32
are outstanding hydrogels since they can be designed to benefit of the well-established advantageous properties shown by naturally based and of those shown by
synthetic materials. For instance, the hybrid hydrogel developed in Bellini et al.
(2018) allowed to combine the valuable properties carried by EPS, such as resistance to dehydration, subsequent rehydration, and biocompatibility, with the stability granted by polyethylene glycol diacrylate. The low cytotoxicity favors the usage
of EPS as carriers of therapeutically relevant compounds (Leite et al. 2017).
The commercial interest in alginate is manifold, especially in wound dressing,
delivery of bioactive agents, and cell carrier activity in tissue engineering applications (Lee and Mooney 2012). Alginate constitutes the main cell wall polysaccharide of brown algae, accounting for up to 45% of the dry weight. The function of
alginate in the cell walls of brown algae is similar to the function of cellulose in the
cell walls of land plants. Alginate denotes a group of industrially important linear
1–4-linked polymers composed of alternating blocks of the C-5-epimers β-Dmannuronic acid (M) and α-L-guluronic acid (G). Alginate is produced from the
polymerization of GDP-mannuronic acid, which is derived from GDP-mannose
according to the reaction GDP-Man + 2NAD+ → GDP-ManA +2 NADH which is
catalyzed by a UDP-glucose/GDP- mannose dehydrogenase. Tenhaken et al. (2011)
and Zhang et al. (2016) biochemically and structurally characterized the genes
encoding the GDP-mannose dehydrogenase in the marine photoautotrophic brown
algae Ectocarpus siliculosus and Saccharina japonica, respectively. The homopolymer is then subjected to multiple modifications which result in the final characteristic organization into M- and G-blocks. The variability observed in the patterns of
M- and G-blocks is ascribed to differences in the epimerization activities of various
mannuronan C-5 epimerases which usually coexist in the same microorganism. The
physical properties of alginate molecules, e.g., gel strength, water-binding capacity,
viscosity, and biocompatibility, are determined by the relative content, the length,
and the spatial distribution of G- and M-blocks. M-rich alginates afford the production of softer gels whereas G-rich alginates more brittle gels.
Furthermore, a demand of biopolymers in environmental applications such as
flocculation, settling, and dewatering of sludge, dyes, and metal removal from
wastewater has renewed the interest in EPS production (Nouha et al. 2018). Indeed,
owing to their excellent metal-binding properties, microbially synthesized EPS are
valuable tools in bioremediation processes. Polysaccharides, phosphate-containing
nucleotides, and protein moieties, rich in negatively charged amino acids, contribute
to EPS anionic properties, which are crucial for the formation of complexes with
metal ions. Among the factors favoring EPS applications in organic pollutant
removal is EPS hydrophobicity, which is known to be influenced by the EPS content
in proteins, the functional groups in the protein fraction, and the protein-to-carbohydrate ratio (Geyik et al. 2016). Heteropolysaccharide EPS from Nostoc flagelliforme possess high intrinsic viscosity, excellent emulsification, and flocculation
activity (Han et al. 2014). Due to their content in sulfate groups and uronic acids,
the EPS derived from cyanobacteria stand out for their strong anionic character,
which is appealing in the removal of metal ions from wastewaters.
A. A. Azim et al.
are outstanding hydrogels since they can be designed to benefit of the well-established advantageous properties shown by naturally based and of those shown by
synthetic materials. For instance, the hybrid hydrogel developed in Bellini et al.
(2018) allowed to combine the valuable properties carried by EPS, such as resistance to dehydration, subsequent rehydration, and biocompatibility, with the stability granted by polyethylene glycol diacrylate. The low cytotoxicity favors the usage
of EPS as carriers of therapeutically relevant compounds (Leite et al. 2017).
The commercial interest in alginate is manifold, especially in wound dressing,
delivery of bioactive agents, and cell carrier activity in tissue engineering applications (Lee and Mooney 2012). Alginate constitutes the main cell wall polysaccharide of brown algae, accounting for up to 45% of the dry weight. The function of
alginate in the cell walls of brown algae is similar to the function of cellulose in the
cell walls of land plants. Alginate denotes a group of industrially important linear
1–4-linked polymers composed of alternating blocks of the C-5-epimers β-Dmannuronic acid (M) and α-L-guluronic acid (G). Alginate is produced from the
polymerization of GDP-mannuronic acid, which is derived from GDP-mannose
according to the reaction GDP-Man + 2NAD+ → GDP-ManA +2 NADH which is
catalyzed by a UDP-glucose/GDP- mannose dehydrogenase. Tenhaken et al. (2011)
and Zhang et al. (2016) biochemically and structurally characterized the genes
encoding the GDP-mannose dehydrogenase in the marine photoautotrophic brown
algae Ectocarpus siliculosus and Saccharina japonica, respectively. The homopolymer is then subjected to multiple modifications which result in the final characteristic organization into M- and G-blocks. The variability observed in the patterns of
M- and G-blocks is ascribed to differences in the epimerization activities of various
mannuronan C-5 epimerases which usually coexist in the same microorganism. The
physical properties of alginate molecules, e.g., gel strength, water-binding capacity,
viscosity, and biocompatibility, are determined by the relative content, the length,
and the spatial distribution of G- and M-blocks. M-rich alginates afford the production of softer gels whereas G-rich alginates more brittle gels.
Furthermore, a demand of biopolymers in environmental applications such as
flocculation, settling, and dewatering of sludge, dyes, and metal removal from
wastewater has renewed the interest in EPS production (Nouha et al. 2018). Indeed,
owing to their excellent metal-binding properties, microbially synthesized EPS are
valuable tools in bioremediation processes. Polysaccharides, phosphate-containing
nucleotides, and protein moieties, rich in negatively charged amino acids, contribute
to EPS anionic properties, which are crucial for the formation of complexes with
metal ions. Among the factors favoring EPS applications in organic pollutant
removal is EPS hydrophobicity, which is known to be influenced by the EPS content
in proteins, the functional groups in the protein fraction, and the protein-to-carbohydrate ratio (Geyik et al. 2016). Heteropolysaccharide EPS from Nostoc flagelliforme possess high intrinsic viscosity, excellent emulsification, and flocculation
activity (Han et al. 2014). Due to their content in sulfate groups and uronic acids,
the EPS derived from cyanobacteria stand out for their strong anionic character,
which is appealing in the removal of metal ions from wastewaters.
A. A. Azim et al.