Compounds. C A R B Y D R A T E S. DISACARIDES and oligosaccharides Aris Kaksis 0.year Riga Stradin s University http://aris.gusc.lv/utritionbiochem/carbohydratesdisacchari.pdf Most carbohydrates in nature contain more than one monosaccharide unit. Those that contain two units are called disaccharides, those that contain three units are called trisaccharides, and so on. The more general term oligosaccharide is often used for carbohydrates that contain from to 0 monosaccharides. Class called oligo (Greek oligos = few ) a few containing. Carbohydrates containing larger numbers of monosaccharides are called polysaccharides, In a disaccharide units are joined by a glycoside bond between the anomeric carbon of one unit an of the other. The important disaccharides are maltose, lactose and sucrose. The aldehyde groups of glucose, mannose and galactose are easily oxidized by copper in alkaline solution. These compounds are therefore designated as reducing sugars. The disaccharides lactose, maltose and isomaltose form aldehyde groups from hemiacetal groups (Figure 7.) that make them reducing sugars. Sucrose, in contrast, is not a reducing sugar. The hemiacetal group of glucose and hemiketal group of fructose form an acetal bond. The absence of a hemiacetal group makes sucrose a nonreducing sugar in alkaline cooper solution and this is a noteworthy property. Trehalose is also nonreducing sugar. A. Lactose Lactose is the principal sugar present in milk. It makes up about 8% of human milk and % of cow's milk. ydrolysis of lactose yields Dglucose and Dgalactose. In lactose, a unit of Dgalactopyranose is joined by a glycoside bond to carbon of Dglucopyranose. Lactose is reducing sugar.,glucoside bond lactose because this is DGalactose unit DGlucose unit DGalactose unit DGlucose unit lactose (from the milk of mammals) for connection Gal()Glc B. Maltose Maltose derives its name from its presence in malt liquors, the juice from sprouted barley, and other cereal grains. Maltose consists of two molecules of Dglucose joined by a glycoside bond between carbon (the anomeric carbon) of one glucose and carbon of the second glucose. Because the oxygen atom on the anomeric carbon of the first glucose unit is alpha, the bond joining the two glucose units is called an,glycoside bond. Following are aworth and chair formulas for maltose, so named because the on the anomeric carbon of the rightmost glucose unit is beta.,glucoside bond maltose because this is DGlucose unit DGlucose unit DGlucose unit DGlucose unit maltose (from the hydrolysis of starch) for connection Glc()Glc Maltose is a reducing sugar, because the anomeric carbon on the right unit of Dglucose is an equilibrium with the free aldehyde and can be oxidized to a carboxylic acid.
C. Isomaltose (isomaltose),glucoside bond,glucoside bond Dglucose unit Dglucose unit Dglucose unit Dglucose unit for connection Glc()Glc D. Sucrose Sucrose (table sugar) is the most abundant disaccharide in the biological world. It is obtained principally from the juice of sugar cane sugar beets. In sucrose, carbon of Dglucose is joined to carbon of Dfructose by an,glycoside bond. Glucose is in a sixmember (pyranose) ring form and fructose is in a fivemember (furanose) ring form. Because the anomeric hemiacetal carbons of both glucose and fructose are involved in formation of the glycoside bond, sucrose is a non reducing sugar. E. Trehalose Dglucose unit Dfructose unit Dglucose unit Dfructose unit sucrose (cane and beet sugar),glucoside bond,glucoside bond Chair structure and for connection Glc()Fruc Dglucose unit Dglucose unit Dglucose unit Dglucose unit for connection Glc()Glc Because the anomeric hemiacetal carbons of both glucose and glucose are involved in formation of the glycoside bond, trehalose is a non reducing sugar.
The Connective Tissue building Units Disaccharides The repeating units of glycosaminoglycans are attached to the core protein by : linked (the predominant form) and linked glycoside bonds. Chondroitin sulfates, heparan sulfate and dermatan sulfate are covalently linked to the core protein by a glycoside bond between xylose and protein serine (Ser). Some molecules of keratan sulfate are covalently linked to the core protein by an glycoside bond between acetylgalactosamine (GlcAc) and protein threonine (Thr). ther molecules of keratan sulfate are attached to the core protein by a glycosidic bond between GlcAc and protein asparagine (Asn C ). These linkage regions contain three or four carbohydrate residues to which the repeating units of disaccharides are attached. GlcUA, Dglucuronic acid; IdUA, Liduronic acid; Glc, Dglucosamine; Gal, Dgalactosamine; Glc, DGlucose; Gal, Dgalactose; Man, Dmannose; Man, Dmannosamine; Xyl, Dxylose; euac, acetyleuraminic acid. Chondroitin sulfate Chondroitin sulfate S C C S C C DGlucuronate unit DGlucuronate unit AcetylDGalactosamineSulfate AcetylDGalactosamineSulfate GlcUA()GalAc() GlcUA()GalAc() Sulfate Sulfate Keratan (Sulfate) ( S ) Dgalactose unit C AcetylDGlucosamine(Sulfate) Gal()GlcAc() (Sulfate) yaluronate polysaccharide unit C DGlucuronate unit C AcetylDGlucosamine GlcUA()GluAc()
eparin eparan sulfate C S S C S Sulfo LIduronate unit S DGlucuronate unit S Acetyl or SulfoDGlucosamine Acetyl or SulfoDGlucosamine ()IdUA()GlcS () ()GlcUA()GlcS () Sulfate Sulfate Sulfate S Dermatan sulfate C C S C LIduronate unit C DGlucuronate unit AcetylDgalactosaminesulfate AcetylDgalactosaminesulfate GlcUA()GalAc() IdUA()GalAc() Sulfate Sulfate Example I. Draw aworth and chair formulas for the alpha anomer of disaccharide in which two units of Dglucopyranose are joined by an,glycoside bond. Solution First draw the structural formula of Dglucopyranose. Then connect the anomeric carbon of this monosaccharide to carbon of second Dglucopyranose glucose unit by an glycoside bond. The resulting molecule is either alpha or beta, depending on the orientation of the group on the reducing end of the disaccharide.,glucoside bond,glucoside bond Problem I. Draw aworth and chair formulas for the beta anomer of a disaccharide in which two units of Dglucopyranose are joined by a,glycoside bond.
