BCH 101 Carbohydrate Lecture Notes PDF

BCH 101 INTRODUCTION TO BIOMOLECULES CARBOHYDRATES BY ADEYEMO, ADESEGUN GIDEON Carbohydrates  DEFINITION Carbohydrates are polyhydroxy aldehydes or ketones or compounds which yield these on hydrolysis.  Functions sources of energy intermediates in...

BCH 101 INTRODUCTION TO BIOMOLECULES CARBOHYDRATES BY ADEYEMO, ADESEGUN GIDEON Carbohydrates  DEFINITION Carbohydrates are polyhydroxy aldehydes or ketones or compounds which yield these on hydrolysis.  Functions sources of energy intermediates in the biosynthesis of other basic biochemical entities (fats and proteins) associated with other entities such as glycosides, vitamins and antibiotics) form structural tissues in plants and in microorganisms (cellulose, lignin, murein) participate in biological transport, cell-cell recognition, activation of growth factors, modulation of the immune system  Carbohydrates glucose provides energy for the brain and ½ of energy for muscles and tissues glycogen is stored glucose glucose is immediate energy glycogen is reserve energy  Classification of carbohydrates Carbohydrates – polyhydroxyaldehydes or polyhydroxy-ketones of formula (CH2O)n, or compounds that can be hydrolyzed to them. (aka sugars or saccharides) Monosaccharides – carbohydrates that cannot be hydrolyzed to simpler carbohydrates; eg. Glucose or fructose. Disaccharides – carbohydrates that can be hydrolyzed into two monosaccharide units; eg. Sucrose, which is hydrolyzed into glucose and fructose. Oligosaccharides – carbohydrates that can be hydrolyzed into a few monosaccharide units. Polysaccharides – carbohydrates that are are polymeric sugars; eg Starch or cellulose. Carbohydrates (glycans) have the following basic composition: I (CH2O)n or H - C - OH I  Monosaccharides - simple sugars with multiple OH groups. Based on number of carbons (3, 4, 5, 6), a monosaccharide is a triose, tetrose, pentose or hexose.  Disaccharides - 2 monosaccharides covalently linked.  Oligosaccharides - a few monosaccharides covalently linked.  Polysaccharides - polymers consisting of chains of monosaccharide or disaccharide units.  Simple Carbohydrates sugars – monosaccharides – single sugars – disaccharides – 2 monosaccharides  Characteristics of Carbohydrates Consist of carbon, hydrogen, & oxygen Energy containing molecules Some provide structure Basic building block is a monosaccharide (CH2O)n ; n = 3,5,6 Two monosaccharides form a disaccharide copyright cmassengale  Monosaccharides also known as simple sugars classified by 1. the number of carbons and 2. whether aldoses or ketoses most (99%) are straight chain compounds D-glyceraldehyde is the simplest of the aldoses (aldotriose) all other sugars have the ending ose (glucose, galactose, ribose, lactose, etc…)  Glucose The chemical formula for glucose is C6H12O6. It is a six sided ring. The structure on the left is a simplified structure of glucose RELATIVE SWEETNESS OF DIFFERENT SUGARS Sucrose 100 Glucose 74 Fructose 174 Lactose 16 Invert Sugar 126 Maltose 32 Galactose 32  Monosaccharides Aldoses (e.g., glucose) have Ketoses (e.g., fructose) have an aldehyde group at one end. a keto group, usually at C2. H O C CH2OH H C OH C O HO C H HO C H H C OH H C OH H C OH H C OH CH2OH CH2OH D-glucose D-fructose  chiral centers by definition are C atoms which have 4 DIFFERENT atoms bonded to it Compounds having same structural formula, but differ in spatial configuration. 􀁺 Asymmetric Carbon atom:Attached to four different atoms or groups. 􀁺 Vant Hoff’s rule: The possible isomers (2n) of a given compound is determined by the number of asymmetric carbon atoms (n). 􀁺 Reference C atom: Penultimate C atom, around which mirror images are formed.  