Week 6-Carbohydrates and Glycobiology PDF

Carbohydrates • 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. • The carbohydrates in these plant products are a dietary staple in most parts of the world, and • The oxidat...

Carbohydrates • 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. • The carbohydrates in these plant products are a dietary staple in most parts of the world, and • The oxidation of carbohydrates is the central energy-yielding pathway in most non-photosynthetic cells. CARBOHYDRATES AND GLYCOBIOLOGY • Carbohydrate polymers called glycans serve as structural and protective elements in the cell walls of bacteria, fungi, and plants and in the connective tissues of animals. • Other carbohydrate polymers lubricate skeletal joints and participate in cell-cell recognition and adhesion. • Some complex carbohydrate polymers covalently attached to proteins or lipids act as signals that determine the intracellular destination or metabolic fate of these hybrid molecules, called glycoconjugates. Assist. Prof. Dr. Derya DİLEK KANÇAĞI Room Number: 511 E-mail: [email protected] Office Hour: Wednesday 13.00-15.00 Carbohydrates Central Principles in Carbohydrates and Glycobiology • Carbohydrates = aldehydes or ketones with at least two hydroxyl groups, or substances that yield such compounds on hydrolysis • Many carbohydrates have the empirical formula (CH2O)n →some also contain nitrogen, phosphorus, or sulfur. Classes of Carbohydrates *Monosaccharides = simple sugars, consist of a single polyhydroxy aldehyde or ketone unit Example: The most abundant monosaccharide in nature is the six-carbon sugar D-glucose, sometimes referred to as dextrose. *Oligosaccharides = short chains of monosaccharide units, or residues, joined by glycosidic bonds *Disaccharides = oligosaccharides with two monosaccharide units Example: Sucrose (table sugar), for example, consists of the six-carbon sugars D-glucose and D-fructose. In cells, most oligosaccharides consisting of three or more units do not occur as free entities but are joined to nonsugar molecules (lipids or proteins) in glycoconjugates. Principle 1 Principle 2 • Carbohydrates can have multiple chiral carbons; the configuration of groups around each carbon atom determines how the compound interacts with other biomolecules. • Monomeric subunits, monosaccharides, serve as the building blocks of large carbohydrate polymers. • As we saw for L-amino acids in proteins, with rare exceptions, biological evolution selected one stereochemical series (D-series) for sugars. • The specific sugar, the way the units are linked, and whether the polymer is branched determine its properties and thus its function. *Polysaccharides = sugar polymers with 10+ monosaccharide units Examples: cellulose (linear), glycogen (branched) → Both cellulose and glycogen consist of recurring units of D-glucose, but they differ in the type of glycosidic linkage and consequently have strikingly different properties and biological roles. Central Principles in Carbohydrates and Glycobiology Central Principles in Carbohydrates and Glycobiology Principle 3 Principle 4 Principle 5 Principle 6 • Storage of low molecular weight metabolites in polymeric form avoids the very high osmolarity that would result from storing them as individual monomers. • The sequences of complex polysaccharides are determined by the intrinsic properties of the biosynthetic enzymes that add each monomeric unit to the growing polymer. • Polysaccharides assume three-dimensional structures with the lowest-energy conformations, determined by covalent bonds, hydrogen bonds, charge interactions, and steric factors. • Molecular complementarity is central to function. • If the glucose in liver glycogen were monomeric, the glucose concentration in liver would be so high that cells would swell and lyse from the entry of water by osmosis. • This is in contrast with DNA, RNA, and proteins, which are synthesized on templates that direct their sequence. • The recognition of oligosaccharides by sugarbinding proteins (lectins) results from a perfect fit between lectin and ligand. • Starch folds into a helical structure stabilized by internal hydrogen bonds; cellulose assumes an extended structure in which intermolecular hydrogen bonds are more important. Central Principles in Carbohydrates and Glycobiology Principle 7 • An almost infinite variety of discrete structures can be built from a small number of monomeric subunits. • Even short polymers, when arranged in different sequences, joined through different