Carbohydrates Lecture Notes PDF

Lecture 8 &9 CARBOHYDRATES TYPES / CLASSES STRUCTURE FUNCTION METABOLISM 1 Carbohydrates also called as glycans and have the I following basic composition: (CH2O)...

Lecture 8 &9 CARBOHYDRATES TYPES / CLASSES STRUCTURE FUNCTION METABOLISM 1 Carbohydrates also called as glycans and have the I following basic composition: (CH2O)n or H - C - OH Carbohydrates are classified into four types: I  Monosaccharides - simple sugars with multiple OH groups. -have an aldehyde group or a keto group;can be defined as polyhydroxy- aldoses or ketoses. -based on number of carbons (3, 4, 5, 6), a monosaccharide is a triose, tetrose ,pentose or hexose.  Disaccharides - Two monosaccharides covalently linked by glycosidic bond.  Oligosaccharides - a few (3-10 units)of monosaccharides covalently linked by glycosidic bonds.  Polysaccharides - polymers consisting of chains of (>10) monosaccharides covalently linked by glycosidic bonds. 2 Monosaccharide is a Triose, Tetrose ,Pentose or Hexose. Triose Tetrose Pentose Hexose 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-fructose D-glucose Sugar Nomenclature The suffix “-ose” designates a carbohydrate. A prefix like; tri , tetra ,penta and hexa designates the number of carbon atoms in the Monosaccharide molecule (e.g., triose and hexose). Monosaccharides have either an aldehyde group or a keto group;so 3 they are named as aldoses or ketoses. Monosaccharides; have either an aldehyde group or a keto group Aldoses have an aldehyde group Ketoses have a keto group, usually at one end, usually at C1. at C2. (e.g., glucose) (e.g., fructose) H O aldehyde 1C CH2OH keto 1 group group H C OH C O 2 2 HO C H HO 3C H 3 H H 4C OH 4C OH H H C OH 5C OH 5 6CH2OH 6CH2OH D-glucose D-fructose 4 Aldohexose/Hexoaldose Ketohexose/Hexoketose  Isomers Stereoisomers; They are monosaccharides molecules having same formula, but differing from each other in structural configuration i.e. Spatial arrangement of H or OH atoms in their structure around chiral atoms. 1 1 1 1 2 2 chiral 3 atoms 3 4 4 5 5 6 6 6 6 C6H12O6 C6H12O6 C6H12O6 C6H12O6 An asymmetric carbon atom (chiral carbon) is a carbon atom that is attached to four different types of atoms or groups. The number of stereoisomers of a monosaccharide is 2n, where n represents the number of asymmetric carbon atoms. A monosaccharide with n chiral centers has 2n stereoisomers 5 4 i.e. Glucose has four chiral carbons (n=4) ;it has 2 =16 stereoisomers. Asymmetric carbon atom (chiral carbon) aldehyde group Carbonyl carbon atom 1 Asymmetric carbon atom 2 3 6  D-Glucose is an aldohexose. It has four chiral centres. So it has 24 =16 Stereoisomers; having same formula ;C6H12O6 but differing in structural configuration. Chemical structures of aldohexose stereoisomers (Fischer projection). Notes: All, allose; Alt, altrose; Glc, glucose; Man, mannose; Gul, gulose; 7 Ido, idose; Gal, galactose; Tal, talose. D &L-Isomers are monosaccharides stereoisomers with more than one chiral center, D or L refer to the asymmetric C atom farthest from the aldehyde or keto group. Most naturally occurring sugars are D isomers. 1 1 1 1 5 5 If the OH on the bottom chiral center points to the right ,the sugar is D 8 If the OH on the bottom chiral center points to the left ,the sugar is L Enantiomers; Enantiomers are non-superimposable mirror-image pairs of stereoisomers. They are D & L stereoisomers of the same monosaccharide which they are mirror images of each other. 5 5 4 4 9 Epimers; are stereoisomers that differ only in configuration about one chiral center. Examples are D-mannose and D-galactose are epimers of D-glucose. While D-mannose is not epimer to D-galactose 1 1 1 2 2 2 3 3 3 4 4 4 5 5 5 6 6 6 D-glucose and D-galactose differ only at fourth carbon [C4 ] position. D-glucose and D-mannose differ only at second carbon[C2 ] position. 