Sugar Nomenclature For sugars with more O H O H than one chiral center, C C D or L refers to the H – C – OH HO – C – H asymmetric C farthest HO – C – H H – C – OH from the aldehyde or H – C – OH HO – C – H keto group. H – C – OH HO – C – H CH2OH CH2OH Most naturally occurring D-glucose L-glucose sugars are D isomers. D & L sugars are mirror O H O H images of one another. C C They have the same H – C – OH HO – C – H name, e.g., D-glucose HO – C – H H – C – OH & L-glucose. H – C – OH HO – C – H H – C – OH HO – C – H Other stereoisomers CH2OH CH2OH have unique names, e.g., glucose, mannose, D-glucose L-glucose galactose, etc. The number of stereoisomers is 2n, where n is the number of asymmetric centers. The 6-C aldoses have 4 asymmetric centers. Thus there are 16 stereoisomers (8 D-sugars and 8 L-sugars).  D vs L Designation CHO CHO D & L designations H C OH HO C H are based on the configuration about CH2OH CH2OH the single asymmetric D-glyceraldehyde L-glyceraldehyde C in glyceraldehyde. CHO CHO The lower H C OH HO C H representations are CH2OH CH2OH Fischer Projections. D-glyceraldehyde L-glyceraldehyde  Enantiomres A special type of isomerism is found in the pairs of structures that are mirror images of each other. These mirror images are called enantiomers, and the two members of the pair are designated as a D- and an L-sugar two monosaccharides differ in configuration around only one specific carbon atom (with the exception of the carbonyl carbon, see below), they are defined as epimers of each other.  * * * * (+)-glucose? An aldohexose CH2CHCHCHCHCH O Emil Fischer (1902) OH OHOHOHOH Four chiral centers, 24 = 16 stereoisomers CHO OH? CH2OH  1 CH2OH 2C O HO C H 1 CH2OH 3 HOH2C 6 O H C OH HO 4 5 H 2 H C OH H 4 3 OH 5 OH H 6 CH2OH D-fructose (linear) -D-fructofuranose Fructose forms either  a 6-member pyranose ring, by reaction of the C2 keto group with the OH on C6, or  a 5-member furanose ring, by reaction of the C2 keto group with the OH on C5. Epimers – stereoisomers that differ only in configuration about one chiral center. CHO CHO H OH HO H HO H HO H H OH H OH H OH H OH CH2OH CH2OH D-glucose D-mannose epimers Sugars are different from one another, only in configuration with regard to a single C atom (other than the reference C atom).  Enantiomers and epimers H H H H C O C O C O C O HO C H HO C H H C OH OH C H HO C H HO C H H C OH OH C H H C OH HO C H CH2OH CH2OH H C OH H C OH these two aldotetroses are enantiomers. They are stereoisomers that are mirror CH2OH CH2OH images of each other these two aldohexoses are C-4 epimers. they differ only in the position of the hydroxyl group on one asymmetric carbo (carbon 4)  Hemiacetal & hemiketal formation H H An aldehyde can C O + R' OH R' O C OH react with an alcohol to form R R a hemiacetal. aldehyde alcohol hemiacetal R R A ketone can react with an C O + "R OH "R O C OH alcohol to form R' R' a hemiketal. ketone alcohol hemiketal Anomers: Stereoisomers formed when ring is formed (, ). H O H OH HO H C C C H C OH H C OH H C OH HO C H O HO C H or O HO C H H C OH H C OH H C OH HO C H C H C H CH 2 OH CH 2 OH CH 2 OH  is same side with ring  Rules for drawing Haworth projections for D-sugars the highest numbered carbon (furthest from the carbonyl) is drawn up. For L-sugars, it is drawn down for D-sugars, the OH group at the anomeric position is drawn down for and up for . For L-sugars is up and  is down  CHO Pentoses and 1 H C OH hexoses can cyclize 2 as the ketone or HO 3 C H D-glucose aldehyde reacts H C OH (linear form) 4 with a distal OH. H C OH 5 Glucose forms an CH2OH 6 intra-molecular 6 CH2OH 6 CH2OH hemiacetal, as the 5 5 C1 aldehyde & C5 H O H H O OH H H OH react, to form a 4 OH H 1 4 OH H 1 6-member pyranose OH 2 OH OH 3 2 H 3 ring, named after H OH H OH pyran. -D-glucose -D-glucose These representations of the cyclic sugars are called Haworth projections. D-glucose can cyclize in two ways forming either furanose or pyranose structures D-ribose and other five-carbon saccharides can form either furanose or pyranose structures  Structural representation of sugars Fisher projection: straight chain representation Haworth projection: simple ring in perspective Conformational representation: chair and boat configurations Different Forms of Glucose copyright cmassengale Oxygen of the hydroxyl group is removed to form deoxy sugars. 