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Chapter 16 Carbohydrates Denniston Topping Caret 5th Edition 16.1 Types of Carbohydrates • Carbohydrates are synthesized by photosynthesis in plants – Grains, cereals, bread, sugar cane • Glucose is major energy source – A gram of digested carbohydrate gives about 4 kcal of energy – Complex carbohydrates are best for diet – USDA recommends about 58% daily calories from carbohydrates 16.1 Types of Carbohydrates Basic Carbohydrate Types • Monosaccharides – e.g., glucose, fructose – One sugar (saccharide) molecule • Disaccharides – e.g., sucrose, lactose – Two monosaccharides linked together – Linkage is called a glycosidic bond • Oligosaccharides – Three to ten monosaccharides linked by glycosidic bonds • Polysaccharides – e.g., starch, glycogen, cellulose – Chains of linked monosaccharide units 16.2 Monosaccharides • Monosaccharides are composed of: – – – – Carbon Hydrogen Oxygen Basic Formula = (CH2O)n n = any integer 3 – 7 • Many monosaccharides also contain chemical modifications – Amino groups – Phosphate groups 16.2 Monosaccharides Naming Monosaccharides Named on the basis of • Functional groups – Ketone carbonyl = ketose – Aldehyde carbonyl = aldose • Number of carbon atoms in the main skeleton – – – – 3 carbons = triose 4 carbons = tetrose 5 carbons = pentose 6 carbons = hexose • Combine both systems gives even more information 16.3 Stereoisomers and Stereochemistry • Prefixes D- and L- in a monosaccharide name identify one of two isomeric forms – These isomers differ in the spatial arrangement of atoms and are stereoisomers • Stereochemistry is the study of different spatial arrangements of atoms • The stereoisomers D- and L- glyceraldehyde are nonsuperimposable mirror image molecules and are called enantiomers (a subset of stereoisomers) 16.3 Stereoisomers and Stereochemistry Enantiomers 16.3 Stereoisomers and Stereochemistry Chirality • A carbon atom that has four different groups bonded to it is called a chiral carbon atom • Any molecule containing a chiral carbon can exist as a pair of enantiomers • Chirality in glyceraldehyde (the simplest carbohydrate) is conveyed by a chiral carbon • Larger biological molecules often have more than one chiral carbon 16.3 Stereoisomers and Stereochemistry Chirality of Glyceraldehyde • Glyceraldehyde has a chiral carbon and thus, has two enantiomers – The D isomer has the -OH on the stereocenter to the right – The L isomer has the -OH on the stereocenter to the left Chiral Carbon: connected to four different atoms or groups Mirror plane O C H H C OH CH2OH the D isomer O C H HO C H CH2OH the L isomer 16.3 Stereoisomers and Stereochemistry Structural Formulas of D- and LGlyceraldehyde 16.3 Stereoisomers and Stereochemistry Optical Activity • Enantiomers are also called optical isomers • Enantiomers interact with plain polarized light to rotate the plane of the light in opposite directions – This interaction with polarized light is called optical activity – Optical activity distinguishes the isomers – It is measured in a device called a polarimeter 16.3 Stereoisomers and Stereochemistry Polarized Light • Normal light vibrates in an infinite number of directions perpendicular to the direction of travel – When the light passes through a polarizing filter (Polaroid sunglasses) only light vibrating in one plane reaches the other side of the filter – A polarimeter allows the determination of the specific rotation of a compound • Measures its ability to rotate plane-polarized light 16.3 Stereoisomers and Stereochemistry Schematic Drawing of a Polarimeter 16.3 Stereoisomers and Stereochemistry The Relationship Between Molecular Structure and Optical Activity • When an enantiomer in a solution is placed in the polarimeter, the plane of rotation of the polarized light is rotated – One enantiomer always rotates light in a clockwise (+) direction • This is the dextrorotatory isomer – The other isomer rotates