linkages, and branched to specific degrees, present unique faces recognized by their molecular partners. Monosaccharides and Disaccharides Stereoisomerism in Sugars The Two Families of Monosaccharides Are Aldoses and Ketoses • 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. • Monosaccharides are colorless, crystalline solids that are freely soluble in water but insoluble in nonpolar solvents. Most have a sweet taste. • Sugar stereoisomers arise because many of the carbon atoms to which the hydroxyl groups are attached are chiral centers • Enzymes that act on sugars are stereospecific • Backbones of monosaccharides: • Unbranched carbon chains with single bonds linking all carbon atoms • One of the carbon atoms is double-bonded to an oxygen atom to form a carbonyl group • Other carbon atoms are bonded to a hydroxyl group • Aldose = carbonyl group is at an end of the carbon chain (in an aldehyde group) The addition of a hydroxyl group from within the same molecule, generates cyclic forms with four or more backbone carbons (the forms that predominate in aqueous solution). This ring closure creates a new chiral center, adding further stereochemical complexity to this class of compounds. • Ketose = carbonyl group is at any other position (in a ketone group) Carbon backbone… Carbon backbone… Trioses Tetroses and Pentoses Hexoses and Heptoses • Trioses = simplest monosaccharides, three carbon backbone • Tetroses = four carbon backbone • Hexoses = six carbon backbone • Pentoses = five carbon backbone • Heptoses = seven carbon backbone What Makes Sugar Sweet? Monosaccharides Have Asymmetric Centers • TAS1R2 and TAS1R3 encode sweet-taste receptors • All monosaccharides (except dihydroxyacetone) contain 1+ chiral carbon atom • Binding of a compatible molecule generates a “sweet” electrical signal in the brain • Enantiomers = two different optical isomers that are mirror images • Requires a steric match • 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 • In general, a molecule with n chiral centers can have 2n stereoisomers Fischer Projection Formulas D Isomers and L Isomers • Used to represent three-dimensional sugar structures on paper • Reference carbon = chiral center most distant from the carbonyl carbon • Two groups of stereoisomers: • Bonds drawn horizontally indicate bonds that project out of the plane of the paper • D isomers = configuration at reference carbon is the same as D-glyceraldehyde • Bonds drawn vertically project behind the plane of the paper • L isomers = configuration at reference carbon is the same as L-glyceraldehyde • On the right (dextro) in a projection formula • Most hexoses of living organisms • On the left (levo) in a projection formula Why D isomers? The basis for this initial preference for one isomer during evolution is unknown; however, once one isomer became prevalent, evolving enzymes able to use that isomer efficiently would have a selective advantage. Numbering Carbons of a Sugar D-Aldoses • Carbons are numbered beginning at the end of the chain near the carbonyl group *One of the carbon atoms is double-bonded to an oxygen atom to form a carbonyl group D-Ketoses Epimers • Epimers = two sugars that differ only in the configuration around one carbon atom The Common Monosaccharides Have Cyclic Structures Hemiacetals and Hemiketals • In aqueous solution, aldotetroses and all monosaccharides with 5+ backbone carbon atoms occur as cyclic structures • Hemiacetals or hemiketals = derivatives formed by a general reaction between alcohols and aldehydes or ketones • in which the carbonyl group has formed a covalent bond with the oxygen of a hydroxyl group in the same sugar molecule. • 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. • Product of the first alcohol molecule addition • A five- or six-membered ring forms if the — OH and carbonyl groups are on the same molecule • Acetal or ketal = product of the second alcohol molecule addition • Forms a glycosidic bond α and β Stereoisomeric Configurations • Reaction with the first alcohol molecule creates an additional chiral center (the carbonyl carbon) • Produces either of two stereoisomeric configurations: α and β • Anomers = isomeric forms of monosaccharides that differ only in their configuration about the hemiacetal or hemiketal carbon atom • Anomeric carbon = the carbonyl carbon atom Formation of the Two Cyclic Forms of D-Glucose • Reaction between the aldehyde group at C-1 and the hydroxyl group at C-5 forms a hemiacetal linkage • Mutarotation = the interconversion of α and β anomers