􀁺 Non reducing and non osazone forming. 􀁺 Important part of nucleic acids.  Simple Carbs monosaccharides – all are 6 carbon hexes 6 carbons 12 hydrogens 6 oxygens arrangement differs – accounts for varying sweetness – glucose, fructose, galactose Three Monosaccharides C6H12O6 copyright cmassengale  H OH H OH HO HO HO HO HO H HO OH H OH H OH H OH H H alpha hemiacetal beta O D-glucopyranoses 4H-Pyran  H H H OH HOHO O HOHO O HO H HO H H OH H H H OH H OH alpha furanose form beta furanose form D-glucofuranoses O furan  OPTICAL ACTIVITY 􀁺Dextrorotatory (+) :If the sugar solution turns the plane of polarized light to right. 􀁺 Levorotatory (–) :If the sugar solution turns the plane of polarized light to left. 􀁺 Racemic mixture:Equimolar mixture of optical isomers has no net rotation.  Glycosidic Bonds The anomeric hydroxyl and a hydroxyl of another sugar or some other compound can join together, splitting out water to form a glycosidic bond: R-OH + HO-R'  R-O-R' + H2O E.g., methanol reacts with the anomeric OH on glucose to form methyl glucoside (methyl-glucopyranose). H OH H OH H O H2O H O HO HO HO H + CH 3 -O H HO H H OH H OH H OH H O CH 3 -D -glucopyranose methanol methyl--D-glucopyranose The anomeric forms of methyl-D-glucoside  Sugar derivatives CH 2OH CH 2OH H O H H O H H H OH H OH H OH OH OH O OH H NH 2 H N C CH 3 H -D-glucosamine -D-N-acetylglucosamine amino sugar - an amino group substitutes for a hydroxyl. An example is glucosamine. The amino group may be acetylated, as in N-acetylglucosamine.  6 CH 2 O H 6 CH 2 O H Disaccharides: H 5 O H H 5 O H H H Maltose, a cleavage 4 OH H 1 4 OH H 1 product of starch OH 3 2 O 3 2 OH (e.g., amylose), is a H OH m altose H OH disaccharide with an (1 4) glycosidic 6 CH 2 O H 6 CH 2O H link between C1 - C4 5 O H H 5 O OH H H OH of 2 glucoses. 4 OH H 1 O 4 OH H 1 H H It is the  anomer OH 3 2 3 2 (C1 O points down). H OH cellobiose H OH Cellobiose, a product of cellulose breakdown, is the otherwise equivalent  anomer (O on C1 points up). The (1 4) glycosidic linkage is represented as a zig-zag, but one glucose is actually flipped over relative to the other. MALTOSE 􀁺 Malt sugar. Produced during the course of digestion of starch by the enzyme amylase. 􀁺Two α-D-glucose units held together by α(1→4) glycosidic bond. Other disaccharides include:  Sucrose, common table sugar, has a glycosidic bond linking the anomeric hydroxyls of glucose & fructose. Because the configuration at the anomeric C of glucose is  (O points down from ring), the linkage is (12). The full name of sucrose is -D-glucopyranosyl-(12)- -D-fructopyranose.)  Lactose, milk sugar, is composed of galactose & glucose, with (14) linkage from the anomeric OH of galactose. Its full name is -D-galactopyranosyl-(1 4)- -D-glucopyranose  SUCROSE Cane sugar. 􀁺 α-D-glucose &β-D-fructose units held together by (α1→β2) glycosidic bond. 􀁺 Reducing groups in both are involved in bond formation, hence non reducing.  Lactose Principal sugar in milk CH2OH CH2OH OH O O O OH OH OH OH OH β-D-galactose & β-D-glucose units held together by β(1→4) glycosidic bond.  􀁺 Reducing:Maltose, Lactose –with free aldehyde or keto group. 􀁺 Non-reducing:Sucrose, Trehalose –no free aldehyde or keto group.  