Carbohydrates-Biochemistry - K. A. Boudreaux, Angelo State University, Lecture notes of Chemistry

Download Carbohydrates-Biochemistry - K. A. Boudreaux, Angelo State University and more Lecture notes Chemistry in PDF only on Docsity! Chapter Objectives: • Learn how to classify carbohydrates. • Learn how to recognize molecules with chiral centers and draw Fischer projections. • Learn how to classify the monosaccharides, and learn their chemical and physical properties. • Learn about the disaccharides and oligosaccharides. • Learn the major types of polysaccharides and their structural and biological features. Chapter 7 Carbohydrates Mr. Kevin A. Boudreaux Angelo State University CHEM 2353 Fundamentals of Organic Chemistry Organic and Biochemistry for Today (Seager & Slabaugh) Biochemistry • Biochemistry is the study of the chemistry of biomolecules and living organisms. • In organic chemistry, we organized our study of carbon-containing molecules by functional group (alcohol, alkene, ketone, carboxylic acid, etc.). • In the first five chapters, we will take a look at several groups of important biological molecules, many of which have more than one functional group: carbohydrates (Ch. 7), lipids (Ch. 8), proteins and enzymes (Ch. 9 and 10), and nucleic acids (Ch. 11). • We will also examine the synthesis of proteins (Ch. 11), nutrition (Ch. 12) and metabolism (Ch. 13, 14), and important body fluids (Ch. 15). 2 5 Carbohydrates • Carbohydrates, or saccharides (saccharo is Greek for “sugar”) are polyhydroxy aldehydes or ketones, or substances that yield such compounds on hydrolysis. C C O H H OH CHO H C C OHH CH2OH OHH glucose COH H C C C OHH CH2OH OHH O CH2OH fructose O CH2OH OH OH OH OH -D-glucose Carbohydrates • Carbohydrates include not only sugar, but also the starches that we find in foods, such as bread, pasta, and rice. 6 • The term “carbohydrate” comes from the observation that when you heat sugars, you get carbon and water (hence, hydrate of carbon). 7 Classes of Carbohydrates • Monosaccharides contain a single polyhydroxy aldehyde or ketone unit (e.g., glucose, fructose). • Disaccharides consist of two monosaccharide units linked together by a covalent bond (e.g., sucrose). • Oligosaccharides contain from 3 to 10 monosaccharide units (e.g., raffinose). 10 Two Forms of Glyceraldehyde • Glyceraldehyde, the simplest carbohydrate, exists in two isomeric forms that are mirror images of each other: Stereoisomers • These forms are stereoisomers of each other. • Glyceraldehyde is a chiral molecule — it cannot be superimposed on its mirror image. The two mirror- image forms of glyceraldehyde are enantiomers of each other. 11 C CHO CH2OH HHO CH OH CH2OH CHO L-glyceraldehyde D-glyceraldehyde Chirality and Handedness • Chiral molecules have the same relationship to each other that your left and right hands have when reflected in a mirror. 12 15 Examples: Chiral Carbons in Carbohydrates • Identify the chiral carbons (if any) in the following carbohydrates: erythrose CH CHO CH OH CH2OH OH C C O H H OH CHO H C C OHH CH2OH OHH glucose O CH2OH OH OH OH OH C CH2OH O CH OH C CH2OH OHH 2n Rule • When a molecule has more than one chiral carbon, each carbon can possibly be arranged in either the right-hand or left-hand form, thus if there are n chiral carbons, there are 2n possible stereoisomers. Maximum number of possible stereoisomers = 2n CH Cl Br F 16 21 = 2 possible stereoisomers CCH3 C OH H CH2OH H OH 22 = 4 possible stereoisomers 17 Examples: Number of Stereoisomers • What is the maximum number of possible stereo- isomers of the following compounds? C C O H H OH CHO H C C OHH CH2OH OHH C CH2OH O CH OH C CH2OH OHH CCH3 CH2CH3 OH H CH H Cl Cl CCH3 C CH3 H CH2OH OH H 20 Naming Stereoisomers • When there is more than one chiral center in a carbohydrate, look at the chiral carbon farthest from the carbonyl group: if the hydroxy group points to right when the carbonyl is “up” it is the D-isomer, and when the hydroxy group points to the left, it is the L-isomer. CHO HHO H OH CHO CH2OH HO H CH2OH OHH D-erythroseL-erythrose 21 Examples: Fischer Projections • Draw Fischer