Lehninger's Biochemistry Carbohydrates Chapter (PDF)

## Carbohydrates and Glycobiology ### 7.1 Monosaccharides and Disaccharides Carbohydrates are the most abundant biomolecules on Earth. Photosynthesis converts more than 100 billion metric tons of CO2 and H2O into cellulose and other plant products each year. Certain carbohydrates (sugar and starc...

## Carbohydrates and Glycobiology ### 7.1 Monosaccharides and Disaccharides Carbohydrates are the most abundant biomolecules on Earth. Photosynthesis converts more than 100 billion metric tons of CO2 and H2O into cellulose and other plant products each year. Certain carbohydrates (sugar and starch) are a dietary staple in most parts of the world and the oxidation of carbohydrates is the central energy-yielding pathway in most nonphotosynthetic cells. Carbohydrate polymers (also called glycans) 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. 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. This chapter introduces the major classes of carbohydrates and glycoconjugates and provides a few examples of their many structural and functional roles. Carbohydrates are polyhydroxy aldehydes or ketones, or substances that yield such compounds on hydrolysis. Many, but not all, carbohydrates have the empirical formula (CH2O)n; some also contain nitrogen, phosphorus, or sulfur. There are three major size classes of carbohydrates: * monosaccharides: simple sugars that consist of a single polyhydroxy aldehyde or ketone unit * oligosaccharides: consist of short chains of monosaccharide units, or residues, joined by characteristic linkages called glycosidic bonds * polysaccharides: sugar polymers containing more than 20 or so monosaccharide units; some have hundreds or thousands of units **Monosaccharides** The simplest of the carbohydrates, the monosaccharides, are either aldehydes or ketones with two or more hydroxyl groups; the six-carbon monosaccharides glucose and fructose have five hydroxyl groups. Many of the carbon atoms to which hydroxyl groups are attached are chiral centers, which give rise to the many sugar stereoisomers found in nature. **The Two Families of Monosaccharides Are Aldoses and Ketoses** Monosaccharides are colorless, crystalline solids that are freely soluble in water but insoluble in nonpolar solvents. Most have a sweet taste. The backbones of common monosaccharides 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 an end of the carbon chain (that is, 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 * dihydroxyacetone, a ketotriose Monosaccharides with four, five, six, and seven carbon atoms in their backbones are called tetroses, pentoses, hexoses, and heptoses, respectively. There are aldoses and ketoses of each of these chain lengths: aldotetroses and ketotetroses, aldopentoses and ketopentoses, and so on. The hexoses, which include the aldohexose D-glucose and the ketohexose D-fructose, are the most common monosaccharides in nature. The aldopentoses D-ribose and 2-deoxy-D-ribose are components of nucleotides and nucleic acids. **Monosaccharides Have Asymmetric Centers** All the monosaccharides except dihydroxyacetone contain one or more asymmetric (chiral) carbon atoms and thus occur in optically active isomeric forms. The simplest aldose, glyceraldehyde, contains one chiral center (the middle carbon atom) and therefore has two different optical isomers, or enantiomers. One of the two enantiomers is designated the D isomer, the other the L isomer. As for other biomolecules with chiral centers, the absolute configurations of sugars are known from x-ray crystallography. #### Nomenclature To represent three-dimensional sugar structures on paper, we often use Fischer projection formulas. In Fischer projection formulas, horizontal bonds project out of the plane of the paper, toward the reader; vertical bonds project behind the plane of the paper, away from the reader. In general, a molecule with *n* chiral centers can have 2*n* stereoisomers. Glyceraldehyde has 21 = 2; the aldohexoses, with four chiral centers, have 24 = 16 stereoisomers. The stereoisomers of monosaccharides of each carbon-chain length can be divided into two groups that differ in the configuration about the chiral center most distant from the carbonyl carbon. * Those in which the configuration at this reference carbon is the same as that of D-glyceraldehyde are designated D isomers, and those with the same configuration as L-glyceraldehyde are L isomers. * When the hydroxyl group on the reference carbon is on the right in a projection formula that has the carbonyl carbon at the top, the sugar is the D isomer; when on the left, it is the L isomer. Of the 16 possible aldohexoses, eight are D forms and eight are L. Most of the hexoses of living organisms are D isomers. **The