11.1 Monosaccharides Are the Simplest Carbohydrates

Carbohydrates are aldoses or ketoses that are rich in hydroxyl groups. An aldose is a carbohydrate with an aldehyde group (as in glyceraldehyde and glucose), whereas a ketose contains a keto group (as in dihydroxyacetone and fructose). A sugar belongs to the d series if the absolute configuration of its asymmetric carbon atom farthest from the aldehyde or keto group is the same as that of d-glyceraldehyde. Most naturally occurring sugars belong to the d series. The C-1 aldehyde in the open-chain form of glucose reacts with the C-5 hydroxyl group to form a six-membered pyranose ring. The C-2 keto group in the open-chain form of fructose reacts with the C-5 hydroxyl group to form a five-membered furanose ring. Pentoses such as ribose and deoxyribose also form furanose rings. An additional asymmetric center is formed at the anomeric carbon atom (C-1 in aldoses and C-2 in ketoses) in these cyclizations. The hydroxyl group attached to the anomeric carbon atom is on the opposite side of the ring from the CH₂OH group attached to the chiral center in the α anomer, whereas it is on the same side of the ring as the CH₂OH group in the β anomer. Not all atoms in the ring lie in the same plane. Rather, pyranose rings usually adopt the chair conformation, and furanose rings usually adopt the envelope conformation. Sugars are joined to alcohols and amines by glycosidic bonds from the anomeric carbon atom. For example, N-glycosidic bonds link sugars to purines and pyrimidines in nucleotides, RNA, and DNA.

11.2 Monosaccharides Are Linked to Form Complex Carbohydrates

Sugars are linked to one another in disaccharides and polysaccharides by O-glycosidic bonds. Sucrose, lactose, and maltose are the common disaccharides. Sucrose (common table sugar) consists of α-glucose and β-fructose joined by a glycosidic linkage between their anomeric carbon atoms. Lactose (in milk) consists of galactose joined to glucose by a β-1,4 linkage. Maltose (from starch) consists of two glucoses joined by an α -1,4 linkage. Starch is a polymeric form of glucose in plants, and glycogen serves a similar role in animals. Most of the glucose units in starch and glycogen are in α-1,4 linkage. Cellulose, the major structural polymer of plant cell walls, consists of glucose units joined by β-1,4 linkages. These β linkages give rise to long straight chains that form fibrils with high tensile strength. In contrast, the αlinkages in starch and glycogen lead to open helices, in keeping with their roles as mobilizable energy stores.

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11.3 Carbohydrates Can Be Linked to Proteins to Form Glycoproteins

Carbohydrates are commonly conjugated to proteins. If the protein component is predominant, the conjugate of protein and carbohydrate is called a glycoprotein. Most secreted proteins, such as the signal molecule erythropoietin, are glycoproteins. Glycoproteins are also prominent on the external surface of the plasma membrane. Proteins bearing covalently linked glycosaminoglycans are proteoglycans. Glycosaminoglycans are polymers of repeating disaccharides. One of the units in each repeat is a derivative of glucosamine or galactosamine. These highly anionic carbohydrates have a high density of carboxylate or sulfate groups. Proteoglycans are found in the extracellular matrices of animals and are key components of cartilage. Mucoproteins, like proteoglycans, are predominantly carbohydrate by weight. The protein component is heavily O-glycosylated with N-acetylgalactosamine joining the oligosaccharide to the protein. Mucoproteins serve as lubricants.

Glycosyltransferases link the oligosaccharide units on proteins either to the side-chain oxygen atom of a serine or threonine residue or to the side-chain amide nitrogen atom of an asparagine residue. Protein glycosylation takes place in the lumen of the endoplasmic reticulum. The N -linked oligosaccharides are synthesized on dolichol phosphate and subsequently transferred to the protein acceptor. Additional sugars are attached in the Golgi complex to form diverse patterns.

11.4 Lectins Are Specific Carbohydrate-Binding Proteins

Carbohydrates on cell surfaces are recognized by proteins called lectins. In animals, the interplay of lectins and their sugar targets guides cell–cell contact. The viral protein hemagglutinin on the surface of the influenza virus recognizes sialic acid residues on the surfaces of cells invaded by the virus.