projections of D and L lactic acid: CH CO2H CH3 OH • Draw Fischer projections of D and L alanine: CH NH2 CH3 CO2H • Given the structure for D-glucose, draw the structure of L- glucose: CHO OHH HHO OHH CH2OH OHH D-glucose 22 Examples: Fischer Projections • Identify the following compounds as D or L isomers, and draw their mirror images. HO H C OHH CH2OH OHH O CH2OH fructose HO H CH2OH OHH H CHO lyxose HO 25 Monosaccharides 26 Classification of Monosaccharides • The monosaccharides are the simplest of the carbohydrates, since they contain only one polyhydroxy aldehyde or ketone unit. • Monosaccharides are classified according to the number of carbon atoms they contain: • The presence of an aldehyde is indicated by the prefix aldo- and a ketone by the prefix keto-. No. of Class of carbons Monosaccharide 3 triose 4 tetrose 5 pentose 6 hexose 27 Classification of Monosaccharides • Thus, glucose is an aldohexose (aldehyde + 6 Cs) and ribulose is a ketopentose (ketone + 5 Cs) C C O H H OH CHO H C C OHH CH2OH OHH glucose an aldohexose C CH2OH O CH OH C CH2OH OHH ribulose a ketopentose 30 The Family of D-aldoses (L-forms not shown) CHO H OH HO H OHH CH2OH OHH CHO H OH H OH OHH CH2OH OHH CHO HO H H OH OHH CH2OH OHH CHO HO H HO H OHH CH2OH OHH CHO H OH H OH HHO CH2OH OHH CHO HO H H OH HHO CH2OH OHH CHO H OH HO H HHO CH2OH OHH D-glucose CHO HO H HO H HHO CH2OH OHH D-talose D-gulose D-idose D-galactose D-mannoseD-allose D-altrose Aldohexoses 2 4 = 16 31 The Family of D-ketoses (L-forms not shown) O CH2OH CH2OH Ketotriose 2 0 = 1 Dihydroxyacetone O CH2OH CH2OH OHH D-erythrulose Ketotetroses 2 1 = 2 CH2OH O H OH CH2OH OHH CH2OH O HO H CH2OH OHH D-xyluloseD-ribulose Ketopentoses 2 2 = 4 32 The Family of D-ketoses (L-forms not shown) CH2OH O H OH HHO CH2OH OHH CH2OH O H OH OHH CH2OH OHH CH2OH O HO H OHH CH2OH OHH CH2OH O HO H HHO CH2OH OHH D-Sorbose D-TagatoseD-Psicose D-Fructose Ketohexoses 2 3 = 8 35 Chemical Properties of Monosaccharides • Monosaccharides do not usually exist in solution in their “open-chain” forms: an alcohol group can add into the carbonyl group in the same molecule to form a pyranose ring containing a stable cyclic hemiacetal or hemiketal. OC C C C C CH2OH H H HO H OH O H H HOH OC C C C C CH2OH H H HO H OH HOH OH H OC C C C C CH2OH H H HO H OH HOH H OH -D-glucose -D-glucose D-glucose cyclic hemiacetals -up -down O a pyranose ring 36 Glucose Anomers • In the pyranose form of glucose, carbon-1 is chiral, and thus two stereoisomers are possible: one in which the OH group points down (-hydroxy group) and one in which the OH group points up (- hydroxy group). These forms are anomers of each other, and carbon-1 is called the anomeric carbon. O CH2OH OH OH OH OH C C O H H OH CHO H C C OHH CH2OH OHH O CH2OH OH OH OH OH -D-glucose 64% -D-glucose 36% D-glucose 0.02% 1 1 1 Haworth structures 37 Fructose Anomers • Fructose closes on itself to form a furanose ring: O a furanose ring O H CH2OH OCH2OH OH OH O CH2OH OH HO CH2OH OH O CH2OH OH HO OH CH2OH D-fructose -D-fructose-D-fructose -hydroxy -hydroxy 40 Oxidation of Monosaccharides • Aldehydes and ketones that have an OH group on the carbon next to the carbonyl group react with a basic solution of Cu2+ (Benedict’s reagent) to form a red-orange precipitate of copper(I) oxide (Cu2O). • Sugars that undergo this reaction are called reducing sugars. (All of the monosaccharides are reducing sugars.) Reducing sugar + Cu2+ oxidation product + Cu2O deep blue solution red-orange ppt Benedict’s Reagent (blue) Copper(I) oxide (red-orange ppt) 41 Formation of Phosphate Esters • Phosphate esters can form at the 6-carbon of aldohexoses and aldoketoses. • Phosphate esters of monosaccharides are found in the sugar-phosphate backbone of DNA and RNA, in ATP, and as intermediates in the metabolism of carbohydrates in the body. O CH2 OH OH OH OH OP O O– –O glucose 6-phosphate O CH2 OH HO OH CH2OH OP–O O– O fructose 6-phosphate 42 Glycoside Formation • The hemiacetal and hemiketal forms of monosaccharides can react with alcohols to form acetal and ketal structures called glycosides. The new carbon-oxygen bond is called the glycosidic linkage. O