Polysaccharides. Группа авторов

      6

      Biopolysaccharides: Properties and Applications

       Sinem Tunçer1,2

       1Vocational School of Health Services, Department of Medical Laboratory Techniques, Bilecik Şeyh Edebali University, Bilecik, Turkey

       2Biotechnology Application and Research Center, Bilecik Şeyh Edebali University, Bilecik, Turkey

       Abstract

      The four basic macromolecules of life are lipids, proteins, polynucleotides, and carbohydrates. Polysaccharides are the most abundant carbohydrate polymers found in nature: they widely exist in animals, plants, microorganisms, and algae. The biological functions of these large and structurally complex polymers are wide-ranging, from those that appear to be relatively subtle, to those that are crucial for survival, growth, development, and maintenance of the organism that synthesizes them. On the other hand, in the past years, attempts to investigate natural polysaccharides-based biomaterials for several diverse applications including tissue engineering, wound healing, drug delivery, food preservation, cosmetics, biofuel production, wastewater treatment, and production of textile fibers have been increased. Undoubtedly, the ever-growing knowledge about the structure, synthesis, and function of biopolysaccharides, as well as their interactions with other biomolecules, will open up new avenues for new applications and innovations of polysaccharides.

      Keywords: Polysaccharides, structure, food, biomedicine, therapeutics, pollution, cosmetics, biofuel

6.1 Structure and Classification of Biopolysaccharides

      Along with lipids, proteins, and polynucleotides, carbohydrates are one of the main classes of biological macromolecules. Polysaccharides are biopolymers of monosaccharides found in every living organism and play critical roles in a wide variety of cellular activities. Comprehensive investigations of polysaccharides are fundamental since the distinct structural features of polysaccharides contribute significantly to their functional properties.

      6.1.1 Structure

Carbohydrates can contain either hydroxy and aldehyde function or hydroxy and ketone functions. Therefore carbohydrates can be defined as polyhydroxy aldehydes/ketones and their simple derivatives [1]. Monosaccharides are the simplest carbohydrates with the general formula C_nH_2nO_n, (n ≥3). In this structure, one of the carbon atoms is double-bonded to an oxygen atom forming a carbonyl (C=O) group and there is one hydroxyl (OH) group on each of the other carbon atoms [2]. Polysaccharides contain 1 to 5 different aldose and/ or ketose monosaccharide units. Structurally, the most frequent monosaccharide units of polysaccharides are found in the six-membered ring structure, although pentoses are not rare, but eight- and nine-carbon-atom sugars are less frequently found [3, 4]. The monosaccharide units are attached by covalent bonds called “glycosidic linkages”. The linkage links the carbon atom in position 1 (C(1)) to a carbon atom of the following unit through an oxygen bridge [4]. Most polysaccharides contain more than 20 monosaccharide units [2]. As specified by the International Union of Pure and Applied Chemistry, the terms glycan (glyc-for sweet or sugar; -an for polymer) and polysaccharide are used as synonyms to describe “compounds consisting of a large number of monosaccharides linked glycosidically” [5]. The number of monosaccharide units in a polysaccharide polymer is called as Degree of Polymerization (DP) and this value varies with polysaccharide type. Most of the naturally occurring polysaccharides have DPs less than 100. However, larger polysaccharides also exist; like cellulose (7,000–15,000 DPs) and the amylopectin component of starch (DPs of more than 100,000) [2]. Depolymerization of polysaccharides can be achieved by specific enzymes, heating, acid-catalyzed hydrolysis, oxidation in an alkaline system, and ultrasonic treatments [3, 6].

In polysaccharides, hydroxyl groups can be acylated (esterified), alkylated (etherified) or oxidized, amino groups can be acylated or deacylated, and carboxyl groups can be converted into amides, amines, and esters. Therefore, structural modifications of polysaccharides can result in even more complex structures [3]. Furthermore, a protein (or a peptide) and a polysaccharide molecule can be linked to each other, like in the structure of proteoglycans which glycosaminoglycan side-chains are found to be attached a core protein [7]. On the other hand, in the lipopolysaccharide structure, the polysaccharide is found to be linked with a lipid molecule along with O-antigen [8]. The diversified structural properties contribute significantly to the functional properties of polysaccharides [9]. In the functional aspect, polysaccharides are ideal natural resources for a broad range of applications, but their activities are not always satisfactory. Thus, based on structure-activity relationships, researchers have modified the structures of natural polysaccharides to obtain functionally improved biopolysaccharides. These molecular modification methods can be classified as chemical, physical, and biological methods. Among these approaches, chemical modification is the most widely used one [10]. Chemical techniques employed to modify polysaccharides include derivative formation, grafting, cross-linking, and polymer-polymer blending. The derivative formation is based on the inclusion of functional groups to polysaccharides by acetylation, carboxymethylation, cyanoethylation, carbamoylethylation, deacetylation, sulfation, phosphorylation, and esterification reactions [11]. In blending, a physical mixture of two or more than two polymers is prepared to obtain the required characters. In the grafting method, different monomers are covalently linked to the polymer. By grafting method, the desired properties of polymer changes, but initial characteristics of the polymer are not affected. In the curing method, via physical forces, an oligomer mixture adheres to a substrate surface to form a coating [12]. Through chemical modifications, spatial structures and physical and chemical features of polysaccharides change and therefore, biological activities of polysaccharides, such as immune regulation, anti-tumor, anti-oxidation properties, can be modified [10]. The mechanism of the physical modification method is based on the truncation of the polysaccharide backbone by using ultrasonic disruption, radiation-induced reaction, or microwave exposure, to obtain lower molecular weight fragments. Finally, in most cases, biological modification of polysaccharides refers to the enzymatic degradation of polysaccharides. Enzymatic modification can be applied to only certain kinds of polysaccharides; however, investigation of different types of enzymes, such as transferase and synthase, will increase the use of the enzymatic modification of polysaccharides [10].

      6.1.2 Classification

      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