Abdullah zurayqat. Bahaa Najjar. Mamoun Ahram

Transcription

1 9 Abdullah zurayqat Bahaa Najjar Mamoun Ahram

2 Polysaccharides Polysaccharides Definition and Structure [Greek poly = many; sacchar = sugar] are complex carbohydrates, composed of 10 to up to several thousand monosaccharides arranged in chains. The most common monosaccharides that appear as parts of polysaccharides are glucose, fructose, galactose and mannose. When all the monosaccharides in a polysaccharide are the same type, the polysaccharide is called a homopolysaccharide or homoglycan, but when more than one type of monosaccharide is present they are called heteropolysaccharides or heteroglycans *let us continue with another sugar: dextran * A storage polysaccharide *Yeast and bacteria alpha -(1-6)-D-glucose with branched chains *Branches: 1-2, 1-3, or 1-4 *dextran work as meshwork, because it's monomers are large with branches * bacteria Use dextran for nutrition and cause Dental Caries Because it isolates teeth From washing 1 P a g e

3 *Cellulose(Fibers) "homo polymer" Cellulose is an organic compound. A polysaccharide consisting of a linear chain of several hundred to many thousands of β (1 4) linked D-glucose units only. *Some animals, particularly ruminants and termites, can digest cellulose with the help of symbiotic micro-organisms that live in their guts. In human nutrition, cellulose acts as a hydrophilic bulking agent for feces and is often referred to as dietary fiber. *note: Humans cannot digest this sugar Because they don't have cellulose enzyme. Important note: when human eats cellulose bind to toxic substance in foods, they get exported from the body, thus preventing them damaging the lower intestine or being reabsorbed there. example: 1- when bind to carcinogens, thus prevent cancer. 2- when bind to cholesterol resulting in less amount in the blood. Note: cellulose is rigid and straight. *pectin (from plants cell wall):** _Forms by D-galacturonic acids " galacturonic acid is a galactose derivative " Alpha *chitin (Found in Exoskeleton in insects): *Structure of the chitin molecule, showing two of the N-acetylglucosamine units that repeat to form long chains in β- (1 4)-linkage Note: repeating in disaccharide in chain Note: chitin is strength because of hydrogen bonds between strands. 2 P a g e

4 Note: glycogen, cellulose, pectin and chitin are not reducing molecule. Another point about glycosides is worth mentioning. We have already seen that the anomeric carbon is frequently involved in the Glycosidic linkage, and also that the test for the presence of sugars specifically for reducing sugars requires a reaction of the group at the anomeric carbon. The internal anomeric carbons in oligosaccharides are not free to give the test for reducing sugars. Only if the end residue is a free hemiacetal rather than a glycoside will there be a positive test for a reducing. The level of detection can be important for such a test. A sample that contains only a few molecules of a large polysaccharide, each molecule with a single reducing end, might well produce a negative test because there are not enough reducing ends to detect. Only 1 carbon can Play as reducing sugar but this carbon makes branches, not 4carbon *Glycosaminoglycan(GAG'S): Feature: They contain amino group " amino sugars" Made of repeated of disaccharide These are negatively charged molecule Some of these have sulfate group or carboxylic group Found in outside of the cells *Glycosaminoglycans (GAGs) or mucopolysaccharides Are long unbranched polysaccharides consisting of a repeating disaccharide unit *. The repeating unit (except for keratin) consists of an amino sugar (Nacetylglucosamine or N-acetylgalactosamine) along with a -uronic sugar (glucuronic acid or iduronic acid) or galactose.*glycosaminoglycans are highly polar and attract water *. They are therefore useful to the body as a lubricant or as a shock absorber. 3 P a g e

5 *proteoglycans: Functions 1)Lubricants 2)Structural components in connective tissue 3)Mediate adhesion of cells to the extracellular matrix 4) Bind factors that stimulate cell proliferation 4 P a g e

6 *Glycoproteins 1)The carbohydrates of glycoproteins are linked to the protein component through either O-Glycosidic or N-Glycosidic bonds 2)The N-Glycosidic linkage is through the amide group of asparagine (Asn, N) 3)The O-Glycosidic linkage is to the hydroxyl of serine (Ser, S), threonine (Thr, T) or hydroxylysine (hlys) **Glycoprotein contain carbohydrate residues in addition to the polypeptide chain. Some of the most important examples of glycoproteins are involved in the immune response; for example, antibodies, bacterial cell wall, which bind to and immobilize antigens (the substances attacking the organism), are glycoproteins. Carbohydrates also play an important role as antigenic determinants, the portions of an antigenic molecule that antibodies recognize and to which they bind. O_Glycosidic or N Glycoside depend on nature of amino acid (where proteins attach with some of sugars) 5 P a g e

7 *Peptidoglycans: Significance of protein-linked sugars **Soluble proteins as well as membrane proteins **Purpose: Protein folding Protein targeting prolonging protein half-life Cell-cell communication Signaling Blood typing Three different structures: A, B, and O The difference: N-acetylgalactosamine (for A) Galactose (for B) None for (O) 6 P a g e

8 Sialic acid 1)Called: N-acetylneuraminate 2)Precursor: the amino sugar, neuraminic acid 3)Location: a terminal residue of oligosaccharide chains of glycoproteins and n glycolipids. **Sialic acid is a generic term for the N- or O-substituted derivatives of neuraminic acid, a monosaccharide with a nine-carbon backbone. *Sialic acids are found widely distributed in animal tissues and to a lesser extent in other organisms, ranging from plants and fungi to yeasts and bacteria, mostly in glycoproteins and gangliosides (they occur at the end of sugar chains connected to the surfaces of cells and soluble proteins). *Importance Of cellulose: 1)helps move the digested food through the intestines, decreasing the transit time through the gut. 2)Potentially toxic substances in food and in bile fluid bind to fiber and are exported from the body, thus preventing them from damaging the lower intestine or being reabsorbed there. 7 P a g e

9 3)People on high-fiber diets also tend to take in less fat and fewer calories. 4) There has been much publicity about fiber in the diet reducing cholesterol. Fiber does bind cholesterol, and it certainly causes some decrease in the amount in the blood. The reduction, expressed as a percentage, is higher in cases in which the original level of cholesterol is higher. There is, however, no definitive evidence that lowering cholesterol via the ingestion of fiber results in less heart disease 8 P a g e