Hein * Pattison * Arena * Best Chapter 27 Carbohydrates Hein * Pattison * Arena * Best Jerry Poteat Science Department Georgia Perimeter College © John Wiley and Sons, Inc. Version 1.0 1 1

Chapter Outline 27.1 Carbohydrates: A First Class of Biochemicals 27.2 Classification of Carbohydrates 27.3 Importance of Carbohydrates 27.4 Monosaccharides 27.5 Structure of Glucose and Other Aldoses 27.6 Cyclic Structure of Glucose; Mutarotation 27.7 Hemiacetals and Acetals 27.8 Structures of Galactose and Fructose 2 2

Chapter Outline 27.9 Pentoses 27.10 Disaccharides 27.11 Structures and Properties of Disaccharides 27.12 Sweeteners and Diet 27.13 Redox Reactions of Monosaccharides 27.14 Polysaccharides Derived from Glucose 27.15 Complex Polysaccharides 3 3

27.1 Carbohydrates: A First Class of Biochemicals 4 4

What are carbohydrates? Carbohydrates are energy-yielding macronutrients in the same class of nutrients as fats and proteins. These polyhydroxy aldehydes or ketones include simple carbohydrates like glyceraldehyde and dihydroxyacetone.

Carbohydrates are important in society because they provide; (1) basic diets in the form of starch and sugar and……. (2) clothing and shelter. Many of the chemical properties of carbohydrates are determined by the chemistry of the hydroxyl and carbonyl functional groups.

27.2 Classification of Carbohydrates

Types of Carbohydrates The four major types of carbohydrates are…….. 1. Monosaccharides 2. Disaccharides 3. Oligosaccharides 4. Polysaccharides

Monosaccharides A monosaccharide is the smallest unit of a carbohydrate that cannot be hydrolyzed to a simpler unit. It is the basic carbohydrate unit of cellular metabolism For example glucose is a monosaccharide. Monosaccharides like glucose are important sources of cellular energy. 9 9

Types of Monosaccharides Monosaccharides can be classified by the …. (a) number of carbon atoms in the molecule ( e.g. a pentose versus a hexose) functional group ( aldoses versus ketoses) configuration ( D versus L isomers) (d) optical activity ( (+) versus (-) isomers ) ring structure ( furanoses versus pyranoses) stereochemistry at an anomeric carbon ( versus  isomers) 10 10

Types of Monosaccharides Number of Carbons The monosaccharides shown below are classified based on the number of carbons in the molecule. 11 11

Types of Monosaccharides Functional Group Mononosaccharides with a –CHO (aldehyde) group are known as aldoses while those with a (keto) group are known as ketoses. 12 12

Types of Monosaccharides Configuration Monosaccharides with the –OH group on the right of the carbon alpha to the terminal ROH carbon are D isomers while those with the –OH group on the left are L isomers. 13 13

Types of Monosaccharides Optical Activity Monosaccharides that rotate plane-polarized light to the right are known as (+) isomers while those that rotate it to the right are (-) isomers. Note: The D and L designations indicate the direction of rotation, e.g. the D isomer of glucose could be either the (+) isomer or it can be the (-) isomer. 14 14

Types of Monosaccharides Ring Structure The cyclic form of monosaccharides that have five atoms in the ring are known as furanoses while those with six atoms are known as pyranoses based on the corresponding heterocyclic ring structures furan and pyran. 15 15

Types of Monosaccharides Anomeric Configuration Monosaccharides that have an –OH below the ring at the anomeric carbon are known as  (alpha) isomers while those with the –OH above the ring are  (beta) isomers. 16 16

Disaccharides A disaccharide yields two monosaccharides when hydrolyzed as shown below.

General Characteristics of Monosaccharides and Disaccharides Disaccharides are often used by plants or animals to transport monosaccharides from one cell to another. Both monosaccharides and disaccharides generally have the ending –ose . For example, glucose, sucrose, and lactose. Both are water-soluble carbohydrates that have a sweet taste and are therefore called sugars.

Oligosaccharides An oligosaccharide is a carbohydrate with at least two but not more than six monosaccharide units linked together. 19 19

Polysaccharides A polysaccharide is a macromolecule ( i.e. polymer) that produces many monosaccharide units when hydrolyzed Polysaccharides are important structural materials in plants and animals. These carbohydrates also serve as a storage depot for monosaccharides which cells use for energy.

