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Due to their diversity, an ideal classification scheme does not exist for polysaccharides. Therefore, a combination of different properties, including the source (e.g., microorganism, higher plant, seaweed), structure (linear, short or highly branched), kinds of monomers (homoglycans and dihetero-, trihetero-, tetrahetero-, and pentaheteroglycans), function (storage or structural), and charge (cationic, anionic or neutral polysaccharides), can be used to describe a polysaccharide. Also, the type of links between monosaccharides (glycosidic bonds on different carbons of the monosaccharides) can be used for classification [13–16]. Biopolysaccharides are characterized based on the combination of these factors which finally determines the biological roles as well as industrial and pharmaceutical applications of the polysaccharides.

Based on structural properties, polysaccharides can be classified as linear, branched, and highly branched (branch on branch). Cellulose, amylose, pectin, alginates are examples for linear polysaccharides while glycogen is a branched biopolymer. Xanthan gum, locust bean gum, and guar gum are short-branched polysaccharides. Gum arabic, amylopectin, and arabinoxylan are highly branched polysaccharides [15, 16]. 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].