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Carbohydrates. Structure and Functions of Di- and Polysaccharides

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Carbohydrates. Structure and Functions of Di- and Polysaccharides Ministry of Public Health of Ukraine Ukrainian Medical Stomatological Academy Department of biological and bioorganic chemistry Carbohydrates. Structure and functions of di- and polysaccharides Assoc. Prof. Bilets M.V. Lecture plan Structure and properties of sucrose, lactose, maltose. Classification of polysaccharides. Structure, biological role of starch. Structure and biological role of glycogen. Structure and biological role of cellulose. Heteropolysaccharides. Glycosaminoglycans. Proteoglycans. Disaccharides There are two functionally different classes of disaccharides: Disaccharides (C12H22O11) are sugars composed of two Reducing disaccharides, in which one monosaccharide, the reducing sugar of the monosaccharide units that are joined by a carbon–oxygen-carbon pair, still has a free hemiacetal unit that can perform as a reducing aldehyde group; linkage known as a glycosidic linkage. This linkage is formed lactose, maltose and cellobiose are examples of reducing disaccharides, each with from the reaction of the anomeric carbon of one cyclic one hemiacetal unit, the other occupied by the glycosidic bond, which prevents it monosaccharide with the OH group of a second monosaccharide. from acting as a reducing agent. They can easily be detected by the Fehling's or Tollens' reagents. Non-reducing disaccharides, in which the component monosaccharides bond The disaccharides differ from one another in their through an acetal linkage between their anomeric centers. This results in neither monosaccharide constituents and in the specific type of monosaccharide being left with a hemiacetal unit that is free to act as a reducing glycosidic linkage connecting them. There are three common agent. Sucrose and trehalose are examples of non-reducing disaccharides because disaccharides: maltose, lactose, and sucrose. their glycosidic bond is between their respective hemiacetal carbon atoms. They do not react with Fehling's Tollens' reagents. http://www.chups.jussieu.fr/polys/biochimie/SGLbioch/POLY.Chp.1.5.h tml https://chem.libretexts.org/Courses/Sacramento_City_College/SCC%3A_Chem_309_- _General_Organic_and_Biochemistry_(Bennett)/Text/14%3A_Carbohydrates/14.6%3A_Disaccharides Maltose Maltose occurs to a limited extent in sprouting grain. It is formed most often by the partial hydrolysis of starch and glycogen. Maltose is about 30% as sweet as sucrose. The human body is unable to metabolize maltose or any other disaccharide directly from the diet because the molecules are too large to pass through the cell membranes of the intestinal wall. In the body, hydrolysis reactions are catalyzed by enzymes such as maltase. Hydrolysis of maltose produces two molecules of D-glucose. Maltose is a reducing sugar. Thus, its two glucose molecules must be linked in such a way as to leave one anomeric carbon that can open to form an aldehyde group. The glucose units in maltose are joined in a head-to-tail fashion through an α-linkage from the first carbon atom of one glucose molecule to the fourth carbon atom of the second glucose molecule (that is, an α-1,4-glycosidic linkage. The bond from the anomeric carbon of the first monosaccharide unit is directed downward, which is why this is known as an α-glycosidic linkage. The OH group on the anomeric carbon of the second glucose can be in either the α or the β position https://chem.libretexts.org/Courses/Sacramento_City_College/SCC%3A_Chem_309_- _General_Organic_and_Biochemistry_(Bennett)/Text/14%3A_Carbohydrates/14.6%3A_Disaccharides Lactose Lactose is known as milk sugar because it occurs in the milk of humans, cows, and other mammals. In fact, the natural synthesis of lactose occurs only in mammary tissue, whereas most other carbohydrates are plant products. Human milk contains about 7.5% lactose, and cow’s milk contains about 4.5%. This sugar is one of the lowest ranking in terms of sweetness, being about one-sixth as sweet as sucrose. Lactose is a reducing sugar composed of one molecule of D- galactose and one molecule of D-glucose joined by a β-1,4- glycosidic bond (the bond from the anomeric carbon of the first monosaccharide unit being directed upward). The two monosaccharides are obtained from lactose by acid hydrolysis or the catalytic action of the enzyme lactase. Many adults and some children suffer from a deficiency of lactase. These individuals are said to be lactose intolerant because they cannot digest the lactose found in milk. A more serious problem is the genetic disease galactosemia, which results from the absence of an enzyme needed to convert galactose to glucose. https://chem.libretexts.org/Courses/Sacramento_City_College/SCC%3A_Chem_309_- http://www.chups.jussieu.fr/polys/biochimie/SGLbioch/POLY.Chp.1.5.html _General_Organic_and_Biochemistry_(Bennett)/Text/14%3A_Carbohydrates/14.6%3A_Disaccharides Sucrose Sucrose, is known as beet sugar, cane sugar, table sugar, or simply sugar. The sucrose molecule is unique among the common disaccharides in having an α-1,β-2-glycosidic (head-to-head) linkage. Because this glycosidic linkage is formed by the OH group on the anomeric carbon of α-D-glucose and the OH group on the anomeric carbon of β-D- fructose, it ties up the anomeric carbons of both glucose and fructose. This linkage gives sucrose certain properties that are quite different from those of maltose and lactose. As long as the sucrose molecule remains intact, neither monosaccharide “uncyclizes” to form an open- chain structure. Thus, sucrose is incapable of mutarotation and exists in only one form both in the solid state and in solution. In addition, sucrose does not undergo reactions that are typical of aldehydes and ketones. Therefore, sucrose is a nonreducing sugar. The hydrolysis of sucrose in dilute acid or through the action of the enzyme sucrase (also known as invertase) gives an equimolar mixture of glucose and fructose. This 1:1 mixture is referred to as invert sugar because it rotates plane-polarized light in the opposite direction than sucrose. The widespread use of sucrose is a contributing factor to obesity and tooth decay. Carbohydrates such as sucrose, are converted to fat when the caloric intake exceeds the body’s requirements, and sucrose causes tooth decay by promoting the formation of plaque that https://chem.libretexts.org/Courses/Sacramento_City_College/SCC%3A_Chem_309_- sticks to teeth. _General_Organic_and_Biochemistry_(Bennett)/Text/14%3A_Carbohydrates/14.6%3A_ http://www.chups.jussieu.fr/polys/biochimie/SGLbioch/POLY.Chp.1.5.htm Disaccharides l Polysaccharides Polysaccharides (glycans), are long chain polymeric carbohydrates composed of monosaccharide units ( more than 20) bound together by glycosidic linkages. This carbohydrate can react with water (hydrolysis) using enzymes as catalyst, which produces constituent sugars (monosaccharides, or oligosaccharides). They range in structure from linear to highly branched. Examples include storage polysaccharides such as starch, glycogenand structural polysaccharides such as cellulose. Polysaccharides can be broadly classified into two classes: Homo-polysaccharides – are made up of one type of monosaccharide units. ex: cellulose, starch, glycogen. Hetero-polysaccharides – are made up of two or more types of monosaccharide units. ex. hyaluronic acid and they provide extracellular support for organisms. https://www.slideshare.net/ArunimaSur/classification-of-polysaccharides-gluconeogenesis-and- glucogenolysis Glycogen Glycogen is a branched biopolymer consisting of linear chains of glucose residues with an average chain length of approximately 8–12 glucose units and 2,000-60,000 residues per one molecule of glycogen. Glycogen has a structure similar to amylopectin (a component of starch), but is more extensively branched and compact than starch. Glucose units are linked together linearly by α(1→4) glycosidic bonds from one glucose to the next. Branches are linked to the chains from which they are branching off by α(1→6) glycosidic bonds between the first glucose of the new branch and a glucose on the stem chain. Each glycogen molecule has a protein, glycogenin, covalently linked to the polysaccharide. Glucose is an osmotic molecule, and can have profound effects on osmotic pressure in high concentrations possibly leading to cell damage or death if stored in the cell without being modified. Glycogen is a non-osmotic molecule, so it can be used as a storing glucose in the cell without disrupting osmotic pressure. The branched structure of glycogen allows you to quickly regulate glucose levels by attaching new glucose residues to the ends of the molecule during hyperglycemia or rapidly cleaving off glucose residues during hypoglycemia. In the liver, glycogen regulates blood glucose level as it breaks down to free glucose that is released from the cell. Liver glycogen stores are enough for https://www.sciencedirect.com/topics/neuroscience/glycogen about 12-18 hours of fasting. In other cells, especially in skeletal muscle, glycogen is broken down to glucose-6-phosphate, which does not leave the cell, but is actively oxidized to produce energy that is used by the cell. Starch Starch is the most important source of glucose in the human diet and accounts for more than 50% of our carbohydrate intake. It occurs in plants in the form of granules, and these are particularly abundant in seeds (especially the cereal grains) and tubers, where they serve as a storage form of carbohydrates. Plants contain a percentage of starch (potatoes 15%, wheat 55%, corn 65%, and rice 75%). Starch is a mixture of
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