Common Monosaccharides Have Cyclic Structures** For simplicity, we have thus far represented the structures of aldoses and ketoses as straight-chain molecules. In fact, in aqueous solution, aldotetroses and all monosaccharides with five or more carbon atoms in the backbone 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, which contain an additional asymmetric carbon atom and thus can exist in two stereoisomeric forms. For example, D-glucose exists in solution as an intramolecular hemiacetal in which the free hydroxyl group at C-5 has reacted with the aldehydic C-1, rendering the latter carbon asymmetric and producing two stereoisomers, designated a and β. The designation a indicates that the hydroxyl group at the anomeric center is, on the same side as the hydroxyl attached at the farthest chiral center, whereas ẞ indicates that these hydroxyl groups are on opposite sides. These six-membered ring compounds are called pyranoses because they resemble the six-membered ring compound pyran. The systematic names for the two ring forms of D-glucose are a-D-glucopyranose and β-D-glucopyranose. #### Isomers Aldoses also exist in cyclic forms having five-membered rings, which, because they resemble the five-membered ring compound furan, are called furanoses. However, the six-membered aldopyranose ring is much more stable than the aldofuranose ring and predominates in aldohexose and aldopentose solutions. Only aldoses having five or more carbon atoms can form pyranose rings. Isomeric forms of monosaccharides that differ only in their configuration about the hemiacetal or hemiketal carbon atom are called anomers. The hemiacetal (or carbonyl) carbon atom is called the anomeric carbon. The a and ẞ anomers of D-glucose interconvert in aqueous solution by a process called mutarotation. Thus, a solution of a-D-glucose and a solution of β-D-glucose eventually form identical equilibrium mixtures having identical optical properties. This mixture consists of about one-third a-D-glucose, two-thirds β-D-glucose, and very small amounts of the linear and five-membered ring (glucofuranose) forms. Ketohexoses also occur in a and ẞ anomeric forms. In these compounds the hydroxyl group at C-5 (or C-6) reacts with the keto group at C-2, forming a furanose (or pyranose) ring containing a hemiketal linkage. D-Fructose readily forms the furanose ring; the more common anomer of this sugar in combined forms or in derivatives is β-D-fructofuranose. Haworth perspective formulas like those in Figure 7-7 are used to show the stereochemistry of ring forms of monosaccharides. However, the six-membered pyranose ring is not planar, but tends to assume either of two "chair" conformations. To interconvert a and ẞ configurations, the bond involving the ring oxygen atom would have to be broken, but interconversion of the two chair forms does not require bond breakage. The specific three-dimensional structures of the monosaccharide units are important in determining the biological properties and functions of some polysaccharides, as we shall see. #### Derivatives In addition to simple hexoses such as glucose, galactose, and mannose, there are a number of sugar derivatives in which a hydroxyl group in the parent compound is replaced with another substituent, or a carbon atom is oxidized to a carboxyl group. * In glucosamine, galactosamine, and mannosamine, the hydroxyl at C-2 of the parent compound is replaced with an amino group. * The amino group is nearly always condensed with acetic acid, as in *N*-acetylglucosamine. This glucosamine derivative is part of many structural polymers, including those of the bacterial cell wall. * Bacterial cell walls also contain a derivative of glucosamine, *N*-acetylmuramic acid, in which lactic acid (a three-carbon carboxylic acid) is ether-linked to the oxygen at C-3 of *N*-acetylglucosamine. * The substitution of a hydrogen for the hydroxyl group at C-6 of L-galactose or L-mannose produces L-fucose or L-rhamnose, respectively. L-Fucose is found in the complex oligosaccharide components of glycoproteins and glycolipids; L-rhamnose is found in plant polysaccharides. Oxidation of the carbonyl (aldehyde) carbon of glucose to the carboxyl level produces gluconic acid; other aldoses yield other aldonic acids. Oxidation of the carbon at the other end of the carbon chain-C-6 of glucose, galactose, or mannose -forms the corresponding uronic acid: glucuronic, galacturonic, or mannuronic acid. Both aldonic and uronic acids form stable intramolecular esters called lactones. In addition to these acidic hexose derivatives, one nine-carbon acidic sugar deserves mention: *N*-acetylneuraminic acid (a sialic acid, but often referred to simply as "sialic acid"), a derivative of *N*-acetylmannosamine, is a component of many