Disaccharides Polysaccharides

Lecture 4Disaccharides Polysaccharides

DiasaccharidesDisaccharides consists of two monosaccharide units (similar or dissimilar) held together by a glycosidic bondThey are crystalline, water-soluble and sweet to tasteThe diasaccharides are of two typesReducing diasacchrides with free aldehyde or keto group e.g. Maltose, lactoseNon reducing disaccharides with no free aldehyde or keto group e.g. sucrose, trehalose

Maltose Composed of two -D-glucose units held together by (1-4) glycosidic bond. Maltose can be hydrolysed by enzyme maltase to liberate two molecules of -D-glucose

Isomaltose The glucose units are held together by (1-6) glycosidic linkage

CellubioseIs a disaccharide identical in structure with maltose except that the former has (1-4) glycosidic linkage.It is formed during the hydrolysis of cellulose

Sucrose (Cane sugar) mostly produced by sugar cane and sugar beets.It is made up of -D-glucose and -D-fructose.The two monosaccharides are held together by glycosidic bond (1-2)It is an important source of dietary carbohydrate.It is sweeter than most other common sugars like fructose glucose and maltose.

It is employed as a sweetening agent in food industryThe intestinal enzyme- sucrase- hydolyses sucrose to glucose and fructose which are absorbed.

Lactose It is more commonly known as milk sugarIt is composed of -D-galactose and -D-glucose( (1-4) glycosidic bond)

Polysaccharides Polysaccharides or simply glycans consists of repeat units of monosaccharides or their derivatives held together by glycosidic bondsThey are primarily concerned with two important functions- structural and storage of energyThey are linear as well as branched polymers

The occurrence of branches in polysaccharide is due to the fact that the glycosidic linkages can be formed at any one of the hydroxyl groups of monosacchraides.

HomopolysaccharidesOn hydrolysis yield only a single type of monosacchraidee.g.Glucans are polymers of glucoseFructosans are polymers of fructose

Heteropolysaccharides On hydrolysis yield a mixture of few monosaccharides or their derivatives

HomopolysaccharidesStarch :It is the carbohydrate reserve of the plants which is the most important dietary source for higher animals High content is found in cereals, roots, tubers, vegetables It is a homopolymer composed of D-glucose units held together by -glycosidic bonds (glucans or glucosans)

It consists of two poysaccharides componentsWater soluble amylose (15-20%)Water insoluble amylopectin (80-85%)Chemically amylose is a long unbranched chain with 200-1000 glucose units held by (1-4) glycosidic linkagesAmylopectin is a branched chain with (1-6) glycosidic bonds at the branching points and (1-4) linkages everywhere else.

Starches are hydrolyzed by amylase to liberate dextrins and finally maltose and glucose unitsAmylase acts specifically on (1-4) glycosidic bondsDextrinsThe breakdown products of starch by the enzyme amylase or dilute acidsStarch is sequentially hydrolyzed through different dextrins and finally to maltose and glucose

DextransThey are polymers of glucose produced by micro-organisms. They are used as plasma volume expanders in transfusionInulinIs a polymer of fructose i.e. fructosanInulin is not utilized by the bodyUsed for accessing kidney functions through measurement of glomerular filtration rate (GFR)

Occurs in a garlic, union Easily soluble in waterGlycogenThe carbohydrate reserve in animals (animals starch)Present in high concentration in liver, followed by muscle, brain etc.Also found in plants that do not possess chlorophyll (yeast, fungi)

Structurally, glycogen is similar to that of amylopectin with more number of branchesGlucose is the repeating unit held together by (1-4) glycosidic bonds and (1-6) glycosidic bonds at branching pointsNumber of glucose units (upto 25000) vary in glycogen depending on the source from which glycogin is obtained

CelluloseOccurs exclusively in plantsPredominant constituent of plant cell-wallTotally absent in animal bodyComposed of -D-Glucose units linked by (1-4) glycosidic bondsCannot be digested by mammals due to lack of enzymes Certain ruminants and herbivores animals contain micro-organisms in the gut which enzymes which can cleave the bond

Hydrolysis of cellulose yields a disaccharide cellobiose, followed by -D-GlucoseCellulose is the major constituent of fiber, the non-digestable carbohydrateThe functions of dietary fibres include: Decreasing the absorption of glucose and cholesterol from the intestines besides increasing the bulk of feces.

ChitinComposed of N-acetyl D-glucosamine units held together by (1-4) glycosidic bondIt is a structural polysaccharide found in the exoskeleton of some invertebrates, for example, insects, crustaceans