Structure-Activity Relationships of Low Molecular Weight Alginate Oligosaccharide Therapy against Pseudomonas aeruginosa

Low molecular weight alginate oligosaccharides have been shown to exhibit anti-microbial activity against a range of multi-drug resistant bacteria, including Pseudomonas aeruginosa. Previous studies suggested that the disruption of calcium (Ca2+)-DNA binding within bacterial biofilms and dysregulation of quorum sensing (QS) were key factors in these observed effects. To further investigate the contribution of Ca2+ binding, G-block (OligoG) and M-block alginate oligosaccharides (OligoM) with comparable average size DPn 19 but contrasting Ca2+ binding properties were prepared. Fourier-transform infrared spectroscopy demonstrated prolonged binding of alginate oligosaccharides to the pseudomonal cell membrane even after hydrodynamic shear treatment. Molecular dynamics simulations and isothermal titration calorimetry revealed that OligoG exhibited stronger interactions with bacterial LPS than OligoM, although this difference was not mirrored by differential reductions in bacterial growth. While confocal laser scanning microscopy showed that both agents demonstrated similar dose-dependent reductions in biofilm formation, OligoG exhibited a stronger QS inhibitory effect and increased potentiation of the antibiotic azithromycin in minimum inhibitory concentration and biofilm assays. This study demonstrates that the anti-microbial effects of alginate oligosaccharides are not purely influenced by Ca2+-dependent processes but also by electrostatic interactions that are common to both G-block and M-block structures.

Strain Construction and Process Development for Efficient Recombinant Production of Mannuronan C-5 Epimerases in Hansenula polymorpha

Alginates are linear polysaccharides produced by brown algae and some bacteria and are composed of β-D-mannuronic acid (M) and α-L-guluronic acid (G). Alginate has numerous present and potential future applications within industrial, medical and pharmaceutical areas and G rich alginates are traditionally most valuable and frequently used due to their gelling and viscosifying properties. Mannuronan C-5 epimerases are enzymes converting M to G at the polymer level during the biosynthesis of alginate. The Azotobacter vinelandii epimerases AlgE1-AlgE7 share a common structure, containing one or two catalytic A-modules (A), and one to seven regulatory R-modules (R). Despite the structural similarity of the epimerases, they create different M-G patterns in the alginate; AlgE4 (AR) creates strictly alternating MG structures whereas AlgE1 (ARRRAR) and AlgE6 (ARRR) create predominantly G-blocks. These enzymes are therefore promising tools for producing in vitro tailor-made alginates. Efficient in vitro epimerization of alginates requires availability of recombinantly produced alginate epimerases, and for this purpose the methylotrophic yeast Hansenula polymorpha is an attractive host organism. The present study investigates whether H. polymorpha is a suitable expression system for future large-scale production of AlgE1, AlgE4, and AlgE6. H. polymorpha expression strains were constructed using synthetic genes with reduced repetitive sequences as well as optimized codon usage. High cell density cultivations revealed that the largest epimerases AlgE1 (147 kDa) and AlgE6 (90 kDa) are subject to proteolytic degradation by proteases secreted by the yeast cells. However, degradation could be controlled to a large extent either by co-expression of chaperones or by adjusting cultivation conditions. The smaller AlgE4 (58 kDa) was stable under all tested conditions. The results obtained thus point toward a future potential for using H. polymorpha in industrial production of mannuronan C-5 epimerases for in vitro tailoring of alginates.

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.

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.

Ancient acquisition of "alginate utilization loci" by human gut microbiota

In bacteria from the phylum Bacteroidetes, the genes coding for enzymes involved in polysaccharide degradation are often colocalized and coregulated in so-called “polysaccharide utilization loci” (PULs). PULs dedicated to the degradation of marine polysaccharides (e.g. laminaran, ulvan, alginate and porphyran) have been characterized in marine bacteria. Interestingly, the gut microbiome of Japanese individuals acquired, by lateral transfer from marine bacteria, the genes involved in the breakdown of porphyran, the cell wall polysaccharide of the red seaweed used in maki. Sequence similarity analyses predict that the human gut microbiome also encodes enzymes for the degradation of alginate, the main cell wall polysaccharide of brown algae. We undertook the functional characterization of diverse polysaccharide lyases from family PL17, frequently found in marine bacteria as well as those of human gut bacteria. We demonstrate here that this family is polyspecific. Our phylogenetic analysis of family PL17 reveals that all alginate lyases, which have all the same specificity and mode of action, cluster together in a very distinct subfamily. The alginate lyases found in human gut bacteria group together in a single clade which is rooted deeply in the PL17 tree. These enzymes were found in PULs containing PL6 enzymes, which also clustered together in the phylogenetic tree of PL6. Together, biochemical and bioinformatics analyses suggest that acquisition of this system appears ancient and, because only traces of two successful transfers were detected upon inspection of PL6 and PL17 families, the pace of acquisition of marine polysaccharide degradation system is probably very slow.