glycoproteins and glycolipids in animals. #### Reducing Sugars Monosaccharides can be oxidized by relatively mild oxidizing agents such as cupric (Cu²⁺) ion. The carbonyl carbon is oxidized to a carboxyl group. Glucose and other sugars capable of reducing cupric ion are called reducing sugars. They reduce cupric ion and are oxidized to aldonic acids. This 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. ### 7.2 Polysaccharides Most carbohydrates found in nature occur as polysaccharides, polymers of medium to high molecular weight. Polysaccharides, also called glycans, differ from each other in the identity of their recurring monosaccharide units, in the length of their chains, in the types of bonds linking the units, and in the degree of branching. * Homopolysaccharides contain only a single monomeric species; * heteropolysaccharides contain two or more different kinds. Some homopolysaccharides serve as storage forms of monosaccharides that are used as fuels; starch and glycogen are homopolysaccharides of this type. Other homopolysaccharides (cellulose and chitin, for example) serve as structural elements in plant cell walls and animal exoskeletons. Heteropolysaccharides provide extracellular support for organisms of all kingdoms. For example, the rigid layer of the bacterial cell envelope (the peptidoglycan) is composed in part of a heteropolysaccharide built from two alternating monosaccharide units. In animal tissues, the extracellular space is occupied by several types of heteropolysaccharides, which form a matrix that holds individual cells together and provides protection, shape, and support to cells, tissues, and organs. #### Storage Polysaccharides The most important storage polysaccharides are starch in plant cells and glycogen in animal cells. Both polysaccharides occur intracellularly as large clusters or granules. Starch and glycogen molecules are heavily hydrated, because they have many exposed hydroxyl groups available to hydrogen-bond with water. Most plant cells have the ability to form starch, and starch storage is especially abundant in tubers (underground stems), such as potatoes, and in seeds. Starch contains two types of glucose polymer, amylose and amylopectin. Amylose consists of long, unbranched chains of D-glucose residues connected by (a1→4) linkages. These chains vary in molecular weight from a few thousand to more than a million. Amylopectin also has a high molecular weight (up to 200 million) but unlike amylose is highly branched. The glycosidic linkages joining successive glucose residues in amylopectin chains are (a1-4); the branch points (occurring every 24 to 30 residues) are (a1→6) linkages. The branches have nonreducing ends, so a glycogen molecule with *n* branches has *n* + 1 nonreducing ends, but only one reducing end. This allows degradation enzymes that act only at nonreducing ends to work simultaneously on the many branches, speeding the conversion of the polymer to monosaccharides. Glycogen is the main storage polysaccharide of animal cells. Like amylopectin, glycogen is a polymer of (al4)-linked subunits of glucose, with (al→6)-linked branches, but glycogen is more extensively branched (on average, every 8 to 12 residues) and more compact than starch. Glycogen is especially abundant in the liver, where it may constitute as much as 7% of the wet weight; it is also present in skeletal muscle. In hepatocytes glycogen is found in large granules, which are themselves clusters of smaller granules composed of single, highly branched glycogen molecules with an average molecular weight of several million. Such glycogen granules also contain, in tightly bound form, the enzymes responsible for the synthesis and degradation of glycogen. #### Structural Polysaccharides Cellulose, is a fibrous, tough, water-insoluble substance that is found in the cell walls of plants, particularly in stalks, stems, trunks, and all the woody portions of the plant body. Cellulose constitutes much of the mass of wood, and cotton is almost pure cellulose. Like amylose, the cellulose molecule is a linear, unbranched homopolysaccharide, consisting of 10,000 to 15,000 D-glucose units. But there is a very important difference: in cellulose the glucose residues have the β configuration, whereas in amylose the glucose is in the a configuration. The glucose residues in cellulose are linked by (β1→4) glycosidic bonds, in contrast to the (a1→4) bonds of amylose. This difference gives cellulose and amylose very different structures and physical properties. Glycogen and starch ingested in the diet are hydrolyzed by α-amylases and glycosidases, enzymes in saliva and the intestine that break (a1→4) glycosidic bonds between glucose units. Most animals cannot use cellulose as a fuel source, because they lack an