CH2OH OH OH OH OH O CH2OH OH OH OH OCH3 O CH2OH OH OH OH + OCH3 methyl -D-glycopyranoside -D-glucose methyl -D-glycopyranoside +CH3OH H + 45 Important Monosaccharides O CH2OH OH OH OH -D-ribose Forms the sugar backbone of ribonucleic acid (RNA) O CH2OH OH OH -D-deoxyribose Forms the sugar backbone of deoxyribonucleic acid (DNA)O CH2OH OH OH OH -D-galactose Incorporated with glucose into lactose (milk sugar) OH 46 Important Monosaccharides O CH2OH OH OH OH OH -D-glucose Also known as dextrose and blood sugar; present in honey and fruits. Glucose is metabolized in the body for energy. Other sugars absorbed into the body must be converted to glucose by the liver. O CH2OH OH HO OH CH2OH -D-fructose Also known as levulose and fruit sugar. Fructose is the sweetest of the monosaccharides. It is present in honey (1:1 ratio with glucose), fruits, and corn syrup. It is often used to sweeten foods, since less fructose is needed to achieve the same degree of sweetness. 47 Disaccharides and Oligosaccharides 50 Important Disaccharides O CH2OH O OH OH OH O CH2OH OH OH OH -D-glucose -D-glucose Maltose Also known as malt sugar. It is produced in germinating grain (such as barley) as starch is broken down during malting, and is formed during the hydrolysis of starch to glucose during digestion. O CH2OH OH OH OH O O CH2OH OH OH OH (1→4) glycosidic linkage -D-galactose -D-glucose Lactose Also known as milk sugar. Lactose constitutes 5% of cow's milk and 7% of human milk. It is digested by the enzyme lactase. Pure lactose is found in whey, the watery byproduct of cheese production. 51 Important Disaccharides O C H 2 O H O C H 2 O H O H O H O H O C H 2 O H H O O H  - D - g l u c o s e  - D - f r u c t o s e  −  →  −  g l y c o s i d i c l i n k a g e HO O HO OH OH OHO OH O OH OH Sucrose Also known as table sugar. Both anomeric carbons of glucose and fructose are tied together in the glycosidic linkage; thus neither ring can open, and sucrose is not a reducing sugar. Sucrose is found in fruits, nectar, sugar cane, and sugar beets; maple syrup contains about 65% sucrose, with glucose and fructose present as well. Caramel is the solid residue formed from heating sucrose. A flavoring agent called invert sugar is produced by the hydrolysis of sucrose under acidic conditions, which breaks it apart into glucose and fructose; invert sugar is sweeter than sucrose because of the fructose. Some of the sugar found in honey is formed in this fashion; invert sugar is also produced in jams and jellies prepared from acid-containing fruits. 52 Oligosaccharides • Oligosaccharides contain from 3 to 10 monosaccharide units. O CH2OH OH OH OH O O CH2OH OH OH -D-galactose -D-glucose Raffinose An oligosaccharide found in peas and beans; largely undigested until reaching the intestinal flora in the large intestine, releasing hydrogen, carbon dioxide, and methane) O HOCH2 O OH HO CH2OH -D-fructose 55 Starch • Starch is a polymer consisting of D-glucose units. • Starches (and other glucose polymers) are usually insoluble in water because of the high molecular weight. – Because they contain large numbers of OH groups, some starches can form thick colloidal dispersions when heated in water (e.g., flour or starch used as a thickening agent in gravies or sauces). • There are two forms of starch: amylose and amylopectin. Starch — Amylose • Amylose consists of long, unbranched chains of glucose (from 1000 to 2000 molecules) connected by (1→4) glycosidic linkages. • 10%-20% of the starch in plants is in this form. • The amylose chain is flexible enough to allow the molecules to twist into the shape of a helix. Because it packs more tightly, it is slower to digest than other starches. 56 O CH2OH OH OH OO O CH2OH OH OH O O CH2OH OH OH O (1→4) glycosidic linkage Amylose Starch — Amylose • Amylose helices can trap molecules of iodine, forming a characteristic deep blue-purple color. (Iodine is often used as a test for the presence of starch.) 