˜ AND ° LINKS The hydroxyl group on the carbon that carries the aldehyde or ketone can rapidly change from one position to the other. These two positions are called ˜ and °. As soon as one sugar is linked to another, the ˜ or ° form is frozen. OH O OH O ° hydroxyl ˜ hydroxyl CH2OH NH2 H O OH OH HO glucosamine CH2OH O OH OH HO CH3 O NH C H N-acetylglucosamine C O OH OH OH HO OH glucuronic acid O CH2OH HO O OH OH CH2OH OH HOCH2 HO CH2OH HO O OH OH OH CH2OH OH HOCH2 HO H O + sucrose ˜glucose °fructoseDISACCHARIDES The carbon that carries the aldehyde or the ketone can react with any hydroxyl group on a second sugar molecule to form a disaccharide. The linkage is called a glycosidic bond. Three common disaccharides are maltose (glucose + glucose) lactose (galactose + glucose) sucrose (glucose + fructose) The reaction forming sucrose is shown here. H2O O O O OLIGOSACCHARIDES AND POLYSACCHARIDES Large linear and branched molecules can be made from simple repeating sugar subunits. Short chains are called oligosaccharides, while long chains are called polysaccharides. Glycogen, for example, is a polysaccharide made entirely of glucose units joined together. branch points glycogen CH2OH NH O CH2OH O CH2OH O OH O HO OH HO O OH NH O O HO CH3 O In many cases a sugar sequence is nonrepetitive. Many different molecules are possible. Such complex oligosaccharides are usually linked to proteins or to lipids, as is this oligosaccharide, which is part of a cell-surface molecule that defnes a particular blood group. COMPLEX OLIGOSACCHARIDES C O CH3 C O CH3 SUGAR DERIVATIVES The hydroxyl groups of a simple monosaccharide such as glucose can be replaced by other groups. For example, 97