enzyme to hydrolyze the (β1→4) linkages. Termites readily digest cellulose (and therefore wood), but only because their intestinal tract harbors a symbiotic microorganism, *Trichonympha*, that secretes cellulase, which hydrolyzes the (β1→4) linkages. Wood-rot fungi and bacteria also produce cellulase. Chitin is a linear homopolysaccharide composed of *N*-acetylglucosamine residues in (β1→4) linkage. The only chemical difference from cellulose is the replacement of the hydroxyl group at C-2 with an acetylated amino group. Chitin forms extended fibers similar to those of cellulose, and like cellulose cannot be digested by vertebrates. Chitin is the principal component of the hard exoskeletons of nearly a million species of arthropods-insects, lobsters, and crabs, for example -and is probably the second most abundant polysaccharide, next to cellulose, in nature; an estimated 1 billion tons of chitin are produced each year in the biosphere. #### Factors Affecting Folding The folding of polysaccharides in three dimensions follows the same principles as those governing polypeptide structure: subunits with a more-or-less rigid structure dictated by covalent bonds form three-dimensional macromolecular structures that are stabilized by weak interactions within or between molecules: hydrogen bonds and hydrophobic and van der Waals interactions, and, for polymers with charged subunits, electrostatic interactions. Because polysaccharides have so many hydroxyl groups, hydrogen bonding has an especially important influence on their structure. Glycogen, starch, and cellulose are composed of pyranoside subunits (having six-membered rings), as are the oligosaccharides of glycoproteins and glycolipids to be discussed later. Such molecules can be represented as a series of rigid pyranose rings connected by an oxygen atom bridging two carbon atoms (the glycosidic bond). There is, in principle, free rotation about both C-O bonds linking the residues, but as in polypeptides, rotation about each bond is limited by steric hindrance by substituents. The three-dimensional structures of these molecules can be described in terms of the dihedral angles, φ and ψ, about the glycosidic bond, analogous to angles φ and ψ made by the peptide bond. The bulkiness of the pyranose ring and its substituents, and electronic effects at the anomeric carbon, place constraints on the angles φ and ψ; thus certain conformations are much more stable than others, as can be shown on a map of energy as a function of φ and ψ. The most stable three-dimensional structure for the (a1-4)-liked chains of starch and glycogen is a tightly coiled helix, stabilized by interchain hydrogen bonds. In amylose (with no branches) this structure is regular enough to allow crystallization and thus determination of the structure by x-ray diffraction. The average plane of each residue along the amylose chain forms a 60° angle with the average plane of the preceding residue, so the helical structure has six residues per turn. For amylose, the core of the helix is of precisely the right dimensions to accommodate iodine as complex ions (I3⁻ and I5⁻), giving an intensely blue complex. This interaction is a common qualitative test for amylose. For cellulose, the most stable conformation is that in which each chair is turned 180° relative to its neighbors, yielding a straight, extended chain. All -OH groups are available for hydrogen bonding with neighboring chains. With several chains lying side by side, a stabilizing network of interchain and intrachain hydrogen bonds produces straight, stable supramolecular fibers of great tensile strength. This property of cellulose has made it a useful substance to civilizations for millennia. Many manufactured products, including papyrus, paper, cardboard, rayon, insulating tiles, and a variety of other useful materials, are derived from cellulose. The water content of these materials is low because extensive interchain hydrogen bonding between cellulose molecules satisfies their capacity for hydrogen-bond formation. ### 7.3 Glycoconjugates: Proteoglycans, Glycoproteins, and Glycolipids In addition to their important roles as stored fuels (starch, glycogen, dextran) and as structural materials (cellulose, chitin, peptidoglycans), polysaccharides and oligosaccharides are information carriers. Some provide communication between cells and their extracellular surroundings; others label proteins for transport to and localization in specific organelles, or for destruction when the protein is malformed or superfluous; and others serve as recognition sites for extracellular signal molecules (growth factors, for example) or extracellular parasites (bacteria or viruses). On almost every eukaryotic cell, specific oligosaccharide chains attached to components