Lecture 4: Carbohydrates PDF

1 Lecture 4 Carbohydrates 2 Monosaccharides Carbohydrates have numerous functions in biochemistry: generating and storing biological energy molecular recognition (as in the immune system) cellular protection (a...

1 Lecture 4 Carbohydrates 2 Monosaccharides Carbohydrates have numerous functions in biochemistry: generating and storing biological energy molecular recognition (as in the immune system) cellular protection (as in bacterial and plant cell walls) cell signaling cell adhesion biological lubricants controlling protein trafficking maintaining biological structure (e.g., cellulose).  Monosaccharides 3 Representative carbohydrates: The three compounds shown here are composed entirely of C, H, and O, with glucose forming the monomer for the oligomer and the polymer. a) Glucose, a monosaccharide. b) Maltose, a disaccharide containing two glucose units.  Monosaccharides 4 Representative carbohydrates: c) A portion of a molecule of amylose, a glucose polymer found in starch.  Monosaccharides 5 The energy cycle of life: In photosynthesis, plants use the energy of sunlight to combine carbon dioxide and water into carbohydrates, releasing oxygen in the process. In respiration, both plants and animals oxidize the carbohydrates made by plants, releasing energy and re-forming CO2 and H2O.  Monosaccharides 6 Trioses are the simplest monosaccharides. The two triose tautomers illustrate the difference between aldose and ketose monosaccharides, also called more descriptively aldotriose and ketotriose, respectively. Carbon numbering begins in all aldoses with the aldehyde carbon and in ketoses with the end carbon closest to the ketone group. The enediol intermediate through which they are interconverted is unstable and cannot be isolated.  Monosaccharides 7 The most compact way to represent enantiomers is to use a Fischer projection. In a Fischer projection the bonds that are drawn horizontally are imagined as coming toward you; those drawn vertically are receding. D and L forms of a monosaccharide are nonsuperimposable mirror images and are called enantiomers. The most important naturally occurring saccharides are the D-enantiomers.  Monosaccharides 8 The enantiomers of glyceraldehyde: The configuration of groups around the chiral carbon 2 distinguishes D-glyceraldehyde from L-glyceraldehyde. The two molecules are mirror images and cannot be superimposed on one another.  Monosaccharides 9 Stereochemistry of aldotetroses: These molecules have two chiral carbons and thus have two diastereomeric forms, threose and erythrose, each with a pair of enantiomers. When monosaccharides contain more than one chiral carbon, the prefix D or L designates the configuration about the carbon farthest from the carbonyl group. Isomers differing in orientation about other carbons are called diastereomers and given different names.  Monosaccharides 10 Monosaccharides with five or more carbons exist preferentially in five- or six-membered ring structures (Haworth projections), resulting from internal hemiacetal formation. Two anomeric forms are possible, a and b.  Monosaccharides 11 12 Monosaccharides The hexoses also exist primarily in ring forms under physiological conditions. As with the aldopentoses, two kinds of rings are found: five- membered furanoses and six-membered pyranoses. In each case, a and b anomers are possible. An example, illustrated by Haworth projections, follows: 13 Monosaccharides The four most common hexoses: These Haworth projections represent the D enantiomers. Only the b anomers are shown. 14 Monosaccharides Hexoses can exist in boat and chair conformations. Usually the chair is more stable. 15 Monosaccharides A summary of terminology describing the structure of sugar molecules: Conformational isomers are distinguished from configurational isomers in that the former can interconvert without breaking and re- forming bonds. Not shown are epimers, stereoisomers differing in their configuration about only one asymmetric carbon atom. 16 Derivatives of the Monosaccharides  Derivatives of the Monosaccharides 17 Sugar phosphates are important intermediates in metabolism, functioning as activated compounds in syntheses. 18 Derivatives of the Monosaccharides Oxidation of monosaccharides can proceed in several ways, depending upon the oxidizing agent used. For example, mild oxidation of an aldose with alkaline Cu(II) (Fehling’s solution) produces the aldonic acids, as in the following example: 19 Derivatives of the Monosaccharides Enzyme-catalyzed oxidation of monosaccharides gives other products, including uronic acids such as glucuronic acid, in which oxidation has occurred at carbon 6. Uronic acids are important constituents of certain natural polysaccharides. 