Introduction to Carbohydrates PDF

Introduction to Carbohydrates Asst. Prof. Betül ÖZBEK [email protected] 2023-2024 Introduction to Carbohydrates o Carbohydrates (saccharides) are the most abundant organic molecules in nature and widely distributed in plants and animals. o They h...

Introduction to Carbohydrates Asst. Prof. Betül ÖZBEK [email protected] 2023-2024 Introduction to Carbohydrates o Carbohydrates (saccharides) are the most abundant organic molecules in nature and widely distributed in plants and animals. o They have important structural and metabolic roles, providing a significant fraction of the dietary calories for most organisms, acting as a storage form of energy in the body, serving as cell membrane components that mediate some forms of intercellular communication. serve as a structural component of many organisms, including the cell walls of bacteria, the exoskeleton of insects, and the fibrous cellulose of plants o The empiric formula for many of the simpler carbohydrates is (CH2O)n, n ≥3. 2 Introduction to Carbohydrates o Glucose is the most important carbohydrate; most dietary carbohydrate is absorbed into the bloodstream as glucose other sugars are converted to glucose in the liver major metabolic fuel of mammals precursor for synthesis of all the other carbohydrates in the body, ✓glycogen for storage, ✓ribose and deoxyribose in nucleic acids (DNA and RNA), ✓galactose for synthesis of lactose in milk, in glycolipids, and ✓in combination with protein in glycoproteins and proteoglycans 3 Introduction to Carbohydrates o Diseases associated with carbohydrate metabolism diabetes mellitus, galactosemia, glycogen storage diseases, lactose intolerance o The glycome is the entire complement of sugars which are produced by the organism, whether free or in more complex molecules. 4 Classification of Carbohydrates 1. Monosaccarides i. depending on the number of carbon atoms Trioses (3C) Tetroses (4C) Pentoses (5C) Hexoses (6C) Heptoses (7C) ii. depending on whether they have an aldehyde or ketone group 2. Disaccharides 3. Oligosaccharides 4. Polysaccharides 5  Classification of Carbohydrates 1. Monosaccharides o Sugars that cannot be hydrolyzed into simpler carbohydrates. o They may be classified depending on the number of carbon (3-7) and as aldoses or ketoses, depending on whether they have an aldehyde or ketone group. o The most abundant monosaccharide in nature is the six-carbon sugar D-glucose. 6 Classification of Carbohydrates 2. Disaccharides o Condensation products of two monosaccharide units (bond: glycosidic), Lactose (glucose + galactose) Maltose (glucose + glucose) Isomaltose (glucose + glucose) Sucrose (glucose + fructose) Trehalose (glucose + glucose) 3. Oligosaccharides o Condensation products of 3 to 10 monosaccharides. Most are not digested by human enzymes (bond: glycosidic). 7 Classification of Carbohydrates 4. Polysaccharides o Condensation products of more than 10 monosaccharide units (bond: glycosidic) glycogen (branched glucose polymers) starches which may be linear or branched polymers dextrins o Foods with a polysaccharide structure, which are not digested by human enzymes and form the main component of dietary fiber. cellulose from plant cell walls (a glucose polymer) and inulin, the storage carbohydrate in some plants (a fructose polymer) 8 Representation of the Structure of D-Glucose 1. Fischer projection (Strait-chain form) 2. Haworth projection 3. Chair form 10 Isomerism of Sugars 1. D- and L- isomerism 2. Optical isomer (d (+) or l (-)) 3. Pyranose and furanose ring structures 4. Alpha- and beta-anomers 5. Epimers 6. Aldose-ketose isomerism 11 1. D- and L- Isomerism of the Sugars o Isomer: Have same chemical formula, but have different structures o One method for determining whether a molecule is D- or L- by looking at the Fischer projection of a molecule. o If the -OH on the bottom-most chiral center (opposite direction to aldehyde structure) is on the right-hand side of the Fischer projection, the molecule is “D“. o If it is on the left-hand side, the molecule is “L”. D- and L- have no relation to the optical rotation of a molecule. Most natural sugars are D- and most natural amino acids are L- isomers! 12 2. Optical Isomerism of the Sugars o The presence of asymmetric (chiral) carbon atoms also confers optical activity on the compound. o When a beam of plane-polarized light is passed through a solution of an optical isomer, it rotates either to the right, dextrorotatory (+), or to the left, levorotatory (–). The direction of rotation of polarized light is independent of the stereochemistry of the sugar (D- or L-). D (+) glucose solution is also called as dextrose. 