of the plasma membrane form a carbohydrate layer (the glycocalyx), several nanometers thick, that serves as an information-rich surface that a cell shows to its surroundings. These oligosaccharides are central players in cell-cell recognition and adhesion, cell migration during development, blood clotting, the immune response, wound healing, and other cellular processes. In most of these cases, the informational carbohydrate is covalently joined to a protein or a lipid to form a glycoconjugate, which is the biologically active molecule. #### Proteoglycans Proteoglycans are macromolecules of the cell surface or extracellular matrix in which one or more sulfated glycosaminoglycan chains are joined covalently to a membrane protein or a secreted protein. The glycosaminoglycan chain can bind to extracellular proteins through electrostatic interactions with the negatively charged groups on the polysaccharide. Proteoglycans are major components of all extracellular matrices. #### Glycoproteins Glycoproteins have one or several oligosaccharides of varying complexity joined covalently to a protein. They are usually found on the outer face of the plasma membrane (as part of the glycocalyx), in the extracellular matrix, and in the blood. Inside cells they are found in specific organelles such as Golgi complexes, secretory granules, and lysosomes. The oligosaccharide portions of glycoproteins are very heterogeneous and, like glycosaminoglycans, they are rich in information, forming highly specific sites for recognition and high-affinity binding by carbohydrate-binding proteins called lectins. Some cytosolic and nuclear proteins can be glycosylated as well. #### Glycolipids Glycolipids are membrane sphingolipids in which the hydrophilic head groups are oligosaccharides. As in glycoproteins, the oligosaccharides act as specific sites for recognition by lectins. The brain and neurons are rich in glycolipids, which help in nerve conduction and myelin formation. Glycolipids also play a role in signal transduction in cells. **Proteoglycans Are Glycosaminoglycan-Containing Macromolecules of the Cell Surface and Extracellular Matrix** Mammalian cells can produce 40 types of proteoglycans. These molecules act as tissue organizers, and they influence various cellular activities, such as growth factor activation and adhesion. The basic proteoglycan unit consists of a "core protein" with covalently attached glycosaminoglycan(s). The point of attachment is a Ser residue, to which the glycosaminoglycan is joined through a tetrasaccharide bridge. The Ser residue is generally in the sequence -Ser-Gly-X-Gly- (where X is any amino acid residue), although not every protein with this sequence has an attached glycosaminoglycan. Many proteoglycans are secreted into the extracellular matrix, but some are integral membrane proteins. For example, the sheet-like extracellular matrix (basal lamina) that separates organized groups of cells from other groups contains a family of core proteins (M, 20,000 to 40,000), each with several covalently attached heparan sulfate chains. There are two major families of membrane heparan sulfate proteoglycans. Syndecans have a single transmembrane domain and an extracellular domain bearing three to five chains of heparan sulfate and, in some cases, chondroitin sulfate (Fig. 7-25a). Glypicans are attached to the membrane by a lipid anchor, a derivative of the membrane lipid phosphatidylinositol. Both syndecans and glypicans can be shed into the extracellular space. A protease in the ECM that cuts close to the membrane surface releases syndecan ectodomains (那些在质膜外部的域), and a phospholipase that breaks the connection to the membrane lipid releases glypicans. Numerous chondroitin sulfate and dermatan sulfate proteoglycans also exist, some as membrane-bound entities, others as secreted products in the ECM. The glycosaminoglycan chains can bind to a variety of extracellular ligands and thereby modulate the ligands' interaction with specific receptors of the cell surface. Detailed studies of heparan sulfate demonstrate a domain structure that is not random; some domains (typically 3 to 8 disaccharide units long) differ from neighboring domains in sequence and in ability to bind to specific proteins. Highly sulfated domains (called NS domains) alternate with domains having unmodified GlcNAc and GlcA residues (N-acetylated, or NA, domains) (Fig. 7-25b). The exact pattern of sulfation in the NS domain depends on the particular proteoglycan; given the number of possible modifications of the GlcNAc-IdoA dimer, at least 32 different disaccharide units are possible. Furthermore, the same core protein can display different heparan sulfate structures when synthesized in different cell types. The NS domains bind specifically to