16 16-1 16 16-2 16 16-3 16 Carbohydrates Learning Objectives 1. What Are the Structures and the Stereochemistry of Monosaccharides?

2. How Do Monosaccharides React? 3. What are Disaccharides ? 4. What Are Some Important Oligosaccharides? 5. What Are the Structures and Functions of Polysaccharides? 6. What Are Glycoproteins? 16-4 16 Carbohydrates Carbohydrate: a polyhydroxyaldehyde or polyhydroxyketone, or a substance that contain C:H:O and H:O ratio is 2:1 (as water) ~ose designate sugars. Monosaccharide: a carbohydrate that cannot be hydrolyzed to a simpler carbohydrate they have the general formula CnH2nOn, where n varies from 3 to 8

Monosaccharide maybe aldose: a monosaccharide containing an aldehyde group ketose: a monosaccharide containing a ketone group 16-5 16 Monosaccharides Monosaccharides are classified by their number of carbon atoms Name triose tetrose pentose hexose heptose octose

Formula C3 H6 O3 C4 H8 O4 C5 H10 O5 C6 H12 O6 C7 H14 O7 C8 H16 O8 16-6 16 Fischer Projections Fischer projection: a two dimensional representation for showing the configuration of tetrahedral horizontal lines represent bonds projecting forward vertical lines represent bonds projecting to the rear CHO H

C OH convert to a Fischer projection CH2 OH D-Glyceraldehyde CHO H OH CH2 OH D-Glyceraldehyde

16-7 16 Monosaccharides There are only two trioses CHO C H2 O H CHOH C= O C H2 O H C H2 O H Glyceraldehyde (an aldotriose)

D ihydroxyacetone (a ketotriose) aldo- and keto- are often omitted and these compounds are referred to simply as trioses; although this designation does not tell the nature of the carbonyl group, it at least tells the number of carbons 16-8 16 D- Aldoses 16-9 16 D-Ketoses

16-10 16 Sugar Hexoses of physiologic importance. D-Glucose D-Fructose Source Importance Clinical Significance Fruit juices. The sugar of Present in the

Hydrolysis of the body. urine (glucosuria) starch, cane carried by in diabetes sugar, blood the mellitus owing to maltose, and principal one raised blood lactose. used by the glucose tissues (hyperglycemia) Fruit juices. Honey. Hydrolysis of cane sugar and of inulin. Can be

changed to glucose in the liver Hereditary fructose intolerance leads to fructose accumulation and hypoglycemia. 16-11 16 Hexoses of physiologic importance. Sugar Source Importance D-Galactose Hydrolysis Can be changed to

of lactose. glucose in the liver and metabolized. Synthesized in the mammary gland to make the lactose of milk. A constituent of glycolipids and glycoproteins. D-Mannose Hydrolysis A constituent of many of plant glycoproteins. mannans and gums. Clinical Significance Failure to

metabolize leads to galactosemia and cataract. 16-12 16 Sucralose ( Splenda ) Discovered in 1976, sucralose is 600 times sweeter than sugar and does not metabolize to produce energy, thus it does not contain calories. It is the only low calorie sweetener that is made from sugar, which has been changed so passes through the body unchanged and unmetabolized. Substituting for three alcohol groups on the sugar molecule with 3 chlorine atoms. 16-13

16 Stevia Stevia is a sweetener and sugar substitute extracted from the leaves of the plant species Stevia rebaudiana which is grown in Brazil and Paraguay. Stevia has no calories, and it is 200 times sweeter than sugar in the same concentration. 16-14 16 Monosaccharides Glyceraldehyde contains a stereocenter (chiral C) and exists as a pair of stereoisomers (enantiomers) CHO CHO

H C OH CH2 OH D-Glyceraldehyde HO C H CH2 OH L-Glyceraldehyde Note: Number of stereoisomers= 2n

n is the number of stereocenters (chiral carbon) 16-15 16 D, L Monosaccharides According to the conventions proposed by Fischer D-monosaccharide: a monosaccharide that, when written as a Fischer projection, has the -OH on its penultimate carbon (the one before the last= the highest numbered chiral carbon) on the right L-monosaccharide: a monosaccharide that, when written as a Fischer projection, has the -OH on its penultimate carbon (the one before the last= the highest numbered chiral carbon) on the left 16-16 16 The four aldotetroses

Enantiomers: stereoisomers that are mirror images . example: D-erythrose and L-erythrose are enantiomers Diastereomers: stereoisomers that are not mirror images . Epimers: diastereomers that differ from each other in configuration of one chiral carbon example: D-erythrose and D-threose are diastereomers Mirror plane CHO H OH H OH

CH2 OH CHO HO H HO H CH2 OH D-Erythrose L-Erythrose Mirror plane CHO HO H H OH CH2 OH

CHO H OH HO H CH2 OH D-Threose L-Threose 16-17 16 D, L Monosaccharides Following are the two most common D-aldotetroses and the two most common D-aldopentoses