Polymer sequencing by molecular machines: a framework for predicting the resolving power of a sliding contact force spectroscopy sequencing method

We evaluate an AFM-based single molecule force spectroscopy method for mapping sequences in otherwise difficult to sequence heteropolymers, including glycosylated proteins and glycans. The sliding contact force spectroscopy (SCFS) method exploits a sliding contact made between a nanopore threaded over a polymer axle and an AFM probe. We find that for sliding α- and β-cyclodextrin nanopores over a wide range of hydrophilic monomers, the free energy of sliding is proportional to the sum of two dimensionless, easily calculable parameters representing the relative partitioning of the monomer inside the nanopore or in the aqueous phase, and the friction arising from sliding the nanopore over the monomer. Using this relationship we calculate sliding energies for nucleic acids, amino acids, glycan and synthetic monomers and predict on the basis of these calculations that SCFS will detect N- and O-glycosylation of proteins and patterns of sidechains in glycans. For these applications, SCFS offers an alternative to sequence mapping by mass spectrometry or newly-emerging nanopore technologies that may be easily implemented using a standard AFM.

Poly-L-Lysine Induces Fibrosis on Alginate Microcapsules via the Induction of Cytokines

Alginate – poly-l-lysine (PLL) microcapsules can be used for transplantation of insulin-producing cells for treatment of type I diabetes. In this work we wanted to study the inflammatory reactions against implanted microcapsules due to PLL. We have seen that by reducing the PLL layer, less overgrowth of the capsule is obtained. By incubating different cell types with PLL and afterwards measuring cell viability with MTT, we found massive cell death at concentrations of PLL higher than 10 μg/ml. Staining with annexin V and propidium iodide showed that PLL induced necrosis but not apoptosis. The proinflammatory cytokine, tumor necrosis factor (TNF), was detected in supernatants from monocytes stimulated with PLL. The TNF response was partly inhibited with antibodies against CD14, which is a well-known receptor for lipopolysaccharide (LPS). Bactericidal permeability increasing protein (BPI) and a lipid A analogue (B-975), which both inhibit LPS, did not inhibit PLL from stimulating monocytes to TNF production. This indicates that PLL and LPS bind to different sites on monocytes, but because they both are inhibited by a p38 MAP kinase inhibitor, they seem to have a common element in the signal transducing pathway. These results suggest that PLL may provoke inflammatory responses either directly or indirectly through its necrosis-inducing abilities. By combining soluble PLL and alginate both the toxic and TNF-inducing effects of PLL were reduced. The implications of these data are to use alginate microcapsules with low amounts of PLL for transplantation purposes.

Microencapsulation of Cells Producing Therapeutic Proteins: Optimizing Cell Growth and Secretion

Microencapsulation of genetically engineered cells may have important applications as delivery systems for therapeutic proteins. However, optimization of the microcapsules with regard to mechanical stability, cell growth, and secretion of proteins is necessary in order to evaluate the future use of this delivery technology. We have explored the growth, survival, and secretion of therapeutic proteins from 293-EBNA cells producing endostatin (293 endo cells) and JJN3 myeloma cells producing hepatocyte growth factor (HGF) that have been embedded in various types of alginate capsules. Parameters that affect capsule integrity such as homogenous and inhomogenous gel cores and addition of an outer poly-l-lysine (PLL)–alginate coating were evaluated in relation to cell functions. When cells were encapsulated, the PLL layer was found to be absolutely required for the capsule integrity. The JJN3 and 293 endo cells displayed completely different growth and distribution patterns of live and dead cells within the microcapsules, as shown by 3D pictures reconstructed from images taken with confocal laser scanning microscopy (CLSM). Encapsulated JJN3 cells showed a bell-shaped growth and HGF secretion curve over a time period of 5 months. The 293 endo cells reached a plateau phase in growth after 23 days postencapsulation; however, after around 30 days a fraction of the microcapsules started to disintegrate. Microcapsule disintegration occurred with time irrespective of capsule and cell type, showing that alginate microcapsules possessing relatively high gel strength are not strong enough to keep proliferating cells within the microcapsules for prolonged time periods. Although this study shows that the stability of an alginate-based cell factory can be increased by a PLL–alginate coating, further improvement is necessary with regard to capsule integrity as well as controlling the cell growth before this technology can be used for therapy.

