Mannuronan C-5-epimerases and their application for in vitro and in vivo design of new alginates useful in biotechnology

In-process epimerisation of alginates from Saccharina latissima, Alaria esculenta and Laminaria hyperborea

Alginates are valued in many industries, due to their versatile properties. These polysaccharides originate from brown algae (Phaeophyceae) and some bacteria of the Azotobacter and Pseudomonas genera, consisting of 1 → 4 linked β-d-mannuronic acid (M), and its C5-epimer α-l-guluronic acid (G). Several applications rely on a high G-content, which confers good gelling properties. Because of its high natural G-content (FG = 0.60-0.75), the alginate from Laminaria hyperborea (LH) has sustained a thriving industry in Norway. Alginates from other sources can be upgraded with mannuronan C-5 epimerases that convert M to G, and this has been demonstrated in many studies, but not applied in the seaweed industry. The present study demonstrates epimerisation directly in the process of alginate extraction from cultivated Saccharina latissima (SL) and Alaria esculenta (AE), and the lamina of LH. Unlike conventional epimerisation, which comprises multiple steps, this in-process protocol can decrease the time and costs necessary for alginate upgrading. In-process epimerisation with AlgE1 enzyme enhanced G-content and hydrogel strength in all examined species, with the greatest effect on SL (FG from 0.44 to 0.76, hydrogel Young's modulus from 22 to 34 kPa). As proof of concept, an upscaled in-process epimerisation of alginate from fresh SL was successfully demonstrated.

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.

Computational modeling of the molecular basis for the calcium-dependence of the mannuronan C-5 epimerase AvAlgE6 from Azotobacter vinelandii

The mannuronan C-5 epimerases catalyze epimerization of β-d-mannuronic acid to α-l-guluronic acid in alginate polymers. The seven extracellular Azotobacter vinelandii epimerases (AvAlgE1-7) are calcium-dependent, and calcium is essential for the structural integrity of their carbohydrate binding R-modules. Ca²⁺ is also found in the crystal structures of the A-modules, where it is suggested to play a structural role. In this study, the structure of the catalytic A-module of the A. vinelandii mannuronan C-5 epimerase AvAlgE6 is used to investigate the role of this Ca²⁺. Molecular dynamics (MD) simulations with and without calcium reveal the possible importance of the bound Ca²⁺ in the hydrophobic packing of β-sheets. In addition, a putative calcium binding site is found in the active site, indicating a potential direct role of this calcium in the catalysis. According to the literature, two of the residues coordinating calcium in this site are essential for the activity. MD simulations of the interaction with bound substrate indicate that the presence of a calcium ion in this binding site increases the binding strength. Further, explicit calculations of the substrate dissociation pathways with umbrella sampling simulations show and energetically higher dissociation barrier when calcium is present. The present study eludes to a putative catalytic role of calcium in the charge neutralizing first step of the enzymatic reaction. In addition to the importance for understanding these enzymes' molecular mechanisms, this could have implications for engineering strategies of the epimerases in industrial alginate processing.

Brown algal cell walls and development

Brown algae are complex multicellular eukaryotes whose cells possess a cell wall, which is an important structure that regulates cell size and shape. Alginate and fucose-containing sulfated polysaccharides (FCSPs) are two carbohydrate types that have major roles in influencing the mechanical properties of the cell wall (i.e. increasing or decreasing wall stiffness), which in turn regulate cell expansion, division, adhesion, and other processes; however, how brown algal cell wall structure regulates its mechanical properties, and how this relationship influences cellular growth and organismal development, is not well-understood. This chapter is focused on reviewing what we currently know about how the roles of alginates and FCSPs in brown algal developmental processes, as well as how they influence the structural and mechanical properties of cell walls. Additionally, we discuss how brown algal mutants may be leveraged to learn more about the underlying mechanisms that regulate cell wall structure, mechanics, and developmental processes, and finally we propose questions to guide future research with the use of emerging technologies.

Mannuronan C-5 Epimerases: Review of Activity Assays, Enzyme Characteristics, Structure, and Mechanism

Mannuronan C-5 epimerases (ManC5-Es) are produced by brown algae and some bacteria, such as Azotobacter and some Pseudomonas species. It can convert the transformation of β-D-mannuronic acid (M) to α-L-guluronic acid (G) in alginate with different patterns of epimerization. Alginate with different compositions and monomer sequences possess different properties and functions, which have been utilized in industries for various purposes. Therefore, ManC5-Es are key enzymes that are involved in the modifications of alginate for fuel, chemical, and industrial applications. Focusing on ManC5-Es, this review introduces and summarizes the methods of ManC5-Es activity assay especially the most widely used nuclear magnetic resonance spectroscopy method, characterization of the ManC5-Es from different origins especially the research progress of its enzymatic properties and product block distributions, and the catalytic mechanism of ManC5-E based on the resolved enzyme structures. Additionally, some potential future research directions are also outlooked.

