Carbohydrate Chemistry PDF

# CARBOHYDRATE CHEMISTRY ## Derivatives of Monosaccharide - The 5 major groups of monosaccharide derivatives: 1. **Sugar Phosphates** 2. **Sugar alcohols** 3. **Sugar acids** 4. **Deoxy Sugars** 5. **Amino Sugars** | | Sugar | | | | | |---|---|---|---|---|---| | **1** | Glucose-6-Phosphate...

# CARBOHYDRATE CHEMISTRY ## Derivatives of Monosaccharide - The 5 major groups of monosaccharide derivatives: 1. **Sugar Phosphates** 2. **Sugar alcohols** 3. **Sugar acids** 4. **Deoxy Sugars** 5. **Amino Sugars** | | Sugar | | | | | |---|---|---|---|---|---| | **1** | Glucose-6-Phosphate | **2** | Sorbitol | **3** | B-D-2-deoxyglucose | | **4** | B-D-2-glucosamine | **5** | B-D-glucuronic acid | | | ## Derivatives of Monosaccharide 1. **Sugar Phosphate** 'ester bond': Phosphorylation reactions are catalyzed by a family of enzymes called protein kinases that use adenosine triphosphate (ATP) as a phosphate donor. 'Phosphate or Pi' 2. **Sugar acids:** are produced by oxidation of carbonyl carbon, last hydroxyl carbon or both. - **Aldonic acids (C1):** oxidation of carbonyl carbon to carboxylic group gives aldonic acid e.g. glucose is oxidized to gluconic acid. - **Uronic acids (C6):** oxidation of last hydroxyl carbon gives uronic acid e.g. glucose is oxidized to glucuronic acid. - **Aldaric acids (C1 & C6):** These are dicarboxylic acids produced by oxidation of both carbonyl carbon and last hydroxyl carbon e.g. glucose Is oxidized to glucaric acid. ## Sugar alcohols - Monosaccharides, both aldoses and ketoses may be reduced at carbonyl carbon, to the corresponding alcohol: - Glucose is reduced to glucitol (sorbitol). - galactose is reduced to galactitol (dulcitol). - mannose is reduced to mannitol. - Fructose is reduced to mannitol and sorbitol. ## Deoxy-sugars: - Are sugars in which one of the hydroxyl groups has been replaced by a hydrogen atom i.e. one oxygen is missed. - Deoxyribose found in ATP & nucleic acid 'DNA'. - L-Fucose (Fuc): 6-Deoxygalactose or methyl pentose: occurring in glycoproteins. ## Amino sugars: - In these sugars, the hydroxyl group attached to carbon number 2 is replaced by an amino or an acetyl-amino group. 'CH CO-NH-sugar' - Amino sugars are important components of glycoproteins, of certain glycol-sphingo-lipids, and of glycosaminoglycans (GAGs), and are also found in some antibiotics. The synthetic pathway of amino sugars is very active in connective tissues, where as much as 20% of glucose flows through this pathway. - The major amino sugars are the hexosamines glucosamine, galactosamine, and mannosamine, and the compound sialic acid (nine-carbon; 3C+6C). ## Amino sugar acids: - Are a condensation of amino sugars and some acids. They are occurring in glycoproteins. - For Example: Sialic acid or N-Acetyl Neuraminic Acid ## Glycosidic Bonds - Naming glycosidic bonds: named according to the numbers of the connected C, and with regard to the position of the anomeric -OH group of the sugar involved in the bond. If this anomeric hydroxyl is in the a configuration, the linkage is an A-bond. If it is in the ẞ configuration, the linkage is a ẞ-bond. - Maltose, for example, is synthesized by forming a glycosidic bond between C1 of A-glucose and C4 of glucose. The linkage is, therefore, an a(1→4) glycosidic bond. ## N- and O-glycosides: - Naming glycosidic bonds 'N- and O-glycosides': These carbohydrates are attached to either the side-chain Oxygen (group is an -OH) of serine or threonine residues by O-glycosidic linkages or to the side chain nitrogen (an -NH, group) of Asparagine residues by N-glycosidic linkages. ## Disaccharide - The disaccharides are sugars composed of two monosaccharide residues linked by a glycoside bond. - They subdivide based on presence or absence of free reducing group into: - **Reducing disaccharides** with free aldehyde or keto group - Example:- Maltose and Lactose - **Non reducing disaccharide** without free aldehyde or keto group - Example:- Sucrose - Hydrolysis of sucrose yields a mixture of glucose and fructose called "invert sugar" because fructose is strongly levorotatory and changes (inverts) the weaker dextrorotatory action of sucrose. ## Sucrose - It is commonly used table sugar and contributes some calories in the diet. - It contains one mole of glucose and one mole of fructose that are linked by a-(1-2) glycosidic linkage. It hydrolyzed into glucose and fructose by the enzyme Sucrase is also called as Invertase. It is a non reducing disaccharide. ## Maltose - It contains two moles of glucose units. - They linked by a-(1-4) glycosidic linkage. - It is a one of the reducing disaccharide, which has free functional group. - It hydrolyzed to two glucose by an enzyme called maltase. ## Isomaltose - It is also contains two moles of glucose. - They linked by a-(1-6) glycosidic linkage. - It is derived from the digestion of starch & glycogen. - It hydrolyzed to glucose in the intestinal tract by an enzyme called isomaltase. ## Lactose - It is present in the milk sugar. It is the source of carbohydrates in breast fed infants. - Its