I. Polysaccharides Glycans A. Starch: Amylose and Amylopectine Starch is the reserve carbohydrate for plants. It is found in all plant seeds and tubers and is the form in which glucose is stored for later use by plants. Starch can be separated into two principal polysaccharides, amylose and amylopectin. While the starch from each plant is unique, most starches contain 9% amylose and 780% amylopectin. Complete hydrolysis of both amylose and amylopectin yields only Dglucose. XRay diffraction studies show that amylose is composed of continuous, unbranched chains of up to 000 Dglucose monomers joined by,glycoside bonds (Figure I.7). Amylopectin has a highly branched structure and contains two types of glycoside bonds. It contains the same type of chains of Dglucose joined by,glycoside bonds as amylose does, but chain lengths vary from only to 0 units (Figure I.8). In addition, there is considerable branching from this linear network. At branch points, new chains are started by,glycoside bonds between carbon of one glucose unit and carbon of another glucose unit. In fact, amylopectin has such a branched structure that it is hardly possible to distinguish between main chains and branch chains of up to > 000 000 Dglucose monomers joined by, and,glycoside bonds (Figure I.8). Why are carbohydrates stored in plants as polysaccharides rather than monosaccharides, a more directly usable form of energy? The answer has to do with osmotic pressure ( = C M R T), which is proportional to molar concentration C M, not the molecular weight of a solute. If we assume that 000 molecules of glucose are assembled in one starch macromolecule, then we can predict that a solution containing g of starch per 0 ml will have only /000 the osmotic pressure = /000 glucose of a solution of g of glucose in the same volume of solution. This feat of packaging is of tremendous advantage because it reduces the strain on various membranes enclosing such macromolecules. amylose (long unbranched chains of glucose units joined by,glycoside bonds) I I I I I I nm Figure I.7 Amylose is a linear polymer of up to 000 monomers of Dglucose joined by,glycoside bonds Iodine I (I I) molecules adsorb inside starch helix and make intensively blue color compound.,glucosidic linkages,glucosidic linkages Figure I.8 Amylopectin is a highly branched polymer with chains of 0 units of Dglucose joined by,glycoside bonds and branch points created by,glycoside bonds.
B. Glycogen Glycogen is the reserve carbohydrate for animals. Like amylopectin, glycogen is a nonlinear polymer of Dglucose joined by, and,glycoside bonds, but it has a lower molecular weight and an even more highly branched structure (Figure I.9). The total amount of glycogen in the body of a wellnourished adult is about 0 g, divided almost equally between liver muscle. UTER REGI protein inner region Glycogenine Figure I.9 Glycogen is a highly branched polymer of Dglucose joined by,glycoside bond. Branch points created by,glycoside bonds contain 08 units of glucose. Glucose transporters via cell membranes from blood into cell glucose uptake in cell by gradient External [Glc]= mm because of phosphorilation by hexokinases membrane channels membrane P Cytosole [Glc]=0.0 µm Glycogene store in liver cell cytosole [Glc]=0. M In uman body muscule and liver cells stored 0g glucose
C. Cellulose Cellulose, the most widely distributed skeletal polysaccharide, constitutes almost half the cell wall material of wood. Cotton is almost pure cellulose. Cellulose is a linear polymer of Dglucose monomers joined by,glycoside bonds (Figure I.0). It has an average molecular weight of 00 000 corresponding to approximately 800 glucose units. Cellulose fibers consist of bundles of parallel polysaccharide chains held together by hydrogen bonding between hydroxyl groups on adjacent chains. This arrangement of parallel chains in bundles and the resulting hydrogen bonding gives cellulose fibers their high mechanical strength. umans and other animals cannot use cellulose as a food. The reason is the our digestive system do not contain glycosidases enzymes that catalyze hydrolysis of glycoside bonds. They contain only glycosidases, hence, the polysaccharides we use as sources of glucose are starch and glycogen. n the other hand, many bacteria and microorganisms do contain glycosidases and can digest cellulose. Termites are fortunate in having such bacteria in their intestines and can use wood as their principal food. Ruminants (cudchewing animals) can also digest grasses and wood because glycosidasecontaining microorganisms are present in their alimentary system.,glucoside linkages cellulose chain Figure I.0 Cellulose is a linear polymer of up to 000 units of Dglucose joined by,glycoside bonds. Chitin triose Dglucose unit Cellobiose Dglucose unit for connection Glc()Glc acetyldglucosamine, derivative of Dglucosamine, is a component of many polysaccharides, including chitin, the hard shelllike exoskeleton of lobsters, crabs, crayfish, shrimp and crustaceans. acetyldglucosamine unit acetyldglucosamine unit acetyldglucosamine unit for connection GlcA()GlcA()GlcA 7