Sulfated Alginates as Heparin Analogues: A Review of Chemical and Functional Properties

Heparin is widely recognized for its potent anticoagulating effects, but has an additional wide range of biological properties due to its high negative charge and heterogeneous molecular structure. This heterogeneity has been one of the factors in motivating the exploration of functional analogues with a more predictable modification pattern and monosaccharide sequence, that can aid in elucidating structure-function relationships and further be structurally customized to fine-tune physical and biological properties toward novel therapeutic applications and biomaterials. Alginates have been of great interest in biomedicine due to their inherent biocompatibility, gentle gelling conditions, and structural versatility from chemo-enzymatic engineering, but display limited interactions with cells and biomolecules that are characteristic of heparin and the other glycosaminoglycans (GAGs) of the extracellular environment. Here, we review the chemistry and physical and biological properties of sulfated alginates as structural and functional heparin analogues, and discuss how they may be utilized in applications where the use of heparin and other sulfated GAGs is challenging and limited.

Biomimetic sulphated alginate hydrogels suppress IL-1β-induced inflammatory responses in human chondrocytes

Loss of articular cartilage from ageing, injury or degenerative disease is commonly associated with inflammation, causing pain and accelerating degradation of the cartilage matrix. Sulphated glycosaminoglycans (GAGs) are involved in the regulation of immune responses in vivo, and analogous polysaccharides are currently being evaluated for tissue engineering matrices to form a biomimetic environment promoting tissue growth while suppressing inflammatory and catabolic activities. Here, we characterise physical properties of sulphated alginate (S-Alg) gels for use in cartilage engineering scaffolds, and study their anti-inflammatory effects on encapsulated chondrocytes stimulated with IL-1β. Sulphation resulted in decreased storage modulus and increased swelling of alginate gels, whereas mixing highly sulphated alginate with unmodified alginate resulted in improved mechanical properties compared to gels from pure S-Alg. S-Alg gels showed extensive anti-inflammatory and anti-catabolic effects on encapsulated chondrocytes induced by IL-1β. Cytokine-stimulated gene expression of pro-inflammatory markers IL-6, IL-8, COX-2 and aggrecanase ADAMTS-5 were significantly lower in the sulphated gels compared to unmodified alginate gels. Moreover, sulphation of the microenvironment suppressed the protein expression of COX-2 and NF-κB as well as the activation of NF-κB and p38-MAPK. The sulphated alginate matrices were found to interact with IL-1β, and proposed to inhibit inflammatory induction by sequestering cytokines from their receptors. This study shows promising potential for sulphated alginates in biomimetic tissue engineering scaffolds, by reducing cytokine-mediated inflammation and providing a protective microenvironment for encapsulated cells.

Sliding Contact Dynamic Force Spectroscopy Method for Interrogating Slowly Forming Polymer Cross-Links

Dynamic single-molecule force spectroscopy (SMFS), conducted most commonly using AFM, has become a widespread and valuable tool for understanding the kinetics and thermodynamics of fundamental molecular processes such as ligand-receptor interactions and protein unfolding. Where slowly forming bonds are responsible for the primary characteristics of a material, as is the case in cross-links in some polymer gels, care must be taken to ensure that a fully equilibrated bond has first formed before its rupture can be interpreted. Here we introduce a method, sliding contact force spectroscopy (SCFS), that effectively eliminates the kinetics of bond formation from the measurement of bond rupture. In addition, it permits bond rupture measurements in systems where one of the binding partners may be introduced into solution prior to binding without tethering to a surface. Taking as an example of a slowly forming bond, the "eggbox" junction cross-links between oligoguluronic acid chains (oligoGs) in the commercially important polysaccharide alginate, we show that SCFS accurately measures the equilibrated bond strength of the cross-link when one chain is introduced into the sample solution without tethering to a surface. The results validate the SCFS technique for performing single-molecule force spectroscopy experiments and show that it has advantages in cases where the bond to be studied forms slowly and where tethering of one of the binding partners is impractical.

