Based on the kind of monomer units, polysaccharides can be categorized as homopolysaccharides and heteropolysaccharides. Polysaccharides containing a single type of monosaccharide units are called homopolysaccharides. Homopolysaccharides can be used for energy storage (starch, glycogen, and dextrans) or have structural roles (cellulose, chitin). Heteropolysaccharides (e.g., glycosaminoglycans; such as hyaluronic acid, heparin, and chondroitin) consist of the different monosaccharide units and more often, they function as structural components of cells or tissues [17, 18]. According to surface charge, polysaccharides can be described as being anionic, cationic or neutral [16]. The presence of the charged groups is one of the factors which determine the solubility of polysaccharides. The charged groups increase the solubility of polysaccharides by enhancing the molecular affinity to water and by preventing the intermolecular association driven by the electrostatic actions posed by the charged group [9]. Cellulose, amylose, and amylopectin are examples of neutral polysaccharides. Gum arabic, alginates, xanthan, and carrageenans are anionic (acidic) polysaccharides while chitosan is a cationic polysaccharide [16].
Similar to all other bio-macromolecules, polysaccharide synthesis is an endergonic reaction: synthesis of polysaccharides from simple sugars requires chemical energy in the form of Adenosine triphosphate (ATP) and Uridine triphosphate (UTP) [19]. Glycosyltransferases, often referred to as polysaccharide synthases, are the enzymes that used for the addition of new monosaccharide units to the growing polysaccharide chain by the formation of glycosidic bonds. These enzymes are specific to the types of monosaccharide units. Since monosaccharides have multiple hydroxyl groups, the formation of various types of glycosidic linkages is possible. Thus, considering the structural diversity of glycosidic linkages, many different glycosyltransferases are needed. Of note, this model of assembly is different from the polypeptide or oligonucleotide assembly which all bond formations are carried out by a single catalytic apparatus [20]. In addition, as distinct from proteins, polysaccharides are said to be polydisperse molecules meaning that they generally do not have definite molecular weights since their synthesis does not require a template molecule. So, molecules of a specific polysaccharide from a single source are found within a general size class (a defined range of molecular weights). Excluding cellulose and a few other plant polysaccharides, only bacterial polysaccharides exist in repeating-unit structures, but the majority of polysaccharides are said to be “polymolecular” which means that fine structures (e.g., proportions of different monosaccharide units, the sequence of monosaccharides, branching frequency, type of glycosidic linkage) of individual molecules within a polysaccharide are different from each other [3, 21]. Besides, because of the inclusion of different non-carbohydrate moieties to the backbone, the structure of a single polysaccharide can show variations between taxa and even it can change depending on the growth conditions of the organism [3].
6.1.3 Structural Characterization Techniques
When it has been obtained in a sufficient degree of purity, structural characteristics of a polysaccharide can be determined by the analysis of (i) monosaccharide units, (ii) linkage types, (iii) anomeric configurations, (iv) noncarbohydrate substituent groups, and (v) molecular weight [3]. However, structural aspects of polysaccharides can be also described on the organizational levels, similar to protein primary, secondary, tertiary, and quaternary structural organizations. Polysaccharide primary structure can be described as a covalent sequence of monosaccharide units [21]. The secondary structure is the geometrically regular arrangement in space that the primary sequence can adopt. The establishment of the tertiary structure occurs via the packing of secondary structure arrangements together. Tertiary structure is stabilized mostly through intermolecular hydrogen bonds, but physical state and temperature can also affect the adoption of an ordered secondary or tertiary structure. Finally, a quaternary structure is the arrangement of single units of tertiary structure within a complex built by non-covalent interaction [21, 22]. Generally, the type of glycosidic linkage affects the molecular conformation more than monosaccharide type. Polysaccharide’s primary structure determines the nature and extent of intramolecular and intermolecular associations within a polysaccharide chain and configures secondary, tertiary, and quaternary structures. To achieve thermodynamically favored conformations, the extent of glycosidic bond rotation is restricted. Therefore, these favored conformations define the proximity of adjacent glycosyl units one to another and determine the three-dimensional configuration of a polysaccharide. For example, amylose, cellulose, and dextran are all linear chains of monosaccharide units, but they are different in the nature of their glycosidic linkages [21].
A systematic methodology and advanced analytical tools are required for separation, purification, modification and application of polysaccharides. Established techniques in the polysaccharide field aim to characterize the structural parameters such as chain conformation, chain length distribution, degree of substitution, degree of branching, crystallinity, and interactions with solvent [23]. The procedure to elucidate the structure of polysaccharides includes the isolation, purification, and molecular weight determination of polysaccharides, and the investigation of monosaccharide units and type of glycosidic linkages via FT-IR spectroscopy, periodate oxidation, partial acid hydrolysis, glycosyl linkage (methylation) analysis, Smith degradation, and GC–MS-based techniques. One-dimensional and two-dimensional NMR spectroscopy are used to describe the sequence of monosaccharides, the anomeric configuration of each sugar residue, and the degree of branching [24]. However, novel and more practical analyses methods, like bio-recognition based methods, immunochemical assays, enzymatic cleavage, are needed to be developed.
6.2 Uses and Applications of Biopolysaccharides
The polysaccharides of living organisms and/or functionalized polysaccharides with biological impacts on living organisms are specified as bioactive polysaccharides [25]. Most bioactive polysaccharides are made from glucose, galactose, fucose, mannose, ribose, arabinose, xylose, glucuronic acid, and galacturonic acid [24].
Since polysaccharides are structurally complex biomolecules and experimental methodologies for studying polysaccharides have been limited, research in polysaccharides has ever fallen behind that on protein and nucleic acids. The opening of the research area into polysaccharides dates back to about 100 years ago. Early research focused mostly on chemical composition and primary structures of polysaccharides and by the 1970s, the combination of carbohydrate chemistry and biochemistry enabled researchers to investigate the potential influences of polysaccharides on cell and molecular biology [24]. In 1988, Dr. Dwek from Oxford University brought the concept of glycobiology and opened a new research area comprising carbohydrate chemistry, immunology, and molecular biology and aiming to determine the functional roles of polysaccharides or carbohydrate chains [24, 26]. In the wider sense, the term “glycobiology” is defined as studying the structure, biosynthesis, biological interactions, and evolution of saccharides that are widespread in nature [1]. Nowadays, it has been known that the functions of polysaccharides are not limited to being the structural support and energy source in life, but they also play important roles in various biological phenomena and physiological processes [24]. Recent advances in bioanalytical technology have enabled researchers to understand and explore the structures and roles of polysaccharides and utilize their functions.
Because of their non-toxic quality and biodegradable and biocompatible characters, bioactive polysaccharides have gained attention as therapeutic agents for pharmacological applications, based on their broad range of biological properties including antioxidative, antimicrobial, antitumor, hypolipidemic, antidiabetic, and hepato-protective actions [25]. Bioactive polysaccharides and polysaccharide-derived polymers are also focused on extensively for the development of novel products useful in the fields of food and feed production, cosmetics, wood products, paper, cellulose derivatives, sustainable fuel production, and textiles [23]. Of note, unraveling the complexity of biopolysaccharides and advances in the use of biopolysaccharides for therapeutic and/or commercial purposes needs multidisciplinary collaboration of scientists from the fields of biology (molecular and cellular biology, phytology, microbiology, glycol-biology) and medicine, nutrition, and food sciences, physics, and chemistry.
6.2.1 Functional Fibers
Carbohydrates that cannot be hydrolyzed by the endogenous enzymes in the upper gastrointestinal system