Alginate oligosaccharides: The structure-function relationships and the directional preparation for application

Alginate oligosaccharides (AOS) are degradation products of alginate extracted from brown algae. With low molecular weight, high water solubility, and good biological activity, AOS present anti-inflammatory, antimicrobial, antioxidant, and antitumor properties. They also exert growth-promoting effects in animals and plants. Three types of AOS, mannuronate oligosaccharides (MAOS), guluronate oligosaccharides (GAOS), and heterozygous mannuronate and guluronate oligosaccharides (HAOS), can be produced from alginate by enzymatic hydrolysis. Thus far, most studies on the applications and biological activities of AOS have been based mainly on a hybrid form of HAOS. To improve the directional production of AOS for practical applications, systematic studies on the structures and related biological activities of AOS are needed. This review provides a summary of current understanding of structure-function relationships and advances in the production of AOS. The current challenges and opportunities in the application of AOS is suggested to guide the precise application of AOS in practice.

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.

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.

RpoS controls the expression and the transport of the AlgE1-7 epimerases in Azotobacter vinelandii

Azotobacter vinelandii produces differentiated cells, called cysts, surrounded by two alginate layers, which are necessary for their desiccation resistance. This alginate contains variable proportions of guluronate residues, resulting from the activity of seven extracytoplasmic epimerases, AlgE1-7. These enzymes are exported by a system secretion encoded by the eexDEF operon; mutants lacking the AlgE1-7 epimerases, the EexDEF or the RpoS sigma factor produce alginate, but are unable to form desiccation resistant cysts. Herein, we found that RpoS was required for full transcription of the algE1-7 and eexDEF genes. We found that the AlgE1-7 protein levels were diminished in the rpoS mutant strain. In addition, the alginate produced in the absence of RpoS was more viscous in the presence of proteases, a phenotype similar to that of the eexD mutant. Primer extension analysis located two promoters for the eexDEF operon, one of them was RpoS-dependent. Thus, during encysting conditions, RpoS coordinates the expression of both the AlgE1-7 epimerases and the EexDEF protein complex responsible for their transport.

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.

Effect of biotechnologically modified alginates on LDH structures

Four alginates possessing different guluronic/mannuronic acid ratios and one acetylated alginate were investigated with respect to their behaviour during intercalation into layered double hydroxides (LDHs). Two alginates were commercial products while the others were synthesised by way of bacterial fermentation and in one sample followed by enzymatic treatment. Intercalation was performed by way of co-precipitation of aluminium nitrate and zinc nitrate in alginate solution at a pH of 8·5–9. The products were characterised by powder X-ray diffraction, elemental analysis, wide-angle X-ray scattering, scanning electron microscopy and magic angle spinning (MAS) NMR spectroscopy. All alginates intercalate well into the Zn2Al-LDH host structure. With an increase in the content of guluronic acid in the alginate, the d-spacing (interlayer distance) in the alginate-LDH compound increases from 1·28 to 1·85 nm. Similarly, acetylation of the carboxylic groups leads to an increased steric volume of such alginate and therefore to a higher d-spacing (1·72 nm). The results indicate that different guluronic/mannuronic acid ratios can be used to trigger the steric size of the alginates and consequently the d-spacing of the alginate-LDHs. 13C CP MAS NMR spectroscopy confirmed an interaction between the carboxylic groups present in the alginate with the inorganic main layer.

Seaweed hydrocolloid production: an update on enzyme assisted extraction and modification technologies

Agar, alginate, and carrageenans are high-value seaweed hydrocolloids, which are used as gelation and thickening agents in different food, pharmaceutical, and biotechnological applications. The annual global production of these hydrocolloids has recently reached 100,000 tons with a gross market value just above US$ 1.1 billion. The techno-functional properties of the seaweed polysaccharides depend strictly on their unique structural make-up, notably degree and position of sulfation and presence of anhydro-bridges. Classical extraction techniques include hot alkali treatments, but recent research has shown promising results with enzymes. Current methods mainly involve use of commercially available enzyme mixtures developed for terrestrial plant material processing. Application of seaweed polysaccharide targeted enzymes allows for selective extraction at mild conditions as well as tailor-made modifications of the hydrocolloids to obtain specific functionalities. This review provides an update of the detailed structural features of κ-, ι-, λ-carrageenans, agars, and alginate, and a thorough discussion of enzyme assisted extraction and processing techniques for these hydrocolloids.