in aqueous solution in which one ring form (say, the α anomer) opens briefly into the linear form, then closes again to produce the β anomer. Pyranoses and Furanoses Haworth Perspective Formulas • Pyranoses = six-membered ring compounds • Haworth perspective formulas = more accurate representation of cyclic sugar structure than Fischer projections • Form when the hydroxyl group at C-6 reacts with the keto group at C-2 • Furanoses = five-membered ring compounds • Form when the hydroxyl group at C-5 reacts with the keto group at C-2 • Ring containing a hemiketal linkage Conformational Formulas of Pyranoses • Six-membered ring is tilted to make its plane almost perpendicular to that of the paper • Bonds closest to the reader are drawn thicker than those farther away Organisms Contain a Variety of Hexose Derivatives There are many 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. • Pyranose rings tend to assume either of two “chair” conformations • Interconvertible without breaking covalent bonds • Requires energy input Aldonic and Uronic Acids Phosphorylated Derivatives • Oxidation of the carbonyl (aldehyde) carbon of glucose to the carboxyl level produces gluconic acid, used in medicine as an innocuous counterion when administering positively charged drugs (such as quinine) or ions (such as Ca2+). • Some sugar intermediates are phosphate esters • Aldonic acids = form following oxidation of the carbonyl carbon of aldoses • Uronic acids = form following oxidation at C-6 • Both form stable intramolecular esters called lactones • In glucosamine, galactosamine, and mannosamine, the hydroxyl at C-2 of the parent compound is replaced with an amino group. The amino group is commonly condensed with acetic acid, as in N-acetylglucosamine. • This glucosamine derivative is part of many structural polymers, including those of the bacterial cell wall. • Substitution of a hydrogen for the hydroxyl group at C-6 of L-galactose or L-mannose produces L-fucose or Lrhamnose, respectively. • L-Fucose is found in the complex oligosaccharide components of glycoproteins and glycolipids; Lrhamnose is found in plant polysaccharides. • Example: Condensation of phosphoric acid with one of the hydroxyl groups of a sugar forms a phosphate ester, as in glucose 6-phosphate, the first metabolite in the pathway by which most organisms oxidize glucose for energy. • Stable at neutral pH and bear a negative charge • Functions to trap sugar inside the cell because most cells do not have membrane transporters for phosphorylated sugars • Phosphorylation also activates sugars for subsequent chemical transformation. Several important phosphorylated derivatives of sugars are components of nucleotides Sugars That Are, or Can Form, Aldehydes Are Reducing Sugars O-Glycosidic Bonds • Reducing sugars = undergo a characteristic redox reaction where free aldehyde groups react with Cu2+ under alkaline condition • O-glycosidic bond = covalent linkage joining two monosaccharides • As the sugar is oxidized from aldehyde to carboxylic acid, Cu2+ is reduced to Cu+, which forms a brickred precipitate. • Example: Glucose, galactose, mannose, ribose, and glyceraldehyde • Ketoses that can rearrange to form aldehydes are also reducing sugars • Example: Fructose and ribulose • Formed when a hydroxyl group of one sugar molecule reacts with the anomeric carbon of the other • Readily hydrolyzed by acid • Formation of a glycosidic bond renders a sugar nonreducing • Reducing end = the end of a disaccharide or polysaccharide chain with a free anomeric carbon Symbols and Abbreviations for Monosaccharides and Derivatives Free anomeric carbon Three Common Disaccharides • Lactose is a reducing disaccharide • The anomeric carbon of the glucose residue is available for oxidation • Sucrose and trehalose are nonreducing sugars Polysaccharides • Most carbohydrates in nature occur as polysaccharides (mr > 20,000) • Also called glycans, differ from each other Polysaccharides • • • • 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 and Heteropolysaccharides Polysaccharides Generally Do Not Have Defined Lengths or Molecular Weights • Homopolysaccharides = contain only a single monomeric sugar species • This distinction between proteins and polysaccharides is a consequence of the mechanisms of assembly • Serve as storage forms and structural elements • Starch and glycogen • Serve as storage forms of monosaccharides that are used as fuels • Cellulose and chitin • Serve as structural elements in plant cell walls and animal exoskeletons. • There