Sucrose 2-0--D-Glucopyranosyl -D-Fructofuranoside CH2OH 1 O CH 2 OH O H OH 2 5 HO HO O 3 4 CH 2 OH OH 6 OH Invert Sugar --- when sucrose in solution, the rotation changes from detrorotatory (+66.5) to levorotatory (-19.8). So, sucrose is called “Invert Sugar”. Sucrose has been hydrolyzed into glucose and fructose. Dehydration Synthesis of a Disaccharide copyright cmassengale Formation of Disaccharides copyright cmassengale  Starches stored in plant cells body hydrolyzes plant starch to glucose  Starch most common storage polysaccharide in plants composed of 10 – 30% amylose and 70- 90% amylopectin depending on the source the chains are of varying length, having molecular weights from several thousands to half a million  CH 2 OH 6 CH OH 2 CH 2 OH CH 2 OH CH 2 OH O 5 O H O H O H H O H H H H H H H H H H H OH H 1 4 OH H 1 OH H OH H OH H O O O O OH OH 2 3 H OH H OH H OH H OH H OH a m y lo s e Polysaccharides: Plants store glucose as amylose or amylopectin, glucose polymers collectively called starch. Glucose storage in polymeric form minimizes osmotic effects. Amylose is a glucose polymer with (14) linkages. The end of the polysaccharide with an anomeric C1 not involved in a glycosidic bond is called the reducing end. Amylose and amylopectin are the 2 forms of starch. Amylopectin is a highly branched structure, with branches occurring every 12 to 30 residues Polysaccharides starch cellulose Starch 20% amylose (water soluble) 80% amylopectin (water insoluble) amylose + H2O  (+)-maltose (+)-maltose + H2O  (+)-glucose starch is a poly glucose (alpha-glucoside to C-4) O O O O O O O O O O O O O O O O  CH 2OH CH 2OH H O H H O H amylopectin H H OH H OH H 1 O OH O H OH H OH CH 2OH CH 2OH 6 CH 2 CH 2OH CH 2OH H O H H O H H 5 O H H O H H O H H H H H H OH H OH H OH H 1 4 OH H OH H 4 O O O O OH OH 3 2 H OH H OH H OH H OH H OH Amylopectin is a glucose polymer with mainly (14) linkages, but it also has branches formed by (16) linkages. Branches are generally longer than shown above. The branches produce a compact structure & provide multiple chain ends at which enzymatic cleavage can occur.  CH 2 OH CH 2 OH glycogen H O H H O H H H OH H OH H 1 O OH O H OH H OH CH 2 OH CH 2 OH 6 CH 2 CH 2 OH CH 2 OH H O H H O H H 5 O H H O H H O H H H H H H OH H OH H OH H 1 4 OH H OH H 4 O O O O OH OH 2 3 H OH H OH H OH H OH H OH Glycogen, the glucose storage polymer in animals, is similar in structure to amylopectin. But glycogen has more (16) branches. The highly branched structure permits rapid glucose release from glycogen stores, e.g., in muscle during exercise. The ability to rapidly mobilize glucose is more essential to animals than to plants.  Glycogen also known as animal starch stored in muscle and liver (mostly) present in cells as granules (high MW) contains both (1,4) links and (1,6) branches at every 8 to 12 glucose unit (more frequent than in starch) complete hydrolysis yields glucose glycogen and iodine gives a red-violet color hydrolyzed by both  and -amylases and by glycogen phosphorylase  Cellulose Polymer of -D-glucose attached by (1,4) linkages Only digested and utilized by ruminants (cows, deers, giraffes, camels) A structural polysaccharide Yields glucose upon complete hydrolysis Partial hydrolysis yields cellobiose Most abundant of all carbohydrates Cotton flax: 97-99% cellulose Wood: ~ 50% cellulose Gives no color with iodine Held together with lignin in woody plant tissues  CH 2 OH 6 CH OH 2 CH 2 OH CH 2 OH CH 2 OH O 5 O O H O H O OH H H H H H H H H OH H 1 O 4 OH H 1 O OH H O OH H O OH H OH H H H H 2 H 3 H OH H OH H OH H OH H OH c e llu lo s e Cellulose, a major constituent of plant cell walls, consists of long linear chains of glucose with (14) linkages. Every other glucose is flipped over, due to  linkages. This promotes intra-chain and inter-chain H-bonds and van der Waals interactions, that cause cellulose chains to be straight & rigid, and pack with a crystalline arrangement in thick bundles - microfibrils. See: Botany online website; Schematic of arrangement of website at Georgia Tech. cellulose chains in a microfibril. Special monosaccharides: amino sugars Constituents of mucopolysaccharides