13 3. Pyranose and Furanose Ring Structures o The ring structures of monosaccharides are similar to the ring structures of either pyran (a six-membered ring) or furan (a five-membered ring) 14 4. Alpha- and Beta-Anomers o The ring structure of an aldose is a hemiacetal, since it is formed by reaction between an aldehyde and an alcohol group. o Similarly, the ring structure of a ketose is a hemiketal. o In the α configuration, the –OH group on the anomeric carbon projects to the same side as the ring in a modified Fischer projection formula and is trans to the CH2OH group in a Haworth projection formula. 15 5. Epimers o Isomers differing as a result of variations in configuration of the -OH and -H on carbon atoms 2, 3, and 4 of glucose are known as epimers. o Biologically, the most important epimers of glucose are mannose (epimerized at carbon 2), galactose (epimerized at carbon 4) 16 6. Aldose-Ketose Isomerism o Chemically, aldoses are reducing compounds, and are sometimes known as reducing sugars. This provides the basis for a simple chemical test for glucose in urine in poorly controlled diabetes mellitus, by reduction of an alkaline copper solution. o Reducing 6C sugars Glucose, Galactose, Mannose 17 Physiological Importance of Monosaccarides o Derivatives of trioses, tetroses, and pentoses and of the seven-carbon sugar sedoheptulose are formed as metabolic intermediates in glycolysis and the pentose phosphate pathway. 18 Physiological Importance of Monosaccarides o Carboxylic acid derivatives of glucose are important, including D-glucuronate (for glucuronide formation and in glycosaminoglycans), L-iduronate (in glycosaminoglycans) and L-gulonate (an intermediate in the uronic acid pathway) 19 Glycosides o Glycosides are formed by condensation between the hydroxyl group of the anomeric carbon of a monosaccharide, and a second compound that may be another monosaccharide or, in the case of an aglycone, not a sugar. o If the hemiacetal portion is glucose, the resulting compound is a glucoside; if galactose, a galactoside; and so on. o If the second group is an amine, an N-glycosidic bond is formed, for example, between adenine and ribose in nucleotides such as ATP. ATP 20 Glycosides o The glycosides that are important in medicine because of their action on the heart (cardiac glycosides), all contain steroids as the aglycone. Digoxin inhibitor of the Na+–K+-ATPase of cell membranes Ouabain o Other glycosides include antibiotics such as streptomycin. 21 Deoxy Sugars o Deoxy sugars are those in which one hydroxyl group has been replaced by hydrogen. An example is deoxyribose in DNA. 22 Amino Sugars (Hexosamines) o Amino sugars (hexosamines) are components of glycoproteins, gangliosides, and glycosaminoglycans. o The amino sugars include D-glucosamine, a constituent of hyaluronic acid. D-galactosamine (also known as chondrosamine), a constituent of chondroitin. o Also several antibiotics contain amino sugars Erythromycine 23 Disaccharides o The disaccharides are sugars composed of two monosaccharide residues linked by a glycoside bond. o The physiologically important disaccharides are maltose, sucrose, and lactose. 24 Quick View of Carbohydrates o Carbohydrate polymers called glycans (polysaccarides) serve as structural and protective elements in the cell walls of bacteria, fungi, and plants, and in the connective tissues of animals. o Other carbohydrate polymers lubricate skeletal joints participate in cell-cell recognition and adhesion o Most of the hexoses of living organisms are D isomers. 25 Polysaccharides o Polysaccharides are physiologically important carbohydrates, they serve storage and structural functions. 1. Starch 2. Glucogen o Other polysaccarides 1. Inulin 2. Cellulose 3. Chitin 4. Pectin 26 Polysaccharides 1. Starch o Starch is a homopolymer of glucose forming an α-glucosidic chain, called a glucosan or glucan. o The two main constituents of starch are amylose (13-20%), which has a nonbranching (linear) helical structure of glucose polymers with α1 → 4 linkages, and amylopectin (80-87%), which consists of branched chains, consists of 24 to 30 glucose residues with α1 → 4 linkages in the chains and by α1 → 6 linkages at the branch points o The extent to which starch in foods is hydrolyzed by amylase. o Dextrins are intermediates in the hydrolysis of starch. 