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Carbohydrates

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Carbohydrates Carbohydrates Objectives: I. Define carbohydrates on the basis of the organic functional groups present on the molecule. II. Define carbohydrates on the basis of size A. Monosaccharides B. Disaccharides C. Oligosaccharides D. Polysaccharides III. Classify the important monosaccharides on the basis of: A. The number of carbons in the molecule. B. Whether it is an aldehyde or a ketone. C. Both IV. Describe the biochemically most important monosaccharides. A. Aldoses (6) versus ketoses (4) B. Significance of stereoisomerism on carbohydrate structure and function. C. Recognize and name the common monosaccharides from their Fischer projection D. From a Fischer projection identify a carbohydrate as a D or L sugar. E. Recognize and name the enantiomer pairs and/or diastereomers of the common monosaccharides from their Fischer projections. V. Describe the differences and similarities in structure and function among glucose, galactose, mannose, fructose, and ribose. VI. What are epimers? A. Which of the monosaccharides are epimers of each other? VII. Cyclic forms of the monosaccharides A. What organic functional group form when an aldose cyclizes? B. What organic functional group form when a ketose cyclizes? C. What are anomers? D. What is the anomeric carbon? E. Which of the monosaccharides are anomers of each other? F. What is Mutarotation? G. Recognize the monosaccharide when given in the Haworth projection of its α- and/or β-cyclic forms. H. Be able to convert the Fischer projection of a monosaccharide into its corresponding Haworth projection or a Haworth projection into its corresponding Fischer projection. VIII.Recognize some of the chemical properties of carbohydrates and relate them to the chemistry of alcohols and carbonyl compounds. A. Oxidation of aldehyde function group to form aldonic acids B. Reduction of carbonyl group to form sugar alcohols C. Oxidation of terminal hydroxyl group to form uronic acids D. Esterification E. Reduction of hydroxyl group to form deoxy sugars F. Substitution of hydroxyl group by an amino group G. Reaction of amino group with a carboxylic acid - amide formation H. Non-enzymatic glycation reactions 1 ©Kevin R. Siebenlist, 2019 IX. Recognize whether a monosaccharide is a reducing sugar or a non reducing sugar A. Discuss the use of the Tollen’s reagent and/or Benedict’s reagent to determine whether an aldose is present. X. Describe the reaction by which monosaccharides are joined to form disaccharides and polysaccharides. A. Describe the glycosidic bond. 1. What type of organic group is formed when a glycosidic bond forms? B. Describe the involvement of the anomeric carbon / anomeric hydroxyl group in the formation of a glycosidic bond. XI. Discuss the structural, chemical, and biochemical properties of the three common disaccharides. A. Describe the nature of the glycosidic bond(s) in the common disaccharides. B. Identify the disaccharide as reducing or non reducing. XII. Homopolysaccharides A. Discuss the structural, chemical, and biochemical properties of the homopolysaccharides produced by bacteria. B. Discuss the structural, chemical, and biochemical properties of the three common homopolysaccharides in eukaryotes. 1. Describe the nature of the glycosidic bond(s) in the common eukaryotic polysaccharides. XIII.Heteropolysaccharides A. With repeating disaccharide units - The Glycosaminoglycans 1. Discuss biochemical properties of the common glycosaminoglycans. a) Chondroitin sulfate. (1) D-Glucuronate (D-Glucuronic acid) and N-Acetyl-D-galactosamine. (2) C-4 or C-6 of N-acetyl galactosamine is often esterified with sulfate. (3) found in tendons, cartilage and bone where it modulates the compressibility of these tissues. b) Dermatan sulfate. (1) L-Iduronate (L-Iduronic acid) and N-Acetyl-D-galactosamine. (2) L-Iduronate is the C-5 epimer of D-Glucuronate (D-Glucuronic acid). (3) N-acetyl-D-galactosamine is often sulfated. (4) found in most tissues, but is especially rich in skin. c) Heparin. (1) D-Glucuronate (D-Glucuronic acid) and D-Glucosamine. (2) both are sulfated. (3) found in all cells, particularly rich Mast Cells where it has anticoagulant activities. d) Hyaluronic acid. (1) D-Glucuronate (D-Glucuronic acid) and N-Acetyl-D-glucosamine. (2) molecule Is Not Sulfated. (3) found in synovial fluid, vitreous humor, loose connective tissue, and cartilage. (4) plays a role in directing cell migration through the extracellular matrix and it modulates the compressibility of cartilage. e) Keratan sulfate. (1) N-Acetyl-D-glucosamine and D-Galactose. (2) N-acetyl-D-glucosamine is often sulfated. (3) found in the cornea of the eye where it plays a role in corneal transparency and 2 ©Kevin R. Siebenlist, 2019 also found in cartilage and tendons. 2. Assembly of glycosaminoglycans into proteoglycans B. With a “random” structure. 