Alginate-modifying enzymes: biological roles and biotechnological uses

Alginate denotes a group of industrially important 1-4-linked biopolymers composed of the C-5-epimers β-D-mannuronic acid (M) and α-L-guluronic acid (G). The polysaccharide is manufactured from brown algae where it constitutes the main structural cell wall polymer. The physical properties of a given alginate molecule, e.g., gel-strength, water-binding capacity, viscosity and biocompatibility, are determined by polymer length, the relative amount and distribution of G residues and the acetyl content, all of which are controlled by alginate modifying enzymes. Alginate has also been isolated from some bacteria belonging to the genera Pseudomonas and Azotobacter, and bacterially synthesized alginate may be O-acetylated at O-2 and/or O-3. Initially, alginate is synthesized as polymannuronic acid, and some M residues are subsequently epimerized to G residues. In bacteria a mannuronan C-5-epimerase (AlgG) and an alginate acetylase (AlgX) are integral parts of the protein complex necessary for alginate polymerization and export. All alginate-producing bacteria use periplasmic alginate lyases to remove alginate molecules aberrantly released to the periplasm. Alginate lyases are also produced by organisms that utilize alginate as carbon source. Most alginate-producing organisms encode more than one mannuronan C-5 epimerase, each introducing its specific pattern of G residues. Acetylation protects against further epimerization and from most alginate lyases. An enzyme from Pseudomonas syringae with alginate deacetylase activity has been reported. Functional and structural studies reveal that alginate lyases and epimerases have related enzyme mechanisms and catalytic sites. Alginate lyases are now utilized as tools for alginate characterization. Secreted epimerases have been shown to function well in vitro, and have been engineered further in order to obtain enzymes that can provide alginates with new and desired properties for use in medical and pharmaceutical applications.

The role of seaweed bioactives in the control of digestion: implications for obesity treatments

Seaweeds are an underutilised nutritional resource that could not only compliment the current western diet but potentially bring additional health benefits over and above their nutritional value.

Produção de alginato por microrganismos

O alginato é um copolímero linear constituído de unidades de ácidos α-L-gulurônicos e β-D-manurônicos e é extensamente utilizado devido as suas propriedades espessantes, estabilizantes e gelificantes. Estas características fazem com que este biopolímero encontre aplicações na indústria de alimentos, na indústria têxtil e de papel, em cosméticos e na área farmacêutica e médica. Atualmente para este conjunto de aplicações sua principal fonte são algas marrons, entretanto, o alginato pode ser obtido a partir de biossíntese, utilizando-se microrganismos do gênero Pseudomonas e Azotobacter. A produção bacteriana de alginato apresenta-se como uma alternativa interessante e sua produção por microrganismos, além de possibilitar a produção de biopolímeros de alta qualidade com características específicas e pré-determinadas, irá diminuir o impacto ambiental nas regiões em que as algas marinhas das quais é extraído são coletadas. Nos últimos anos, vários estudos relacionados à produção de alginato por microrganismos foram realizados com o objetivo de avaliar sua produção e rota metabólica de biossíntese, para caracterizar o material produzido e para determinar as potencialidades de aplicação deste novo material. O rápido desenvolvimento de aplicações do alginato na área médica e farmacêutica, bem como a descoberta de propriedades imunológicas únicas deste material tem aumentado o interesse no desenvolvimento de processos para produzi-lo. Neste artigo são abordados aspectos relacionados à produção e as características do alginato bacteriano e também reportadas às potencialidades e aplicações inovadoras nas quais este material vem sendo utilizado.

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.

