Carbohydrates and Glycobiology

Representative Monosaccharides

Most of the hexoses of living organisms are D isomers

Mannose and Galactose are epimers of Glucose

Structures of the D-Aldoses

Structures of the D-Ketoses

Formation of Hemiacetals and Hemiketals Aldotetroses and all monosaccharides with five or more carbon atoms occur predominantly as cyclic ring structures in which the carbonyl group has formed a covalent bond with the oxygen of a hydroxyl group along the chain. The formation of these ring structures is the result of a general reaction between alcohols and aldehydes or ketones to form derivatives called hemiacetals or hemiketals (Fig. 7-5). Actually, two molecules of an alcohol can add to a carbonyl carbon. The product of the first reaction for an aldose is a hemiacetal, and the product of the first reaction for a ketose is a hemiketal. If the -OH and carbonyl groups are from the same molecule, a five- or six-membered ring results. The addition of the second alcohol molecule produces the full acetal or ketal, and the bond formed is a glycosidic linkage. When the two reacting molecules are both monosaccharides, the acetal or ketal produced is a disaccharide.

Formation of the two cyclic forms of D-glucose In aqueous solution monosaccharides with five or more carbons occur as cyclic structures. Two stereoisomers are produced and called alpha (a) and beta (b).

Disaccharides contain a glycosidic bond

Common Disaccharides

Starch Structure Amylopectin and Glycogen

Cellulose

French Fries versus Fiber Humans digest starch and glycogen ingested in their diet using α-amylases, enzymes that hydrolyze (α 14) glycosidic bonds. Humans cannot hydrolyze (β14) linkages of cellulose. Therefore cellulose is not a fuel source for humans. It is fiber. Certain microorganisms have cellulases, enzymes that hydrolyze (β 14) linkages of cellulose. Cattle have these organisms in their rumen. Termites have them in their intestinal tract.

Chitin is the principal component of the exoskeleton of arthropods like insects, lobsters, and crabs Linear homopolysaccharide of N-acetylglucosamine in β(14) linkage

Agarose is a heteropolysaccharide found in the cell walls of some seaweed

Cellular location of carbohydrates

Proteoglycan aggregate of the extracellular matrix

Lipopolysaccharide of the outer membrane of the bacterium Salmonella typhimurium

Role of lectin-ligand interaction in leukocyte movement to the site of an infection or injury