contains one mole of galactose and one mole of glucose that are linked by B-(1-4) glycosidic linkage. It is also a reducing disaccharide. - It hydrolyzed into galactose and glucose by the enzyme lactase in human and by B-D-galactosidase in bacteria. ## Polysaccharides - Polysaccharides are long chains of monosaccharides joined together by glycosidic bonds (linkages). Polysaccharides have been classified into two groups : - (i) **homopolysaccharides** (also called homoglycans) containing only one type of monosaccharide glycose units, and - (ii) **heteropolysaccharides** (also called heteroglycans) containing atheist two different monosaccharides (or glycose) units. Most heteropoly- saccharides do not contain more than two types of glycose units. These polysaccharides are formed due to condensation of many molecules of monosaccharides With simultaneous release of water molecules. Because each monosaccharide has several free hydroxyl groups that can be used for condensation with another monosaccharide: the number of possible structures of a polysaccharide involving the same monosaccharide units. can large. ## Homopolysaccharides - Homopolysaccharides (homoglycans) consist of same monosaccharide units (e.g. glucose, fructose, galactose, mannose. xylose, etc.). - Cellulose, glycogen, and starch are the best-known examples. However, they have very different properties. - Glycogen is found in animals While, starch and cellulose are in plants. - All of them are composed of only glucose residues. - While glycogen and starch are used for storing energy, cellulose makes the extracellular matrix (ECM) or cell wall in plants. - Different homopolysaccharides, their repeat units and the linkage type in each case. ## Homopolysaccharides: 1. Starch - Starch: a homopolymer composed of D-glucose units or (D-glucopyranose units) held by A- glycosidic bonds, and occurs in many plants. - Composition of starch granule: It consists of two polymeric (1) water soluble amylose (15 -20%) and (ii) water insoluble amylopectin (80-85%). - Starch consists of amylose and amylopectin in the ratio of 1: 4. - Chemically, Amylose is a linear polysaccharide linked by a(1-4) linkages while amylopectin is a branched polysaccharide [The linear linkage is a(1-4) while the branched linkage is a(1-6)]. - Amylopectin: Approx. 80 branches, One branch after every 24 to 30 D-Glucose units. ## 2. Glycogen - Glycogen is the main storage polysaccharide occurring in animal cells, so it referred as animal starch. It is present in high concentration in liver, followed by muscle, brain etc. - Liver glycogen response to blood glucose (BG) levels: - `BG → glycogen breakdown → ↑ BG` - Muscle glycogen can be broken down for energy for the muscle. - Its structure is very similar to amylopectin, in that main chain linkages between D-glucose units are (14) and the linkages at branch points are (16). - Branch points occur more frequently in glycogen (about every 8 to 12 residues) than in amylopectin. ## 3. Cellulose - The main component of plant cell wall. - An unbranched polymer (a linear homopolysaccharide ) of glucose linked by B-(1-4) glycosidic linkages. 'Due to the presence of B linkages, cellulose chains fold quite differently than chains of D- glucose in the starches and glycogen' - Since humans lack an enzyme cellulase that can hydrolyse B-(1-4) glycosidic linkages, cellulose cannot be digested and absorbed. 'This enzyme are produced by many cellulolytic microorganisms'. - has no food value unlike starch. ## 4. Chitin - A linear homopolysaccharide, It is a polymer of units of N-acetyl glucosamine which is linked by ẞ(1-4) glycosidic bond. - It is similar to cellulose in both structure and function. - The only chemical difference from cellulose is the replacement of the hydroxyl group at C-2 with an acetylated amino group. - Chitin is the principal component of the hard exoskeletons of nearly a million species of Arthropods (insects and crabs). ## 5. Dextrans and Dextrins - Dextrans are bacterial and yeast polysaccharides made up of (16) -linked poly-D-glucose; all have (13) branches, and some also have (12) or (14) branches. - Dental plaque, formed by bacteria growing on the surface of teeth, is rich in dextrans. - Dextrins, When starch is partially hydrolysed by the action of acids or enzymes, it is broken down into a number of products of lower molecular weight known as dextrins. ## 6. Inulin - Inulin composed of 2 - 60 fructose molecules connected by ẞ-(2→1) glycoside bonds. - Inulin structure and functions Inulin as a reserve carbohydrate in many plants (in the in the roots of the artichoke, dandelion and in the bulbs of onion and garlic). - Acids hydrolyze of Inulin given to D-fructose; similarly it is also hydrolysed by the enzyme inulinase, which accompanies it in plants. It has no dietary importance in human beings as inulinase is absent in human. - Biomedical Importance: - It is used in physiological investigation for determination of the rate of glomerular filtration rate (GFR). - It has been also used for estimation of body water (ECF) volume.