Monosaccharides: Structure, Classification, and Reactions

1. Section 7.1: Monosaccharides Carbohydrates Chapter 7 Overview

2. Section 7.1: Monosaccharides • 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

3. Section 7.1: Monosaccharides General Formulas for the Aldose and Ketose Forms of Monosaccharides • 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

4. Section 7.1: Monosaccharides Glyceraldehyde (an Aldotriose) and Dihydroxyacetone (a Ketotriose) • 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

5. Section 7.1: Monosaccharides • Monosaccharide Stereoisomers • An increase in the number of chiral carbons increases the number of possible optical isomers • 2n where n is the number of chiral carbons

6. Section 7.1: Monosaccharides • 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

7. Section 7.1: Monosaccharides • Diastereomers are stereoisomers that are not enantiomers • Diastereomers that differ at a single chiral carbon are epimers (e.g., D-glucose and D-galactose) The Optical Isomers D- and L- Ribose and D- and L- Arabinose

8. Section 7.1: Monosaccharides Formation of Hemiacetals and Hemiketals • 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

9. Section 7.1: Monosaccharides Monosaccharide Structure • 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)

10. Section 7.1: Monosaccharides Haworth Structures of the Anomers of D-Glucose • 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 7.8 Furan and Pyran

11. Section 7.1: Monosaccharides Fischer and Haworth Representations of D-Fructose • Cyclic form of fructose is fructofuranose, while glucose in the pyranose form is glucopyranose

12. Section 7.1: Monosaccharides • 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 a- and b- D-glucose

13. Section 7.1: Monosaccharides • 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 Equilibrium Mixture of D-Glucose

14. Section 7.1: Monosaccharides • 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

15. Section 7.1: Monosaccharides Oxidation Products of Glucose • 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 CH2OH group yields uronic acid

16. Section 7.1: Monosaccharides Oxidation Products of Glucose • 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

17. Section 7.1: Monosaccharides Structure of Ascorbic Acid • Lactones are readily produced in nature, for example L-ascorbicacid (vitamin C) • Vitamin C is a powerful reducing agent that protects cells from reactive oxygen and nitrogen species

18. Section 7.1: Monosaccharides Figure 7.14 Reaction of Glucose with Benedict’s Reagent • 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

19. Section 7.1: Monosaccharides Laboratory Reduction of Glucose to Form D-Glucitol (Sorbitol) • 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)

20. Section 7.1: Monosaccharides Isomerization of D-Glucose to Form D-Mannose and D-Fructose • 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

21. Section 7.1: Monosaccharides • The transformation of glucose to fructose is an aldose-ketose interconversion • The transformation of glucose to mannose is referred to as epimerization Isomerization of D-Glucose to Form D-Mannose and D-Fructose

22. Section 7.1: Monosaccharides • 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

23. Section 7.1: Monosaccharides Formation of Acetals and Ketals • 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

24. Section 7.1: Monosaccharides Methyl Glucoside Formation • 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

25. Section 7.1: Monosaccharides • 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

26. Section 7.1: Monosaccharides The Maillard Reaction • 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)

27. Section 7.1: Monosaccharides The Maillard Reaction • 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