is no template for polysaccharide synthesis • The program for polysaccharide synthesis is intrinsic to the enzymes that catalyze the polymerization of monomer units • Heteropolysaccharides = contain 2+ kinds of monomers • Provide extracellular support • The rigid layer of the bacterial cell envelope (the peptidoglycan) Glycogen and starch are composed of pyranoside (sixmembered ring) subunits Some Homopolysaccharides Are Storage Forms of Fuel Starch and Glycogen • Storage polysaccharides = starch in plant cells and glycogen in animal cells • Starch = contains two types of glucose polymer, amylose and amylopectin • Both polysaccharides occur intracellularly as large clusters or granules. • Starch storage is especially abundant in tubers (underground stems), such as potatoes, and in seeds. • Starch and glycogen molecules are heavily hydrated because they have many exposed hydroxyl groups available to hydrogen bond • Amylose = long, unbranched chains of D-glucose residues connected by (α1→4) linkages • Amylopectin = larger than amylose with (α1→4) linkages between glucose residues and highly branched due to (α1→6) linkages (occurring every 24 to 30 residues) • Glycogen = Like amylopectin, glycogen ispolymer of (α1→4)-linked glucose subunits, with (α1→6)-linked branches • • • • More extensively branched (a branch every 8 to 12 residues) More compact than starch The main storage polysaccharide of animal cells 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. Structure of Starch and Glycogen Glycogen… • Because each branch of glycogen ends with a nonreducing sugar unit, a glycogen molecule with n branches has n + 1 nonreducing ends, but only one reducing end. • When glycogen is used as an energy source, glucose units are removed one at a time from the nonreducing ends. • Degradative enzymes that act only at nonreducing ends can work simultaneously on the many branches, speeding the conversion of the polymer to monosaccharides. Cellulose and chitin are composed of pyranoside (sixmembered ring) subunits Storage of Glucose as Polymers Avoids High Osmolarity Some Homopolysaccharides Serve Structural Roles • Hepatocytes in the fed state store glycogen equivalent to a glucose concentration of 0.4 M. • Cellulose = tough, fibrous, water-insoluble substance in the cell walls of plants, particularly in stalks, stems, trunks, and all the woody portions of the plant body. • The actual concentration of glycogen, which contributes little to the osmolarity of the cytosol, is about 0.01 μM. • 0.4 M glucose in the cytosol would elevate the osmolarity • • • • • Cellulose constitutes much of the mass of wood, and cotton is almost pure cellulose. Linear, unbranched homopolysaccharide, consisting of 10,000 to 15,000 D-glucose units Glucose residues have the β configuration Linked by (β1→4) glycosidic bonds Animals do not have the enzyme to hydrolyze (β1→4) glycosidic bonds • The resulting osmotic entry of water might rupture the cell This difference causes individual molecules of cellulose and amylose to fold differently in space, giving them very different macroscopic structures and physical properties. Chitin Steric Factors and Hydrogen Bonding Influence Homopolysaccharide Folding • Chitin = linear homopolysaccharide composed of N-acetylglucosamine residues in (β1→4) linkage • Three-dimensional structures stabilized by weak interactions within or between molecules • The only chemical difference from cellulose is the replacement of the hydroxyl group at C-2 with an acetylated amino group, which makes chitin more hydrophobic and water-resistant than cellulose. • Chitin forms extended fibers similar to those of cellulose, and like cellulose cannot be digested by vertebrates. • Hydrogen bonding is especially important due to the high number of hydroxyl groups in polysaccharides • Such molecules can be represented as a series of rigid pyranose rings connected by an oxygen atom bridging two carbon atoms (the glycosidic bond). • Free rotation about both C—O bonds linking the residues is limited by steric hindrance by substituents Different Energetic Conformation of a Disaccharide Helical Structure of Starch and Glycogen • Bulkiness and electronic effects at the anomeric carbon place constraints on φ and ψ • Most stable three-dimensional structure for the (α1→4)-linked chains of starch and glycogen, stabilized by interchain hydrogen bonds. • Six residues/turn Linear Structure of Cellulose Peptidoglycan Reinforces the Bacterial Cell Wall • Most stable conformation is a straight, extended chain • Peptidoglycan = rigid component of bacterial cell walls • Each chair is turned 180° relative to its neighbors • 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 • Heteropolymer of alternating (β1→4)-linked N-acetylglucosamine and N-acetylmuramic acid residues • The linear polymers lie side by side in the cell wall, cross-linked by short peptides, the exact structure of which depends on the bacterial species. Industry • The peptide cross-links weld the polysaccharide chains into a strong sheath (peptidoglycan) that envelops the entire cell and prevents cellular swelling and lysis due to the osmotic entry of water. Glycosaminoglycans Are Heteropolysaccharides of the Extracellular Matrix Repeating Units of Glycosaminoglycans of ECM • Extracellular matrix (ECM) = gel-like material in the extracellular space of tissues that holds cells together and provides a porous pathway for nutrient and O2 diffusion • Glycosaminoglycans = heteropolysaccharides in ECM • Composed of an interlocking meshwork of heteropolysaccharides (ground substance) and fibrous proteins such as fibrillar collagens, elastins, and fibronectins. • The basement membrane is a specialized ECM that underlies epithelial cells; it consists of specialized collagens, laminins, and heteropolysaccharides. • Linear polymers composed of repeating disaccharide units • One monosaccharide is always either Nacetylglucosamine or N-acetylgalactosamine and the other is usually a uronic acid • Unique to animals and bacteria (not found in plants) • Some contain esterified sulfate groups The combination of sulfate groups and the carboxylate groups of the uronic acid residues gives glycosaminoglycans a very high density of negative charge. To minimize the repulsive forces among neighboring charged groups; these molecules assume an extended conformation in solution, forming a rodlike helix in which the negatively charged carboxylate groups occur on alternate sides of the helix Types of Glycosaminoglycans Chondroitin Sulfate, Dermatan Sulfate, Keratan Sulfate • Hyaluronan (hyaluronic acid) = alternating residues of D-glucuronic acid and N-acetylglucosamine • Chondroitin sulfate (Greek chondros, “cartilage”) contributes to the tensile strength of cartilage, tendons, ligaments, heart valves, and the walls of the aorta. • Chondroitin sulfate, dermatan sulfate, keratan sulfate, and heparan sulfate differ from hyaluronan in three respects: • Generally, much shorter polymers • Covalently linked to specific proteins (proteoglycans) • One or both monomer units differ from hyaluronan • Provide viscosity, adhesiveness, and tensile strength to the extracellular matrix • Hyaluronidase, an enzyme secreted by some pathogenic bacteria, can hydrolyze the glycosidic linkages of hyaluronan, rendering tissues more susceptible to bacterial invasion. • In many animal species, a similar enzyme in sperm hydrolyzes the outer glycosaminoglycan coat around an ovum, allowing sperm penetration. Heparan Sulfate • Dermatan sulfate (Greek derma, “skin”) contributes to the pliability of skin and is also present in blood vessels and heart valves. • In this polymer, many of the glucuronate residues present in chondroitin sulfate are replaced by their C-5 epimer, L-iduronate (IdoA). • Keratan sulfates (Greek keras, “horn”) have no uronic acid, and their sulfate content is variable. They are present in cornea, cartilage, bone, and a variety of horny structures formed from dead cells: horn, hair, hoofs, nails, and claws. Structure and Roles of Some Polysaccharides • Contains variable, nonrandom arrangements of sulfated and nonsulfated sugars • Sulfated residues gives the molecule the ability to interact specifically with proteins • Heparin is a highly sulfated, intracellular form of heparan sulfate produced primarily by mast cells (a type of leukocyte, or immune cell). • Used as a therapeutic agent to inhibit coagulation of blood through its capacity to bind the protease inhibitor antithrombin Polysaccharides and Oligosaccharides as Information Carriers. • Communication between cells and their extracellular surroundings • Labelling proteins • For transport to and localization in specific organelles, or • For destruction when the protein is malformed or superfluous; • Recognition sites for Glycoconjugates: Proteoglycans, Glycoproteins, and Glycolipids • 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 the cell shows to its surroundings. • • • • • • Cell-cell recognition and adhesion, Cell migration during development, Blood clotting, The immune Response, Wound healing, and