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Fundamentals of Glycan Structure 2

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Fundamentals of Glycan Structure 2 Fundamentals of Glycan Structure 1 Learning Objectives How are glycans named? What are the different constituents of a glycan? How are these represented? What conformations do sugar residues adopt in solution Why do glycan conformations matter? 2 Fundamentals of Glycan Structure Carbohydrate Nomenclature Monosaccharides Structure Fisher Representation Cyclic Form Chair Form Mutarotation Monosaccharide Derivatives Reducing Sugars Uronic Acids Other Derivatives Monosaccharide Conformation Inter‐Glycosidic Bond Normal Sucrose Lactose Sequence Specificity and Recognition Branching 3 Carbohydrate Nomenclature The word ‘carbohydrate’ implies “hydrate of carbon” … Cn(H2O)m Glucose (a monosaccharide) C6H12O6 … C6(H2O)6 Sucrose (a disaccharide) C12H22O11 … C12(H2O)11 Cellulose (a polysaccharide) (C6H12O6)n… (C6(H2O)6)n Not all carbohydrates have this formula … some have nitrogen Glucosamine (glucose + amine) …. C6H13O5N… ‐NH2 at the 2‐position of glucose N‐acetyl galactosamine (galactose + amine + acetyl group) …. C8H15O6N … ‐ NHCOCH3 at the 2‐position of galactose Typical prefixes and suffixes used in naming carbohydrates Suffix = ‘‐ose’ & prefix = ‘tri‐’, ‘tetr‐’, ‘pent‐’, ‘hex‐’ Pentose (a five carbon monosaccharide) or hexose (a six carbon monosaccharide) Functional group types Monosaccharides with an aldehyde group are called aldoses … e.g., glyceraldehyde Those with a keto group are called ketoses … e.g., dihydroxyacetone 4 Monosaccharides Structure Have a general formula CnH2nOn and contain a carbonyl group Common monosaccharides have 3 9 carbons Two molecules form the class of smallest monosaccharides … glyceraldehyde and dihydroxyacetone CHO CH2OH CHOH O CH2OH CH2OH Glyceraldehyde Dihydroxyacetone 5 Monosaccharides Glyceraldehyde has a chiral carbon CHO *CHOH * =chiral carbon … 2 stereoisomers CH2OH 3‐Dimensional arrangement CHO CHO HO H HOCH2 H CH2OH OH (R) (S) 2‐Dimensional representation CHO CHO CHO CHO H OH H OH HO H HO H CH2OH CH2OH CH2OH CH2OH (R) (S) (D) (L) 6 Monosaccharides Emil Fischer’s representation Arbitrarily assigned the dextrorotatory enantiomer as D‐ glyceraldehyde, which fortuitously proved correct CHO CHO H OH HO H CH2OH CH2OH (D) (+) (L) (-) Note: This does not mean that all D‐saccharides are dextrorotatory; likewise it does not mean that all D‐saccharides are also (R) in the Cahn‐Ingold‐Prelog system 7 Monosaccharides ‘D’ monosaccharides CHO H OH CHO CHO CH OH H OH 2 HO H D-glyceraldehye H OH H OH CH2OH CH2OH D-erythrose D-threose CHO CHO CHO CHO H OH HO H H OH HO H H OH H OH HO H HO H H OH H OH H OH H OH CH2OH CH2OH CH2OH CH2OH D-ribose D-arabinose D-xylose D-lyxose CHO CHO CHO CHO CHO CHO CHO CHO H OH HO H H OH HO H H OH HO H H OH HO H H OH H OH HO H HO H H OH H OH HO H HO H H OH H OH H OH H OH HO H HO H HO H HO H H OH H OH H OH H OH H OH H OH H OH H OH CH OH CH OH CH OH CH OH CH OH CH OH CH2OH 2 CH2OH 2 2 2 2 2 D-allose D-altrose D-glucose D-mannose D-gulose D-idose D-galactose D-talose ‘D’ monosaccharides have the same configuration on their penultimate carbon as ‘D’ glyceraldehyde Exactly similar series for ‘L’ monosaccharides starting with ‘L’ glyceraldehyde Likewise, similar series exists for ‘L’ and ‘D’ ketoses starting from dihydroxyacetone 8 Monosaccharides ‘L’ ketoses CH2OH O CH2OH Dihydroxyacetone (no D- or L-form) CH2OH O HO H CH2OH L-erythrulose CH2OH CH2OH O O H OH HO H HO H HO H CH2OH CH2OH L-xylulose L-ribulose CH OH CH OH CH2OH CH2OH 2 2 O O O O H OH HO H H OH HO H H OH H OH HO H HO H HO H HO H HO H HO H CH OH CH OH CH2OH CH2OH 2 2 L-psicose L-fructose L-sorbose L-tagatose 9 Monosaccharides Some Useful Information on Monosaccharides D‐glucose is also called dextrose because it is dextrorotatory. D‐glucose is the most abundant monosaccharide; it is present in most fruit juices D‐glucose is also called blood sugar …… it is present in about 65 – 100 mg per 100 mL of blood The dextrose solution for iv drip contains about 5% glucose + 0.15% saline (KCl) Fructose is one of the monosaccharides present in disaccharide sucrose (table sugar) … the other is D‐glucose Galactose is part of lactose (milk sugar) 10 Monosaccharides Monosaccharides Are Typically not Linear (or Open‐Chain) Remember from organic chemistry: Aldehydes and ketones react with alcohols