CHAPTER 07A

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Chapter 7:

From McKee and McKee, Biochemistry , 4th Edition, © 2009 Oxford University Press Chapter 7 Section 7.1: Monosaccharides Section 7.2: Disaccharides Section 7.3: Polysaccharides Section 7.4: Glycoconjugates Section 7.5: The Sugar Code Biochemistry in Perspective Carbohydrates Overview

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From McKee and McKee, Biochemistry , 4th Edition, © 2009 Oxford University Press Carbohydrates are the most abundant biomolecule in nature Have a wide variety of cellular functions: energy, structure, communication, and precursors for other biomolecules They are a direct link between solar energy and chemical bond energy Section 7.1: Monosaccharides

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Section 7.1: Monosaccharides From McKee and McKee, Biochemistry , 4th Edition, © 2009 Oxford University Press Monosaccharides , or simple sugars, are polyhydroxy aldehydes or ketones Sugars with an aldehyde functional group are aldoses Sugars with an ketone functional group are ketoses Figure 7.1 General Formulas for the Aldose and Ketose Forms of Monosaccharides

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Section 7.1: Monosaccharides From McKee and McKee, Biochemistry , 4th Edition, © 2009 Oxford University Press Carbohydrates are also classified by the number of carbon atoms they contain Trioses, tetroses, pentoses, and hexoses Most abundent in living cells are hexoses and pentoses Class names often combine information about carbon number and functional group Figure 7.2 Glyceraldehyde (an Aldotriose) and Dihydroxyacetone (a Ketotriose)

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Section 7.1: Monosaccharides From McKee and McKee, Biochemistry , 4th Edition, © 2009 Oxford University Press Monosaccharide Stereoisomers An increase in the number of chiral carbons increases the number of possible optical isomers 2 n where n is the number of chiral carbons Figure 7.3 The D Family of Aldoses

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Section 7.1: Monosaccharides From McKee and McKee, Biochemistry , 4th Edition, © 2009 Oxford University Press In optical isomers, the reference carbon is the asymmetric carbon farthest from the carbonyl carbon Almost all naturally occurring monosaccharides are the D form All can be considered to be derived from D -glyceraldehyde or nonchiral dihydroxyacetone Figure 7.3 The D Family of Aldoses

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Section 7.1: Monosaccharides From McKee and McKee, Biochemistry , 4th Edition, © 2009 Oxford University Press Diastereomers are stereoisomers that are not enantiomers Diastereomers that differ at a single chiral carbon are epimers (e.g., D -glucose and D -galactose) Figure 7.4 The Optical Isomers D- and L- Ribose and D- and L- Arabinose

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Section 7.1: Monosaccharides From McKee and McKee, Biochemistry , 4th Edition, © 2009 Oxford University Press Cyclic Structure of Monosaccharides Sugars with four or more carbons exist primarily in cyclic forms Ring formation occurs because aldehyde and ketone groups react reversibly with hydroxyl groups in an aqueous solution to form hemiacetals and hemiketals Figure 7.5 Formation of Hemiacetals and Hemiketals

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Section 7.1: Monosaccharides From McKee and McKee, Biochemistry , 4th Edition, © 2009 Oxford University Press The two possible diastereomers that form because of cyclization are called anomers Hydroxyl group on hemiacetal occurs on carbon 1 and can be in the up position (above ring) or down position (below ring) In the D -sugar form, when the anomer hydroxyl is up it gives a b -anomeric form (left in Fischer projection) while down gives the a -anomeric form (right) Figure 7.6 Monosaccharide Structure

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Section 7.1: Monosaccharides From McKee and McKee, Biochemistry , 4th Edition, © 2009 Oxford University Press Haworth Structures- these structures more accurately depict bond angle and length in ring structures than the original Fischer structures Five-membered rings are called furanoses and six-membered rings are pyranoses Figure 7.7 Haworth Structures of the Anomers of D -Glucose Figure 7.8 Furan and Pyran

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Section 7.1: Monosaccharides From McKee and McKee, Biochemistry , 4th Edition, © 2009 Oxford University Press Cyclic form of fructose is fructofuranose, while glucose in the pyranose form is glucopyranose Figure 7.9 Fischer and Haworth Representations of D -Fructose

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Section 7.1: Monosaccharides From McKee and McKee, Biochemistry , 4th Edition, © 2009 Oxford University Press Conformational Structures Conformational structures are more accurate than Haworth, because they show the puckered nature of sugar rings X-ray and bond angle analysis Space-filling models also give useful information Figure 7.10 a - and b - D -glucose

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Section 7.1: Monosaccharides From McKee and McKee, Biochemistry , 4th Edition, © 2009 Oxford University Press Mutarotation The a - and b -forms of monosaccharides are readily interconverted in aqueous environments This spontaneous process, mutarotation , produces an equilibrium mixture of a - and b -forms in both furanose and pyranos ring structures Open chain form can participate in redox reactions Figure 7.11 Equilibrium Mixture of D -Glucose

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Section 7.1: Monosaccharides From McKee and McKee, Biochemistry , 4th Edition, © 2009 Oxford University Press Reaction of Monosaccharides The carbonyl and hydroxyl groups can undergo several chemical reactions Most important include: oxidation, reduction, isomerization, esterification , glycoside formation, and glycosylation reactions