other cellular processes. Glycoconjugate Proteoglycans • Glycoconjugate = biologically active molecule consisting of an informational carbohydrate joined to a protein or lipid • Proteoglycans = macromolecules of the cell surface or ECM consisting of 1+ sulfated glycosaminoglycan chain(s) joined covalently to a membrane protein or secreted protein • Major component of all extracellular matrices Glycoproteins Glycolipids and Glycosphingolipids • Glycoproteins = have one or several oligosaccharides joined covalently to a protein • Glycolipids = plasma membrane components in which the hydrophilic head groups are oligosaccharides • Found on the outer face of the plasma membrane, in ECM, in blood, and in organelles (Golgi complexes, secretory granules, and lysosomes) • Oligosaccharide portions are heterogenous and rich in information forming highly specific sites for recognition and high-affinity binding by carbohydrate-binding proteins called lectins. • Glycosphingolipids = class of glycolipids with specific backbone structure • Neurons are rich in glycosphingolipids • Play a role in signal transduction • Some cytosolic and nuclear proteins can be glycosylated as well. Proteoglycans Are Glycosaminoglycan-Containing Macromolecules of the Cell Surface and Extracellular Matrix • Mammalian cells can produce dozens of types of proteoglycans. • Many are secreted into the ECM, where they act as tissue organizers and influence cellular activities, such as growth factor activation and adhesion. • Proteoglycan unit= “core protein” with covalently attached glycosaminoglycan(s) • Tetrasaccharide linker connects to glycosaminoglycan to a Ser residue of the protein • Some proteoglycans are integral membrane proteins. NS Domains Two Families of Membrane Heparan Sulfate Proteoglycans • Syndecans = single transmembrane domain and an extracellular domain bearing 3–5 chains of heparan sulfate and sometimes chondroitin sulfate protease • Glypicans = attached to the membrane by a GPI anchor (a glycosylated derivative of the membrane lipid phosphatidylinositol) phospholipase These mechanisms provide a way for a cell to change its surface features quickly. Proteoglycan shedding is involved in cell-cell recognition and adhesion, and in the proliferation and differentiation of cells. Four Types of Protein Interactions with NS Domains of Heparan Sulfate • NS domains = highly sulfated domains that alternate with domains having unmodified GlcNAc and GlcA residues Heparan Sulfate Enhancement of the Binding of Thrombin to Antithrombin Proteoglycan Aggregates • Antithrombin binds to and inhibits the protease thrombin only in the presence of heparan sulfate • Proteoglycan aggregates = supramolecular assemblies of many core proteins all bound to a single molecule of hyaluronan • Both proteins are rich in Arg and Lys residues • Aggrecan interacts strongly with collagen in the ECM of cartilage contributing to the development, tensile strength, and resilience of this connective tissue. • Interact electrostatically with the sulfates of the glycosaminoglycans Fibronectin and Integrins Matrix proteins such as collagen, elastin, and fibronectin, forming a cross-linked meshwork that gives the whole ECM strength and resilience. • Fibronectin = has separate domains to bind fibrin, heparan sulfate, and collagen • contain the conserved RGD sequence (Arg–Gly–Asp) to bind integrins • Integrins = mediate signaling between cell interior and ECM molecules Purpose of Interactions between Cells and the ECM • Interactions between cells and the ECM: • Anchor cells to the ECM, providing the strength and elasticity of skin and joints • Provide paths that direct the migration of cells in developing tissue • Convey information in both directions across the plasma membrane Glycoproteins Have Covalently Attached Oligosaccharides Examples of Glycoproteins • Glycoproteins are carbohydrate-protein conjugates in which the glycans are branched and are much smaller and more structurally diverse than the huge glycosaminoglycans of proteoglycans. • Mucins = secreted or membrane glycoproteins • Two types of attachments: • Proteins of the blood • Can contain large numbers of O-linked oligosaccharide chains • Present in most secretions • Examples: immunoglobulins (antibodies), follicle-stimulating hormone, luteinizing hormone, and thyroid-stimulating hormone • O-linked = a glycoside bond joins the anomeric carbon of a carbohydrate to the — OH of a Ser or Thr residue • N-linked = an N-glycosyl bond joins the anomeric carbon of a sugar to the