Carbohydrates

Contents Dr. Nikhat Siddiqi 2 Functions of carbohydrates Classification of carbohydrates Isomers and epimers Stereochemistry Hemiacetal and hemiketals Fischer’s projection formula Haworth projection formula Chain and ring forms Chair conformations

Dr. Nikhat Siddiqi 3 Carbohydrates are one of the four major classes of biomolecules along with proteins, nucleic acids, and lipids. Carbohydrates are aldehyde or ketone compounds with multiple hydroxyl groups. They make up most of the organic matter on earth because of their extensive roles in all forms of life.

Functions of Carbohydrates Dr. Nikhat Siddiqi 4 Carbohydrates serve as energy stores, fuels, and metabolic intermediates. Second, ribose and deoxyribose sugars form part of the structural framework of RNA and DNA.RNADNA

Functions of Carbohydrates Dr. Nikhat Siddiqi 5 Third, polysaccharides are structural elements in the cell walls of bacteria and plants. Eg., Cellulose in plants Chitin in insects

Functions of Carbohydrates Dr. Nikhat Siddiqi 6 Fourth, carbohydrates are linked to many proteins and lipids, where they play key roles in mediating interactions among cells and interactions between cells and other elements in the cellular environment.

Classification Dr. Nikhat Siddiqi 7 Monosaccharides Disaccharides Oligosaccharides Polysaccharides According to the number of sugar units.

Monosaccharides Dr. Nikhat Siddiqi 8 Monosaccharides, the simplest carbohydrates, are aldehydes or ketones that have two or more hydroxyl groups; The empirical formula of many is (C-H 2 O)n, literally a “carbon hydrate.”

Monosaccharides Dr. Nikhat Siddiqi 9 Monosaccharides are important fuel molecules as well as building blocks for nucleic acids. The smallest monosaccharides, for which n = 3, are dihydroxyacetone and d- and l-glyceraldehyde.

Monosaccharides Dr. Nikhat Siddiqi 10 They are referred to as trioses (tri- for 3). Dihydroxyacetone is called a ketose because it contains a keto group, whereas glyceraldehyde is called an aldose because it contains an aldehyde group.

Dr. Nikhat Siddiqi 11 All carbohydrates contain at least one asymmetrical (chiral) carbon and are, therefore, optically active. In addition, carbohydrates can exist in either of two conformations, as determined by the orientation of the hydroxyl group about the asymmetric carbon farthest from the carbonyl.

Dr. Nikhat Siddiqi 12 With a few exceptions, those carbohydrates that are of physiological significance exist in the D- conformation. The mirror-image conformations, called enantiomers, are in the L-conformation.

Isomers and epimers Dr. Nikhat Siddiqi 13 Compounds that have the same chemical formula but have different structures are called isomers. For example, fructose, glucose, mannose, and galactose are all isomers of each other, having the same chemical formula, C 6 H 12 O 6.

Isomers and epimers Dr. Nikhat Siddiqi 14 Carbohydrate isomers that differ in configuration around only one specific carbon atom are defined as epimers of each other. For example, glucose and galactose are C-4 epimers—their structures differ only in the position of the -OH group at carbon 4.

C-2 and C-4 epimers and an isomer of glucose. Dr. Nikhat Siddiqi 15

Dr. Nikhat Siddiqi 16 Glucose and mannose are C-2 epimers. However, galactose and mannose are NOT epimers—they differ in the position of -OH groups at two carbons (2 and 4) and are, therefore, defined only as isomers.

Dr. Nikhat Siddiqi 17 The names of monosaccharides are frequently abbreviated; most common are three-letter abbreviations for simple monosaccharides (e.g., Gal, Glc, Man, Xyl, Fuc).monosaccharides Glycan-A generic term for any sugar or assembly of sugars, in free form or attached to another molecule, used interchangeably with saccharide or carbohydrate.sugarsugarssaccharidecarbohydrate

Stereochemistry Dr. Nikhat Siddiqi 18 Saccharides with identical functional groups but with different spatial configurations have different chemical and biological properties. Stereochemisty is the study of the arrangement of atoms in three-dimensional space.

Stereochemistry Dr. Nikhat Siddiqi 19 Stereoisomers are compounds in which the atoms are linked in the same order but differ in their spatial arrangement. Compounds that are mirror images of each other but are not identical, comparable to left and right shoes, are called enantiomers.

Enantiomers Dr. Nikhat Siddiqi 20 A special type of isomerism is found in the pairs of structures that are mirror images of each other. The vast majority of the sugars in humans are D-sugars. Enzymes known as racemases are able to interconvert D- and L-isomers.

Enantiomers Dr. Nikhat Siddiqi 21 These mirror images are called enantiomers, and the two members of the pair are designated as a D- and an L- sugar. In the D isomeric form, the OH group on the asymmetric carbon (a carbon linked to four different atoms or groups) farthest from the carbonyl carbon is on the right, while in the L-isomer it is on the left.

Monosaccharides Dr. Nikhat Siddiqi 22 The monosaccharides commonly found in humans are classified according to the number of carbons they contain in their backbone structures. The major monosaccharides contain four to six carbon atoms.

Classification Dr. Nikhat Siddiqi 23 # CarbonsNameExample 3TrioseGlyceraldehyde, Dihydroxyacetone 4TetroseErythrose 5PentoseRibose, Ribulose, Xylulose 6HexoseGlucose, Galactose, Mannose, Fructose 7HeptoseSedoheptulose 9NonoseNeuraminic acid, also called sialic acid

d-Aldoses containing three, four, five, and six carbon atoms Dr. Nikhat Siddiqi 24

d-Aldoses containing three, four, five, and six carbon atoms Dr. Nikhat Siddiqi 25

Dr. Nikhat Siddiqi 26 d-Aldoses contain an aldehyde group (shown in blue) and have the absolute configuration of d-glyceraldehyde at the asymmetric center (shown in red) farthest from the aldehyde group.

