Chemistry of Carbohydrates PDF

2 | Chemistry of Carbohydrates Chapter 7: Chapter 1: Carbohydrates and Chemistry of Glycobiology Carbohydrates Carbohydrates ▶ Named so because many have formula Cn(H2O)n ▶ Produced from CO2 and H2...

2 | Chemistry of Carbohydrates Chapter 7: Chapter 1: Carbohydrates and Chemistry of Glycobiology Carbohydrates Carbohydrates ▶ Named so because many have formula Cn(H2O)n ▶ Produced from CO2 and H2O via photosynthesis in plants ▶ Range from as small as glyceraldehyde (Mw = 90 g/mol) to as large as amylopectin (Mw > 200,000,000 g/mol) ▶ Fulfill a variety of functions, including: ▶ energy source and energy storage ▶ structural component of cell walls and exoskeletons ▶ informational molecules in cell-cell signaling ▶ Can be covalently linked with proteins and lipids  Common CHO and their Food Sources Glycogen Cellulose Carbohydrates ▶ Basic nomenclature: ▶ number of carbon atoms in the carbohydrate + -ose ▶ example: three carbons = triose ▶ Common functional groups: ▶ All carbohydrates initially had a carbonyl functional group. ▶ aldehydes = aldose ▶ ketones = ketose  Monosaccharides Found in Food Six Seven Monosaccharides Can Be Constitutional Isomers An aldose is a carbohydrate with aldehyde functionality. A ketose is a carbohydrate with ketone functionality.  Carbohydrates Can Be Stereoisomers Epimers are stereoisomers that differ at only one chiral center. Epimers are NOT mirror images, and therefore are NOT enantiomers. Epimers are diastereomers; diastereomers have different physical properties (i.e., water solubility, melting temp). example: D-Threose is the C-2 epimer of D-erythrose. Both are D sugars because they both have the same orientation around the last chiral carbon in the chain.  Epimers D-Mannose and D-galactose are both epimers of D-glucose. D-Mannose and D-galactose vary at more than one chiral center and are diastereomers, but not epimers. Monosaccharides Have Straight and Ring Structures -D-Glucopyranose  Cyclization of Monosaccharides The nucleophilic alcohol attacks the electrophilic carbonyl carbon, allowing formation of a hemiacetal. As a result, the linear carbohydrate forms a ring structure. At the completion of this structure, the carbonyl carbon is reduced to an alcohol The orientation of the alcohol around the carbon is variable and transient.  Cyclization of Monosaccharides ▶ Pentosesand hexoses readily undergo intramolecular cyclization. ▶ The former carbonyl carbon becomes a new chiral center, called the anomeric carbon. ▶ When the former carbonyl oxygen becomes a hydroxyl group, the position of this group determines if the anomer is  or . ▶ Ifthe hydroxyl group is on the opposite side (trans) of the ring as the CH2OH moiety, the configuration is . ▶ Ifthe hydroxyl group is on the same side (cis) of the ring as the CH2OH moiety, the configuration is .  Cyclization of Monosaccharides: Pyranoses and Furanoses ▶ Six-membered oxygen-containing rings are called pyranoses after the pyran ring structure. ▶ Five-membered oxygen-containing rings are called furanoses after the furan ring structure. ▶ Theanomeric carbon is usually drawn on the right side.  Conformational Formulas of Cyclized Monosaccharides Cyclohexane rings have “chair” or “boat” conformations. Pyranose rings favor “chair” conformations. Multiple “chair” conformations are possible but require energy for interconversion (~46 kJ/mole). Reducing Sugars ▶ Reducing sugars have a free anomeric carbon. ▶ Aldehyde can reduce Cu2+ to Cu+ (Fehling’s test). ▶ Aldehyde can reduce Ag+ to Ag0 (Tollens’ test). ▶ It allows detection of reducing sugars, such as glucose. ▶ Modern detection techniques use colorimetric and electrochemical tests.  Colorimetric Glucose Analysis ▶ Enzymatic methods are used to -D-Gluconolactone -D-Glucose quantify reducing sugars such as CH2OH CH2OH glucose. O OH Glucose oxidase O ▶ The enzyme glucose oxidase OH OH O catalyzes the conversion of HO HO glucose to glucono--lactone OH O2 OH and hydrogen peroxide. ▶ Hydrogen peroxide oxidizes H2O2 NH organic molecules into highly OCH3 NH2 colored compounds. 