CARBOHYDRATES PART 2 ECDA Aug 2009

DISACCHARIDES

Two monosaccharide units are linked together by glycosidic bond to create larger structures Disaccharides are created through dehydration synthesis (removal of H 2 O). They are condensation products of two monosaccharides The physiologically important disaccharides are maltose, sucrose, and lactose.

D ISACCHARIDES DisaccharideDescription Component monosaccharides sucrosecommon table sugarglucose α1 → 2 fructose maltose product of starch hydrolysis glucose α1 → 4 glucose trehalosefound in fungiglucose α1 → 1 glucose lactosemain sugar in milk galactose β1 → 4 glucose melibiosefound in legumes galactose α1 → 6 glucose

D ISACCHARIDES MALTOSE Also known as malt sugar Obtained by enzyme –catalyzed hydrolysis of starch Composed of 2 D-glucopyranoses 1,4-alpha-glycoside bond (1-4-α-linkages) Reducing sugar, contains hemiacetal function and can mutarotate Easily digested by humans

D ISACCHARIDES CELLOBIOSE Formed from partial hydrolysis of cellulose Composed of 2 D-glucopyranoses 1,4-beta-glycoside bond (1,4-β-linkage) Reducing sugar, contains hemiacetal function and can mutarotate Cannot be digested by humans Only difference between maltose and cellobiose is linkage

D ISACCHARIDES

LACTOSE Milk sugar (both human and cow) Milk sours when lactose is converted to lactic acid (tastes sour) Composed of D-galactose and D-glucose 1,4-beta-glycoside bond (1,4-β-linkage) Digested by most humans Lactose intolerant individuals often lack the enzyme lactase, which hydrolyzes the glycosidic linkage Reducing sugar

D ISACCHARIDES

SUCROSE Table sugar (from sugar beets or sugar cane) Composed of one glucose and one fructose molecule 1,2-glycoside linkage (beta-D-fructoside and alpha-D-glucoside) Not a reducing sugar; no free hemiacetal, this implies both glucose and fructose must be glycosides

D ISACCHARIDES

OLIGOSACCHARIDES

Oligosaccharides are condensation products of three to ten monosaccharides Ex. Raffinose

POLYSACCHARIDES

Polysaccharides are condensation products of more than ten monosaccharide units linked by glycosidic bonds

POLYSACCHARIDES May be classified into two types: 1. Homopolysaccharides- gives only one type of monomer unit upon complete hydrolysis Ex. Glucans (glucose is the only monosaccharide constituent) in starch Cellulose Amylose 2. Heteropolysaccharides- more than one type of monomer unit Ex. Glycosaminoglycans Glycoproteins

POLYSACCHARIDES HOMOPOLYSACCHARIDES 1. Cellulose - Is the major structural component of plant cell walls - The basic repeating unit is cellobiose (β1-4 linkage) - Two features: - In most animals therefore, cellulose cannot be utilized as an energy source; instead it is used to facilitate waste disposal - Can form hydrogen bonds with water in solutions, and thus can accommodate a large amount of water - Its presence in the diet lubricates and aids movement and removal of fecal matter

POLYSACCHARIDES

HOMOPOLYSACCHARIDES 2. Amylose - The repeating units is maltose - The α1  4 linkage results in a coiled helical configuration which is hollow thus allowing large molecules like iodine to be accommodated inside

POLYSACCHARIDES

HOMOPOLYSACCHARIDES 3. Amylopectin - This is the other component of starch is a branched structure made up of two types of glucosidic linkage: - α1  4 bond which links linear polyglucose together - α1  6 bond which links polyglucose branches made up of residues to “main” polyglucose chain.

POLYSACCHARIDES

HOMOPOLYSACCHARIDES 4. Glycogen - this is the main storage polysaccharide found in animal cells - these are found in cellular structures known as glycogen granules and serve both to store glucose as well convert it to an insoluble form which will not affect cellular density - the structure is similar to amylopectin but is more highly branched. Branching occurs every residues

POLYSACCHARIDES

HETEROPOLYSACCHARIDES 1. Glycosaminoglycans - these are important polysaccharides which functions mainly as the main structural component of mucins, connective tissues, and the extracellular matrix - these are normally found linked with proteins as a proteoglycan structure

POLYSACCHARIDES GLYCOSAMINOGLYCANS Proteoglycans provide ground or packing substance of connective tissue Hyaluronic acid – in connective tissues, vitreous humor, synovial fluids Chondroitin sulfate – connective tissues Heparan – in blood Dermatan sulfate – connective tissues; skin Keratan sulfate – connective tissues; skin; cornea

POLYSACCHARIDES Chondroitin sulfateHeparin

POLYSACCHARIDES May be further classified into: I.Storage Polysaccharides- reserved food materials Ex. Starch, glycogen, inulin II.Structural Polysaccharides- gives shape, elasticity, and rigidity to plant and animal tissues as well as protection and support to unicellular organisms Ex. Chitin, cellulose, hemicellulose

POLYSACCHARIDES STARCH a homopolymer of glucose forming an α- glycosidic chain called glucan 2 main constituents are amylose (15-20%) with non-branching helical structure and amylopectin (80-85%) with branched chains composed of glucose residues united by 1  4 linkages in the chains and by 1  6 linkages branch points

POLYSACCHARIDES STARCH (Amylose)

POLYSACCHARIDES STARCH (Amylopectin)

POLYSACCHARIDES GLYCOGEN Storage polysaccharide in animals More highly branched structure than amylopectin, with chains of 12 to 14 α- D-glucopyranose residues, with branching by means of α (1  6) glycosidic bonds

POLYSACCHARIDES Glycogen

POLYSACCHARIDES CHITIN Structural polysaccharide in the exoskeleton of crustaceans and insects, and also mushrooms Consists of N-acetyl-D-glucosamine units joined by β (1  4)-glycosidic linkages

POLYSACCHARIDES Chitin

POLYSACCHARIDES GLYCOPROTEINS Proteins containing branched or unbranched oligosaccharide chains The sialic acids are N- or O-acyl derivatives of neuraminic acid constituents of both glycoproteins and gangliosides Neuraminic acid is a 9-carbon sugar derived from mannosamine (an epimer of glucosamine) and pyruvate

POLYSACCHARIDES GLYCOPROTEINS These occur mainly in cell membranes and in extracellular matrix In cell membranes, glycoproteins are concentrated mainly at the outer half of the membrane bilayer where they comprise the glycocalyx of the cell The carbohydrate units, mostly N-acetylglucosamine and N-acetylgalactosamine are covalently linked with ser and thr or asn as in the mucopolysaccharide

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