Use of protein trans-splicing to produce active and segmentally (2)H, (15)N labeled mannuronan C5-epimerase AlgE4

Alginate epimerases are large multidomain proteins capable of epimerising C5 on beta-D-mannuronic acid (M) turning it into alpha-L-guluronic acid (G) in a polymeric alginate. Azotobacter vinelandii secretes a family of seven epimerases, each of which is capable of producing alginates with characteristic G distribution patterns. All seven epimerases consist of two types of modules, denoted A and R, in varying numbers. Attempts to study these enzymes with solution-state NMR are hampered by their size-the smallest epimerase, AlgE4, consisting of one A- and one R-module, is 58 kDa, resulting in heavy signal overlap impairing the interpretation of NMR spectra. Thus we obtained segmentally (2)H, (15)N labeled AlgE4 isotopomeres (A-[(2)H, (15)N]-R and [(2)H, (15)N]-A-R) by protein trans-splicing using the naturally split intein of Nostoc punctiforme. The NMR spectra of native AlgE4 and the ligated versions coincide well proving the conservation of protein structure. The activity of the ligated AlgE4 was verified by two different enzyme activity assays, demonstrating that ligated AlgE4 displays the same catalytic activity as wild-type AlgE4.

1H, 15N, 13C resonance assignment of the AlgE6R1 subunit from the Azotobacter vinelandii mannuronan C5-epimerase

Isotopically labelled, (13)C/(15)N from of recombinant subunit of the first R-module from alginate C5-epimerase 6 (AlgE6R1) from Azotobacter vinelandii mannuronan C5-epimerase was produced. We report here the (1)H, (15)N, (13)C resonance assignment of this subunit from AlgE6 epimerase.

Molecular Engineering as an Approach to Design New Functional Properties of Alginate

Through enzymatic modification, we are now able to manipulate the composition and sequential nanostructures of alginate, one of the most versatile gelling polymers found in nature. Here we report the application of a set of processive polymer-modifying epimerases for the preparation of novel alginates with highly improved functional properties essential for numerous applications as gel matrices. Gels of enzymatically engineered alginate were found to be more elastic and compact, less permeable, and extremely stable under physiological conditions, offering significant advantages over native alginates. As a result, this study shows that, by controlling alginate nanostructure, its macroscopic properties can be highly controlled. The ability to tailor alginate has a great impact on the wide use of this biomaterial in industry and medicine. More importantly, this adds more knowledge to the link between polymer nanostructure and macroscopic properties and may serve as a model system for other polymer-based materials.

Molecular and bioengineering strategies to improve alginate and polydydroxyalkanoate production by Azotobacter vinelandii

Several aspects of alginate and PHB synthesis in Azotobacter vinelandii at a molecular level have been elucidated in articles published during the last ten years. It is now clear that alginate and PHB synthesis are under a very complex genetic control. Genetic modification of A. vinelandii has produced a number of very interesting mutants which have particular traits for alginate production. One of these mutants has been shown to produce the alginate with the highest mean molecular mass so far reported. Recent work has also shed light on the factors determining molecular mass distribution; the most important of these being identified as; dissolved oxygen tension and specific growth rate. The use of specific mutants has been very useful for the correct analysis and interpretation of the factors affecting polymerization. Recent scale-up/down work on alginate production has shown that oxygen limitation is crucial for producing alginate of high molecular mass, a condition which is optimized in shake flasks and which can now be reproduced in stirred fermenters. It is clear that the phenotypes of mutants grown on plates are not necessarily reproducible when the strains are tested in lab or bench scale fermenters. In the case of PHB, A. vinelandii has shown itself able to produce relatively large amounts of this polymer of high molecular weight on cheap substrates, even allowing for simple extraction processes. The development of fermentation strategies has also shown promising results in terms of improving productivity. The understanding of the regulatory mechanisms involved in the control of PHB synthesis, and of its metabolic relationships, has increased considerably, making way for new potential strategies for the further improvement of PHB production. Overall, the use of a multidisciplinary approach, integrating molecular and bioengineering aspects is a necessity for optimizing alginate and PHB production in A. vinelandii.

Identification and characterization of an Azotobacter vinelandii type I secretion system responsible for export of the AlgE-type mannuronan C-5-epimerases

Alginate is a linear copolymer of beta-d-mannuronic acid and its C-5-epimer, alpha-l-guluronic acid. During biosynthesis, the polymer is first made as mannuronan, and various fractions of the monomers are then epimerized to guluronic acid by mannuronan C-5-epimerases. The Azotobacter vinelandii genome encodes a family of seven extracellular such epimerases (AlgE1 to AlgE7) which display motifs characteristic for proteins secreted via a type I pathway. Putative ATPase-binding cassette regions from the genome draft sequence of the A. vinelandii OP strain and experimentally verified type I transporters from other species were compared. This analysis led to the identification of one putative A. vinelandii type I system (eexDEF). The corresponding genes were individually disrupted in A. vinelandii strain E, and Western blot analysis using polyclonal antibodies against all AlgE epimerases showed that these proteins were present in wild-type culture supernatants but absent from the eex mutant supernatants. Consistent with this, the wild-type strain and the eex mutants produced alginate with about 20% guluronic acid and almost pure mannuronan (< or =2% guluronic acid), respectively. The A. vinelandii wild type is able to enter a particular desiccation-tolerant resting stage designated cyst. At this stage, the cells are surrounded by a rigid coat in which alginate is a major constituent. Such a coat was formed by wild-type cells in a particular growth medium but was missing in the eex mutants. These mutants were also found to be unable to survive desiccation. The reason for this is probably that continuous stretches of guluronic acid residues are needed for alginate gel formation to take place.