27.3 Importance of Carbohydrates 21 21

Why are carbohydrates so important? Carbohydrates are important because they are widely available and because they have exceptional utility. 22 22

27.4 Monosaccharides 23 23

The most important monosaccharides are the pentoses and hexoses as shown in the diagram below . 24 24

Glucose Glucose and galactose are both aldohexoses. An aldohexose is a hexose with an aldehyde group. Glucose is the most important monosaccharide because of its role in cellular metabolism.

Glucose The concentration of glucose in blood is normally about 80-100 mg/100 mL Glucose is known by either of the following names dextrose (from dextrorotatory), grape sugar (found in grapes) , or blood sugar (because it is transported in the blood). 26 26

Galactose Galactose, like glucose, is an aldohexose and is a component of the disaccharide lactose. Fructose Fructose is a ketohexose (i.e. it is a hexose with a keto group). Fructose is known as levulose and is found in fruit juices and honey. It is the sweetest common sugar being about two times sweeter than glucose. 27 27

Structures of the Hexoses (Glucose, Galactose, and Fructose) These structures are shown as Fischer projection formulas where each line intersection represents a carbon atom. 28 28

Structures of the Pentoses (Ribose and Deoxyribose) 29 29

27.5 Structure of Glucose and Other Aldoses 30 30 30 30

Glucose Enantiomers The enantiomers of glucose can be represented by Fischer projection formulas. 31 31 31

Fischer projection formulas are drawn with these characteristics: The keto or aldehyde group is placed at the top of the projection. (2) Each interior carbon atom is shown as an intersection point between two lines ( ) (3) The H atom and –OH group are written to left or right of the projection . 32 32 32 32

This is an example of a modified structural formula of D-glucose written as a Fischer projection formula. 33 33 33 33

The enantiomers of glyceraldehyde are also known as epimers. Epimers are stereoisomers that differ only in the configuration at a single chiral carbon atom. In the case of glyceraldehyde this atom is carbon 2. 34 34

Fischer projections can also be represented by a three-dimensional representation. 35 35

(continued from previous slide) Fischer projections are shown for the family of D-aldoses in Figure 27.1 and the Kiliani-Fischer synthesis is shown in Figure 27.2. 36 36

Figure 27. 1 The D-family of aldoses Figure 27. 1 The D-family of aldoses. The red –OH group indicate the new chiral carbon added in case from top to bottom of the diagram.

Figure 27.2 An example of the Kiliani-Fischer synthesis in which two aldotetrose molecules are formed from an aldotriose molecule.

27.6 Cyclic Structure of Glucose; Mutarotation 39 39 39 39

What is mutarotation? Mutarotation is the change in specific rotation of an anomer as it is converted into an equilibrium mixture of the  and  forms as shown in Figure 27.3. Figure 27.3 Fischer projections and Haworth formulas showing mutarotation of D-glucose. Fischer projections 40 40

Figure 27.3 ( continued) Haworth formulas 41 41

What is an anomer? An anomer is the  or  form of a monosaccharide as shown here. 42 42

Haworth Formulas Haworth formulas are structural formulas that represent cyclic sugars. In the case of glucose the formula is drawn as a flat hexagon with H and –OH written above and below the plane of the ring. Haworth formulas are sometimes shown in a abbreviated form as shown here.

Most naturally occurring monosaccharides however occur in the chair conformation as shown in the ball-and-stick model below. The spacefilling model is a more accurate representation of a sugar molecule. Ball-and-stick model Spacefilling model Figure 27. 4 Three-dimensional representations of the chair form of a-D-glucopyranose 44 44

27.7 Hemiacetals and Acetals 45 45

Hemiacetals Hemiacetals are structures that contain an alkoxy group and a hydroxyl group on the same carbon atom. 46 46

The cyclic structures of monosaccharides are intramolecular hemiacetals. Five and six -membered ring hemiacetals are stable but these rings can open in aqueous solution to the straight-chain aldehyde. 47 47

Acetals Acetals are structures that contain two alkoxy groups on the same carbon atom. 48 48

Glycosides Cyclic acetals are known as glycosides and glycosides are derivatives of hemiacetals. 49 49

Glycosides Glycosides like the methyl isomers shown below are less reactive than the corresponding monosaccharide. The methyl isomers shown below will not undergo mutarotation. 50 50

27.8 Structures of Galactose and Fructose 51 51

Structure of Galactose Galactose has the same structure as glucose except the configuration at carbon four is reversed as shown here. 52 52

Structure of Galactose Galactose is an aldohexose like glucose and like glucose it also exists in the alpha and beta cyclic pyranose forms.