Sulfated alginate microspheres associate with factor H and dampen the inflammatory cytokine response

Alginate microspheres show promise for cell-encapsulation therapy but encounter challenges related to biocompatibility. In the present work we designed novel microbeads and microcapsules based on sulfated polyalternating MG alginate (SMG) and explored their inflammatory properties using a human whole blood model. SMG was either incorporated within the alginate microbeads or used as a secondary coat on poly-l-lysine (PLL)-containing microcapsules, resulting in reduction of the inflammatory cytokines (IL-1β, TNF, IL-6, IL-8, MIP-1α). The sulfated alginate microbeads exhibited a complement inert nature with no induction of terminal complement complex (TCC) above the values in freshly drawn blood and low surface accumulation of C3/C3b/iC3b. Conversely, SMG as a coating material lead to substantial TCC amounts and surface C3/C3b/iC3b. A common thread was an increased association of the complement inhibitor factor H to the alginate microbeads and microcapsules containing sulfated alginates. Factor H was also found to associate to non-sulfated alginate microbeads in lower amounts, indicating factor H binding as an inherent property of alginate. We conclude that the dampening effect on the cytokine response and increased factor H association points to sulfated alginate as a promising strategy for improving the biocompatibility of alginate microspheres.

STATEMENT OF SIGNIFICANCE

Alginate microspheres are candidate devices for cell encapsulation therapy. The concept is challenged by the inflammatory host response, and modification strategies for improved biocompatibility are urgently needed. One potential strategy is using sulfated alginates, acting as versatile heparin analogues with similar anti-inflammatory properties. We designed novel alginate microspheres using sulfated alginate with an alternating sequence mimicking glycosominoglycans. Evaluation in a physiologically relevant human whole blood model revealed a reduction of inflammatory cytokines by a sulfated alginate coating, and sulfated alginate microbeads were complement inert. These effects were correlated with a strong factor H association, which may represent the mechanistic explanation. This novel approach could improve the biocompatibility of alginate microspheres in vivo and present a new strategy toward clinical use.

Functional Exploration of the Polysaccharide Lyase Family PL6

Alginate, the main cell-wall polysaccharide of brown algae, is composed of two residues: mannuronic acid (M-residues) and, its C5-epimer, guluronic acid (G-residues). Alginate lyases define a class of enzymes that cleave the glycosidic bond of alginate by β-elimination. They are classified according to their ability to recognize the distribution of M- and G-residues and are named M-, G- or MG-lyases. In the CAZy database, alginate lyases have been grouped by sequence similarity into seven distinct polysaccharide lyase families. The polysaccharide lyase family PL6 is subdivided into three subfamilies. Subfamily PL6_1 includes three biochemically characterized enzymes (two alginate lyases and one dermatan sulfatase lyase). No characterized enzymes have been described in the two other subfamilies (PL6_2 and PL6_3). To improve the prediction of polysaccharide-lyase activity in the PL6 family, we re-examined the classification of the PL6 family and biochemically characterized a set of enzymes reflecting the diversity of the protein sequences. Our results show that subfamily PL6_1 includes two dermatan sulfates lyases and several alginate lyases that have various substrate specificities and modes of action. In contrast, subfamilies PL6_2 and PL6_3 were found to contain only endo-poly-MG-lyases.

Sulfated hydrogel matrices direct mitogenicity and maintenance of chondrocyte phenotype through activation of FGF signaling

Deciphering the roles of chemical and physical features of the extracellular matrix (ECM) is vital for developing biomimetic materials with desired cellular responses in regenerative medicine. Here, we demonstrate that sulfation of biopolymers, mimicking the proteoglycans in native tissues, induces mitogenicity, chondrogenic phenotype, and suppresses catabolic activity of chondrocytes, a cell type that resides in a highly sulfated tissue. We show through tunable modification of alginate that increased sulfation of the microenvironment promotes FGF signaling-mediated proliferation of chondrocytes in a three-dimensional (3D) matrix independent of stiffness, swelling, and porosity. Furthermore, we show for the first time that a biomimetic hydrogel acts as a 3D signaling matrix to mediate a heparan sulfate/heparin-like interaction between FGF and its receptor leading to signaling cascades inducing cell proliferation, cartilage matrix production, and suppression of de-differentiation markers. Collectively, this study reveals important insights on mimicking the ECM to guide self-renewal of cells via manipulation of distinct signaling mechanisms.