the light in a counterclockwise (-) direction • It is the levorotatory isomer • Under identical conditions, the enantiomers always rotate light to exactly the same degree, but in opposite directions 16.3 Stereoisomers and Stereochemistry Fischer Projection Formulas • A Fischer projection uses lines crossing through a chiral carbon to represent bonds – Projecting out of the page (horizontal lines) – Projecting into the page (vertical lines) • Compare the wedge to the Fischer diagrams 16.3 Stereoisomers and Stereochemistry Fischer Projections of Monosaccharides O 1C 2 1 CH2OH H H C OH H 3C OH 2C 3 O HO C H CH2OH 4 D-erythrose an aldotetrose H 4C OH 5 H C OH 6CH2OH D-fructose a ketohexose 16.3 Stereoisomers and Stereochemistry Aldose or Ketose? 16.3 Stereoisomers and Stereochemistry The D- and L-System • Monosaccharides are drawn in Fischer projections – With the most oxidized carbon closest to the top – The carbons are numbered from the top – If the chiral carbon with the highest number has the OH to the right, the sugar is D – If the OH is to the left, the sugar is L • Most common sugars are in the D form CHO CHO H C OH H CH2OH OH CH2OH the D isomer CHO CHO HO C H HO CH2OH H CH2OH the L isomer 16.4 Biological Monosaccharides • Glucose is the most important sugar in the human body – Found in many foods – Several common names include: dextrose and blood sugar – Its concentration in the blood is regulated by insulin and glucagon • Under physiological conditions, glucose exists in a cyclic hemiacetal form where the C-5 hydroxyl reacts with the C-1 aldehyde group – Two isomers are formed which differ in the location of the -OH on the acetal carbon, C-1 • An aldohexose with molecular formula C6H12O6 16.4 Biological Monosaccharides Cyclic Form of Glucose • The cyclic form of glucose is shown as a Haworth projection arrows show electron movement CH2OH O H H H H OH H O HO H OH CH2OH O H form H H (alpha) OH H HO OH H OH Pyranose CH2OH ring form O OH H H form OH H (beta) HO H H OH 16.4 Biological Monosaccharides Cyclization of Glucose 16.4 Biological Monosaccharides Fructose • Fructose is also called: – Levulose – Fruit sugar • Found in large amounts in: – Honey – Corn syrup – Fruits • The sweetest of all sugars • Ketohexose 1 CH2OH CO 3 HO 4C H H 5C OH H 6C OH CH2OH 2 D-fructose CH2OH CH OH O 2 OH H OHH CH2OH OH O CH2OH H OHH 16.4 Biological Monosaccharides Galactose Galactose is the principal sugar found in mammalian milk Aldohexose very similar to glucose -D-galactosamine is a component of the blood group antigens 1 CHO 2 H C OH 3 HO C H 4 HO C H 5 H C OH 6 CH2OH CH2OH HO OH H OH H OH H H OH -D-galactose 16.4 Biological Monosaccharides Galactose Orientation Glucose and galactose differ only in the orientation of one hydroxyl group 16.4 Biological Monosaccharides Ribose and Deoxyribose, Five-Carbon Sugars • Components of many biologically important molecules • Exists mainly in the cyclic form • Aldopentose CHO CH2OH H H C OH O H C OH H H OH H H C OH OH OH CH2OH -D-ribose Deoxyribose has an -H here replacing the -OH 16.4 Biological Monosaccharides Reducing Sugars • The aldehyde groups of aldoses are oxidized by Benedict’s reagent, an alkaline copper(II) solution • The blue color of the reagent fades as reaction occurs reducing Cu2+ to Cu+ with a red-orange precipitate forming as Cu2O results • Test can measure glucose in urine O H O C H C OH +2 Cu2+ CH2OH O C H C OH CH2OH + Cu2O (red-orange) 16.4 Biological Monosaccharides Reducing Sugars • All monosaccharides and the disaccharides except sucrose are reducing sugars • Ketoses can isomerize to aldoses and react also CH2OH CO HO C H H C OH H C OH CH2OH D-fructose HO CH O CH C OH H C OH HO C H HO C H H C OH H C OH H C OH H C OH CH2OH CH2OH enediol D-glucose A Reduced Sugar 16.4 Biological Monosaccharides • The most important reduced sugar is deoxyribose O H C H C H H C OH H C OH CH2OH D-deoxyribose CH2OH O OH H H H H OH H -D-2-deoxyribose 16.5 