extracellular proteins and signaling molecules to alter their activities. The change in activity may result from a conformational change in the protein that is induced by the binding, or it may be due to the ability of adjacent domains of heparan sulfate to bind to two different proteins, bringing them into close proximity and enhancing protein-protein interactions. A third general mechanism of action is the binding of extracellular signal molecules (growth factors, for example) to heparan sulfate, which increases their local concentrations and enhances their interaction with growth factor receptors in the cell surface; in this case, the heparan sulfate acts as a coreceptor (Fig. 7-26c). For example, fibroblast growth factor (FGF), an extracellular protein signal that stimulates cell division, first binds to heparan sulfate moieties of syndecan molecules in the target cell's plasma membrane. Syndecan presents FGF to the FGF plasma membrane receptor, and only then can FGF interact productively with its receptor to trigger cell division. Finally, in another type of mechanism, the NS domains interact-electrostatically and otherwise-with a variety of soluble molecules outside the cell, maintaining high local concentrations at the cell surface (Fig. 7-26d). The importance of correctly synthesizing sulfated domains in heparan sulfate is demonstrated in mutant ("knockout") mice lacking the enzyme that sulfates the C-2 hydroxyl of IdoA. These animals are born without kidneys and with very severe developmental abnormalities of the skeleton and eyes. Other studies demonstrate that membrane proteoglycans are important in lipoprotein clearance in the liver. There is growing evidence that the path taken by developing axons in the nervous system, and thus the wiring circuitry, is influenced by proteoglycans containing heparan sulfates and chondroitin sulfate, which provide directional cues for axon outgrowth. Some proteoglycans can form proteoglycan aggregates, enormous supramolecular assemblies of many core proteins all bound to a single molecule of hyaluronan. Aggrecan core protein (M, ~250,000) has multiple chains of chondroitin sulfate and keratan sulfate, joined to Ser residues in the core protein through trisaccharide linkers, to give an aggrecan monomer of M~2×10^6. When a hundred or more of these "decorated" core proteins bind a single, extended molecule of hyaluronate (Fig. 7-27), the resulting proteoglycan aggregate (M > 2 × 10^8) and its associated water of hydration occupy a volume about equal to that of a bacterial cell. Aggrecan interacts strongly with collagen in the extracellular matrix of cartilage, contributing to the development, tensile strength, and resiliency of this connective tissue. Interwoven with these enormous extracellular proteoglycans are fibrous matrix proteins such as collagen, elastin, and fibronectin, forming a cross-linked meshwork that gives the whole extracellular matrix strength and resilience. Some of these proteins are multiadhesive, a single protein having binding sites for several different matrix molecules. Fibronectin, for example, has separate domains that bind fibrin, heparan sulfate, collagen, and a family of plasma membrane proteins called integrins that mediate signaling between the cell interior and the extracellular matrix. The overall picture of cell-matrix interactions that emerges (Fig. 7-28) shows an array of interactions between cellular and extracellular molecules. These interactions serve not merely to anchor cells to the extracellular matrix but also to provide paths that direct the migration of cells in developing tissue and to convey information in both directions across the plasma membrane. #### Glycoproteins Glycoproteins are carbohydrate-protein conjugates in which the glycans are smaller, branched, and more structurally diverse than the glycosaminoglycans of proteoglycans. The carbohydrate is attached at its anomeric carbon through a glycosidic link *O*-linked to the -OH of a Ser or Thr residue; or it may be *N*-linked to the amide nitrogen of an Asn residue (Figure 7-29). Some glycoproteins have a single oligosaccharide chain, but many have more than one; the carbohydrate may constitute from 1% to 70% or more of the glycoprotein by mass. Mucins are secreted or membrane glycoproteins that can contain large numbers of *O*-linked oligosaccharide chains. Mucins are present in most secretions; they give mucus its characteristic slipperiness. About half of all proteins of mammals are glycosylated, and about 1% of all mammalian genes encode enzymes involved in the synthesis and attachment of these oligosaccharide chains. Sequences for the attachment of *O*-linked chains tend to be rich in Gly, Val, and Pro residues. In contrast the attachment of *N*-linked chains