10  Pentoses and Hexoses can cyclize Pentoses and hexoses can cyclize; taking forms called pyranoses or furanoses because they resemble the organic compounds , Pyran or Furan Pyran; 6-member ring Furan; 5-member ring Is an organic compound, Is an organic compound, consisting of a six-membered consisting of a five-membered non-aromatic ring, with: aromatic ring with: 5 carbon atoms and 1oxygen 4 carbon atoms and 1 oxygen atom. atom. Formula; C5H6O Formula; C4H4O 11  Hemiacetal & Hemiketal Formation Pentoses and hexoses can cyclize by forming intra-molecular hemiacetal or hemiketal bonds (similar when an aldehyde or a ketone react with an alcohol to form a hemiacetal or a hemiketal). They take forms that resembles the organic compounds , Pyran or Furan cyclization of aldose cyclization of ketose Hemiacetal bond Hemiketal bond 1 1 5 2 1 5 2 1 5 5 Glucose Glucopyranose Fructose Fructofuranose When an aldose cyclizes, the hydroxyl When a ketose cyclizes, the hydroxyl group on the second to last carbon group on the second to last carbon undergoes an intramolecular reaction with undergoes an intramolecular reaction with the carbonyl group of the aldose forming the carbonyl group of the ketose forming intra-molecular hemiacetal bond. intra-molecular hemiketal bond.12 Glucose Fisher 1 CHO Glucose is a hexoaldose projection H C OH that can cyclize by 2 forming intra- HO C H D-glucose 3 molecular hemiacetal Haworth H C OH (linear form) 4 projection bond. They take forms H C OH 5 that resembles the Hemiacetal bond CH2OH organic compounds , 6 Pyran 6 CH2OH 6 CH2OH 5 O 5 O Glucose forms an intra- H H H OH H H molecular hemiacetal, as 4 OH H 1 4 OH H 1 the C1 aldehyde & C5 OH OH OH H OH react, to form a 6- 3 2 3 2 member pyranose ring, H OH H OH named after pyran. -D-glucose -D-glucose The cyclic form of sugars are called Haworth projections,while the 13 open chain form of sugars Fisher projections.  Fructose 1 CH2OH Hemiketal bond is a Hexoketose. 2C O HO C H 1 CH2OH 3 Hexoketoses can cyclize HOH2C 6 O by forming intra- H C OH 4 5 H HO 2 molecular hemiketal H C OH H 5 4 3 OH bonds. They take forms OH H that resembles the 6 CH2OH organic compounds , D-fructose (linear) -D-fructofuranose Furan. Fructose forms a 5- member furanose ring, by reaction of the C2 keto group with the OH on C5. 14  Anomers are cyclic monosaccharides differing from each other in the configuration of C1 if they are aldoses or in the configuration at C2 if they are ketoses. 6 CH2OH 6 CH2OH 5 O 5 O H H H OH There are 2 forms of anomers, H H 4 1 4 1 namely α and β anomers, based OH H OH H OH OH OH H on the direction of -OH group on 3 2 3 2 (C1) of aldose or (C2) of ketose H OH H OH -D-glucose -D-glucose on the cyclic sugar. The α-D-glucose and β-D-glucose are anomers Cyclization of glucose produces a new asymmetric center at C1; the Two cyclic stereoisomers are called anomers; α & β ;The key difference between alpha (α) and beta (β) glucose is the orientation of hydroxyl (-OH) group attached to the first carbon atom. α -glucose has its -OH perpendicular to the ring. (down to the ring) β-glucose has its -OH parallel to the ring (is written up to the ring) Mutarotation;The two anomeric forms ,α & β ,interconvert in water solutions 15 by a process called Mutarotation. Chair Configuration H OH H OH 4 6 H O H O HO 5 HO HO 2 H HO OH 3 H OH 1 H OH H OH H H -D-glucopyranose -D-glucopyranose Because of the tetrahedral nature of carbon bonds, pyranose sugars actually assume a "chair" or "boat" configuration, depending on the sugar. The representation above reflects the chair configuration of the 16 glucopyranose ring more accurately than the Haworth projection. Carbohydrates Monosaccharides Glucose Fructose Disaccharides Maltose Cellobiose Sucrose Lactose Polysaccharides Starch (amylose & amylopectin) Cellulose Glycogen 17  Glycosidic Bonds The anomeric hydroxyl of one monosaccharide and a hydroxyl of another monosaccharide can join together, splitting out water to form a glycosidic bond: (1 4) glycosidic bond in Maltose & Amylose (12) glycosidic bond in Sucrose, β(1-4) glycosidic bond in Lactose , Cellobiose ,Cellulose (1 4) and (16) glycosidic bonds in Amylopectin Formation of Glycosidic Bonds is dehydration process 6 R-OH +6 HO -R'  R-O-R' 6 + H2O 6 5 5 5 5 4 1 4 1 2 2 2 3 3 3 3 2 18 (1 4) Glycosidic Bonds Disaccharides: 6 CH2OH 6 CH2OH 5 O 5 O H H H H Maltose, is a disaccharide H H with an (1 4) glycosidic link 1 4 1 4 OH H OH H between C1 - C4 OH of two OH O OH 2 2 3 (1 4) 3 glucoses. H glucose OH H OH maltose It is the  anomer (O on C1 glucose points down). It is classified as 6 CH2OH 6 CH2OH a reducing sugar. 