Structure of Fructose Fructose is a ketohexose and like glucose it also exists in the open-chain and cyclic forms as shown here. ( open-chain form) ( cyclic form)

27.9 Pentoses 55 55 55 55

Structure of Ribose and Deoxyribose D-Ribose and its derivative D-2-dexoyribose are pentoses found in nuclei acids RNA and DNA. Notice that the 2-deoxy in D-2-deoxyribose means an oxygen is omitted from the D-ribose molecule at carbon two. 56 56

Structure of Ribulose Ribulose is a ketose that is related to ribose. It is a biological intermediate used by cells to make other monosaccharides. 57 57

27.10 Disaccharides 58 58 58 58

Disaccharides Disaccharides are carbohydrates consisting of two monosaccharides. The two monosaccharides are connected by a glycosidic linkage as shown here for the disaccharide lactose . lactose 59 59

Disaccharides Sucrose and lactose are important disaccharides found in the free state in nature. Sucrose is known as table sugar while lactose is known as milk sugar. Both undergo hydrolysis in the presence of an acid or the enzymes sucrase or lactase respectively. 60 60

Disaccharides Maltose is not found in the free state but is the product when a polysaccharide is degraded during the sprouting of grain. Maltose undergoes hydrolysis in the presence of acid or maltase to produce two molecules of glucose. 61 61

27.11 Structures and Properties of Disaccharides 62 62 62 62

Formation of Maltose

Structure of Lactose Shown here are Haworth structures using the stacked position convention and the bent structure convention. stacked position bent structure 64 64

This is a Haworth projection formula of the disaccharide sucrose 65 65

27.12 Sweeteners and Diet 66 66 66 66

Importance of Sucrose as a Sweetener Sucrose represents 40-60% of all sweeteners and is 20-30% of the average caloric intake in the United States because of its low price and sweet taste. Sucrose is hydrolyzed to prevent crystallization in certain food preparations and in these cases it is known as invert sugar. 67 67

Artificial Sweeteners Artificial sweeteners have been developed with the intent to balance the concerns for safety, relative sweetness, and aftertaste. Many artificial sweeteners have a higher relative sweetness than the common sweeteners like sucrose, glucose or fructose as shown in Table 27.1 68 68

69 69

Artificial Sweeteners The compounds shown below are examples of artificial sweeteners. 70 70

The history of sodium cyclamate illustrates the difficulty in balancing consumer safety with the needs of the consumer market. This sweetener was banned in 1970 because of research that indicated risks of cancer from consuming the sweetener. Natural sweeteners being developed as substitutes for sucrose include sweeteners derived from citrus fruits and stevoside. 71 71

27.13 Redox Reactions of Monosaccharides 72 72 72 72

Oxidation of Aldohexoses The aldehyde group in monosaccharides can be oxidized to monocarboxylic acids with a mild oxidizing agent. For example glucose is oxidized to gluconic acid in the presence of bromine water.

Oxidation of Aldohexoses Dicarboxylic acids are formed when aldohexoses are treated with stronger oxidizing agents. For example glucose is oxidized to glucaric acid in the presence of nitric acid. 74 74

Reduction of Aldohexoses Hexahydric alcohols ( six –OH groups) are formed when aldohexoses are treated with reducing agents. For example glucose is reduced to glucitol (sorbitol) in the presence of H2/Pt. 75 75

Redox Tests for Carbohydrates A reducing sugar is a compound that will reduce Ag + → Ag or Cu2+ → Cu+. A reducing sugar will have one of the following groups; An aldehyde group ( e.g. glyceraldehyde) A hydroxyketone ( e.g. fructose) A cyclic hemiacetal group ( e.g. glucose or maltose) 76 76

Redox Tests for Carbohydrates The Benedict, Barfoed, and Fehling tests are based on the formation of a brick red copper(I) oxide precipitate as a positive result while the Tollens test is based on the formation of a silver mirror. The Barfoed test is more sensitive in that it can distinguish a reducing monosaccharide from a reducing disaccharide. 77 77

