University of Guyana Biochemistry II - Lecture 2: Carbohydrates and Glycobiology PDF

UNIVERSITY OF GUYANA FACULTY OF NATURAL SCIENCES DEPARTMENT OF BIOLOGY BIO 3110 BIOCHEMISTRY II – Intermediary Metabolism LECTURE 2: CARBOHYDRATES AND GLYCOBIOLOGY  Insoluble carbohydrate polymers serve as structural and protective elements in the cell w...

UNIVERSITY OF GUYANA FACULTY OF NATURAL SCIENCES DEPARTMENT OF BIOLOGY BIO 3110 BIOCHEMISTRY II – Intermediary Metabolism LECTURE 2: CARBOHYDRATES AND GLYCOBIOLOGY  Insoluble carbohydrate polymers serve as structural and protective elements in the cell walls of bacteria and plants and in the connective tissues of animals. Other carbohydrate polymers lubricate skeletal joints and participate in recognition and adhesion between cells. More complex carbohydrate polymers covalently attached to proteins or lipids act as signals that determine the intracellular location or metabolic fate of these hybrid molecules, called glycoconjugates. Aldoses and Ketoses – the two families of monosaccharides SweetTooth? Monosaccharides are colourless, crystalline solids that are freely soluble in water but insoluble in nonpolar solvents. Most have a sweet taste. The backbones of common monosaccharide molecules are unbranched carbon chains in which all the carbon atoms are linked by single bonds. In the open-chain form, one of the carbon atoms is double-bonded to an oxygen atom to form a carbonyl group; each of the other carbon atoms has a hydroxyl group. If the carbonyl group is at the end of the carbon chain (i.e., in an aldehyde group) the monosaccharide is an aldose; if the carbonyl group is at any other position (in a ketone group) the monosaccharide is a ketose. The simplest monosaccharides are the two three-carbon trioses: glyceraldehyde, an aldotriose, and dihydroxyacetone, a ketotriose Sterioisomers Enantiomers  In Fischer projection formulas, horizontal bonds proje ct out of the plane of the paper,toward the reader, vertical bonds project to the back of the paper The stereoisomers are mirror images of each other. Perspective formulas,solid wedge-shaped Ball and stick models show the actual configuration of molecules bonds point toward the reader, dashed wedges point away from the reader. Epimers Structures to know  epimers epimers epimers epimers Common Monosaccharides have cyclic structures 5 Hemiacetals & Hemiketals Cyclization of Monosaccharides Hydroxymethyl Pyranoses & Furanoses  Monosaccharides Are Reducing Agents Monosaccharides can be oxidized by relatively mild oxidizing agents such as ferric (Fe3) or cupric (Cu2) ion. The carbonyl carbon is oxidized to a carboxyl group. Glucose and other sugars capable of reducing ferric or cupric ion are called reducing sugars. This property is the basis of Fehling’s reaction, a qualitative test for the presence of reducing sugar. By measuring the amount of oxidizing agent reduced by a solution of a sugar, it is also possible to estimate the concentration of that sugar. For many years this test was used to detect and measure elevated glucose levels in blood and urine in the diagnosis of diabetes mellitus. Now, more sensitive methods for measuring blood glucose employ an enzyme, glucose oxidase. Disaccharides Contain a Glycosidic Bond C1 C4 Maltose: a reducing sugar Reducing end Non-reducing Sugars Sucrose is a disaccharide of glucose and fructose. It is formed by plants but not by animals. In contrast to maltose and lactose, sucrose contains no free anomeric ca rbon atom; the anomeric carbons of both monosaccharide units are involved in the glycosidic bond. Sucrose is therefore a non- reducing sugar, and its stability—its resistance to oxidation—makes it a suitable molecule for the storage and transport of energy in plants Polysaccharides Starch and Glycogen: Homopolysaccharides Metabolism of Glycogen and Starch  amylose Amylopectin Starch or glycogen Mixture of Amylose and Amylopectin in Starch  Because each branch in glycogen ends with a non-reducing sugar unit, a glycogen molecule has as many non-reducing ends as it has branches, but only one reducing end. When glycogen is used as an energy source, glucose units are removed one at a time from the non-reducing ends. Degradative enzymes that act only at non-reducing ends can work simultaneously on the many branches, speeding the conversion of the polymer to monosaccharides Acetylated amino group END OF LECTURE 2