20 Derivatives of the Monosaccharides Free aldonic acids, such as gluconic acid, are in equilibrium in solution with lactones, which are cyclic esters Found in the pentose-phosphate pathway.  Derivatives of the Monosaccharides 21 Reduction of the carbonyl group on a sugar gives rise to the class of polyhydroxy compounds called alditols. Important naturally occurring ones are erythritol, D-mannitol, and D-glucitol, often called sorbitol. 22 Derivatives of the Monosaccharides Two amino derivatives of simple sugars are widely distributed in natural polysaccharides: glucosamine and galactosamine, derived from glucose and galactose, respectively. 23 Derivatives of the Monosaccharides Glucosamine an amino sugar and a prominent precursor in the biochemical synthesis of glycosylated proteins and lipids part of the structure of the polysaccharides chitosan and chitin appears to be safe for use as a dietary supplement; effectiveness has not been established for any condition in the US it is one of the most common non-vitamin, non- mineral, dietary supplements used by adults Glucosamine is marketed to support the structure and function of joints, and the marketing is targeted to people suffering from osteoarthritis Glucosamine, along with commonly used chondroitin, should not be used to treat patients who have symptomatic osteoarthritis of the knee, as evidence shows that these treatments fail to provide relief for that condition 24 Derivatives of the Monosaccharides N-Acetylgalactosamine (GalNAc) - in humans it is the terminal carbohydrate forming the antigen of blood group A and is necessary for intercellular communication, and is concentrated in sensory nerve structures of both humans and animals. Sialic acid plays an important role in the transmission of signals between cells. It occurs in the gray matter of the brain. It is present in cell membrane glycoproteins. 25 Derivatives of the Monosaccharides Amino sugars are found in many polysaccharides. 26 Derivatives of the Monosaccharides N-Acetylglucosamine - monosaccharide and a derivative of glucose, an amide between glucosamine and acetic acid. It is part of bacterial cell wall, which is built from alternating units of GlcNAc and N-acetylmuramic acid. This layered structure is called peptidoglycan.  Derivatives of the Monosaccharides 27 Elimination of water between the anomeric hydroxyl of a cyclic monosaccharide and the hydroxyl group of another compound yields an O-glycoside. The acetal bond formed is referred to as a glycosidic bond. A simple example is the formation of methyl-a-D-glucopyranoside: 28 Oligosaccharides Two naturally occurring glycosides: Ouabain (cardiac glycoside, Na+/K+ ATPase inhibitor, also known as g-strophanthin) and amygdalin (cyanide!) are highly toxic glycosides produced by plants. 29 Oligosaccharides Two naturally occurring glycosides: Ouabain (cardiac glycoside, Na+/K+ ATPase inhibitor) and amygdalin (cyanide!) are highly toxic glycosides produced by plants.  Oligosaccharides 30 Distinguishing Features of Different Disaccharides: Four major features distinguish disaccharides from one another: 1.The two specific sugar monomers involved, and their stereoconfigurations. The monomers may be of the same kind, as the two D-glucopyranose residues in maltose, or they may be different, as the D-glucopyranose and D-fructofuranose residues in sucrose. 2. The carbons involved in the linkage. The most common linkages are 1à 1 (as in trehalose), 1à 2 (as in sucrose), 1à 4 (as in lactose, maltose, and cellobiose), and 1à 6 (as in gentiobiose). Note that all of these disaccharides involve the anomeric hydroxyl of at least one sugar as a participant in the bond.  Oligosaccharides 31 Distinguishing Features of Different Disaccharides: 3. The order of the two monomer units, if they are different kinds. The glycosidic linkage involves the anomeric carbon on one sugar, but in most cases the other is free. The two ends of the molecule can be distinguished by their chemical reactivity. For example, the glucose residue in lactose, having a free anomeric carbon and thus a potential free aldehyde group, could be oxidized by Fehling’s solution; the galactose residue could not be. Lactose is therefore a reducing sugar, and the glucose residue is at its reducing end. The other end is called the nonreducing end. In sucrose, neither residue has a potential free aldehyde group; both anomeric carbons are involved in the glycosidic bond. Therefore, sucrose is a nonreducing sugar.  