2 H 2O OCH3 ▶ Concentrations of such compounds is measured colorimetrically. Peroxidase OCH3 NH OCH3 NH2 Oxidized Reduced Electrochemical detection is o-dianisidine (bright orange) o-dianisidine (faint orange) used in portable glucose sensors. Disaccharides and the Glycosidic Bond ▶ Two sugar molecules can be joined via a glycosidic bond between an anomeric carbon and a hydroxyl carbon. ▶ The glycosidic bond (an acetal) between monomers is more stable and less reactive than the hemiacetal at the second monomer. ▶ The second monomer, with the hemiacetal, is reducing. ▶ The anomeric carbon involved in the glycosidic linkage is nonreducing. ▶ Disacharides can be named by the organization and linkage or a common name. ▶ The disaccharide formed upon condensation of two glucose molecules via a 1  4 bond is described as α-d-glucopyranosyl-(14)-D- glucopyranose. ▶ The common name for this disaccharide is maltose. Nonreducing Disaccharides ▶ Two sugar molecules can be also joined via a glycosidic bond between two anomeric carbons. ▶ The product has two acetal groups and no hemiacetals. ▶ There are no reducing ends; this is a nonreducing sugar. Polysaccharides ▶ Natural carbohydrates are usually found as polymers. ▶ These polysaccharides can be: ▶ homopolysaccharides (one monomer unit) ▶ heteropolysaccharides (multiple monomer units) ▶ linear (one type of glycosidic bond) ▶ branched (multiple types of glycosidic bonds) ▶ Polysaccharides do not have a defined molecular weight. ▶ This is in contrast to proteins because, unlike proteins, no template is used to make polysaccharides. ▶ Polysaccharides are often in a state of flux; monomer units are added and removed as needed by the organism. Common Polysaccharides  Homopolymers of Glucose: Glycogen Glycogen is a branched homopolysaccharide of glucose. – Glucose monomers form (1 4) linked chains. – There are branch points with (1 6) linkers every 8–12 residues. – Molecular weight reaches several millions. – It functions as the main storage polysaccharide in animals.  Homopolymers of Glucose: Starch Starch is a mixture of two homopolysaccharides of glucose. Amylose is an unbranched polymer of (1 4) linked residues. Amylopectin is branched like glycogen, but the branch points with (1 6) linkers occur every 24–30 residues. Molecular weight of amylopectin is up to 200 million. Starch is the main storage polysaccharide in plants. Glycosidic Linkages in Glycogen and Starch Mixture of Amylose and Amylopectin in Starch Dextrins are Hydrolysis Products of Starch ▶ Low-molecular-weight carbohydrates ▶ Polymers or D-glucose linked by α-(1→4) or α- (1→6) glycosidic bonds ▶ Produced by the partial hydrolysis of starch or glycogen (salivary amylase and similar enzymes) ▶ Can be produced in the gut and in cooking starch or glycogen ▶ Can be used as water-soluble adhesives (glues) because of formation of a sticky solution in water ▶ Further hydrolysis leads limit dextrins (erythrodextrin) and later to achrodextrins ▶ Achrodextrins are further hydrolyzed to maltose and glucose  Metabolism of Glycogen and Starch ▶ Glycogenand starch are insoluble due to their high molecular weight and often form granules in cells. ▶ Granulescontain enzymes that synthesize and degrade these polymers. ▶ Glycogenand amylopectin have one reducing end but many nonreducing ends. ▶ Enzymaticprocessing occurs simultaneously in many nonreducing ends.  Homopolymers of Glucose: Cellulose ▶ Cellulose is a linear homopolysaccharide of glucose. ▶ Glucose monomers form (1 4) linked chains. ▶ Hydrogen bonds form between adjacent monomers. ▶ There are additional H-bonds between chains. ▶ Structure is now tough and water insoluble. ▶ It is the most abundant polysaccharide in nature. ▶ Cotton is nearly pure fibrous cellulose.  