1 Chem 352 - Lecture 7 Carbohydrates Question for the Day: Unlike amino acids, which owe their diversity to a diverse array of functional groups, monosaccharides feature primarily two functional groups, hydroxyl groups and either a ketone or aldehyde group. What, then, do monosaccharides owe their diversity to? Introduction to Carbohydrates 2 Carbohydrates are one of the four major classes of biological molecules, which include, ✦ Proteins ✦ Nucleic acids ✦ Carbohydrates ✦ Lipids Chem 352, Lecture 7 - Carbohydrates X Introduction to Carbohydrates 3 ✦ Carbohydrates represent a major source of energy for living organisms. ✦ They also play major structural, protective and communication roles. glucose Chem 352, Lecture 7 - Carbohydrates X Introduction to Carbohydrates 4 Carbohydrates are chemically simple, but can be structurally complex ✦ The simple sugars have the basis stoichiometric empirical formula(CH2O)n, where n = 3 to 7 Like amino acid, simple sugars (monosaccharides) can combine to form polymers. ✦ monosaccharides (monomer) ✦ oligosaccharides (several monomers linked together) ✦ polysaccharides (many monomers linked together Chem 352, Lecture 7 - Carbohydrates X 5 9.1 Monosaccharides Monosaccharides 6 Question: Monosaccharides are What is the name ending that is ✦ either Aldoses typically used to designate a carbohydrate? • polyhydroxylaldehydes A. -ase ✦ or Ketoses • polyhydroxylketones B. -ose Classes based on number of carbonsC. -ol ✦ triose (3) D. -al ✦ tetrose (4) E. -amine ✦ pentose (5) ✦ hexose (6) ✦ heptose (7) Chem 352, Lecture 7 - Carbohydrates X Monosaccharides 7 Trioses ✦ L and D Glyceraldehyde • Contains a chiral carbon • Fischer projections ✦ Dihydroxyacetone Chem 352, Lecture 7 - Carbohydrates X Monosaccharides 8 There are different ways to designate the stereochemistry of a chiral center: ✦ R vs S • Rectus versus Sinister - based on the atomic mass of the substituents ✦ d (+) vs l (-) • dextrorotatory versus levorotatory - based on the bending of plane polarized light ✦ D vs L • Based on how glyceraldehyde bends plane polarized light Chem 352, Lecture 7 - Carbohydrates X Monosaccharides 9 Trioses ✦ L and D Glyceraldehyde enantiomers • Contains a chiral carbon • Fischer projections ✦ Dihydroxyacetone Chem 352, Lecture 7 - Carbohydrates X These are FischerMonosaccharides 10 Clicker Question:Projections Aldohexoses•Aldoses contain 4 chiral carbons. Therefore, how many different stereoisomers are members of this group of ✦ Trioses through hexoses monosaccharides? A. 4 B. 16 C. 8 D. 12 E. 32 Chem 352, Lecture 7 - Carbohydrates X 11 L-enantiomersMonosaccharides Trioses through hexoses ✦ This figure shows only the D–enantantiomers ✦ The L-enantiomers are mirror images of the D- enantiomers. ✦ Members of an enantiomeric pair are are distinguished using the chirial carbon that is furthest from the carbonyl group. ✦ Most of the monosaccharides that we will encounter are the D-enantiomers. L D D L L D L D Chem 352, Lecture 7 - Carbohydrates X Monosaccharides 12 Question:Question:•Types of optical isomers WhatWhich is the of relationshipfollowing monosaccharides between D-glucose is an and epimer D-mannose? of glucose: ✦ Enantiomers are stereoisomers that are mirror images A. Enantiomersof one another B. Epimers✦ Epimer are stereoisomers having more than one chiral carbon that differ from one another at just one chiral C. Diastereomerscarbon. D. Not stereoisomers A.✦ DiastereomersB. are stereoisomersC. havingD. more than one chiral carbon that differ from one another at multiple chiral carbons. Chem 352, Lecture 7 - Carbohydrates X Monosaccharides 13 Ketoses ✦ Trioses through hexoses Chem 352, Lecture 7 - Carbohydrates X Monosaccharides 14 •The ones to remember ✦ Aldoses • triose ✦ D-glyceraldehyde • tetrose ‣ D-erythrose • pentoses ✦ D-ribose • hexoses ✦ D-glucose ✦ D-mannose ✦ D-galactose Chem 352, Lecture 7 - Carbohydrates X Monosaccharides 15 •The ones to remember ✦ Ketoses • trisoes ✦ dihydroxyacetone • pentoses ✦ D-ribulose ✦ D-xylulose • hexoses ✦ D-fructose Chem 352, Lecture 7 - Carbohydrates X Monosaccharides 16 •Cyclization of aldoses and ketoses ✦ An aldehyde can react spontaneously with an alcohol to form a hemiacetal. ✦ A ketone can react spontaneously with an alcohol to form a hemiketal. ✦ Both of these reactions are addition reactions that lead to the formation of a new chiral carbon. Chem 352, Lecture 7 - Carbohydrates X Monosaccharides 17 •Cyclization of aldoses and ketoses ✦ For aldoses and ketoses, this reaction occurs intramolecularly and leads to a cyclic molecule. ✦ The chiral hemiacetal or hemiketal carbon is called the anomeric carbon. ≈≈ 0% 0% • The new stereoisomers are designated α (-OH down) and β anomers (-OH up). ✦ Haworth projections are used to represent the cyclic 21.5% 