Carbohydrates. Learning Objective

Transcription

1 , one of the four major classes of biomolecules, are aldehyde or ketone compounds with multiple hydroxyl groups. They function as energy stores, metabolic intermediates and important fuels for the body. Learning Objective After Interacting with this learning object, learner will be able to: Describe Monosaccharides. Describe Disaccharides and Polysaccharides. Define Glycoconjugates.

2 Monosaccharides All monosaccharide sugars are optically active due to the presence of asymmetric carbon atoms. The simplest aldotriose, D-glyceraldehyde, has one asymmetric centre giving rise to the D and L enantiomers, which are mirror images of each other. A compound will have 2n isomers, where n is the number of asymmetric centres..

3 Monosaccharides Simple D-aldose sugars can have anywhere from three to seven carbon atoms with an aldehyde functional group. All the D-sugars have the same absolute configuration as that of Dglyceraldehyde at their asymmetric centre that is farthest away from the carbonyl carbon. The most commonly observed sugars include D-glucose, Dmannose and D-galactose.

4 Monosaccharides Those sugars that differ in configuration from each other about only one asymmetric carbon atom are referred to as epimers. D-Glucose and D-mannose are epimers at the second carbon atom while D-glucose and D-galactose are epimers at the fourth carbon atom. D-mannose and D-galactose however are only diastereomers since they differ in configuration at two asymmetric centres.

5 Monosaccharides Ketoses can be three, four, five or six carbon sugars with a ketone functional group. Dihydroxyacetone, the simplest ketose having 3 carbon atoms does not possess an asymmetric centre. The D configuration is therefore based on the absolute configuration of Derythrulose, the four carbon ketose and is designated based on the asymmetric centre farthest away from the ketone group.

6 Monosaccharides The aldehyde or ketone group of monosaccharides reacts with the alcohol groups to form intramolecular hemiacetals or ketals. This gives rise to stable five or sixmembered rings known as furanose and pyranose respectively. The functional group carbon atom is designated as C-1 and is known as the anomeric carbon. The configuration of the groups about the anomeric carbon gives rise to the alpha and beta configurations. The chair form of the six membered pyranose ring is more stable due to minimal steric hindrance at the axial positions since they are occupied by small hydrogen atoms.

7 Monosaccharides Although glucose is more stable in the six membered pyranose configuration, stability of fructose is greater as a fivemembered furanose ring.

8 Monosaccharides A simple test for identifying sugars such as glucose is by the Fehling s test. The free aldehyde group provides the sugars with a reducing nature thereby bringing about reduction of a solution of cupric ions. This results in an easily identifiable brown precipitate.

9 Monosaccharides The reactive anomeric carbon atom can be modified by reaction with alcohols or amines to form adducts. Reaction of glucose with methanol gives the corresponding methyl glucopyranoside with the formation of an O-glycosidic bond. When the anomeric carbon is linked to an amine via its nitrogen atom, it results in formation of an Nglycosidic bond. Several other modified monosaccharides such as fucose, N-acetyl glucosamine etc are present which serve various structural and functional roles.

10 Disaccharides & Polysaccharides Disaccharides are formed by the condensation reaction between two monosaccharide units. The release of a molecule of water results in the formation of a glycosidic bond between the two residues. Shown in this example is the formation of maltose, a disaccharide that is composed of two units of glucose linked by an -1 4 glycosidic bond. Maltose is hydrolyzed into its individual units by the enzyme maltase.

11 Disaccharides & Polysaccharides Table sugar or sucrose is a disaccharide composed of one unit of glucose and one of fructose linked by a 1 2 bond. It is a non-reducing sugar since the aldehyde group of glucose and ketone group of fructose are involved in formation of the glycosidic linkage. It can be cleaved by the enzyme sucrase. Lactose, the sugar component of milk, is made up of one unit of galactose and one of glucose linked by an 1 4 linkage. It can be cleaved by the enzyme lactase, also known as galactosidase.

12 Disaccharides & Polysaccharides Starch granules, which form the major nutritional reserve of plants, are composed of two components amylose and amylopectin. Amylose consists of linear, unbranched chains of D-glucose residues linked by -1,4 glycosidic linkages. Amylopectin, however, is a branched polymer with an -1,6 glycosidic linkage present around every 30 residues. Starch is rapidly degraded by the enzyme amylase.