Oligosaccharides 32 Distinguishing Features of Different Disaccharides: 4. The configuration of the anomeric hydroxyl group of each residue. This feature is especially important for the anomeric carbon(s) involved in the glycosidic bond. The configuration may be either a or b. This difference may seem small, but it has a major effect on the shape of the molecule, and the difference in shape is recognized readily by enzymes. For example, different enzymes are needed to catalyze the hydrolysis of maltose and cellobiose, even though both are dimers of D-glucopyranose. 33 Oligosaccharides Note the convention used to draw glycosidic bonds. The “bent bonds” allow the Haworth projections of the monomers to be drawn in parallel. The “corners” do not imply extra carbon atoms, as they often do in organic structure representations. 34 Oligosaccharides 35 Oligosaccharides  Oligosaccharides 36 Writing the Structure of Disaccharides: A convenient way to describe the structures of these and more complex oligosaccharides - the rules are as follows: 1.The sequence is written starting with the non-reducing end at the left. 2.Anomeric and enantiomeric forms are designated by prefixes (e.g., a-, D-). 3.The ring configuration is indicated by a suffix (p for pyranose, f for furanose). 4.The atoms between which glycosidic bonds are formed are indicated by numbers in parentheses between residue designations (e.g., (1 à 4) means a bond from carbon 1 of the residue on the left to carbon 4 of the residue on the right).  Oligosaccharides 37 Writing the Structure of Disaccharides: For example, we can write the structure of sucrose (glucose and fructose) as: a-D-Glcp(1à2)-b-D-Fruf Often, the D- and p or f are omitted under normal circumstances. For example, the structure of maltose can be written as: Glca(1à4)Glc  Oligosaccharides 38 Formation of the glycosidic bond between two monomers in an oligosaccharide is a condensation reaction, involving the elimination of a molecule of water. Thus, we might expect the synthesis of lactose to proceed as follows: Like the phosphodiester bond in nucleic acid and amide bond in proteins, the glycosidic bond is metastable. Enzymes control its hydrolysis.  Stability and Formation of Glycosidic Bonds 39 The ΔG°ʹ for glycosidic bond formation is about +15 kJ/mol, therefore activation is needed In lactose biosynthesis, the activated sugar is UDP-galactose UDP-galactose is used as a high-energy derivative of galactose that condenses with glucose to form lactose  Oligosaccharides 40 Enzymatic formation of lactose: The reaction shown occurs in the formation of milk in mammary tissue. Galactose is phosphorylated by ATP, then transferred to uridine diphosphate (UDP). UDP-galactose transfers galactose to glucose, with the accompanying cleavage of a phosphate bond. The reaction is catalyzed by the enzyme lactose synthase. 41 Polysaccharides The principal storage polysaccharides are amylose and amylopectin, which together constitute starch in plants, and glycogen, which is stored in animal cells. Both starch and glycogen are stored in granules within cells Glycogen is deposited in the liver, which acts as a central energy storage organ in many animals. Glycogen is also abundant in muscle tissue, where it is more immediately available for energy release. Glycogen and the components of starch—amylose and amylopectin—are storage polysaccharides. Amylose is linear; amylopectin and glycogen are branched.  Homopolysaccharides and 42 Heteropolysaccharides Polysaccharides can be distinguished into homopolysaccharides and heterosaccharides: Homopolysaccharides are made of one type of monomer, while heterosaccharides are made of more than one type of monomer Functionally, polysaccharides can be divided into - energy storage polysaccharides (e.g., starch and glycogen) - structural polysaccharides (e.g., cellulose) - lubricants (e.g., some glycoaminoglycans)  43 ENERGY STORAGE POLYSACCHARIDES Starch (amylose and amylopectin) and glycogen  Polysaccharides 44 Amylopectin, a branched glucan: The branches in glycogen are somewhat more frequent and shorter than those in amylopectin, and glycogen is usually of higher molecular weight, but in most respects the structures of these two polysaccharides are very similar. 45 Polysaccharides The secondary structure of amylose: The orientation of successive glucose residues favors helix generation. Note the large interior core. Hydrogen bonds (not shown) stabilize the helix.  