Chapter 16 - Carbohydrates MB(1).pptx

CHAPTER 16: CARBOHYDRATES CHEM311 Summary Sugar structure, stereochemistry and representation Monosaccharide reactions Disaccharides Oligo- and polysaccharides Glycoproteins Types of Carbohydrates Carbohydrates are synthesized by photosynthesis in plants Grains, cereals, bread, sugar cane Glucose is...

CHAPTER 16: CARBOHYDRATES CHEM311 Summary Sugar structure, stereochemistry and representation Monosaccharide reactions Disaccharides Oligo- and polysaccharides Glycoproteins 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 45-65% daily calories from 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 Polysaccharides e.g., starch, glycogen, cellulose Chains of linked monosaccharide units 4 Monosaccharides Monosaccharides are composed of: Carbon Hydrogen Oxygen Basic Formula = (CH2O)n n = any integer 3 – 7 5 Monosaccharides A monosaccharide is a single sugar molecule. It is also called a simple sugar. It has a backbone of 3 to 7 carbon atoms. Examples: Glucose (blood sugar), fructose (fruit sugar), and galactose Hexoses – six carbon atoms Ribose and deoxyribose (sugars contained in nucleotides, the monomer of DNA) Pentoses – five carbon atoms 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 Combining both systems gives even more information Enantiomers Nonsuperimposable mirror images: Enantiomers 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 Chirality of Glyceraldehyde Glyceraldehyde has a chiral carbon and thus, has two enantiomers Glyceraldehyde is a chiral molecule The D isomer has the -OH on the stereocenter to the right The L isomer has the -OH on the stereocenter to the left 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 Fischer Projections of Monosaccharides Biological Monosaccharides Glucose is the most important sugar in the human body Found in many foods Many common names: 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 carbonyl 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 Cyclic Form of Glucose The cyclic form of glucose is shown as a Haworth projection The two isomers formed differ from one another in the location of the –OH attached to the hemiacetal carbon These α and β isomers are called anomers. Fructose Fructose is also called: Levulose Fruit sugar Found in: Honey Corn syrup Fruits The sweetest sugar Ketohexose Galactose Galactose is the principal sugar found in mammalian milk Aldohexose very similar to glucose β-D-galactosamine is a component of the blood group antigens Galactose Orientation Glucose and galactose differ only in the orientation of one hydroxyl group Ribose and Deoxyribose, Five-Carbon Sugars Ribose - components of many biologically important molecules Aldopentose The only difference between the ribose and β-D-2deoxyribose is the absence of the –OH group on C-2 of β-D-2-deoxyribose Benedict’s Reagent The aldehyde groups of aldoses are oxidized by Benedict’s reagent, an alkaline Cu2+ 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 Reducing Sugars All monosaccharides and the disaccharides except sucrose are reducing sugars Ketoses can isomerize to aldoses via enediol reaction Biologically Important Disaccharides The anomeric -OH can react with another -OH on an alcohol or sugar, forming a glycosidic bond Water is lost to form an acetal Different types pf Glycosidic bonds Lactose Glycosidic Bond 23 Maltose Maltose is formed by linking two D-glucose molecules to give a 1,4 glycosidic linkage Maltose is malt sugar Formed as an intermediate in starch hydrolysis Synthesis and Degradation of Maltose monosaccharide +monosaccharide ⟵ disaccharide+water Lactose Lactose is formed by joining b-D-galactose to Dglucose to give a b-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 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 Sucrose Sucrose is formed by linking α-D-glucose with β-D-fructose to give a 1,2 glycosidic linkage 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 Glycosidic Bond Formed in Sucrose Polysaccharides Homopolysaccharides – composed of one type of monosaccharide Heteropolysaccharides – made up of two or more different monosaccharides Starch Storage polymers of α linked glucose found in plants 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 Polysaccharides: Energy-Storage and Structural Molecules A polysaccharide is a polymer of monosaccharides. Examples: Starch provides energy storage in plants. Glycogen provides energy storage in animals. Cellulose is found in the cell walls of plants. Most abundant organic molecule on earth Animals are unable to digest cellulose. Polysaccharides: Energy-Storage and Structural Molecules (photos): (a): ©Jeremy Burgess/SPL/Science Source; (b): ©Don Fawcett/Science Source Jump to Polysaccharides: Energy-Storage and Structural Molecules (2) Long Description Polysaccharides: Energy-Storage and Structural Molecules (photo): ©Scimat/Science Source Jump to Polysaccharides: EnergyStorage and Structural Molecules (3) Long Description Comparison of Amylose to Amylopectin Amylose : Amylopectin : 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 Glycogen and Amylopectin Structures Glycogen and Amylopectin are α(1-4) chains with α(1-6) branches Cellulose Cellulose is the major structural polymer in plants It is a liner homopolymer composed of β-Dglucose units linked β-1,4 The repeating disaccharide of cellulose is βcellobiose 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. Structure of Cellulose Glucagon and Insulin Glucagon is secreted in response to low blood glucose levels Carbohydrate metabolism Inhibits glycogen synthesis while stimulating glycogenolysis and gluconeogenesis Effects of Insulin and Glucagon Glycolipids: important in cell recognition ABO Blood type antigens