Summary of Redox Tests 78 78

Redox Tests for Carbohydrates Many clinical tests that monitor glucose are based on the reaction shown here. 79 79

Reduction of Hemiacetals Sugars with the hemiacetal structure can be reduced under alkaline conditions because the ring opens as shown below forming an aldehyde group. Therefore glucose, lactose, and maltose have the hemiacetal structure and are reducing sugars but the disaccharide sucrose is not a reducing sugar because it does not have the hemiacetal structure. 80 80

27.14 Polysaccharides Derived from Glucose 81 81

Glucose Based Polysaccharides There are three types of naturally occurring polysaccharides; cellulose, glycogen, and starch as shown below. 82 82

Starch, glycogen, and cellulose all yield D-glucose when hydrolyzed as shown here. 83 83

Starch Starch is a polysaccharide composed of amylose and amylopectin. Amylose is a large molecule consisting of unbranched chains composed of about 25-1300 a-D-glucose units joined by a-1,4-glycosidic linkages. Amylopectin is a large molecule with branched chains composed of a-1,4-glycosidic linkages in the main chain and a-1,6-glycosidic linkages at branch points as seen in Figure 27.7.

Figure 27.7 Molecular Structure of Amylose No Branching Glucose units in Amylose Amylose Figure 27.7 Molecular Structure of Amylose

Figure 27.7 (continued) Molecular Structure of amylopectin. Branching Glucose units in Amylopectin Amylopectin Figure 27.7 (continued) Molecular Structure of amylopectin.

Hydrolysis of Starch An important reaction during digestion is the hydrolysis of starchy foods as shown below. Starch is not soluble in cold water and will form a colloidal dispersion in hot water. Starch solutions form a blue-black color in the presence of free iodine. 87 87

Glycogen Glycogen is a carbohydrate polymer that is stored in the liver and muscle tissues in animals. Glycogen has a structure similar to amylopectin except it is more highly branched with the a-1,6-glycosidic linkages occurring more frequently along the polymer chain.

Cellulose Cellulose, like starch and glycogen, is a glucose based polymer. However the glucose units in cellulose are join by -1,4-glycosidic linkages instead of -1,4-glycosidic linkages.

Cellulose This change in stereochemistry at the anomeric carbon allows extensive hydrogen bonding in cellulose as shown in Figure 27.9. Cellulose is the most abundant organic substance found in nature and it is the chief structural component of plants and wood. 90 90

Haworth formula Figure 27.9 Two representations of cellulose. In the three-dimensional model note the hydrogen bonding that links the extended cellulose polymers to form cellulose fibers. Three-dimensional model of cellulose

27.15 Complex Polysaccharides 92 92

Polysaccharides Complex polysaccharides are found in animal tissue to include glycosaminoglycans and antigens. Glycosaminoglycans are part of the connective tissue found in joints such as the knee. These complex polysaccharides act as shock absorbers between bones. 93 93 93 93

Antigens Antigens are even more complex polysaccharides and act as labels to help the immune system differentiate an animal own cells from invading bacteria. Antigens are found on red blood cells and are used in the ABO classification system as shown in Figure 27.10. 94 94 94 94

Figure 27.10 These are antigens that are used in the ABO blood group classification system. 95 95 95 95

Chapter 27 Summary Carbohydrates are polyhydroxy substances that are classified as monosaccharides, disaccharides, oligosaccharides or polysaccharides. Important monosaccharides include ribose, deoxyribose, glucose, and fructose. Important disaccharides include sucrose, lactose, and maltose. Important polysaccharides include glycogen, starch, and cellulose. Monosaccharides that differ in the configuration at a single carbon atom are called epimers. A glycoside is a cyclic acetal derived from a monosaccharide. 96 96

Chapter 27 Summary Glucose exists in the straight-chain and cyclic forms. The isomers formed when glucose forms a ring structure are known as anomers. The interconversion between glucose anomers is known as mutarotation. The hydrolysis of sucrose ( table sugar) produces invert sugar which is a mixture of glucose and fructose. Reducing sugars like glucose can reduce copper(II) to copper(I) or silver ions to the silver metal. Reducing sugars will generally have an aldehyde group, an -hydroxy ketone group, or a hemiacetal group. 97 97