Dr. Nikhat Siddiqi 27 d-Ribose, the carbohydrate component of RNA, is a five- carbon aldose.RNA d-Glucose, d-mannose, and d-galactose are abundant six- carbon aldoses. d-glucose and d-mannose differ in configuration only at C-2. Sugars differing in configuration at a single asymmetric center are called epimers. Thus, d-glucose and d-mannose are epimeric at C-2; d-glucose and d-galactose are epimeric at C- 4.

Ketose Dr. Nikhat Siddiqi 28 Dihydroxyacetone is the simplest ketose. d-Fructose is the most abundant ketohexose.

d -Ketoses containing three- four, five, and six carbon Dr. Nikhat Siddiqi 29

Pentoses and Hexoses Cyclize to Form Furanose and Pyranose Rings Dr. Nikhat Siddiqi 30

Dr. Nikhat Siddiqi 31 The predominant forms of ribose, glucose, fructose, and many other sugars in solution are not open chains. Rather, the open-chain forms of these sugars cyclize into rings. In general, an aldehyde can react with an alcohol to form a hemiacetal.

Dr. Nikhat Siddiqi 32 For an aldohexose such as glucose, the C-1 aldehyde in the open-chain form of glucose reacts with the C-5 hydroxyl group to form an intramolecular hemiacetal. The resulting cyclic hemiacetal, a six- membered ring, is called pyranose because of its similarity to pyran.

Dr. Nikhat Siddiqi 33 Similarly, a ketone can react with an alcohol to form a hemiketal.

Dr. Nikhat Siddiqi 34 The C-2 keto group in the open-chain form of a ketohexose, such as fructose, can form an intramolecular hemiketal by reacting with either the C-6 hydroxyl group to form a six- membered cyclic hemiketal or the C-5 hydroxyl group to form a five-membered cyclic hemiketal. The five-membered ring is called a furanose because of its similarity to furan

Furan & Pyran Dr. Nikhat Siddiqi 35

Dr. Nikhat Siddiqi 36 The depictions of glucopyranose and fructofuranose shown are Haworth projections.

Chain and Ring forms Dr. Nikhat Siddiqi 37 Many simple sugars can exist in a chain form or a ring form, as illustrated by the hexoses above. The ring form is favored in aqueous solutions, and the mechanism of ring formation is similar for most sugars. The glucose ring form is created when the oxygen on carbon number 5 links with the carbon comprising the carbonyl group (carbon number 1) and transfers its hydrogen to the carbonyl oxygen to create a hydroxyl group.

Chain and Ring forms Dr. Nikhat Siddiqi 38 The rearrangement produces alpha glucose when the hydroxyl group is on the opposite side of the -CH 2 OH group, or beta glucose when the hydroxyl group is on the same side as the -CH 2 OH group. Isomers, such as these, which differ only in their configuration about their carbonyl carbon atom are called anomers.

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Dr. Nikhat Siddiqi 40 a-D-glucose and b-D-glucose differ by the positioning of hydroxyl group on C1 carbon Both a-D-glucose serves as basic units of glycogen and starch b-D-glucose is the basic unit of cellulose

Chain and Ring forms Dr. Nikhat Siddiqi 41 The little D in the name derives from the fact that natural glucose is dextrorotary, i.e., it rotates polarized light to the right, but it now denotes a specific configuration. Monosaccharides forming a five-sided ring, like ribose, are called furanoses. Those forming six-sided rings, like glucose, are called pyranoses.

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Dr. Nikhat Siddiqi 44 The Haworth representations are preferably drawn with the ring oxygen atom at the top (for furanose) or the top right- hand corner (for pyranose) of the structure; the numbering of the ring carbons increases in a clockwise direction.furanosepyranose

Chair Conformation Dr. Nikhat Siddiqi 45 The planar Haworth structures are distorted representations of the actual molecules. The preferred conformation of a pyranose ring is the chair conformation, similar to the structure of cyclohexane. pyranose

β -D-Glucose in Haworth projection and in its 4 C 1 and 1 C 4 chair conformationsHaworth projection Dr. Nikhat Siddiqi 46

Dr. Nikhat Siddiqi 47 An additional asymmetric center is created when a cyclic hemiacetal is formed. In glucose, C-1, the carbonyl carbon atom in the open-chain form, becomes an asymmetric center. Thus, two ring structures can be formed: α -d-glucopyranose and β - d-glucopyranose.

Anomers Dr. Nikhat Siddiqi 48 For d sugars drawn as Haworth projections, the designation α means that the hydroxyl group attached to C- 1 is below the plane of the ring; β means that it is above the plane of the ring. The C-1 carbon atom is called the anomeric carbon atom, and the α and β forms are called anomers.

Dr. Nikhat Siddiqi 49 The same nomenclature applies to the furanose ring form of fructose, except that α and β refer to the hydroxyl groups attached to C-2, the anomeric carbon atom.

Reducing sugars Dr. Nikhat Siddiqi 50 If the oxygen on the anomeric carbon of a sugar is not attached to any other structure, that sugar can act as a reducing agent and is termed a reducing sugar. Such sugars can react with chromogenic agents (for example, Benedict's reagent or Fehling's solution) causing the reagent to be reduced and colored, with the anomeric carbon of the sugar becoming oxidized.

Dr. Nikhat Siddiqi 51 The End