Bacterial alginates: from biosynthesis to applications

Alginate is a polysaccharide belonging to the family of linear (unbranched), non-repeating copolymers, consisting of variable amounts of beta-D-mannuronic acid and its C5-epimer alpha- L-guluronic acid linked via beta-1,4-glycosidic bonds. Like DNA, alginate is a negatively charged polymer, imparting material properties ranging from viscous solutions to gel-like structures in the presence of divalent cations. Bacterial alginates are synthesized by only two bacterial genera, Pseudomonas and Azotobacter, and have been extensively studied over the last 40 years. While primarily synthesized in form of polymannuronic acid, alginate undergoes chemical modifications comprising acetylation and epimerization, which occurs during periplasmic transfer and before final export through the outer membrane. Alginate with its unique material properties and characteristics has been increasingly considered as biomaterial for medical applications. The genetic modification of alginate producing microorganisms could enable biotechnological production of new alginates with unique, tailor-made properties, suitable for medical and industrial applications.

Metabolic engineering of microbes for oligosaccharide and polysaccharide synthesis

Metabolic engineering has recently been embraced as an effective tool for developing whole-cell biocatalysts for oligosaccharide and polysaccharide synthesis. Microbial catalysts now provide a practical means to derive many valuable oligosaccharides, previously inaccessible through other methods, in sufficient quantities to support research and clinical applications. The synthesis process based upon these microbes is scalable as it avoids expensive starting materials. Most impressive is the high product concentrations (up to 188 g/L) achieved through microbe-catalyzed synthesis. The overall cost for selected molecules has been brought to a reasonable range (estimated $ 30–50/g).

Microbial synthesis of oligosaccharides and polysaccharides is a carbon-intensive and energy-intensive process, presenting some unique challenges in metabolic engineering. Unlike nicotinamide cofactors, the required sugar nucleotides are products of multiple interacting pathways, adding significant complexity to the metabolic engineering effort. Besides the challenge of providing the necessary mammalian-originated glycosyltransferases in active form, an adequate uptake of sugar acceptors can be an issue when another sugar is necessary as a carbon and energy source. These challenges are analyzed, and various strategies used to overcome these difficulties are reviewed in this article. Despite the impressive success of the microbial coupling strategy, there is a need to develop a single strain that can achieve at least the same efficiency. Host selection and the manner with which the synthesis interacts with the central metabolism are two important factors in the design of microbial catalysts. Additionally, unlike in vitro enzymatic synthesis, product degradation and byproduct formation are challenges of whole-cell systems that require additional engineering. A systematic approach that accounts for various and often conflicting requirements of the synthesis holds the key to deriving an efficient catalyst.

Metabolic engineering strategies applied to selected polysaccharides (hyaluronan, alginate, and exopolysaccharides for food use) are reviewed in this article to highlight the recent progress in this area and similarity to challenges in oligosaccharide synthesis. Many naturally occurring microbes possess highly efficient mechanisms for polysaccharide synthesis. These mechanisms could potentially be engineered into a microbe for oligosaccharide and polysaccharide synthesis with enhanced efficiency.

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).

NMR Structure of the R-module

In the bacterium Azotobacter vinelandii, a family of seven secreted and calcium-dependent mannuronan C-5 epimerases (AlgE1-7) has been identified. These epimerases are responsible for the epimerization of beta-d-mannuronic acid to alpha-l-guluronic acid in alginate polymers. The epimerases consist of two types of structural modules, designated A (one or two copies) and R (one to seven copies). The structure of the catalytically active A-module from the smallest epimerase AlgE4 (consisting of AR) has been solved recently. This paper describes the NMR structure of the R-module from AlgE4 and its titration with a substrate analogue and paramagnetic thulium ions. The R-module folds into a right-handed parallel beta-roll. The overall shape of the R-module is an elongated molecule with a positively charged patch that interacts with the substrate. Titration of the R-module with thulium indicated possible calcium binding sites in the loops formed by the nonarepeat sequences in the N-terminal part of the molecule and the importance of calcium binding for the stability of the R-module. Structure calculations showed that calcium ions can be incorporated in these loops without structural violations and changes. Based on the structure and the electrostatic surface potential of both the A- and R-module from AlgE4, a model for the appearance of the whole protein is proposed.