Energy Landscape of Alginate-Epimerase Interactions Assessed by Optical Tweezers and Atomic Force Microscopy

Mannuronan C-5 epimerases are a family of enzymes that catalyze epimerization of alginates at the polymer level. This group of enzymes thus enables the tailor-making of various alginate residue sequences to attain various functional properties, e.g. viscosity, gelation and ion binding. Here, the interactions between epimerases AlgE4 and AlgE6 and alginate substrates as well as epimerization products were determined. The interactions of the various epimerase–polysaccharide pairs were determined over an extended range of force loading rates by the combined use of optical tweezers and atomic force microscopy. When studying systems that in nature are not subjected to external forces the access to observations obtained at low loading rates, as provided by optical tweezers, is a great advantage since the low loading rate region for these systems reflect the properties of the rate limiting energy barrier. The AlgE epimerases have a modular structure comprising both A and R modules, and the role of each of these modules in the epimerization process were examined through studies of the A- module of AlgE6, AlgE6A. Dynamic strength spectra obtained through combination of atomic force microscopy and the optical tweezers revealed the existence of two energy barriers in the alginate-epimerase complexes, of which one was not revealed in previous AFM based studies of these complexes. Furthermore, based on these spectra estimates of the locations of energy transition states (x_β), lifetimes in the absence of external perturbation (τ⁰) and free energies (ΔG^#) were determined for the different epimerase–alginate complexes. This is the first determination of ΔG^# for these complexes. The values determined were up to 8 k_BT for the outer barrier, and smaller values for the inner barriers. The size of the free energies determined are consistent with the interpretation that the enzyme and substrate are thus not tightly locked at all times but are able to relocate. Together with the observed different affinities determined for AlgE4-polymannuronic acid (poly-M) and AlgE4-polyalternating alginate (poly-MG) macromolecular pairs these data give important contribution to the growing understanding of the mechanisms underlying the processive mode of these enzymes.

The Impact of Chain Length and Flexibility in the Interaction between Sulfated Alginates and HGF and FGF-2

Alginate is a promising polysaccharide for use in biomaterials as it is biologically inert. One way to functionalize alginate is by chemical sulfation to emulate sulfated glycosaminoglycans, which interact with a variety of proteins critical for tissue development and homeostasis. In the present work we studied the impact of chain length and flexibility of sulfated alginates for interactions with FGF-2 and HGF. Both growth factors interact with defined sequences of heparan sulfate (HS) at the cell surface or in the extracellular matrix. Whereas FGF-2 interacts with a pentasaccharide sequence containing a critical 2-O-sulfated iduronic acid, HGF has been suggested to require a highly sulfated HS/heparin octasaccharide. Here, oligosaccharides of alternating mannuronic and guluronic acid (MG) were sulfated and assessed by their relative efficacy at releasing growth factor bound to the surface of myeloma cells. 8-mers of sulfated MG (SMG) alginate showed significant HGF release compared to shorter fragments, while the maximum efficacy was achieved at a chain length average of 14 monosaccharides. FGF-2 release required a higher concentration of the SMG fragments, and the 14-mer was less potent compared to an equally sulfated high-molecular weight SMG. Sulfated mannuronan (SM) was subjected to periodate oxidation to increase chain flexibility. To assess the change in flexibility, the persistence length was estimated by SEC-MALLS analysis and the Bohdanecky approach to the worm-like chain model. A high degree of oxidation of SM resulted in approximately twice as potent HGF release compared to the nonoxidized SM alginate. The release of FGF-2 also increased with the degree of oxidation, but to a lower degree compared to that of HGF. It was found that the SM alginates were more efficient at releasing FGF-2 than the SMG alginates, indicating a greater dependence on monosaccharide identity and charge orientation over chain flexibility and charge density.