5 H O H 5 O OH Cereals;barley, certain fruits and H H 4 1 O 4 1 sweet potatoes contain naturally OH H OH H high amounts of this sugar. OH H H 2 3 2 (1 4) 3 Its a cleavage product of starch H OH H OH (e.g., amylose). glucose cellobiose glucose Cellobiose, is the  anomer (O on C1 points up). It is made up of two glucose joined byβ(1-4)-glycosidic bond. The (1 4) glycosidic linkage is represented as a zig-zag, but one glucose is actually flipped over relative to the other. It is classified as a reducing sugar. The cellobiose is found in natural foods – honey, and maize stems.Its a cleavage 19 product of cellulose.  Sucrose, common table sugar, naturally found in sugar cane and sugar beet ,has a glycosidic bond linking the anomeric hydroxyls of glucose & fructose. Because the configuration at the anomeric C1 of glucose is  (O points down from ring), the linkage is (12). The full name of sucrose is -D-glucopyranosyl-(12)--D- fructofuranose.). It is classified as a non reducing sugar. 6 6 6 1 5 5 5 4 1 4 1 4 1 2 5 3 2 6 3 2 (14) 3 2 (12) 3 4 Sucrose Lactose  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. 20 It is classified as a reducing sugar.  Polysaccharides: Starch; plants store glucose as amylose or amylopectin, glucose polymers called starch. Glucose is stored in a polymeric form (starch) to minimize osmotic effects of the cell. 1-Amylose CH2OH 6CH OH CH2OH CH2OH CH2OH 2 O 5 O H O H O H H O H H H H H H H H H H H 1 OH H 1 4 OH H 1 OH H OH H OH H O O O O OH OH 2 (1 4) 3 H OH H OH H OH H OH H OH amylose Amylose is a glucose polymer with (14) linkages. It is non reducing sugar. The end of the polysaccharide with an anomeric C1 not involved in a glycosidic bond is called the reducing end. 21 Starch: 2-Amylopectin CH 2OH CH 2OH H O H H O H amylopectin H H OH H OH H 1 O OH O (16) 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 (1 4) 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. It is non reducing sugar The branches produce a compact structure & provide multiple chain ends at which enzymatic cleavage can occur. 22 Polysaccharides: Cellulose CH2OH 6CH OH CH2OH CH2OH CH2OH 2 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 (14) H OH H OH H OH cellulose Cellulose, a major constituent of plant cell walls, consists of long linear chains of glucose unites with (14) linkages. It is non reducing sugar Every other glucose is flipped over, due to  linkages. This promotes intra-chain and inter-chain Hydrogen-bonds 23 Polysaccharides: Glycogen CH 2OH CH2OH H O O glycogen H H H H H OH H OH H 1 O OH O (16) H OH H OH CH 2OH CH2OH 6 CH2 CH2OH CH2OH 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 (1 4) H H OH H OH H OH OH H OH Glycogen, the glucose storage polymer in animals (a glucose polymer with mainly (14) linkages, but it also has branches formed by (16) linkages.) , is similar in structure to amylopectin. It is non reducing sugar 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 24 to plants. GLYCO-CONJUGATES: There are two types of glyco-conjugates : 1. GLYCOLIPIDS : They are made up of carbohydrates and lipids 2. GLYCOPROTEINS : They are made up of carbohydrate and proteins. 25 Functions of Carbohydrate In Biological System 1.They are instant sources of energy. ( example is glucose) 2. They are also present as stored sources of energy (energy stores). (examples are starch and glycogen) 3.Carbohydrates attached to lipids and protein acts as cellular signals & for the purpose of adhesion between cells ( examples are Glycolipids and Glycoproteins). 4.They are also present as structural components ( examples are cellulose and chitin). Chitin is the building component of the exoskeletons of crustaceans & insects Crustacean; Cellulose in cell wall shrimp 26 lobster of plant cell insect spider DIGESTION OF CARBOHYDRATES 27 28 DIGESTION OF CARBOHYDRATES Nahidah AlFuraih  Human diseases related to carbohydrate metabolism: 1) Diabetes mellitus 2) Lactose intolerance 3) Fructose intolerance (dietary and hereditary). 4) Galactosemia (life-threatening health problems) 5) Glycogen storage disease 30  Thank you Starch , Cellulose & glycogen are polymers Sucrose is a dimer Glucose is a monomer 31