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Chapter 27

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Chapter 27

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  1. Chapter 27 Introduction to General, Organic, and Biochemistry, 10e John Wiley & Sons, Inc Morris Hein, Scott Pattison, and Susan Arena Carbohydrates Green plants turn H2O, CO2, and sunlight into carbohydrates.

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

  3. 27.9Pentoses 27.10Disaccharides 27.11Structures and Properties of Disaccharides 27.12 Sweeteners and Diet 27.13Redox Reactions of Monosaccharides 27.14Polysaccharides Derived from Glucose 27.15Complex Polysaccharides Chapter 27 Summary Course Outline 3 3

  4. Carbohydrates are a principal class of energy-yielding nutrients that are in great abundance. The other two classes are fats and proteins. Carbohydrates are polyhydroxy aldehydes or ketones (aldehydes and ketones with many hydroxyl groups). The simplest carbohydrates are glyceraldehyde and dihydroxyacetone. Carbohydrates: A First Class of Biochemicals 4

  5. 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 in the molecules. Carbohydrates: A First Class of Biochemicals 5

  6. Classification of Carbohydrates The four major types of carbohydrates are: 1) Monosaccharides 2) Disaccharides 3) Oligosaccharides 4) Polysaccharides These classifications are based on the units (monosaccharides) in the molecules. 6

  7. A monosaccharide is the smallest unit of a carbohydrate that cannot be hydrolyzed to a simpler carbohydrate unit. It is the basic carbohydrate unit of cellular metabolism. Glucose is a monosaccharide. Monosaccharides like glucose are important sources of cellular energy. Classification of Carbohydrates 7

  8. Classification of Carbohydrates A disaccharideyields two monosaccharides—either the same or different—when hydrolyzed. Disaccharides are often used by plants and animals to transport monosaccharides from one cell to another. 8

  9. Classification of Carbohydrates The monosaccharides and disaccharides generally have names ending in –ose like glucose, sucrose, and lactose. Monosaccharides and disaccharides are water-soluble carbohydrates, have a characteristically sweet taste, and are often called sugars. 9

  10. Classification of Carbohydrates An oligosaccharide is a carbohydrate with at least two but not more than six monosaccharide units linked together. A polysaccharideis a macromolecular substance that can be hydrolyzed to yield many monosaccharide units. 10

  11. Classification of Carbohydrates Polysaccharides are important structural materials in plants and animals. These carbohydrates also serve as a storage depot for monosaccharides which cells use for energy. 11

  12. Carbohydrates can also be classified bythe: a) Number of carbon atoms in the molecule b) Functional group present in the molecule c) Spatial orientation of the molecule d) Optical activity of the molecule Classification of Carbohydrates 12

  13. Classification of Carbohydrates The monosaccharides shown below are classified based on the number of carbon atoms in the molecules. Monosaccharides commonly have three to seven carbon atoms. 13

  14. Classification of Carbohydrates Mononosaccharides can also be classified based on whether they have the aldehyde or ketone functional group. Monosaccharides with a –CHO (aldehyde) group are known as aldoses whilethose with a –C=O (ketone) group are known as ketoses. The ketone group is usually on carbon #2. 14

  15. Classification of Carbohydrates Monosaccharides can be classified based on their spatial orientation (stereochemistry). A monosaccharide can be classified as a D or L isomer, depending on the spatial orientation of the –H and –OH groups attached to the carbon atom adjacent to the terminal primary alcohol group. The D isomer is represented when the –OH is written to the right of this carbon in the Fischer projection formula. The L isomer is represented when this –OH is written to the left. 15

  16. Classification of Carbohydrates 16

  17. Classification of Carbohydrates The letters D and L do not refer to the direction of optical rotation of a carbohydrate. Monosaccharides that rotate plane-polarized light to the right are known as (+) isomers while those that rotate light to the left are (−) isomers. 17

  18. Classification of Carbohydrates The D andL forms of any specific compound are enantiomers. D-glucose and L-glucose are enantiomers. Notice that orientation of the hydroxyl groups on the carbon atoms adjacent to the terminal primary alcohol groups. 18

  19. Your Turn! Write the projection formula for a D-aldopentose. 19

  20. Your Turn! This is one example of a D-aldopentose. This molecule is a D-isomer because of the orientation of the hydroxyl group (see arrow). The molecule is an aldose because it is an aldehyde and a pentose because it contains five carbon atoms. 20

  21. Your Turn! Classify the following as D- or L-monosaccharides and as aldoses or ketoses. 21

  22. Your Turn! Classify the following as D- or L-monosaccharides and as aldoses or ketoses. 22

  23. Importance of Carbohydrates • Carbohydrates are the most abundant organic chemical in nature. Why are they so important in biochemistry? • They are used by essentially all cells as an energy source. • They are easily transported between and within cells. • They provide essential structural support for both plant and animal cells. 23