amide nitrogen of an Asn residue • Milk proteins • Example: major whey protein α-lactalbumin About half of all proteins of mammals are glycosylated. Glycomics The Biological Advantages of Adding Oligosaccharides to Proteins • Glycomics = the systematic characterization of all carbohydrate components of a given cell or tissue, including those attached to proteins and to lipids • Covalently attached oligosaccharides: • For glycoproteins, this also means determining which proteins are glycosylated and where in the amino acid sequence each oligosaccharide is attached. • Influence the folding and stability of the proteins • Provide critical information about the targeting of newly synthesized proteins • Allow specific recognition by other proteins Glycolipids and Lipopolysaccharides Are Membrane Components • Gangliosides = membrane lipids of eukaryotic cells in which the polar head group is a complex oligosaccharide containing a sialic acid and other monosaccharide residues Carbohydrates as Informational Molecules: The Sugar Code • Lipopolysaccharides = dominant surface feature of the outer membrane of gramnegative bacteria Glycolipids and glycosphingolipids in plants and animals and lipopolysaccharides in bacteria are components of the cell envelope, with covalently attached oligosaccharide chains exposed on the cell’s outer surface. The Challenge of Glycobiology Oligosaccharide Structures Are Information-Dense • Glycobiology = the study of the structure and function of glycoconjugates • Branched structures, not found in nucleic acids or proteins, are common in oligosaccharides • The challenge is to understand how cells use specific oligosaccharides to encode information about: • Almost limitless variety of oligosaccharides due to differences in: • • • • Intracellular targeting of proteins Cell-cell interactions Cell differentiation and tissue development Extracellular signals • Stereochemistry and position of glycosidic bonds • Type and orientation of substituent groups • The number and type of branches Lectins Are Proteins That Read the Sugar Code and Mediate Many Biological Processes Selectins • Lectins = bind carbohydrates with high specificity and with moderate to high affinity • Selectins = family of plasma membrane lectins that mediate cell-cell recognition and adhesion in a wide range of cellular processes • Functions: • • • • • Move immune cells through the capillary wall • Mediate inflammatory responses • Mediate the rejection of transplanted organs Cell-cell recognition Signaling Adhesion Intracellular targeting of newly synthesized proteins Role of Lectin-Ligand Interactions in Leukocyte Movement Lectin-Carbohydrate Interactions Are Highly Specific and Often Multivalent Interactions of Sugar Residues Due to the Hydrophobic Effect • Subtle molecular complementarity allows interaction only with the lectin’s correct carbohydrate binding partners • Many sugars have a more polar side and a less polar side • Often a divalent metal ion such as Ca2+ or Mn2+ is part of the binding site. • Lectin multivalency = single lectin molecule has multiple carbohydrate binding domains • Increases effective affinity The more polar side hydrogen-bonds with the lectin, while the less polar side undergoes interactions with nonpolar amino acid residues through the hydrophobic effect. The sum of all these interactions produces highaffinity binding and high specificity of lectins for their carbohydrate ligands. Biological Interactions Mediated by the Sugar Code Working with Carbohydrates Oligosaccharide analysis is complicated by the fact that, unlike nucleic acids and proteins, oligosaccharides can be branched and are joined by a variety of linkages. The high charge density of many oligosaccharides and polysaccharides, and the relative lability of the sulfate esters in glycosaminoglycans, present further difficulties. Methods of Carbohydrate Analysis Determining Oligosaccharide and Polysaccharide Structures • More complex than protein and nucleic acid analysis • Can employ a variety of methods to determine sequence, configuration at anomeric and other carbons, and positions of glycosidic bonds: • Traditional chemical and enzymatic approaches • Mass spectrometry • High-resolution NMR spectroscopy Solid-Phase Synthetic Methods • Carbohydrate chemists can synthesize short segments of almost any glycosaminoglycan • Solid-phase oligosaccharide synthesis: • Based on the same principles as peptide synthesis • Yields defined oligosaccharides • Useful in exploring lectin-oligosaccharide interactions