13 Disaccharides & Polysaccharides Glycogen is the storage form of glucose in animal cells. It is structurally similar to start with glucose residues being joined by -1,4 glycosidic linkages. Glycogen has more extensive branching than starch with branch points being observed around every 10 residues. These allow the molecules to be stored in a very compact way in the cells.

14 Glycoconjugates Glycosaminoglycans are heteropolysaccharide components of extracellular matrix spaces along with other fibrous proteins. They are linear polymers that are composed of disaccharide repeating units of which one residue is always a derivative of an amino sugar such as N-acetyl glucosamine or N-acetyl galactosamine. They also contain a negatively charged sulphate or carboxylate group.common glycosaminoglycans include chondrotin sulphate, hyaluronate, heparin etc.

15 Glycoconjugates Glycosyl transferases are specific enzymes that bring about transfer of activated sugar residues onto other substrates. The activated sugar linked to a nucleotide moiety such as UDP is cleaved and covalently linked with the substrate which could either be another monosaccharide unit, a polysaccharide or the serine or aspargine side chains of proteins.

16 Glycoconjugates Glycosaminoglycans can be linked to proteins to form various proteoglycans. These molecules have a variety of functions in tissue organization, development of specialized tissues and for modulation of ligand interactions with cell surface receptors. Aggrecan is a proteoglycan aggregate consisting of many core proteins bound to a single hyaluronate molecules; they serve as shock absorbers in cartilage.

17 Glycoconjugates Carbohydrate groups are often covalently attached to proteins to form glycoproteins. The sugar residues are typically attached to the amide nitrogen atom of the aspargine side chain or to the oxygen atom of the serine or threonine side chain. These glycoproteins are components of cell membranes and have a variety of functions in cell adhesion processes. The ABO blood groups arise due to differing carbohydrate structures on the surface of the blood cells.

18 Glycoconjugates Carbohydrate moieties can also be covalently linked with various lipids. Glycolipids are membrane components bearing a hydrophilic head group and a hydrophobic lipid tail. Lipopolysaccharides are a predominant feature on the outer membrane of gram negative bacteria, which consist of fatty acid chains bound to sugar residues.

19 Monosaccharides 1. Monosaccharide: Monosaccharides comprise the simplest group of carbohydrates with the empirical formula (C-H2O)n. They can be either aldehydes or ketones having two or more hydroxyl groups. These monosaccharides serve as important fuel molecules and as the basic building unit for nucleic acids. 2. Aldotriose: The smallest monosaccharide having an aldehyde group and a total of 3 carbon atoms is referred to as an aldotriose. Glyceraldehyde is the simplest aldotriose. 3. Ketotriose: The smallest monosaccharide that has a ketone group with a total of 3 carbon atoms is referred to as a ketotriose. Dihydroxyacetone is the simplest ketotriose. 4. Asymmetric centre: A carbon atom that has four different groups attached to it in a tetrahedral arrangement is said to be asymmetric or chiral and gives rise to the phenomenon of optical isomerism. All monosaccharides have multiple asymmetric carbon atoms, giving rise to 2n isomers for each monosaccharide, where n refers to the number of asymmetric centres. 5. Enantiomers: Molecules with a chiral centre have a non-superimposable mirror image and the two forms of this molecule are known as enantiomers. They are designated as D and L or R (rectus) and S (sinister) depending on the arrangement of groups around the asymmetric carbon atom. R and S nomenclature is based on priority of atomic numbers of atoms directly attached to the central asymmetric centre.

20 Monosaccharides 6. Aldopentose: A monosaccharide having an aldehyde functional group with a total of five carbon atoms is referred to as an aldopentose. D-ribose, which is an important component of all nucleic acids is one such aldopentose. 9. Epimers: Sugars that differ in configuration from each other at just one asymmetric carbon atom, are referred to as epimers. Glucose and mannose are epimers at C-2 while glucose and galactose are epimers at C Aldohexose: A monosaccharide having an aldehyde functional group with a total of six carbon atoms is referred to as an aldohexose. D-glucose is one of the most common aldohexoses. 10. Anomers: The aldehyde or ketone functional groups can react with an alcohol to form a hemiacetal or ketal. This intramolecular reaction occurs in sugars, thereby allowing them to form cyclic structures. These are commonly represented by means of the Haworth s projections. Upon cyclization, the aldehyde or ketone carbon becomes C1 and is referred to as the anomeric carbon atom. The configuration of groups about the anomeric carbon can result in either the alpha or beta structures, which are referred to as anomers. 8. Ketohexose: A sugar having a ketone functional group with a total of six carbon atoms is referred to as a ketohexose. Dfructose is the most abundant ketohexose. Ketoses have fewer asymmetric centres compared to aldoses.