depends on the consensus sequence *N*- (P) - [ST]. One class of glycoproteins found in the cytoplasm and the nucleus is unique in that the glycosylated positions in the protein carry oligosaccharide chains containing only single residues of *N*-acetylglucosamine, in *O*-glycosidic linkage to the hydroxyl group of Ser side chains. This modification is reversible and often occurs on the same Ser residues that are phosphorylated at some stage in the protein's activity. The two modifications are mutually exclusive, and this type of glycosylation may prove to be important in the regulation of protein activity. #### Glycolipids Glycoproteins are not the only cellular components that bear complex oligosaccharide chains; lipids also have covalently bound oligosaccharides. Gangliosides are membrane lipids of eukaryotic cells in which the polar head group, the part of the lipid that forms the outer surface of the membrane, is a complex oligosaccharide containing a sialic acid (Fig. 7-9) and other monosaccharide residues. Some of the oligosaccharide moieties of gangliosides, such as those that determine human blood groups, are identical with those found in certain glycoproteins, which therefore also contribute to blood group type. Like the oligosaccharide moieties of glycoproteins, those of membrane lipids are generally, perhaps always, found on the outer face of the plasma membrane. Lipopolysaccharides are the dominant surface feature of the outer membrane of gram-negative bacteria such as Escherichia coli and Salmonella typhimurium. These molecules are prime targets of the antibodies produced by the vertebrate immune system in response to bacterial infection and are therefore important determinants of the serotype of bacterial strains. ### 7.4 Carbohydrates as Informational Molecules: The Sugar Code Glycobiology, the study of the structure and function of glycoconjugates, is one of the most active and exciting areas of biochemistry and cell biology. As is becoming increasingly clear, cells use specific oligosaccharides to encode important information about intracellular targeting of proteins, cell-cell interactions, cell differentiation and tissue *development, and extracellular signals. Improved methods for the analysis of oligosaccharide and polysaccharide structure have revealed remarkable complexity and diversity in the oligosaccharides of glycoproteins and glycolipids. **Lectins Are Proteins That Read the Sugar Code and Mediate Many Biological Processes** * Lectins, found in all organisms, are proteins that bind carbohydrates with high specificity and affinity (see Table 7-3). * Lectins serve in a wide variety of cell-cell recognition, signaling, and adhesion processes and in intracellular targeting of newly synthesized proteins. * Plant lectins, abundant in seeds, probably serve as deterrents to insects and other predators. * In the laboratory, purified plant lectins are useful reagents for detecting and separating glycans and glycoproteins with different oligosaccharide moieties. #### Function of Lectins * Some peptide hormones that circulate in the blood have oligosaccharide moieties that strongly influence their circulatory half-life. * Luteinizing hormone and thyrotropin (polypeptide hormones produced in the adrenal cortex) have *N*-linked oligosaccharides that end with the disaccharide GalNAc4S(β1→4) GlcNAc, which is recognized by a lectin (receptor) of hepatocytes. * Receptor-hormone interaction mediates the uptake and destruction of luteinizing hormone and thyrotropin, reducing their concentration in the blood. The residues of Neu5Ac (a sialic acid) situated at the ends of the oligosaccharide chains of many plasma glycoproteins (Fig. 7-29) protect those proteins from uptake and degradation in the liver. For example, ceruloplasmin, a copper-containing serum glycoprotein, has several oligosaccharide chains ending in Neu5Ac. The mechanism that removes sialic acid residues from serum glycoproteins is unclear. It may be due to the activity of the enzyme sialidase (also called neuraminidase) produced by invading organisms or to a steady, slow release by extracellular enzymes. The plasma membrane of hepatocytes has lectin molecules (asialoglycoprotein receptors; "asialo-" indicating "without sialic acid") that specifically bind oligosaccharide chains with galactose residues no longer "protected" by a terminal Neu5Ac residue. A similar mechanism is apparently responsible for removing "old" erythrocytes from the mammalian bloodstream. Newly synthesized erythrocytes have several membrane glycoproteins with oligosaccharide chains that end in Neu5Ac. When the sialic acid residues are removed by withdrawing a sample of blood from experimental animals, treating it with sialidase in vitro, and reintroducing it into the