57 60 Cellulose • Cellulose is a polymer consisting of long, unbranched chains of D-glucose connected by (1→4) glycosidic linkages; it may contain from 300 to 3000 glucose units in one molecule. O CH2OH OH OH O O O CH2OH OH OH O O CH2OH OH OH O (1→4) glycosidic linkage Cellulose 61 Cellulose • Because of the -linkages, cellulose has a different overall shape from amylose, forming extended straight chains which hydrogen bond to each other, resulting in a very rigid structure. • Cellulose is the most important structural polysaccharide, and is the single most abundant organic compound on earth. It is the material in plant cell walls that provides strength and rigidity; wood is 50% cellulose. Plant cell wall 62 Cellulose • Most animals lack the enzymes needed to digest cellulose, but it does provide roughage (dietary fiber) to stimulate contraction of the intestines and help pass food through the digestive system. – Some animals, such as cows, sheep, and goats (ruminants), process cellulose using colonies of bacteria in the digestive system which are capable of breaking down cellulose, and a series of stomachs to give cellulose a longer time to digest. – Some other animals have a longer intestinal tract (e.g., horses), and others reprocess digested food (e.g., rabbits) to allow more time for the breakdown of cellulose to occur. • Cellulose is important industrially, from its presence in wood, paper, cotton, cellophane, rayon, linen, nitrocellulose (guncotton), photographic films (cellulose acetate), etc. Chitin • Chitin is a polymer of N-acetylglucosamine, an amide derivative of the amino sugar glucosamine, in which one of the OH groups is converted to an amine (NH2) group. The polymer is extremely strong because of the increased hydrogen bonding provided by the amide groups. 65 O CH2OH OH OH NH2 OH Glucosamine O CH2OH OH OH HN OH N-Acetylglucosamine CH3 O O CH2OH OH HN O O O CH2OH OH HN O O CH2OH OH HN O ChitinCH3 O CH3 O CH3 O Chitin • Chitin is the main component of the cell walls of fungi, the exoskeletons of arthropods such as crustaceans and insects, and the beaks of cephalopods. The chitin is often embedded in either a protein matrix, or in calcium carbonate crystals. Since this matrix cannot expand easily, it must be shed by molting as the animal grows. 66 67 How Sweet It Is! Sugar Substitutes 70 N S H Cyclamate (Sodium N-cyclohexylsulfamate) Discovered by Michael Sveda in 1937, who noticed that a cigar that he was smoking in the lab tasted especially sweet. It is about 30-50 times sweeter than sugar, and is often used in combination with other artificial sweeteners. It is heat-stable. Cyclamate was banned by the FDA in 1970, although it is still used in other countries. O- Na+ O O O S N– K+ O CH3 O O Acesulfame-K (Sunette, Sweet One) Discovered accidentally at Hoechst AG when a chemist accidentally dipped his fingers into the chemicals that he was working with, and later licked his finger to pick up a piece of paper. It is 200 times sweeter than sugar, noncaloric, and is heat-stable and can be used during cooking. It is often used in combination with other artificial sweetners, so that each one masks the others' aftertaste. 71 O CH2OH Cl OH OH "galactose" Sucralose (Splenda) A non-caloric artificial sweetener approved by the FDA in 1998. The glucose in sucrose is replaced by a galactose, and three of the OH groups are replaced by Cl atoms. It is about 600 times sweeter than sucrose, and is non- caloric and heat-stable. O CH2Cl O OH HO CH2Cl "fructose" CH2OH H OH HO H H OH H OH CH2OH Sorbitol A sugar alcohol; incompletely absorbed during digestion, and contribute fewer calories than carbohydrates; found naturally in fruits; used commercially in sugar-free candies, cookies, chewing gum, etc. 72 Stevia OH O HO Steviol O O O O O O O OH HO HO OH OH OH OH HO OH HO OH Stevioside (a steviol glycoside) Steviol (Truvia, PureVia) Stevia rebaudiana (sweetleaf) is a plant in the sunflower family widely grown for its sweet leaves. The leaves contain a compound called steviol, which is attached via glycoside linkages to glucose molecules to form stevioside (250-300 times sweeter than sugar) and rebaudioside A (aka rebiana, contains an additional glucose group, 350-450 times sweeter than sugar). The FDA put rebaudioside A on the GRAS list in 2009.