Structural and functional characterization of the R-modules in alginate C-5 epimerases AlgE4 and AlgE6 from Azotobacter vinelandii

The bacterium Azotobacter vinelandii produces a family of seven secreted and calcium-dependent mannuronan C-5 epimerases (AlgE1-7). These epimerases are responsible for the epimerization of β-D-mannuronic acid (M) to α-L-guluronic acid (G) in alginate polymers. The epimerases display a modular structure composed of one or two catalytic A-modules and from one to seven R-modules having an activating effect on the A-module. In this study, we have determined the NMR structure of the three individual R-modules from AlgE6 (AR1R2R3) and the overall structure of both AlgE4 (AR) and AlgE6 using small angle x-ray scattering. Furthermore, the alginate binding ability of the R-modules of AlgE4 and AlgE6 has been studied with NMR and isothermal titration calorimetry. The AlgE6 R-modules fold into an elongated parallel β-roll with a shallow, positively charged groove across the module. Small angle x-ray scattering analyses of AlgE4 and AlgE6 show an overall elongated shape with some degree of flexibility between the modules for both enzymes. Titration of the R-modules with defined alginate oligomers shows strong interaction between AlgE4R and both oligo-M and MG, whereas no interaction was detected between these oligomers and the individual R-modules from AlgE6. A combination of all three R-modules from AlgE6 shows weak interaction with long M-oligomers. Exchanging the R-modules between AlgE4 and AlgE6 resulted in a novel epimerase called AlgE64 with increased G-block forming ability compared with AlgE6.

Lyase-catalyzed degradation of alginate in the gelled state: effect of gelling ions and lyase specificity

Lyase-catalyzed degradation has been proposed as a more cell-friendly alternative to dissolution of alginate gels than using chelating agents. In this study, we investigated the effect of lyase specificity on degradation of alginate gels, including the effect of crosslinking ions with different affinity for the polymer. Degradation kinetics and products were analyzed. In particular, the degradation products were characterized using novel methods for alginate sequence determination by chromatography. Lyase-catalyzed gel disruption worked well for gels crosslinked with calcium, but was less effective when barium was included in the gel formulation. The importance of crosslinking of long G-blocks in maintaining the structural integrity of the gels was identified. The failure to degrade these long G-blocks, either due to protection of the G-blocks by strong ionic crosslinking or due to lack of lyase activity on G-G linkages, resulted in retained resistance to mechanical disruption of the gel.

Binding and leakage of barium in alginate microbeads

Microbeads of alginate crosslinked with Ca(2+) and/or Ba(2+) are popular matrices in cell-based therapy. The aim of this study was to quantify the binding of barium in alginate microbeads and its leakage under in vitro and accumulation under in vivo conditions. Low concentrations of barium (1 mM) in combination with calcium (50 mM) and high concentrations of barium (20 mM) in gelling solutions were used for preparation of microbeads made of high-G and high-M alginates. High-G microbeads accumulated barium from gelling solution and contained higher concentrations of divalent ions for both low- and high-Ba exposure compared with high-G microbeads exposed to calcium solely and to high-M microbeads for all gelling conditions. Although most of the unbound divalent ions were removed during the wash and culture steps, leakage of barium was still detected during storage. Barium accumulation in blood and femur bone of mice implanted with high-G beads was found to be dose-dependent. Estimated barium leakage relevant to transplantation to diabetic patients with islets in alginate microbeads showed that the leakage was 2.5 times lower than the tolerable intake value given by WHO for high-G microbeads made using low barium concentration. The similar estimate gave 1.5 times higher than is the tolerable intake value for the high-G microbeads made using high barium concentration. To reduce the risk of barium accumulation that may be of safety concern, the microbeads made of high-G alginate gelled with a combination of calcium and low concentration of barium ions is recommended for islet transplantation.

1H, 13C and 15N resonances of the AlgE62 subunit from Azotobacter vinelandii mannuronan C5-epimerase

The 17.7 kDa R2 module from Azotobacter vinelandii mannronan C5-epimerase AlgE6 has been isotopically labeled ((13)C,(15)N) and recombinantly expressed. Here we report the (1)H, (13)C, (15)N resonance assignment of AlgE6R2.