circulation, the treated erythrocytes disappear from the bloodstream within a few hours; erythrocytes with intact oligosaccharides (withdrawn and reintroduced without sialidase treatment) continue to circulate for days. Cell surface lectins are important in the development of some human diseases-both human lectins and the lectins of infectious agents. Selectins are a family of plasma membrane lectins that mediate cell-cell recognition and adhesion in a wide range of cellular processes. One such process is the movement of immune cells (neutrophils) through the capillary wall, from blood to tissues, at sites of infection or inflammation (Fig. 7-31). At an infection site, P-selectin on the surface of capillary endothelial cells interacts with a specific oligosaccharide of the glycoproteins of circulating neutrophils. This interaction slows the neutrophils as they adhere to and roll along the endothelial lining of the capillaries. A second interaction, between integrin molecules (p. 455) in the neutrophil plasma membrane and an adhesion protein on the endothelial cell surface, now stops the neutrophil and allows it to move through the capillary wall into the infected tissues to initiate the immune attack. Two other selectins participate in this "lymphocyte homing": E-selectin on the endothelial cell and L-selectin on the neutrophil bind their cognate oligosaccharides on the neutrophil and endothelial cell, respectively. Human selectins mediate the inflammatory responses in rheumatoid arthritis, asthma, psoriasis, multiple sclerosis, and the rejection of transplanted organs, and thus there is great interest in developing drugs that inhibit selectin-mediated cell adhesion. Many carcinomas express an antigen normally present only in fetal cells (sialyl Lewis x, or sialyl Lex) that, when shed into the circulation, facilitates tumor cell survival and metastasis. Carbohydrate derivatives that mimic the sialyl Lex portion of sialoglycoproteins or that alter the biosynthesis of the oligosaccharide might prove effective as selectin-specific drugs for treating chronic inflammation or metastatic disease. Several animal viruses, including the influenza virus, attach to their host cells through interactions with oligosaccharides displayed on the host cell surface. The lectin of the influenza virus, known as the HA (hemagglutinin) protein, is essential for viral entry and infection. After the virus has entered a host cell and has been replicated, the newly synthesized viral particles bud out of the cell, wrapped in a portion of its plasma membrane. A viral sialidase (neuraminidase) trims the terminal sialic acid residue from the host cell's oligosaccharides, releasing the viral particles from their interaction with the cell and preventing their aggregation with one another. Another round of infection can now begin. The antiviral drugs oseltamivir (Tamiflu) and zanamivir (Relenza) are used clinically in the treatment of influenza. These drugs are sugar analogs; they inhibit the viral sialidase by competing with the host cell's oligosaccharides for binding. This prevents the release of viruses from the infected cell and also causes viral particles to aggregate, both of which block another cycle of infection. Lectins on the surface of the herpes simplex viruses HSV-1 and HSV-2 (the causative agents of oral and genital herpes, respectively) bind specifically to heparan sulfate on the host cell surface as a first step in the infection cycle; infection requires precisely the right pattern of sulfation on this polymer. Analogs of heparan sulfate that mimic its interaction with the viruses are being investigated as possible antiviral drugs, interfering with interactions between virus and cell. Some microbial pathogens have lectins that mediate bacterial adhesion to host cells or the entry of toxin into cells. For example, Helicobacter pylori--shown by Barry J. Marshall and J. Robin Warren in the 1980s to be responsible for most gastric ulcers--adheres to the inner surface of the stomach as bacterial membrane lectins interact with specific oligosaccharides of membrane glycoproteins of the gastric epithelial cells (Fig. 7-32). Among the binding sites recognized by H. pylori is the oligosaccharide Lewis b (Le), when it is part of the type O blood group determinant. This observation helps to explain the severalfold greater incidence of gastric ulcers in people of blood type O than in those of type A or B. Chemically synthesized analogs of the Le^b oligosaccharide may prove useful in treating this type of ulcer. Administered orally, they could prevent bacterial adhesion (and thus infection) by competing with the gastric glycoproteins for binding to the bacterial lectin. Some of the most devastating of the human parasitic diseases, widespread