Biologically Important Disaccharides • The anomeric -OH can react with another -OH on an alcohol or sugar • Process is forming a glycosidic bond • Water is lost to form an acetal CH2OH H O OH H OH H HO H H OH + CH3 OH CH2OH H O O CH3 H OH H HO H H OH + H2O 16.5 Biologically Important Disaccharides Maltose • Maltose is formed by linking two -Dglucose molecules to give a 1,4 glycosidic linkage • Maltose is malt sugar • Formed as an intermediate in starch hydrolysis • Reducing sugar due to the hemiacetal hydroxyl CH2OH CH2OH O O H H H H H H OH H OH H O OH HO H OH H OH 16.5 Biologically Important Disaccharides Formation of Maltose 16.5 Biologically Important Disaccharides Lactose • Lactose is formed by joining -D-galactose to -D-glucose to give a -1,4-glycoside • Lactose is milk sugar – For use as an energy source, must be hydrolyzed to glucose and galactose – Lactose intolerance results from lack of lactase to hydrolyze the glycosidic link of lactose 16.5 Biologically Important Disaccharides Lactose Glycosidic Bond 16.5 Biologically Important Disaccharides Galactosemia • In order for lactose to be used as an energy source, galactose must be converted to a phosphorylated glucose molecule • When enzymes necessary for this conversion are absent, the genetic disease galactosemia results • People who lack the enzyme lactase (~20%) are unable to digest lactose and have the condition lactose intolerance 16.5 Biologically Important Disaccharides Sucrose • Sucrose is formed by linking -D-glucose with -D-fructose to give a 1,2 glycosidic linkage – Nonreducing – negative reaction in Benedict test – The glycosidic O is part of an acetal and a ketal • Important plant carbohydrate – Water soluble – Easily transported in plant circulatory system • Cannot by synthesized by animals • Sucrose called: – – – – Table sugar Cane sugar Beet sugar Linked to dental caries 16.5 Biologically Important Disaccharides Glycosidic Bond Formed in Sucrose 16.5 Biologically Important Disaccharides Cellobiose • Cellobiose is formed by linking two -Dglucose molecules to give a 1,4-glycosidic link • It is a product of hydrolyzed cellulose CH2OH CH2OH O O H H OH H H O OH H OH H H HO H H OH H OH 16.6 Polysaccharides Starch • Starches are storage forms of glucose found in plants • They are polymers of linked glucose • If the links are: – Only 1,4 links, the polymer is linear = amylose • Amylose usually assumes a helical configuration with six glucose units per turn • Comprises about 80% of plant starch – Both 1,4 and 1,6 links then, the polymer structure is branched = amylopectin • Highly branched with branches of approximately 20-25 glucose units 16.6 Polysaccharides Structure of Amylose 16.6 Polysaccharides Comparison of Amylose to Amylopectin CH2 OH CH2 OH CH2 OH CH2 OH O H H O H H O H H O H H H H H H ( OH H H H OH OH OH H O O O O O) H OH H OH H OH H OH amylose(-1,4 links) CH2 OH O H H H ( OH H O O H OH CH2 OH O H H H H ( OH H O O H OH CH2 OH O H H H OH H amylopectin (-1,6 link) O H OH CH2 CH2 OH CH2 OH O H H O H H O H H H H OH H OH H OH H O O O) H OH H OH H OH 16.6 Polysaccharides Cellulose • Cellulose is the major structural polymer in plants • It is a liner homopolymer composed of -Dglucose units linked -1,4 • The repeating disaccharide of cellulose is cellobiose • Animals lack the enzymes necessary to hydrolyze cellulose • The bacteria in ruminants (e.g., cows) can digest cellulose so that they can eat grass, etc. 16.6 Polysaccharides Structure of Cellulose -(1->4) glycosidic bond CH2OH O H O CH2OH H OH H CH2OH O H H O H O H H OH OH H O H H OH H H H OH O H OH 16.6 Polysaccharides Glycogen and Amylopectin Structures Glycogen and Amylopectin are (1-4) chains with with (1-6) branches Amylopectin Glycogen 16.6 Polysaccharides Glycogen • The major glucose storage carbohydrate in animals is glycogen • A highly branched chain polymer like amylopectin – More frequent branching – 10 monomers • Glycogen is stored in: – Liver – Muscle cells