Alginates: Existing and Potential Biotechnological and Medical Applications

The present chapter on alginates in biotechnology and medicine comprises a description of traditional uses where alginates have been used as devices in e.g. wound dressings, as well as an in-depth introduction to possible future applications. The latter area is based on new scientific information on the immunostimulating properties of certain alginate sequences, as well as on the popularity gained by the alginate as immobilization matrix for cells. The latter opens up the use of alginate encapsulated cells for specific cell transplantation provided that the properties of the capsules are properly understood and controlled.

NMR assignments of 1H, 13C and 15N resonances of the C-terminal subunit from Azotobacter vinelandii mannuronan C5-epimerase 6 (AlgE6R3)

The 19.9 kDa C-terminal module (R3) from Azotobacter vinelandii mannronan C5-epimerase AlgE6 has been (13)C, (15)N isotopically labelled and recombinantly expressed. We report here the (1)H, (13)C, (15)N resonance assignment of AlgE6R3.

Isolation of mutant alginate lyases with cleavage specificity for di-guluronic acid linkages

Alginates are commercially valuable and complex polysaccharides composed of varying amounts and distribution patterns of 1-4-linked β-D-mannuronic acid (M) and α-L-guluronic acid (G). This structural variability strongly affects polymer physicochemical properties and thereby both commercial applications and biological functions. One promising approach to alginate fine structure elucidation involves the use of alginate lyases, which degrade the polysaccharide by cleaving the glycosidic linkages through a β-elimination reaction. For such studies one would ideally like to have different lyases, each of which cleaves only one of the four possible linkages in alginates: G-G, G-M, M-G, and M-M. So far no lyase specific for only G-G linkages has been described, and here we report the construction of such an enzyme by mutating the gene encoding Klebsiella pneumoniae lyase AlyA (a polysaccharide lyase family 7 lyase), which cleaves both G-G and G-M linkages. After error-prone PCR mutagenesis and high throughput screening of ∼7000 lyase mutants, enzyme variants with a strongly improved G-G specificity were identified. Furthermore, in the absence of Ca(2+), one of these lyases (AlyA5) was found to display no detectable activity against G-M linkages. G-G linkages were cleaved with ∼10% of the optimal activity under the same conditions. The substitutions conferring altered specificity to the mutant enzymes are located in conserved regions in the polysaccharide lyase family 7 alginate lyases. Structure-function analyses by comparison with the known three-dimensional structure of Sphingomonas sp. A1 lyase A1-II' suggests that the improved G-G specificity might be caused by increased affinity for nonproductive binding of the alternating G-M structure.

Mode of action and subsite studies of the guluronan block-forming mannuronan C-5 epimerases AlgE1 and AlgE6

AlgE1, AlgE5 and AlgE6 are members of a family of mannuronan C-5 epimerases encoded by the bacterium Azotobacter vinelandii, and are active in the biosynthesis of alginate, where they catalyse the post-polymerization conversion of beta-D-mannuronic acid (M) residues into alpha-L-guluronic acid residues (G). All enzymes show preference for introducing G-residues neighbouring a pre-existing G. They also have the capacity to convert single M residues flanked by G, thus 'condensing' G-blocks to form almost homopolymeric guluronan. Analysis of the length and distribution of G-blocks based on specific enzyme degradation combined with size-exclusion chromatography, electrospray ionization MS, HPAEC-PAD (high-performance anion-exchange chromatography and pulsed amperometric detection), MALDI (matrix-assisted laser-desorption ionization)-MS and NMR revealed large differences in block length and distribution generated by AlgE1 and AlgE6, probably reflecting their different degree of processivity. When acting on polyMG as substrates, AlgE1 initially forms only long homopolymeric G-blocks >50, while AlgE6 gives shorter blocks with a broader block size distribution. Analyses of the AlgE1 and AlgE6 subsite specificities by the same methodology showed that a mannuronan octamer and heptamer respectively were the minimum substrate chain lengths needed to accommodate enzyme activities. The fourth M residue from the non-reducing end is epimerized first by both enzymes. When acting on MG-oligomers, AlgE1 needed a decamer while AlgE6 an octamer to accommodate activity. By performing FIA (flow injection analysis)-MS on the lyase digests of epimerized and standard MG-oligomers, the M residue in position 5 from the non-reducing end was preferentially attacked by both enzymes, creating an MGMGGG-sequence (underlined and boldface indicate the epimerized residue).