially important polysaccharide. Mannuronan C5-epimerase (MC5E) is an alginate-modifying enzyme that catalyzes the conversion of β-D-mannuronate (M) to its C5 epimer α-L-guluronate (G) in alginate. Both the biological activities and physical properties of alginate are determined by M/G ratios and distribution patterns. Therefore, MC5E is regarded as a biotechnological tool for modifying and processing alginate. Various MC5Es derived from brown algae, Pseudomonas and Azotobacter have been isolated and characterized. With the rapid development of structural biology, the crystal structures and catalytic mechanisms of several MC5Es have been elucidated. It is necessary to comprehensively understand the research status of this alginate-modifying enzyme. In this review, the properties and potential applications of MC5Es isolated from different kinds of organisms are summarized and reviewed. Moreover, future research directions of MC5Es as well as strategies to enhance their properties are elucidated, highlighted, and prospected.

Impact of Alginate Mannuronic-Guluronic Acid Contents and pH on Protein Binding Capacity and Complex Size

Alginates, serving as hydrocolloids in the food and pharma industries, form particles at pH < 4.5 with positively charged proteins, such as β-lactoglobulin (β-Lg). Alginates are linear anionic polysaccharides composed of 1,4-linked β-d-mannuronate (M) and α-l-guluronate (G) residues. The impact of M and G contents and pH is investigated to correlate with the formation and size of β-Lg alginate complexes under relevant ionic strength. It is concluded, using three alginates of M/G ratios 0.6, 1.1, and 1.8 and similar molecular mass, that β-Lg binding capacity is higher at pH 4.0 than at pH 2.65 and for high M content. By contrast, the largest particles are obtained at pH 2.65 and with high G content. At pH 4.0 and 2.65, the stoichiometry was 28-48 and 3-10 β-Lg molecules bound per alginate, respectively, increasing with higher M content. The findings will contribute to the design of formation of the desired alginate-protein particles in the acidic pH range.

Exploiting Mannuronan C-5 Epimerases in Commercial Alginate Production

Alginates are one of the major polysaccharide constituents of marine brown algae in commercial manufacturing. However, the content and composition of alginates differ according to the distinct parts of these macroalgae and have a direct impact on the concentration of guluronate and subsequent commercial value of the final product. The Azotobacter vinelandii mannuronan C-5 epimerases AlgE1 and AlgE4 were used to determine their potential value in tailoring the production of high guluronate low-molecular-weight alginates from two sources of high mannuronic acid alginates, the naturally occurring harvested brown algae (Ascophyllum nodosum, Durvillea potatorum, Laminaria hyperborea and Lessonia nigrescens) and a pure mannuronic acid alginate derived from fermented production of the mutant strain of Pseudomonas fluorescens NCIMB 10,525. The mannuronan C-5 epimerases used in this study increased the content of guluronate from 32% up to 81% in both the harvested seaweed and bacterial fermented alginate sources. The guluronate-rich alginate oligomers subsequently derived from these two different sources showed structural identity as determined by proton nuclear magnetic resonance (1H NMR), high-performance anion-exchange chromatography with pulsed amperometric detection (HPAEC-PAD) and size-exclusion chromatography with online multi-angle static laser light scattering (SEC-MALS). Functional identity was determined by minimum inhibitory concentration (MIC) assays with selected bacteria and antibiotics using the previously documented low-molecular-weight guluronate enriched alginate OligoG CF-5/20 as a comparator. The alginates produced using either source showed similar antibiotic potentiation effects to the drug candidate OligoG CF-5/20 currently in development as a mucolytic and anti-biofilm agent. These findings clearly illustrate the value of using epimerases to provide an alternative production route for novel low-molecular-weight alginates.

Functional characterization of three Azotobacter chroococcum alginate-modifying enzymes related to the Azotobacter vinelandii AlgE mannuronan C-5-epimerase family

Bacterial alginate initially consists of 1–4-linked β-D-mannuronic acid residues (M) which can be later epimerized to α-L-guluronic acid (G). The family of AlgE mannuronan C-5-epimerases from Azotobacter vinelandii has been extensively studied, and three genes putatively encoding AlgE-type epimerases have recently been identified in the genome of Azotobacter chroococcum. The three A. chroococcum genes, here designated AcalgE1, AcalgE2 and AcalgE3, were recombinantly expressed in Escherichia coli and the gene products were partially purified. The catalytic activities of the enzymes were stimulated by the addition of calcium ions in vitro. AcAlgE1 displayed epimerase activity and was able to introduce long G-blocks in the alginate substrate, preferentially by attacking M residues next to pre-existing G residues. AcAlgE2 and AcAlgE3 were found to display lyase activities with a substrate preference toward M-alginate. AcAlgE2 solely accepted M residues in the positions − 1 and + 2 relative to the cleavage site, while AcAlgE3 could accept either M or G residues in these two positions. Both AcAlgE2 and AcAlgE3 were bifunctional and could also catalyze epimerization of M to G. Together, we demonstrate that A. chroococcum encodes three different AlgE-like alginate-modifying enzymes and the biotechnological and biological impact of these findings are discussed.

Identification of a Pivotal Residue for Determining the Block Structure-Forming Properties of Alginate C-5 Epimerases

Alginate is a linear copolymer composed of 1→4 linked β-d-mannuronic acid (M) and its epimer α-l-guluronic acid (G). The polysaccharide is first produced as homopolymeric mannuronan and subsequently, at the polymer level, C-5 epimerases convert M residues to G residues. The bacterium Azotobacter vinelandii encodes a family of seven secreted and calcium ion-dependent mannuronan C-5 epimerases (AlgE1-AlgE7). These epimerases consist of two types of structural modules: the A-modules, which contain the catalytic site, and the R-modules, which influence activity through substrate and calcium binding. In this study, we rationally designed new hybrid mannuronan C-5 epimerases constituting the A-module from AlgE6 and the R-module from AlgE4. This led to a better understanding of the molecular mechanism determining differences in MG- and GG-block-forming properties of the enzymes. A long loop with either tyrosine or phenylalanine extruding from the β-helix of the enzyme proved essential in defining the final alginate block structure, probably by affecting substrate binding. Normal mode analysis of the A-module from AlgE6 supports the results.