1. O-Linked vs N-Linked 2. Lectins & Selectins 3. A, B, O Blood group heteropolysaccharides (A, B, O Blood group antigens) kkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkk General Considerations The CARBOHYDRATES are widely distributed in nature, and by mass they are the most abundant biomolecules on earth. Carbohydrates are also called SACCHARIDES or SUGARS. In terms of organic functional groups present on the molecule, carbohydrates are polyhydroxyl aldehydes or ketones, or molecules that yield polyhydroxyl aldehydes or polyhydroxyl ketones upon hydrolysis. MONOSACCHARIDES are the monomeric units of carbohydrates. They all share the general formula, Cn(H2O)n, where n ≥ 3. From this formula the origin of the name “carbohydrate” is obvious - Carbon + Water. DISACCHARIDES contain two monomeric units covalently linked. OLIGOSACCHARIDES are polymers of monosaccharides containing ≤ 20 monomeric units. POLYSACCHARIDES are polymers of monosaccharides containing > 20 monomeric units. GLYCOCONJUGATES are carbohydrate derivatives in which oligosaccharides or polysaccharides are covalently linked to peptides, proteins, or lipids. Glycoconjugates include the: • PEPTIDOGLYCANS and PROTEOGLYCANS- more sugar than peptide/protein by mass. • GLYCOPROTEINS - more protein than sugar by mass. • GLYCOLIPIDS - more lipid than sugar by mass. The Monosaccharides The monosaccharides are all polyhydroxyl aldehydes or ketones. They can be classified by the number of carbon atoms they contain -TRIOSE, TETROSE, PENTOSE, HEXOSE, etc. NONOSES, monosaccharides with 9 carbons, are the largest monosaccharides found in nature. Monosaccharides can be classified by the type / location of carbonyl group they contain; an aldehyde group - ALDOSE; or a ketone group - KETOSE. A combination of these terms is can be used to describe monosaccharides more precisely - e.g., a KETOTETROSE has 4 carbons and a ketone functional group. Aldoses and ketoses with the same number of carbon atoms are isomers of each other. They have the same empirical formula Cn(H2O)n, but different structural formulas. Most monosaccharides end in the suffix “OSE”. The two simplest monosaccharides are GLYCERALDEHYDE, an aldotriose, and DIHYDROXYACETONE, a 3 ©Kevin R. Siebenlist, 2019 ketotriose. H O H O C C CH2OH HO CH H COH CO CH2OH CH2OH CH2OH L-Glyceraldehyde D-Glyceraldehyde Dihydroxyacetone Glyceraldehyde is a chiral molecule. The second carbon atom of this molecule is a chiral (asymmetric) carbon. Since glyceraldehyde contains a chiral carbon it can exist as a pair of enantiomers, a D-form (R) and an L-form (S). All of the aldoses and the ketoses with four or more carbons contain at least one chiral carbon and therefore can exist as at least one pair of enantiomers. Like the amino acids, only one enantiomeric form of the carbohydrates is primarily used by cells. The D-form of the carbohydrates is the biologically active form. L-forms are used to a limited extent in specialized structures or for specialized functions. The larger aldoses and ketoses can be modeled as extensions of glyceraldehyde or dihydroxyacetone. A secondary alcohol functional group H-C-OH or its mirror image HO-C-H is inserted between the carbonyl carbon and the adjacent secondary alcohol group. When these groups are inserted, two possible stereoisomers result; one with the -OH group pointing to the right the other with the -OH group pointing left. Of the 15 possible D-form aldoses with 6 or fewer carbons the ones most often encountered in living organisms are: D-GLYCERALDEHYDE is a aldotriose, D-ERYTHROSE is a aldotetrose, D-RIBOSE (Rib) is a aldopentose, and D-GLUCOSE (Glc), D-GALACTOSE (Gal), & D-MANNOSE (Man) are aldohexoses. H O H O C C H O H C OH C H C OH H C OH H C OH H C OH H C OH CH2OH CH OH 2 CH2OH D-Glyceraldehyde D-Erythrose D-Ribose H O H O H O C C C H C OH H C OH HO C H HO C H HO C H HO C H H C OH HO C H H C OH H C OH H C OH H C OH CH2OH CH2OH CH2OH D-Glucose D-Galactose D-Mannose 4 ©Kevin R. Siebenlist, 2019 CH2OH CH OH C O 2 CH2OH C O C O HO C H CH2OH H C OH HO C H H C OH C O H C OH H C OH H C OH CH OH CH OH CH OH 2 2 CH2OH 2 Dihydroxyacetone D-Ribulose D-Xylulose D-Fructose Of the 8 possible D-form ketoses with 6 or fewer carbon atoms the ones most often encountered are: DIHYDROXYACETONE is a ketotriose, D-RIBULOSE & D-XYLULOSE are ketopentoses, and D-FRUCTOSE (Fru) is a ketohexose. While the ketone functional group can be on any interior carbon of the molecule, all of the naturally occurring ketoses have the carbonyl group on carbon two. Except for dihydroxyacetone all of the monosaccharides depicted here contain one or more chiral carbon atoms. All of the sugars presented here are the D-enantiomeric form. When more than one chiral carbon is present in the molecule how is the D-form determined? If the configuration of the chiral carbon furthest from the carbonyl carbon; if the configuration of the last chiral carbon is identical to that of D- glyceraldehyde (i.e., it points to the right in the Fischer Projection), it is a D-monosaccharide.
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