Mapping enzymatic functionalities of mannuronan C-5 epimerases and their modular units by dynamic force spectroscopy

Alginates are (1-->4)-linked structural copolyuronans consisting of beta-D-mannuronic acid (M) and its C-5 epimer alpha-L-guluronic acid (G). The residue sequence variation is introduced in a unique postpolymerisation step catalysed by a family of C-5 epimerases named AlgE enzymes. The seven known AlgE's are composed of two modules, designated A and R, present in different number. The molecular details of the structure-function relationship of these seven epimerases, introducing specific residue sequences, are not understood. In this study, single-molecular pair interactions between alginate and AlgE enzymes were investigated using dynamic force spectroscopy. The AlgE enzymes AlgE4 and AlgE6, the recombinant construct PKA1 composed of A- and R-modules from various AlgE's, as well as separate R- and A-modules were studied. The strength of the protein-mannuronan interaction, when applying a loading rate of 0.6 nN/s, varied from 73 pN (AlgE4) to 144 pN (A-module). The determined potential width, that is, the distance from the activation barrier to the bound substrate molecule, was 0.23 nm for AlgE4, 0.19 nm for AlgE6 and 0.1 nm for the A-module. No attraction was observed between the R-module and the substrate. The observations indicate that the A-module contains the substrate binding site and that the R-module modulates the enzyme-substrate binding strength. The observed AlgE4-polymer residence times, two orders of magnitude longer than expected from kcat reported for AlgE4, not observed for PKA1, led us to propose a processive mode of action of AlgE4.

Alginate plasma expander maintains perfusion and plasma viscosity during extreme hemodilution

Extreme hemodilution was performed in the hamster chamber window model using 6% Dextran 70, lowering systemic hematocrit by 60%. Animals were subsequently divided into three groups and hemodiluted to a hematocrit of 11% using 6% Dextran 70, 6% Dextran 500, and a 4% Dextran 70 + 0.7% alginate solution ( n = 6 each group). Final plasma viscosities were 1.4 ± 0.2, 2.2 ± 0.1, and 2.7 ± 0.2 cp, respectively, ( P < 0.05, high viscosity vs. low viscosity). Blood viscosities were 2.1 ± 0.2, 2.9 ± 0.4, and 3.9 ± 0.3 cp, respectively. The lowest blood and plasma viscosity group had a significantly lower functional capillary density, 37 ± 16%, whereas the two high-viscosity solutions were 71 ± 15% and 76 ± 12% ( P < 0.05, high viscosity vs. low viscosity), respectively. Arteriolar and venular flow in the Dextran 500 and alginate groups was higher than baseline (i.e., normal nontreated animals), whereas the low-viscosity group showed a reduction in flow. These microvascular changes were paralleled by changes in base excess, which was negative for the Dextran 70 group and positive for the other groups. However, tissue Po2 was uniformly low for all groups (average of 1.4 mmHg). Calculation of tissue oxygen consumption in the window chamber based on the microvascular data, flow, and intravascular Po2 showed that only the alginate + Dextran 70 solution-exchanged animals returned to baseline oxygen consumption, whereas the other groups were lower than baseline ( P < 0.05). These results show that hemodilution performed with high-viscosity plasma expanders yields systemic arterial pressures and functional capillary densities that are significantly higher ( P < 0.05) than those obtained with 6% Dextran 70, a fluid whose viscosity is similar to that of plasma. A condition for obtaining these results is that the oncotic pressure of the plasma expander be titrated to near normal, so that autotransfusion of fluid from the tissue into the vascular compartment does not reduce the effects of increasing plasma viscosity and increased shear stress on the microvascular wall.