to give hemi‐acetals OH H O OH H + R' OR' R R CHO H H O OH H H H OH O H H CH CH H OH 3 3 anomeric carbon CH3 6CH OH 6 6 1 CHO 2 CH2OH CH2OH H OH 5 OH 5 O H 5 O H O H H OH HO H H H H 1 1 + 1 H OH OH H OH H OH H H H 5 OH OH OH OH OH H H H H 6CH2OH OH OH OH -D-glucopyranose -D-glucopyranose (-D-glucose) (-D-glucose) HAWORTH Projections What is an anomer? How are ‐ and ‐anomers defined? What is an epimer? Monosaccharides Cyclic Structure of Ketoses Similar cyclic structures exist for five membered saccharides … e.g., ribose. These five membered cyclic hemiacetals are well known for nucleic acids … e.g., RNA (ribonucleic acid) and DNA (deoxyribonucleic acid) 1CH OH 2 6 O 1CH OH HOCH CH OH HOCH OH HOCH2 OH 2 2 O 2 2 O HO H 5 2 H HO O H HO H HO H H + H H OH OH CH2OH 5 H OH OH H OH H OH H 6 CH2OH -D-fructofuranose -D-fructofuranose D-fructose (-D-fructose) (-D-fructose) HAWORTH Projections CH OH HOCH OH HOCH2 O 2 2 O H H H H H H OH CH2OH OH H OH H 2-deoxy--D-fructofuranose 2-deoxy--D-fructofuranose (2-deoxy--D-fructose) (2-deoxy--D-fructose) 12 Monosaccharides The Cyclic Form Assumes Different Conformations in Solution CH2OH CH2OH O O H H H OH H H OH H OH H OH OH OH H H OH H OH 6 6 HOH C HOH2C O 2 O HO 5 H HO 5 H HO H 2 H HO H 2 OH OH 1 OH 1 H OH H H -D-glucopyranose -D-glucopyranose (-D-glucose) (-D-glucose) Chair Conformations Why is ‐D‐glucose the most common sugar in nature? 13 Monosaccharides The Cyclic Form of D‐Galactose CH2OH CH2OH O O OH H OH OH H H OH H OH H H OH H H H OH H OH OH OH HOH C HOH2C O 2 O H H H H HO H H HO H OH OH OH H OH H H -D-galactopyranose -D-galactopyranose (-D-galactose) (-D-galactose) Chair Conformations 14 Monosaccharides Mutarotation Arises From Cyclic and Open‐Chain Form Interconversion CHO H OH HO H H OH H OH CH2OH D-glucose -D-glucopyranose -D-glucopyranose (-D-glucose) (-D-glucose) Optical activity O O []D = +112.2 []D = +18.7 O At equilibrium …. []D = +52.7 (Not an average! But weighted average; 63.6% of the ‐anomer) 15 Monosaccharide Derivatives Reducing Sugars and Reducing End CHO COOH H OH H OH HO H HO H H OH Mild Oxidizing Agent H OH e.g., Tollen’s reagent H OH + H OH (Ag / NH3 –H2O CH2OH CH2OH D‐Glucose D‐Gluconic Acid Reducing sugars and diabetes Hemoglobin (Hb) is glycated ….. HbA1c levels should be < 6.5% Reaction of high glucose with NH2 group at the N‐terminus (Val) of Hb Non‐enzymatic Reflects Glc levels for the past 3 months (typical life of erythrocytes) 16 Monosaccharide Derivatives Uronic Acids CHO H OH HO H H OH Specific enzymes in the body H OH CH2OH D‐Glucose D‐Glucuronic Acid Biological relevance Several other uronic acids occur in our body, e.g., D‐mannuronic acid (from D‐ mannose), L‐iduronic acid (from L‐idose), D‐galacturonic acid Components of many polysaccharides Add to the conformational complexity , e.g., L‐iduronic acid can exist in 2 O 1 4 several conformations other than chair – SO, S2, C4 and C1 17 Monosaccharide Derivatives Glycosamines Several aminosugars are known including D‐glucosamine, D‐mannosamine, D‐ galactosamine and N‐acetyl‐D‐glucosamine. These have a nitrogen at position 2 CHO CHO CHO CHO H N H H NH H NHCOCH H NH2 2 2 3 HO H HO H HO H HO H H OH H OH HO H H OH H OH H OH H OH H OH CH OH CH OH CH OH CH2OH 2 2 2 D-glucosamine D-mannosamine D-galactosamine N-acetyl-D-glucosamine Other Derivatives Glycan Esters The –OH groups can be esterified enzymatically, e.g., phosphate esters, acyl esters, and sulfate esters Deoxy Sugars The –OH group can be replaced with –H, e.g., 2‐deoxyribose (C‐2 deoxy), fucose (C‐6 deoxy), etc. Methylated Sugars The –OH group can be methylated (methyltransferases), e.g., –OMe group at C‐1 18 Monosaccharide Conformation Finding the Most Stable Conformation of a Monosaccharide CHO H OH HO H H OH H OH CH2OH D‐Glucose 19 Monosaccharide Conformation Conformation of L‐Iduronic Acid CHO H OH HO H H OH H OH CH2OH D‐Glucose L‐Iduronic Acid 2 4 6 5 4 5 1 1 4 6 3 3 1 2 2 4 1 SO C1 C4 20 Generation of Oligosaccharides The Inter‐Glycosidic Bond The bond between two monosaccharides is the inter‐glycosidic bond Fundamental linkage for generation of oligosaccharides Any hydroxylicated molecule (e.g., amino acids Tyr, Ser, Thr; OR alcohols cholesterol, cholic acid; OR saccharides) can form glycosidic bond Most oligosaccharides form glycosidic bond between the anomeric carbon (C‐1) and other carbons (C‐2, C‐3, C‐4, and C‐6).
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