OH CH2 OH D-ErythroseD-Threose D-Ribose CHO H H H OH H OH CH2 OH 2-Deoxy-Dribose 16-18

16 The three most common D-aldohexoses. Note that the third of these is an amino sugar. also shown is the most common 2-keto-D-hexose H HO H H CHO CHO OH H OH H HO H


HO H OH H OH OH H OH CH2 OH CH2 OH D-Glucose D-Galactose D-Glucosamine D-Fructose 16-19 16 Cyclic Structure Monosaccharides especially pentoses and

hexoses have -OH and C=O groups in the same molecule and exist almost entirely as five- and six-membered cyclic hemiacetals or hemiketal Hemiacetal : bond between C=O on C1 (aldehyde) and -OH on last chiral C Hemiketal: bond between C=O on C2 (ketone) and -OH on last chiral C Cyclic strusture will display extra chiral carbon (anomeric carbon) anomeric carbon: the new stereocenter resulting from cyclic hemiacetal or hemiketal formation anomers: monosaccharides that differ in configuration only at their anomeric carbons 16-20 16 Haworth Projections

Haworth projections five- and six-membered hemiacetals are represented as planar pentagons or hexagons most commonly written with the anomeric carbon on the right and the hemiacetal oxygen to the back right the designation -means that -OH on the anomeric carbon is cis to the terminal -CH2OH (above the plane); - means that it is trans to the terminal -CH2OH (below the plane) The OH groups on the right in Fischer below in Haworth. Those to the left in Fischer upward in Haworth. Counting is clockwise starting from anomeric carbon 16-21 16 Haworth Projections 16-22

16 Haworth Projections a six-membered hemiacetal ring is shown by the infix - pyran a five-membered hemiacetal ring is shown by the infix - furanO Furan O Pyran Pyranose: a six-membered ring sugar Furanose: a five-membered ring sugar 16-23 16 Haworth Projections Five-membered rings are so close to being planar that Haworth projections are adequate to represent furanoses.

HOCH2 OH() HOCH2 H O O H H H H H H OH() H HO OH HO OH -D-Ribofuranose

-D-Ribofuranose (-D-Ribose) (-D-Ribose) 16-24 Comparison of the Fischer and 16 Haworth Representations 16-25 Comparison of the Fischer and 16 Haworth Representations 16-26 16 Oxidation Reducing sugar: one that reduces an oxidizing agent

Reducing sugar: has free aldehyde or ketone group oxidation of a cyclic hemiacetal form gives a lactone when the oxidizing agent is Tollens reagent (used to detect reducing sugars), Ag precipitates as a silver mirror CH2 OH OH H H + Ag(NH3 ) 2 + OH H HO OH H OH A cyclic hemiacetal CH2 OH

H OHHO O H OH H O + Ag H OH A lactone (a cyclic ester) 16-27 16 Ascorbic Acid (Vitamin C)

L-Ascorbic acid (vitamin C) is synthesized both biochemically and industrially from oxidation of D-glucose CHO both biochemial H OH CH2 OH and industrial HO H H OH O syntheses H OH O H H

OH HO OH CH2 OH L-Ascorbic acid D-Glucose (Vitamin C) 16-28 16 Ascorbic Acid (Vitamin C) L-Ascorbic acid is very easily oxidized to L- dehydroascorbic acid both are physiologically active and are found in most body fluids CH2 OH

H CH2 OH OH O H HO O OH L-Ascorbic acid (Vitamin C) oxidation H reduction

OH O H O O O L-Dehydroascorbic acid 16-29 16 Reduction The carbonyl group of a monosaccharide can be reduced to a hydroxyl group by a variety of reducing agents

reduction of the CHO group of a monosaccharide gives a polyhydroxy compound called an alditol H HO H H CHO OH H OH + H2 OH CH2 OH D-Glucose Ni H

HO H H CH2 OH OH H Commercial sweetener in sugarless gum and candy OH OH CH2 OH D-Glucitol (D-Sorbitol) 16-30 16 Reduction Reactions

16-31 16 Formation of deoxy sugars 16-32 16 Phosphoric Esters Phosphoric esters are particularly important in the metabolism of sugars phosphoric esters are frequently formed by transfer of a phosphate group from ATP CH2 OH O O O O H OH enzy

H + - O-P-OPOPO-Adenosine me OH H O O O HO H 2ATP CH OPO 2 3 H OH O O O H -D-glucose OH

H + - OPOPO-Adenosine OH H O O HO H ADP H OH -D-glucose-6-phosphate 16-33 16 Formation of Glycosides Glycoside: a carbohydrate in which the -OH of the anomeric carbon is replaced by -OR those derived from furanoses are furanosides; those

derived from pyranoses are pyranosides O-glycosidic bond: the bond in the glycoside, between the anomeric carbon and the -OR group CH2 OH H HO CH2 OH OH H OH H H OH OH

H + CH3 OH (methyl alcohol) -D-glucopyranose HO glycosidic bond OH H OH H OCH3

+ H2 O H OH Methyl-Dglucopyranoside 16-34 16 Amino Sugars N-Acetyl-D-glucosamine N-Acetylmuramic acid CH2 OH H HO O OH