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Section 7.1: Monosaccharides From McKee and McKee, Biochemistry , 4th Edition, © 2009 Oxford University Press Oxidation- monosaccharides may readily undergo several oxidation reactions in the presence of metal ions or certain enzymes Oxidation of the aldehyde group yields aldonic acid Oxidation of the terminal CH 2 OH group yields uronic acid Figure 7.12 Oxidation Products of Glucose

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Section 7.1: Monosaccharides From McKee and McKee, Biochemistry , 4th Edition, © 2009 Oxford University Press Oxidation of both groups yields aldaric acid A lactone can be produced if the carbonyl groups of aldonic or uronic acids react with an OH group in the same molecule Figure 7.12 Oxidation Products of Glucose

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Section 7.1: Monosaccharides From McKee and McKee, Biochemistry , 4th Edition, © 2009 Oxford University Press Lactones are readily produced in nature, for example L -ascorbic acid (vitamin C) Vitamin C is a powerful reducing agent that protects cells from reactive oxygen and nitrogen species Figure 7.13 Structure of Ascorbic Acid

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Section 7.1: Monosaccharides From McKee and McKee, Biochemistry , 4th Edition, © 2009 Oxford University Press Sugars that can be reduced by weak, oxidizing agents such as Benedict’s reagent are called reducing sugars Needs open chain so all monosaccharides are reducing sugars Figure 7.14 Reaction of Glucose with Benedict’s Reagent

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Section 7.1: Monosaccharides From McKee and McKee, Biochemistry , 4th Edition, © 2009 Oxford University Press Reduction- Sugar alcohols ( alditols ) are produced by the reduction of aldehyde and ketone groups of monosaccharides Sugar alcohols are used in commercial food processing and in pharmaceuticals (e.g., sorbitol can be used to prevent moisture loss) Figure 7.15 Laboratory Reduction of Glucose to Form D -Glucitol (Sorbitol)

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Section 7.1: Monosaccharides From McKee and McKee, Biochemistry , 4th Edition, © 2009 Oxford University Press Isomerization- monosaccharides can undergo several types of isomerization D -glucose incubated in an alkaline solution for several hours produces two isomers: D -mannose and D -fructose Both involve an enediol intermediate Figure 7.16 Isomerization of D -Glucose to Form D -Mannose and D -Fructose

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Section 7.1: Monosaccharides From McKee and McKee, Biochemistry , 4th Edition, © 2009 Oxford University Press The transformation of glucose to fructose is an aldose-ketose interconversion The transformation of glucose to mannose is referred to as epimerization Figure 7.16 Isomerization of D -Glucose to Form D -Mannose and D -Fructose

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Section 7.1: Monosaccharides From McKee and McKee, Biochemistry , 4th Edition, © 2009 Oxford University Press Esterification- free OH groups of carbohydrates can be converted to esters by reactions with acids Esterification often dramatically changes a sugar’s chemical and physical properties Sulfate esters of carbohydrate molecules are found predominantly in the proteoglycan components of connective tissue Participate in forming of salt bridges between carbohydrate chains

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Section 7.1: Monosaccharides From McKee and McKee, Biochemistry , 4th Edition, © 2009 Oxford University Press Glycoside Formation- hemiacetals and hemiketals react with alcohols to form the corresponding acetal and ketal When the cyclic hemiacetal or hemiketal form of the monosaccharide reacts with an alcohol, the new linkage is a glycosidic linkage and the compound a glycoside Figure 7.17 Formation of Acetals and Ketals

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Section 7.1: Monosaccharides From McKee and McKee, Biochemistry , 4th Edition, © 2009 Oxford University Press Naming of glycosides specifies the sugar component Acetals of glucose and fructose are glucoside and fructoside If an acetal linkage is formed between the hemiacetal hydroxyl of one monosaccharide and the hydroxyl of another, this forms a disaccharide In polysaccharides , large numbers of monosaccharides are linked together through acetal linkages Figure 7.18 Methyl Glucoside Formation

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Section 7.1: Monosaccharides From McKee and McKee, Biochemistry , 4th Edition, © 2009 Oxford University Press Glycosylation Reactions attach sugars or glycans (sugar polymers) to proteins or lipids Catalyzed by glycosyl transferases, glycosidic bonds are formed between anomeric carbons in certain glycans and oxygen or nitrogen of other types of molecules, resulting in N- or O-glycosidic bonds N-Glycosidic linkages form between oligosaccharides and the side chain amide nitrogen of certain asparagine residues O-Glycosidic linkages attach glycans to the side chain hydroxyl of serine or threonine residues, or the hydroxyl oxygens of membrane lipids

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Section 7.1: Monosaccharides From McKee and McKee, Biochemistry , 4th Edition, © 2009 Oxford University Press Glycation is the reaction of reducing sugars with nucleophilic nitrogen atoms in a nonenzymatic reaction Most researched example of the glycation reaction is the nonenzymatic glycation of protein ( Maillard reaction ) Figure 7.20 The Maillard Reaction

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Section 7.1: Monosaccharides From McKee and McKee, Biochemistry , 4th Edition, © 2009 Oxford University Press The Schiff base that forms rearranges to a stable ketoamine, called the Amadori product Can further react to form advanced glycation end products (AGEs) Promote inflammatory processes Figure 7.20 The Maillard Reaction

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