27 Polysaccharides glycogenin 2. Glycogen o Glycogen is the storage polysaccharide (formed from glucose monomers) in animals. o It is a more highly branched structure than amylopectin, with chains of 12 to 15 α-D-glucopyranose residues (in α1 → 4 glucosidic linkage) with branching by means of α1 → 6 glucosidic bonds. o Tissues store glycogen i. Muscle ii. Liver 28 Why don't we store glucose in its monomeric form? o Hepatocytes in the fed state store glycogen equivalent to a glucose concentration of 0.4 M. o The actual concentration of glycogen, which contributes little to the osmolarity of the cytosol, is about 0.01 μM. o If the cytosol contained 0.4 M glucose, the osmolarity would be threateningly elevated, leading to osmotic entry of water that might rupture the cell. o Storage of low molecular weight metabolites in polymeric form avoids the very high osmolarity that would result from storing them as individual monomers. 29 Polysaccharides 3. Inulin o Inulin is a polysaccharide of fructose (a fructosan) found in tubers and roots of dahlias, artichokes, and dandelions. o It is readily soluble in water and is used to determine the glomerular filtration rate, o It is not hydrolyzed by intestinal enzymes, so has no nutritional value. 30 Polysaccharides 4. Cellulose o Cellulose is the chief constituent of plant cell walls. o It is insoluble and consists of β-D-glucopyranose units linked by β1 → 4 Have linear chains of D-glucose. o Mammals lack any enzyme that hydrolyzes the β1 → 4 bonds, and so cannot digest cellulose. o It is the major component of dietary fiber. 31 Polysaccharides 5. Chitin o Chitin is a structural polysaccharide in the exoskeleton of crustaceans and insects, and also in mushrooms. 6. Pectin o Pectin occurs in fruits. 32 Glycemic Index o The glycemic index of a starchy food is a measure of its digestibility, based on the extent to which it raises the blood glucose concentration compared with an equivalent amount of glucose or a reference food such as white bread or boiled rice. o Glycemic index ranges from 1 to 0. 1 (or 100%) for starches that are readily hydrolyzed in the small intestine 0 for those that are not hydrolyzed at all 33 Glycoconjugates: Proteoglycans, Glycoproteins, and Glycolipids o On almost every eukaryotic cell, specific oligosaccharide chains attached to components of the plasma membrane form a carbohydrate layer (the glycocalyx), that serves as an information-rich surface that the cell shows to its surroundings. o These oligosaccharides are central players in cell-cell recognition and adhesion, cell migration during development, blood clotting, the immune response, wound healing 34 Glycoconjugates: Proteoglycans, Glycoproteins, and Glycolipids o The informational carbohydrate is covalently joined to a protein or a lipid (they do not occur as free entities) to form a glycoconjugate, which is the biologically active molecule. o Carbohydrate chains are attached to the amino terminal portion outside the external surface. o Carbohydrate presence on the outer surface of the plasma membrane (the glycocalyx) has been shown with the use of plant lectins (proteins that bind specific glycosyl residues). o Glycophorin is a major integral membrane glycoprotein of human erythrocytes which increases the flip-flop frequency of membrane phospholipids. o Carbohydrates are also present in apoprotein B of plasma lipoproteins. 35 Glycosaminoglycans (GAGs) o Glycosaminoglycans (mucopolysaccharides) are complex carbohydrates containing i. amino sugars and ii. uronic acids o Glycosaminoglycans (GAGs) may be attached to a protein molecule to form a proteoglycan. o Proteoglycans hold large quantities of water and occupy space, thus cushioning or lubricating other structures. o Proteoglycans provides hydration and swelling pressure to the tissue enabling it to withstand compressional forces. 36 Proteoglycans and Glycosaminoglycans 37 Glycosaminoglycans o There are at least seven GAGs: 1) hyaluronic acid (hyaluronan), 2) chondroitin sulfate, 3) keratan sulfate I 4) keratan sulfate II, 5) heparin, 6) heparan sulfate, and 7) dermatan sulfate 38 Glycoproteins o Glycoproteins (also known as mucoproteins) are proteins containing branched or unbranched oligosaccharide chains, including fucose. Carbohydrate (oligosaccharide chains or glycans) covalently bound to amino acids; glycosylation (the enzymic attachment of sugars) is the most frequent posttranslational modification of proteins. o They occur in cell membranes and many proteins are glycosylated. 39 Glycoproteins o The sialic acids are N- or O- acyl derivatives of neuraminic acid. Neuraminic acid is a nine-carbon sugar (nonose) derived from mannosamine (an epimer of glucosamine) and pyruvate. Sialic acids are constituents of both glycoproteins and gangliosides. 40 Glycoproteins o Immunoglobulins (antibodies) o Some certain hormones, such as follicle-stimulating hormone (FSH), luteinizing hormone (LH), thyroid-stimulating hormone (TSH), are glycoproteins. 41 42