O NH-C-CH3 CH2 OH O CH3 -CH COO- NH C O CH3 OH The bond involved is N-glycosidic bond 16-35 16 Glycosidic linkages Glycosidic linkages can take various forms; the

anomeric carbon of one sugar to any of the -OH groups of another sugar to forma an - or glycosidic linkage 16-36 16 Disaccharides Sucrose table sugar; obtained from the juice of sugar cane and sugar beet one unit of D-glucose and one unit of D-fructose joined by an -1,2-glycosidic bond nonreducing sugar: no reducing ends 16-37 16 Disaccharides Lactose about 5% - 8% in human milk, 4% - 5% in cows milk

one unit of D-galactose and one unit of D-glucose joined by a -1,4-glycosidic bond reducing sugar: has reducing ends Lactose intolerance 16-38 16 Disaccharides Maltose two units of D-glucose joined by an -1,4-glycosidic bond Resulted from hydrolysis of starch reducing sugar: has reducing ends 16-39 16 Disaccharides Cellobiose Two units of -glucose linked by -(1-4) glycosidic

bond 16-40 16 Reducing disaccharides 16-41 16 Polysaccharides Cellulose: the major structural component of plants, especially wood and plant fibers a linear polymer of approximately 2800 D-glucose units per molecule joined by -1,4-glycosidic bonds fully extended conformation with alternating 180 flips of glucose units extensive intra- and intermolecular hydrogen bonding between chains HO-CH2

4 O HO O 1 OH HO O 4 HO-CH2 OH 1 O HO-CH2 4

O HO O 1 OH 16-42 16 Starch A polymers of -D-glucose units and used for energy storage in plants, 2 forms amylose: continuous, helical unbranched chains of up

to 4000 -D-glucose units joined by -1,4-glycosidic bonds amylopectin: a highly branched helical polymer consisting of 24-30 units of D-glucose joined by _1,4-glycosidic bonds and branches created by -1,6-glycosidic bonds amylases catalyze hydrolysis of -1,4-glycosidic bonds debranching enzymes catalyze the hydrolysis of -1,6-glycosidic bonds 16-43 16 Starch 16-44 16 Polysaccharides

Branching in amylopectin and glycogen 16-45 16 Polysaccharides Chitin: the major structural component of the exoskeletons of invertebrates, such as insects and crustaceans; also occurs in cell walls of algae, fungi, and yeasts composed of units of N-acetyl--D-glucosamine joined by -1,4-glycosidic bonds (insert bottom of Fig 13.23) 16-46 16

16-47 16 Polysaccharides Bacterial cell walls: prokaryotic cell walls are constructed on the framework of the repeating unit NAM-NAG joined by -1,4-glycosidic bonds 16-48 16 Bacterial Cell Walls The N-acetyl-D-glucoseamine and N-acetylmuramic acid polysaccharide is in turn cross-linked by small peptides in Staphylococcus aureus, the cross link is a tetrapeptide this tetrapeptide is unusual in that it contains two amino acids of the D-series, namely D-Ala and D-Gln

each tetrapeptide is cross linked to an adjacent tetrapeptide by a pentapeptide of five glycine units Peptidoglycan: is the resulted cross linking of polysaccharides by peptides 16-49 16 Bacterial Cell Walls 16-50 16 Bacterial Cell Walls The peptidoglycan of a bacterial cell wall Staphylococcus aureus

16-51 16 Polysaccharides Glycosaminoglycans: polysaccharides based on a repeating disaccharide where one of the monomers is an amino sugar and the other has a negative charge due to acid such as a sulfate or carboxylate group heparin: natural anticoagulant hyaluronic acid: a component of the vitreous humor of the eye and the lubricating fluid of joints chondroitin sulfate and keratan sulfate: components of connective tissue 16-52 16 Heparin

The repeating unit of heparin 16-53 16 Hyaluronic Acid The repeating unit of hyaluronic acid 16-54 16 Chondroitin sulfate The repeating unit of chondroitin sulfate 16-55 16 Glycoproteins Glycoproteins contain carbohydrate units covalently bonded to a polypeptide chain antibodies are glycoproteins

carbohydrates play a role as antigenic determinants, the portions of the antigenic molecule that antibodies recognize and to which they bond. 16-56 16 Blood Group Substances Membranes of animal plasma cells have large numbers of relatively small carbohydrates bound to them: these membrane-bound carbohydrates act as antigenic determinants among the first antigenic determinants discovered were the blood group substances in the ABO system, individuals are classified according to four blood types: A, B, AB, and O at the cellular level, the biochemical basis for this

classification is a group of relatively small membranebound carbohydrates 16-57 16 ABO Blood Classification 16-58 16 ABO Blood Classification in type A, the nonreducing end is NAGal in type B it is Gal in type AB, both types are present in Type O, neither of these terminal residues is present 16-59

16 End Chapter 16 16-60

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