Construction and analyses of hybrid Azotobacter vinelandii mannuronan C-5 epimerases with new epimerization pattern characteristics

The secreted mannuronan C-5 epimerases from Azotobacter vinelandii form a family of seven homologous modular type enzymes, which appear to have evolved through duplications and point mutations in the individual modules. The catalytic A modules of these enzymes are responsible for generating the characteristic sequence distribution patterns of G residues in the industrially important polymer alginate by epimerizing M (beta-D-mannuronic acid) moieties to G (alpha-L-guluronic acid). Forty-six different hybrid enzymes were constructed by exchanging parts of the sequences encoding the A modules of AlgE2 (generates consecutive stretches of G residues) and AlgE4 (generates alternating structures). These hybrid enzymes introduce a variety of new monomer-sequence patterns into their substrates, and some regions important for the subsite specificity or processivity of the enzymes were identified. By using time-resolved NMR spectroscopy, it became clear that the rates for introducing alternating structures and consecutive stretches of G residues are different for each enzyme, and that it is the ratio between these rates that determines the overall epimerization pattern. These findings open up new possibilities in biotechnology and in studies of the many biological functions of alginates.

The Pseudomonas syringae Genome Encodes a Combined Mannuronan C-5-epimerase and O-Acetylhydrolase, Which Strongly Enhances the Predicted Gel-forming Properties of Alginates

Alginates are industrially important, linear copolymers of beta-d-mannuronic acid (M) and its C-5-epimer alpha-l-guluronic acid (G). The G residues originate from a postpolymerization reaction catalyzed by mannuronan C-5-epimerases (MEs), leading to extensive variability in M/G ratios and distribution patterns. Alginates containing long continuous stretches of G residues (G blocks) can form strong gels, a polymer type not found in alginate-producing bacteria belonging to the genus Pseudomonas. Here we show that the Pseudomonas syringae genome encodes a Ca(2+)-dependent ME (PsmE) that efficiently forms such G blocks in vitro. The deduced PsmE protein consists of 1610 amino acids and is a modular enzyme related to the previously characterized family of Azotobacter vinelandii ME (AlgE1-7). A- and R-like modules with sequence similarity to those in the AlgE enzymes are found in PsmE, and the A module of PsmE (PsmEA) was found to be sufficient for epimerization. Interestingly, an R module from AlgE4 stimulated Ps-mEA activity. PsmE contains two regions designated M and RTX, both presumably involved in the binding of Ca(2+). Bacterial alginates are partly acetylated, and such modified residues cannot be epimerized. Based on a detailed computer-assisted analysis and experimental studies another PsmE region, designated N, was found to encode an acetylhydrolase. By the combined action of N and A PsmE was capable of redesigning an extensively acetylated alginate low in G from a non gel-forming to a gel-forming state. Such a property has to our knowledge not been previously reported for an enzyme acting on a polysaccharide.

Improvement of the biocompatibility of alginate/poly-L-lysine/alginate microcapsules by the use of epimerized alginate as a coating

Alginate/poly-L-lysine(PLL)/alginate capsules are used widely for the microencapsulation of cells. Alginate consists of guluronic acid and mannuronic acid, the ratio and sequence of which affect the properties of the alginate. Using C5-epimerases, mannuronic acid can be converted to guluronic acid in the alginate polymer. Such an enzyme, AlgE4, was used to convert blocks of mannuronic acid (M-blocks) to blocks of alternating sequence (MG-blocks). The aims of this study were 1) to investigate whether the use of epimerized alginate as a coating could improve the biocompatibility of alginate/PLL/alginate capsules and 2) to study the biocompatibility of simple alginate beads prepared with epimerized alginate. Four different capsules, two of which contained epimerized alginate, were investigated after implantation in C57BL/6 mice for 1 week. The biocompatibility of alginate/PLL/alginate capsules, as measured by retrieval rates of the capsules and DNA contents and glucose oxidation rates of the cellular overgrowth, was improved when an epimerized coating alginate was used. There were, however, no statistically significant differences in the biocompatibility of simple alginate beads made from epimerized alginate when compared with non-epimerized alginate beads. In general, such beads produced without a PLL coating swelled to a higher extent than the conventional alginate/PLL/alginate capsules. In conclusion, the use of an epimerized coating on alginate-PLL-alginate can improve the biocompatibility of such capsules but still cannot completely eliminate the detrimental effects of PLL on the biocompatibility of the capsules.

