Disaccharides: Reducing and Non-Reducing

Disaccharides

• Disaccharides consist of two monosaccharides joined by a glycosidic linkage
• Disaccharides are classified as reducing or non-reducing

Reducing Disaccharides

• Maltose(Malt sugar) is formed from two molecules of D-glucopyranose united by an α 1,4-glucosidic linkage
• Maltose exists in both α and β forms
• Maltose is the main product of starch digestion by amylase
• Maltose is hydrolyzed by maltase enzyme or acids into two molecules of D-glucose
• Isomaltose consists of two molecules of D-glucopyranose united by an α 1,6-glucosidic linkage
• Isomaltose is a hydrolysis product of starch and glycogen by amylase and represents the molecule's branching point
• Lactose(Milk sugar) is made of β-D-galactopyranose and D-glucopyranose joined by a β 1,4-galactosidic linkage
• Lactose is hydrolyzed by lactase enzyme or acids into D-glucose and D-galactose

Non-Reducing Disaccharides

• Sucrose (Cane/Table sugar) is found in sugar cane and beets
• Sucrose is formed from β-D-fructofuranose and α-D-glucopyranose united by an α 1,2-glucosidic or β 2,1-fructosidic linkage
• Invertases present in some insects like honeybees catalyze sucrose hydrolysis
• Sucrose is not a hemiacetal, and glucose and fructose must be glycosides
• The two sugars are joined by a glycoside link between the anomeric carbons of both sugars C1 of glucose and C2 of fructose

Oligosaccharides

• These are polymers comprised of 3-10 monosaccharide units
• They aren't a substantial dietary source for humans due to their indigestibility
• They are present as a constituent of many glycolipids and glycoproteins, which include oligosaccharides that are constituents of the ABO blood group substance, immunoglobulins, and glycolipids and glycoproteins of cell membranes

Polysaccharides

• Polysaccharides are composed of more than 10 monosaccharide units linked by glycosidic bonds
• Polysaccharides are nonreducing due to the condensation of monosaccharide units involving the carbonyl groups of the sugars, which leaves only one free carbonyl group at the end of a large molecule

Homopolysaccharides

• Polysaccharides consist entirely of one type of monosaccharide unit
• Glucans are formed of D-glucose units and include starch, dextrins, glycogen, and cellulose
• Starch is the main storage form of carbohydrates in chlorophyll-containing plants and exists in large amounts in cereals, tubers, and legumes
• Starch granules contain two forms: amylose (15-20%) in the inner part and amylopectin (80-85%) in the outer part
• Amylose molecules comprise of about 300-400 (up to 1000) α -D-glucopyranose molecules connected by α 1,4-glucosidic linkages, forming an unbranched chain
• Amylopectin is a branched polysaccharide where each branch is formed of 24-30 α -D-glucopyranose units
• Amylopectin's glucose units are connected by α1,4-glucosidic linkages within the branches and by α 1,6-glucosidic linkages at the branching points
• Dextrins are produced during starch hydrolysis by salivary or pancreatic amylase
• Glycogen is the storage form of carbohydrates in animals (animal starch), found mainly in skeletal muscles and liver
• Cellulose forms the principal part of plant cell walls.
• It is formed of a long unbranched chain of β-D-glucopyranose units connected by β1,4-glucosidic linkages
• Cellulose is insoluble in water, doesn't react with iodine
• It is non-hydrolyzable by amylase because it contains a β1,4-glucosidic linkage
• Plant cellulose is important as it increases the bulk of food, stimulates intestinal contractions, and prevents constipation

Heteropolysaccharides

• Includes glycosaminoglycans (GAGs), formerly called mucopolysaccharides
• They feature unbranched, long chains (typically >50 sugar units)
• They consist of repeating disaccharide units, usually made up of an amino sugar and a uronic acid
• GAGs and proteoglycans are mainly carbohydrates (95%) and only (5%) proteins
• GAGs are present mainly in the extracellular matrix (ECM) or ground substance in association with other extracellular proteins
• Glycoproteins are proteins to which oligosaccharide chains are covalently bound
• Glycoproteins have less carbohydrate content than proteoglycans and are deficient in uronic acids and the repeating disaccharide units of proteoglycans
• Eight monosaccharides are commonly present in glycoprotein oligosaccharide chains: galactose, glucose, mannose, xylose, fucose, N-acetylglucosamine, N-acetylgalactosamine, and N-acetylneuraminic acid
• Glycoproteins are found in mucous fluids, tissues, blood, and cell membranes
• Glycoprotein carbohydrate content is variable, and can be 1% in collagen, 4% in IgG (Immunoglobulin G), 50% in mucins, and 85% in ABO blood group antigens

ABO Blood Group Antigens

• Human blood groups A, B, AB, and O depend on the oligosaccharide part of the glycoprotein or glycolipid on the surface of erythrocytic cells

Representing Carbohydrate Stereochemistry: Fischer Projections

• In 1891, German chemist Emil Fischer suggested a method based on a tetrahedral carbon atom's projection onto a flat surface
• A Fischer projection can be rotated by 180° but not by 90° or 270° to maintain the Fischer convention
• A 90° rotation breaks the Fischer convention by exchanging the groups that go into the plane with those that come out
• A Fischer projection can have one group held steady while the other three rotate in either direction, thereby rotating around a single bond without changing the stereochemistry

Reactions of Monosaccharides

• Carbohydrates are converted into ethers using an alkyl halide in the presence of base via the Williamson ether synthesis
• Treatment of an aldose or ketose with NaBH4 reduces it to a polyalcohol, an alditol
• Like other aldehydes, aldoses are easily oxidized, yielding carboxylic acids called aldonic acids
• The oxidation of an aldose with either Ag+ in aqueous ammonia (Tollens' reagent) or Cu2+ with aqueous sodium citrate (Benedict's reagent) forms the basis of tests for what are called reducing sugars
• Upon warming with dilute HNO3 (nitric acid), an aldose is oxidized to a dicarboxylic acid called an aldaric acid
• Oxidation of only the -CH2OH end of the aldose without affecting the -CHO group yields a monocarboxylic or uronic acid
• This reaction can only be done enzymatically, as no chemical reagent is known to accomplish this selective oxidation in the laboratory

Chain Lengthening: The Kiliani-Fischer Synthesis

• Heinrich Kiliani realized in 1886 that aldoses react with HCN to form cyanohydrins
• Emil Fischer converted the cyanohydrin nitrile group into an aldehyde

Chain Shortening: The Wohl Degradation

• The Wohl degradation is nearly the opposite of the Kiliani-Fischer sequence
• The aldose aldehyde carbonyl group is converted into a nitrile, and the resulting cyanohydrin loses HCN under basic conditions via a nucleophilic addition reaction
• An aldose reacts with hydroxylamine to form an imine or oxime, followed by dehydration of the oxime with acetic anhydride

Ruff Degradation

• The Ruff degradation is a two-step process that starts with bromine-water oxidation of the aldose to its aldonic acid
• Treating the aldonic acid with hydrogen peroxide and ferric sulfate oxidizes the carboxyl group to CO2, yielding an aldose with one fewer carbon atom
• The Ruff degradation is mainly used for structure determination and synthesis of new sugars