21 Disaccharides & Polysaccharides 1. Disaccharide: Two monosaccharide units joined together by means of an O-glycosidic linkage forms a disaccharide. Three of the most common disaccharides include maltose, sucrose and lactose. 2. Polysaccharide: Several monosaccharide units joined together by glycosidic bonds form a polysaccharide. These help in maintaining structural integrity and serve as fuel reserves in organisms. Some of the most common polysaccharides include starch, the nutritional reservoir in plants, glycogen the fuel storage form in animal cells and cellulose, which is the most important structural elements in plants. 3. Glycosidic bond: The bond formed by interaction between the hydroxyl group of one monosaccharide with the hydroxyl group, aldehyde or ketone group of another monosaccharide, with the subsequent elimination of water is known as the glycosidic bond. 4. Hemiacetal: The interaction between an aldehyde or ketone group with a hydroxyl group with the elimination of water results in the formation of a hemiacetal or ketal. In sugars, intramolecular hemiacetals and ketals are formed by cyclization of the sugar molecules. 5. Homopolymer: When all the monosaccharide units of a polysaccharide are the same, it is said to be a homopolymer. The most common homopolymers are starch and glycogen. 6. Heteropolymer: When the repeating monosaccharide units of the polysaccharide are different, they are said to be heteropolymers.

22 Disaccharides & Polysaccharides 7. Starch: The nutritional reserve of plants is starch, which is composed of two components amylose and amylopectin. Amylose consists of linear, unbranched chains of D-glucose residues linked by -1,4 glycosidic linkages. Amylopectin, however, is a branched polymer with an -1,6 glycosidic linkage present around every 30 residues. Starch is rapidly degraded by the enzyme amylase. 8. Cellulose: Cellulose plays a major structural role in plants and consists of linear chains of glucose residues linked together by -1,4 glycosidic bonds. These chains formed by -linkages have very high tensile strength and can be digested by the enzyme cellulase, which is not inherent in mammals. 9. Glycogen: Glycogen is the storage form of glucose in animal cells. It is structurally similar to starch, with glucose residues being joined by -1,4 glycosidic linkages. Glycogen has more extensive branching than starch with branch points being observed around every 10 residues. These allow the molecules to be stored in a very compact way in the cells. 10. Chitin: Chitin is another structural polysaccharide that is a homopolymer of Nacetyl-D-glucosamine residues joined together by -1,4 glycosidic linkages. It is commonly found on the exoskeleton of insects.

23 Disaccharides & Polysaccharides 1. Glycosaminoglycan: Glycosaminoglycans are heteropolysaccharide repeating units found on animal cell surfaces and in the extracellular matrix. They are composed of disaccharide units that contain a derivative of an amino sugar and at least one of the sugars in the unit has a negatively charged carboxylate or sulphate group. 2. Proteoglycan: These are structural elements that are composed of glycosaminoglyan units linked to proteins. The proteoglycan, aggrecan, is a major component of cartilage along with the protein, collagen, where it serves as a shock absorber. Degradation of aggrecan and collagen can lead to osteoarthritis. 3. Glycoprotein: Carbohydrate groups are often covalently attached to proteins to form glycoproteins. The sugar residues are typically attached to the amide nitrogen atom of the aspargine side chain or to the oxygen atom of the serine or threonine side chain. These glycoproteins are components of cell membranes and have a variety of functions in cell adhesion processes. 4. Glycolipid: Carbohydrate moieties can also be covalently linked with various lipids. Glycolipids are membrane components bearing a hydrophilic head group and a hydrophobic lipid tail. 5. Glycosyl transferase: These are a specific class of enzymes that are responsible for the transfer or sugar residues onto other substrates. They transfer the sugar in its activated state such as UDP-glucose to other molecules such as other monosaccharides, polysaccharides or amino acid side chains of proteins.

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