58.5% 6.5% 13.5% form of monosaccharides.36%36% 64%64% Chem 352, Lecture 7 - Carbohydrates X Monosaccharides 18 •Cyclization of aldoses and ketoses ✦ The six-member rings are called pyranose rings ✦ The five-member rings are called furanose rings. Chem 352, Lecture 7 - Carbohydrates X Monosaccharides 19 •Cyclization of aldoses and ketoses ✦ pyranose rings • D-glucopyranose (aldohexose) • D-mannopyranose (aldohexose) • D-galactopyranose (aldohexose) ✦ furanose rings • D-fructofuranose (ketohexose) • D-ribofuranose (aldopentose) Chem 352, Lecture 7 - Carbohydrates X Monosaccharides 20 Clicker Question: •Conformations of Monosaccharides Which of following conformations for β-D-glucosopyranose is predicted✦ Monosaccharides to be more stable: can have different conformations. A. B. Chem 352, Lecture 7 - Carbohydrates X 21 9.2 Derivatives of the Monosaccharides Monosaccharides 22 Derivatives of monosaccharides are produced by chemical modifications. ✦ Phosphate esters ✦ Deoxy sugars also called sialic acid • One of the hydroxyl groups is replaced with a hydrogen ✦ Amino sugars • One of the hydroxyl groups is replaced with an amino group. Chem 352, Lecture 7 - Carbohydrates X Monosaccharides 23 Sugar alcohols ✦ The aldehyde or ketone group is reduced to an alcohol. ✦ Some are used as natural artificial sweeteners. erythritol xylitol Chem 352, Lecture 7 - Carbohydrates X Monosaccharides 24 Sugar acids ✦ The aldehyde can be oxidized to an acid to produce aldonic acids. • Which in turn can cyclize to form lactones. ✦ The 6-hydroxyl group on aldohexoses can also be oxidized to an acid to produce uronic acids . Chem 352, Lecture 7 - Carbohydrates X Glycosides 25 The hemiacetal or hemiketal carbon can go on to react with the hydroxyl group from another molecule to form an acetal or ketal. ✦ The bond formed is called a glycosidic bond. ✦ Glycosidic bonds are used to connect one monosaccharide to another. Chem 352, Lecture 7 - Carbohydrates X Glycosides 26 ✦ Unlike hemiacetals and hemiketals, acetals and ketals cannot open and close dynamically, • The glycosidic bond blocks a pyranose or furanose ring from reopening again. ✦ In cells, glycosidic bond formation is enzyme catalyzed and requires a source of free energy. Chem 352, Lecture 7 - Carbohydrates X Monosaccharides 27 Cyclized aldoses and ketoses tend to get locked into the following ring configurations ✦ pyranose rings • 21.5%D-glucopyranose58.5% (aldohexose)6.5% 13.5% • D-mannopyranose (aldohexose) • D-galactopyranose (aldohexose) ✦ furanose rings • D-fructofuranose (ketohexose) • D-ribofuranose (aldopentose) Chem 352, Lecture 7 - Carbohydrates X 28 9.3 Oligosaccharides Disaccharides 29 The glycosidic bond can be used to connect two monosaccharides together to form disaccharides. Important disaccharides include: ✦ Maltose (obtained from starch) α(1-4) glycosidic bond β-anonomer Chem 352, Lecture 7 - Carbohydrates 3 Disaccharides 30 TheClickerQuestion: glycosidic Question: bond can be used to connect two monosacchridesWhichWhat typeanomer of glycosidicof togetherlactose bond is shown?to does form lactose a disaccharides. have? ImportantA. α (1-4) disaccharides include: • B.Lactoseβ(1-6) C. β(1-4) (milk sugar) (1-4) glycosidic bond D. α(1-5) β α-anonomer Chem 352, Lecture 7 - Carbohydrates 4 Disaccharides 31 The glycosidic bond can be used to connect two monosaccharides together to form disaccharides. Important disaccharides include: • Cellobiose (obtained from cellulose) Chem 352, Lecture 7 - Carbohydrates 5 Disaccharides 32 The glycosidic bond can be used to connect two monosaccharides together to form disaccharides. Important disaccharides include: • Sucrose (table sugar) α1-β2 glycosidic bond Chem 352, Lecture 7 - Carbohydrates 6 Disaccharides 33 Question: Because a hemiacetal or hemiketal can easily open to expose Whicheither anend aldehyde of the disaccharideor α-hydroxy maltose ketone, isthey the can reducing still serve end? as reducing agents for the Cu2+ ions found in Fehling’s and Benedict’s reagents. A. B. O O R C H + Cu2+ R C OH + Cu+ CuSO4(aq) Cu2O(s) clear blue red ppt. ‣ This reaction is used to distinguish the two monosaccharides in a disaccharide as the reducing and the nonreducing ends. Chem 352, Lecture 7 - Carbohydrates 7 Disaccharides 34 Not all disaccharides have a reducing end ✦ For example, the disaccharide sucrose contains both and acetal and a ketal, but no hemiacetal or hemiketal. Chem 352, Lecture 7 - Carbohydrates 8 Glycosides 35 Monosaccharides also form glycosidic bonds to non- sugar. ✦ For example, nucleotides, such as • ATP • UDP-glucose • NAD and NADP • FMN and FAD ✦ And sterols, such as Adenosine triphosphate • Ouabain Flavin mononucleotide (FMN) Flavin adenine dinucleotide (FAD) Chem 352, Lecture 7 - Carbohydrates 9 Glycosides 36 Ribitol