Polysaccharides 46 Cellulose structure: The b(1à4) linkages of cellulose generate a planar structure. The parallel cellulose chains are linked together by a network of hydrogen bonds.  Polysaccharides 47 Cellulose and chitin are examples of structural polysaccharides. Unlike starches, which have a(1à4) links, these fibrous polymers have b(1à4) linkages. 48 Polysaccharides The major structural polysaccharides in vertebrate animals are the glycosaminoglycans, formerly called mucopolysaccharides. Important examples are the chondroitin sulfates and keratan sulfates of connective tissue, the dermatan sulfates of skin, and hyaluronic acid. All are polymers of repeating disaccharide units, in which one of the sugars is either N-acetylgalactosamine or N-acetylglucosamine or one of their derivatives. All are acidic (anionic), through the presence of either sulfate or carboxylate groups.  Polysaccharides 49 Repeating structures of some glycosaminoglycans: In each case, the repeating unit is a disaccharide, of which two are shown for each structure. 50 Polysaccharides Chondroitin sulfate is usually found attached to proteins as part of a proteoglycan. A chondroitin chain can have over 100 individual sugars, each of which can be sulfated in variable positions and quantities. Chondroitin sulfate is an important structural component of cartilage and provides much of its resistance to compression. Along with glucosamine, chondroitin sulfate has become a widely used dietary supplement for treatment of osteoarthritis. 51 Polysaccharides Keratan sulfates - large, highly hydrated molecules which in joints can act as a cushion to absorb mechanical shock. It has been found especially in the cornea, cartilage, and bone. It is also synthesized in the central nervous system where it participates both in development and in the glial scar formation following an injury. The Extracellular Matrix 52 Polysaccharides Hyaluronic acid - distributed widely throughout connective, epithelial, and neural tissues. It is unique among glycosaminoglycans in that it is nonsulfated, forms in the plasma membrane instead of the Golgi apparatus, and can be very large, with its molecular weight often reaching the millions. One of the chief components of the extracellular matrix, hyaluronan contributes significantly to cell proliferation and migration 53 Polysaccharides A highly sulfated glycosaminoglycan is heparin. Heparin appears to be a natural anticoagulant and is found in many body tissues. It binds strongly to a blood protein, antiprothrombin III, and the complex inhibits enzymes of the blood clotting process. Therefore, heparin is used in medicine to inhibit clotting in blood vessels.  Polysaccharides 54 The cell wall of a Gram-positive bacterium, S. aureus, consists of a thick peptidoglycan layer made up of polysaccharide chains and short peptides. The peptides are cross-linked by glycine pentapeptides.  Polysaccharides 55 The cell wall of a Gram-negative bacterium, E. coli, has a thin peptidoglycan layer and an outer lipid membrane. The cross-links here are between tetrapeptides attached to the N-acetylmuramic acid (NAM) residues in adjacent chains  Polysaccharides 56 The structure of a lipotechoic acid: D-Alanyl and NAG groups are arranged irregularly on the chain, which is anchored in the membrane by lipid. Lipoteichoic acid is a major constituent of the cell wall of gram-positive bacteria. It has antigenic properties being able to stimulate specific immune response. Released during lysozyme-induced bacteriolysis or as a result of the activity of beta-lactam antibiotics. It determines the development of inflammatory reactions, in extreme cases up to the development of sepsis. 57 Glycoproteins Oligosaccharides and proteins can be linked to form glycoproteins in two ways: o O-linked glycans are attached via threonine or serine hydroxyls. o N-linked glycans via asparagine amino groups. 58 Glycoproteins The ABO blood group antigens: The O oligosaccharide does not induce antibodies in most humans. The A and B antigens are formed by addition of GalNAc or Gal, respectively, to the O oligosaccharide. Each of the A and B antigens can induce a specific antibody. R can represent either a protein molecule or a lipid molecule. 59 Glycoproteins The blood group substances are a set of antigenic oligosaccharides attached to the surfaces of red cells. 60 Dextran Dextran is a complex, branched glucan (made of many glucose molecules) composed of chains of varying lengths (from 3 to 2000 kilodaltons). It is used medicinally as an antithrombotic (anti-platelet), to reduce blood viscosity, and as a volume expander in hypovolaemia. 