The catalytic activities of the bifunctional Azotobacter vinelandii mannuronan C-5-epimerase and alginate lyase AlgE7 probably originate from the same active site in the enzyme

The Azotobacter vinelandii genome encodes a family of seven secreted Ca(2+)-dependent epimerases (AlgE1--7) catalyzing the polymer level epimerization of beta-D-mannuronic acid (M) to alpha-L-guluronic acid (G) in the commercially important polysaccharide alginate. AlgE1--7 are composed of two types of protein modules, A and R, and the A-modules have previously been found to be sufficient for epimerization. AlgE7 is both an epimerase and an alginase, and here we show that the lyase activity is Ca(2+)-dependent and also responds similarly to the epimerases in the presence of other divalent cations. The AlgE7 lyase degraded M-rich alginates and a relatively G-rich alginate from the brown algae Macrocystis pyrifera most effectively, producing oligomers of 4 (mannuronan) to 7 units. The sequences cleaved were mainly G/MM and/or G/GM. Since G-moieties dominated at the reducing ends even when mannuronan was used as substrate, the AlgE7 epimerase probably stimulates the lyase pathway, indicating a complex interplay between the two activities. A truncated form of AlgE1 (AlgE1-1) was converted to a combined epimerase and lyase by replacing the 5'-798 base pairs in the algE1-1 gene with the corresponding A-module-encoding DNA sequence from algE7. Furthermore, substitution of an aspartic acid residue at position 152 with glycine in AlgE7A eliminated almost all of both the lyase and epimerase activities. Epimerization and lyase activity are believed to be mechanistically related, and the results reported here strongly support this hypothesis by suggesting that the same enzymatic site can catalyze both reactions.

Hexuronyl C5-epimerases in alginate and glycosaminoglycan biosynthesis

The sugar residues in most polysaccharides are incorporated as their corresponding monomers during polymerization. Here we summarize the three known exceptions to this rule, involving the biosynthesis of alginate, and the glycosaminoglycans, heparin/heparan sulfate and dermatan sulfate. Alginate is synthesized by brown seaweeds and certain bacteria, while glycosaminoglycans are produced by most animal species. In all cases one of the incorporated sugar monomers are being C5-epimerized at the polymer level, from D-mannuronic acid to L-guluronic acid in alginate, and from D-glucuronic acid to L-iduronic acid in glycosaminoglycans. Alginate epimerization modulates the mechanical properties of seaweed tissues, whereas in bacteria it seems to serve a wide range of purposes. The conformational flexibility of iduronic acid units in glycosaminoglycans promotes apposition to, and thus functional interactions with a variety of proteins at cell surfaces and in the extracellular matrix. In the bacterium Azotobacter vinelandii the alginates are being epimerized at the cell surface or in the extracellular environment by a family of evolutionary strongly related modular type and Ca(2+)-dependent epimerases (AlgE1-7). Each of these enzymes introduces a specific distribution pattern of guluronic acid residues along the polymer chains, explaining the wide structural variability observed in alginates isolated from nature. Glycosaminoglycans are synthesized in the Golgi system, through a series of reactions that include the C5-epimerization reaction along with extensive sulfation of the polymers. The single, Ca(2+)-independent, epimerase in heparin/heparan sulfate biosynthesis and the Ca(2+)-dependent dermatan sulfate epimerase(s) also generate variable epimerization patterns, depending on other polymer-modification reactions. The alginate and heparin epimerases appear unrelated at the amino acid sequence level, and have probably evolved through independent evolutionary pathways; however, hydrophobic cluster analysis indicates limited similarity. Seaweed alginates are widely used in industry, while heparin is well established in the clinic as an anticoagulant.

Mannuronan C-5 epimerases and cellular differentiation of Azotobacter vinelandii

Differentiation in Azotobacter vinelandii involves the encystment of the vegetative cell under adverse environmental circumstances and the germination of the resting cell into the vegetative state when growth conditions are satisfactory again. Morphologically, the encystment process involves the development of a protective coat around the resting cell. This coat partly consists of multiple layers of alginate, which is a copolymer of beta-D-mannuronic acid (M) and alpha-L-guluronic acid (G). Alginate contributes to coat rigidity by virtue of a high content of GG blocks. Such block structures are generated through a family of mannuronan C-5 epimerases that convert M to G after polymerization. Results from immunodetection and light microscopy, using stains that distinguish between different cyst components and types, indicate a correlation between cyst coat organization and the amount and appearance of mannuronan C-5 epimerases in the extracellular medium and attached to the cells. Specific roles of individual members of the epimerase family are indicated. Calcium and magnesium ions appear to have different roles in the structural organization of the cyst coat. Also reported is a new gene sharing strong sequence homology with parts of the epimerase-encoded R-modules. This gene is located within the epimerase gene cluster of Azotobacter vinelandii.