  24. The most important monosaccharides are pentoses and hexoses as seen on the next slide . . . Monosaccharides 24

  25. Monosaccharides 25

  26. Monosaccharides Glucose is the most important of the monosaccharides. It is an aldohexose and is found in the free state in plant and animal tissue. Glucose is also known as dextrose and grape sugar. Glucose is a component of the disaccharides sucrose, maltose, and lactose and is the monomer in the polysaccharides amylose, amylopectin, cellulose, and glycogen. Glucose is the key sugar of the body and is carried by the bloodstream to all body parts. 26

  27. Monosaccharides Galactose is an aldohexose like glucose and occurs, along with glucose, in lactose and in many oligo- and polysaccharides. Galactose is synthesized in the mammary glands to make the disaccharide lactose (milk sugar). It is also a constituent of glycolipids and glycoproteins in many cell membranes. 27

  28. Monosaccharides Fructose, also known as levulose, it is a ketohexose that occurs in fruit juices, honey, and, along with glucose, is a constituent of the disaccharide sucrose. Fructose is the major constituent of the polysaccharide inulin, a starchlike substance present in many plants such as dahlia tubers, chicory roots, and Jerusalem artichokes. Fructose is the sweetest of all the common sugars, being about twice as sweet as glucose. This accounts for the sweetness of high-fructose corn syrup and honey. 28

  29. Monosaccharides The Fischer projections of D-glucose, D-galactose and D-fructose are shown here. 29

  30. Monosaccharides The Fischer projections of the pentoses D-ribose and D-2-deoxyribose are shown here. 30

  31. Structure of Glucose and Other Aldoses • The structures of monosaccharides are frequently drawn as Fischer projections. These drawings have the following characteristics. • The keto or aldehyde group is placed at the top of the projection. • Each interior carbon atom is generally shown as an intersection point between two lines. • The –H atom and –OH group attached to interior carbons are written to left or right of the projection. 31 31

  32. Structure of Glucose and Other Aldoses The simplest carbohydrate is glyceraldehyde. The Fischer projections of D-glyceraldehyde is shown here. 32

  33. Structure of Glucose and Other Aldoses This is the Fischer projections of L-glyceraldehyde. 33

  34. Structure of Glucose and Other Aldoses 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 the chiral carbon atom is #2. 34

  35. Structure of Glucose and Other Aldoses The enantiomers of glucose can also be represented by Fischer projection formulas. 35

  36. Structure of Glucose and Other Aldoses Glucose is an aldohexose with four chiral carbon atoms. According to the 2nformula glucose should have 16 optical isomers. Eight have the D-configuration and eight have the L-configuration. Remember that D- and L- isomers are enantiomers. The D-isomers of the aldoses (with 3-6 carbon atoms) are shown on Figure 27.1 on the following slide . . . 36

  37. 37

  38. Your Turn! Draw the enantiomers of D-allose and D-talose using Figure 27.1 on the previous slide. 38

  39. Your Turn! Draw the enantiomers of D-allose and D-talose using Figure 27.1 on the previous slide. The enantiomer of D-allose is L-allose. These molecules are mirror images. 39

  40. Your Turn! Draw the enantiomers of D-allose and D-talose using Figure 27.1 on the previous slide. The enantiomer of D-talose is L-talose. These molecules are mirror images. 40

  41. Structure of Glucose and Other Aldoses The 16 aldohexoses have all been synthesized, but only D-glucose, D-mannose, and D-galactose appear to be of considerable biological importance. The laboratory conversion of one aldose into another aldose containing one more carbon atom is known as the Kiliani–Fischer synthesis. 41

  42. Structure of Glucose and Other Aldoses • The Kiliani–Fischer synthesis involves: • the addition of HCN to form a cyanohydrin • hydrolysis of the –CN group to –COOH; and • reduction with sodium amalgam, Na(Hg), to form the aldehyde. • The synthesis of two aldotetroses from an aldotriose is shown on the following slide . . . 42

  43. Structure of Glucose and Other Aldoses 43

  44. Your Turn! Show the conversion of D-threose (a tetrose) to two D-pentoses using the Kiliani–Fischer synthesis. 44

  45. Your Turn! 45

  46. Cyclic Structure of Glucose; Mutatoration Straight open-chain D-glucose is so reactive that almost all molecules quickly rearrange their bonds to form two new structures. These structures are six-membered rings called pyranosesugars. 46

  47. Cyclic Structure of Glucose; Mutatoration An interesting phenomenon occurs when the α- and β-forms of glucose are put into separate solutions and allowed to stand for several hours. The phenomena that occurs is called mutarotation. 47

  48. Cyclic Structure of Glucose; Mutatoration During mutarotation the two cylic forms convert into each other through the open-chain form. The resulting equilibrium solution contains about 36% α and 64% βmolecules with a trace of open-chain molecules. 48

  49. Cyclic Structure of Glucose; Mutatoration The cyclic forms only differ in the stereo arrangement of the carbon atom involved in the mutarotation (carbon #1). This carbon atom is called the anomeric carbon. 49

  50. Cyclic Structure of Glucose; Mutatoration Two cyclic isomers that differ only in their stereo arrangement about the carbon involved in mutarotation are called anomers. The α- and β- forms are anomers. 50