61 Glycoproteins The structure of the influenza virus: The 13,600-nucleotide RNA genome is packaged within the sphere, about 120 nm in diameter. The spikes on the virion exterior include the hemagglutinin molecule and neuraminidase molecules. 62 Glycoproteins The structure of the influenza virus: Neuraminidase - a glycoprotein enzyme responsible for the breakdown of sialic acid, found in influenza viruses. This enzyme enables viruses to leave cells by breaking down the cell membrane of the infected cell. 63 Glycoproteins The structure of the influenza virus: Hemagglutinin - a glycoprotein with antigenic properties located on the surface of influenza viruses. The function of this protein is to attach a virus molecule to the surface of an infected cell. The name hemagglutinin comes from the ability of this glycoprotein to cause agglutination of erythrocytes. 64 Glycoproteins Rational design of neuraminidase inhibitors: Structures of sialic acid, zanamivir, and oseltamivir. Partial model of the neuraminidase-zanamivir complex, showing amino acid residues that are close to the binding site for the inhibitor. 65 Oligosaccharides as Cell Markers Some animal cells have an extremely thick coating of polysaccharides called a glycocalyx (literally “sugar coat”). Glycocalyx oligosaccharides interact with other substances: o With bacteria in the intestine. o With collagen of the intercellular matrix in some other tissues.  Oligosaccharides as Cell Markers 66 Cell surface recognition factors: a) Schematic view of a lipid membrane. b) Electron micrograph of the surface of an intestinal epithelial cell. The cellular projections, called microvilli, are covered on their outer surface by a layer of branched polysaccharide chains attached to proteins in the cell membrane. This carbohydrate layer, called the glycocalyx, is found on many animal cell surfaces.  Glycoconjugates of Interest 67 Biosynthesis of the lipid-linked oligosaccharide intermediate: The five sequential mannosyl transfer reactions from GDP-mannose are catalyzed by separate glycosyltransferases, as are the four mannosyl transfers from dolichol phosphate mannose. The latter is synthesized in turn from GDP- mannose. The acceptor site on the polypeptide chain is an asparagine residue two positions to the N side of a serine or threonine. The whole process occurs in the endoplasmic reticulum, with translocation to the lumen of the ER occurring after transfer of the fifth mannose residue.  Glycoconjugates of Interest 68 Schematic pathway of oligosaccharide processing on newly synthesized glycoproteins:  Glycoconjugates of Interest 69 Synthesis of the linear peptidoglycan molecule of S. aureus: The peptidoglycan molecule is synthesized by addition of N-acetylglucosamine and five glycyl residues to N-acetylmuramylpentapeptide, with undecaprenol phosphate acting as carrier. Sites of inhibition by the antibiotics bacitracin and vancomycin are identified. Following synthesis, the peptidoglycan is transported through the cell membrane to the cell wall and added to the end of a chain in the peptidoglycan layer.  Glycoconjugates of Interest 70 Biosynthesis of the repeating oligosaccharide unit of the O antigen of Salmonella typhimurium: The first four reactions occur within the inner membrane. Transfer of the activated tetrasaccharide unit to the unactivated terminus of a growing polysaccharide unit occurs on the outside of the outer membrane.  Glycoconjugates of Interest 71 The cross-linking reaction in peptidoglycan synthesis (left) in bacteria cel wall and inhibition of the transpeptidase enzyme, E, by penicillin (right): Cross-links between adjacent peptidoglycan chains are formed by the action of a transpeptidase enzyme, as shown at the left. At the right is shown how penicillin, a structural analog of the natural substrate, reacts with the active form of the enzyme to form an inactive covalent complex that resembles the enzyme–substrate complex.  Glycoconjugates of Interest 72 Penicillin inhibits the completion of the synthesis of peptidoglycans (structural components). It specifically inhibits the activity of enzymes that are needed for the cross-linking of peptidoglycans by binding to proteins with the β-lactam ring. This causes the cell wall to weaken due to fewer cross-links and means water uncontrollably flows into the cell because it cannot maintain the correct osmotic gradient. This results in cell lysis.