Cellulose Metabolism ▶ The fibrous structure and water insolubility make cellulose a difficult substrate to act upon. ▶ Most animals cannot use cellulose as a fuel source because they lack the enzyme to hydrolyze (1 4) linkages. ▶ Fungi, bacteria, and protozoa secrete cellulase, which allows them to use wood as source of glucose. ▶ Ruminants and termites live symbiotically with microorganisms that produce cellulase and are able to absorb the freed glucose into their bloodstreams. ▶ Cellulases hold promise in the fermentation of biomass into biofuels.  Chitin Is a Homopolysaccharide Chitin is a linear homopolysaccharide of N-acetylglucosamine. – N-acetylglucosamine monomers form (1 4)-linked chains. – forms extended fibers that are similar to those of cellulose – hard, insoluble, cannot be digested by vertebrates – structure is tough but flexible, and water insoluble – found in cell walls in mushrooms and in exoskeletons of insects, spiders, crabs, and other arthropods Inulin Is Made from Fructose ▶ Plant polysaccharide n~35 ▶ Made from fructose units ▶ Chain-terminating glucose ▶ Sweet taste ▶ Present in roots for energy storage ▶ Most plants that synthesize inulin do not store energy in the form of starch ▶ Good source of dietary fiber  Agar and Agarose ▶ Agar is a branched heteropolysaccharide composed of agarose and agaropectin. ▶ Agar serves as a component of cell wall in some seaweeds. ▶ Agar solutions form gels that are commonly used in the laboratory as a surface for growing bacteria. ▶ Agarose solutions form gels that are commonly used in the laboratory for separation DNA by electrophoresis. Agarose Is a Heteropolysaccharide  Glycosaminoglycans ▶ Linear polymers of repeating disaccharide units ▶ One monomer is either: ▶ N-acetyl-glucosamine or ▶ N-acetyl-galactosamine ▶ Negatively charged ▶ uronic acids (C6 oxidation) ▶ sulfate esters ▶ Extended hydrated molecule ▶ minimizes charge repulsion ▶ Forms meshwork with fibrous proteins to form extracellular matrix ▶ connective tissue ▶ lubrication of joints  Heparin and Heparan Sulfate ▶ Heparin is linear polymer, 3–40 kDa. ▶ Heparan sulfate is heparin-like polysaccharide but attached to proteins. ▶ Highest negative-charge density biomolecules ▶ Prevent blood clotting by activating protease inhibitor antithrombin ▶ Binding to various cells regulates development and formation of blood vessels. ▶ Can also bind to viruses and bacteria and decrease their virulence  TABLE 7-2 Structures and Roles of Some Polysaccharides Size (number of monosaccharide Primer Typea Repeating unitb units) Roles/significance Starch Energy storage: in plants Amylose Homo- (α1S4) Glc, linear 50–5,000 Amylopectin Homo- (α1S4) Glc, with (α1S6) Up to 106 Glc branches every 24–30 residues Glycogen Homo- (α1S4) Glc, with (α1S6) Up to 50,000 Energy storage: in bacteria and animal Glc branches every 8–12 cells residues Cellulose Homo- (β1S4) Glc Up to 15,000 Structural: in plants, gives rigidity and strength to cell walls Chitin Homo- (β1S4) GlcNAc Very large Structural: in insects, spiders, crustaceans, gives rigidity and strength to exoskeletons Dextran Homo- (α1S6) Glc, with (α1S3) Wide range Structural: in bacteria, extracellular branches adhesive Peptidoglycan Hetero-; peptides 4)Mur2Ac(β1S4) Very large Structural: in bacteria, gives rigidity and attached GlcNAc(β1 strength to cell envelope Agarose Hetero- 3)D-Gal (β1S4)3,6- 1,000 Structural: in algae, cell wall material anhydro-L-Gal(α1 Hyaluronan (a Hetero-; acidic 4)GlcA (β1S3) GlcNAc(β1 Up to 100,000 Structural: in vertebrates, extracellular glycosaminoglycan) matrix of skin and connective tissue; viscosity and lubrication in joints aEach polymer is classified as a homopolysaccharide (homo-) or heteropolysaccharide (hetero-). bThe abbreviated names for the peptidoglycan, agarose, and hyaluronan repeating units indicate that the polymer contains repeats of this disaccharide unit. For example, in peptidoglycan, the GlcNAc of one disaccharide unit is (β1S4)-linked to the first residue of the next disaccharide unit.