Biochemistry - 02 Carbohydrates

BIOCHEMISTRY MODULE 2 - CARBOHYDRATES LESSON 1 - DEFINITION, SOURCES, PROPERTIES AND FUNCTIONS OF CARBOHYDRATES • CARBOHYDRATES • • • • • • • • • • • Called sugars and starches, are polyhydroxy aldehydes or ketones, or compounds that can be hydrolyzed to them. They are synthesized in green plants through photosynthesis, whereby energy from the sun is stored as chemical energy in carbohydrates. In the body, they are used for bursts of energy needed during exercise in the form of glucose. Sources of Carbohydrates o sucrose [table sugar], o glucose, o fructose, o lactose, o maltose and o starches (found in pasta, bread, and grains). o These carbohydrates can be digested by the body and provide energy for cells. There are other carbohydrates that the human body doesn't digest, including insoluble fiber, o cellulose from plants, and o chitin from insects and other arthropods. Sources of Carbohydrates o Healthy sources of carbohydrates ▪ include both food sources-animal and plant products, such as fresh fruits, vegetables, corn, potatoes, milk and milk pro ducts. o Unhealthy source ▪ include soda, white bread, artificial sugar, pastries, and other highly processed foods. Carbohydrate foods are an important source of fiber and other nutrients. Sugars and starches provide glucose, the main energy source for the body. Major sources of Healthy Carbohydrates o Vegetables, fruits, whole grains, milk, and milk products are the major food sources of carbohydrates Good Carbs: o Vegetables: All of them. ... o Whole fruits: Apples, bananas, strawberries, etc. o Legumes: Lentils, kidney beans, peas, etc. o Nuts: Almonds, walnuts, hazelnuts, macadamia nuts, peanuts, etc. o Seeds: Chia seeds, pumpkin seeds. o Whole grains: Choose grains that are truly whole, as in pure oats, quinoa, brown rice, etc. Carbohydrates have six major functions in the body: o providing energy o regulating blood glucose o sparing the use of proteins for energy o breaking down fatty acids o providing dietary fiber and o natural sweetener for foods Glucose energy - stored as glycogen, with the majority of it in the muscle and liver. Primary Function of Carbohydrates - to supply energy to all cells in the body. Many cells prefer glucose as a source of energy versus other compounds like fatty acids. Some cells, such as red blood cells, are only able to produce cellular energy from glucose. What is a GOOD Carbohydrate? o Carbohydrates are all about energy and are found in foods like fruits, vegetables, bread, pasta, and dairy products. Your body uses these foods to make glucose, which is your body's main energy source. ... "Carbohydrates provide the body with energy it needs and is good source of many vitamins and minerals. o Those kinds of carbohydrates are described as good carbs or low glycemic foods. Good carbs include vegetables, low glycemic fruits such as berries and apples, as well as legumes and unprocessed high fiber whole grains. LESSON 2 - CLASSIFICATION OF CARBOHYDRATES CARBOHYDRATES • • • • • • Also called Carbs, are defined as aldehydic or ketonic compounds with some number of oxydrilic groups Many of them, but not all, have general formula (CH2O)n (only molecules with n>4 are considered carbohydrates); some, in addition to carbon (C), oxygen (O), and hydrogen (H), include nitrogen or sulfur. On the basis of the number of forming units, three major classes of carbohydrates can be defined: monosaccharides, oligosaccharides, and polysaccharides. Monosaccharides o Simply sugars are formed by only one polyhydroxy aldehydic or ketonic unit. o The most abundant monosaccharide is D-glucose, also called dextrose Oligosaccharides o Formed by short chains of monosaccharide units (from 2 to 20) linked one to the next by chemical bonds, called glycosidic bonds. o The most abundant oligosaccharides are disaccharides, formed by two monosaccharides, and especially in the human diet the most important are sucrose (common table sugar), lactose, and maltose. o Within cells many oligosaccharides formed by three or more units do not find themselves as free molecules but linked to other ones, lipids or proteins, to form glycoconjugates. Polysaccharides o Polymers consisting of 20 to 107 monosaccharide units; they differ each other for the monosaccharides recurring in the structure, for the length and the degree of branching of chains or for the type of links between units. o Whereas in the plant kingdom several types of polysaccharides are present, in vertebrates there is only a small number. o Polysaccharides are defined as: o homopolysaccharides if they contain only one type of monosaccharide as starch, glycogen, and chitin; o heteropolysaccharides, instead, contain two or more different kinds (e.g. hyaluronic acid). the term “saccharide” derives from the Greek word “sakcharon”, which means sugar. PHYSIOLOGICAL CLASSIFICATION OF CARBOHYDRATES • • On the basis of their degree of polymerization, they can be classified as: o simple: mono- and disaccharides (also known as “sugars”) and tri- and tetrasaccharides (oligosaccharides); o complex: the polysaccharides. A further classification lays the foundations on the possibility of being used directly for energy purpose, so: o Available ▪ As glucose, fructose, galactose between monosaccharides, sucrose, lactose, maltose and maltodextrin bet ween oligosaccharides, and starch and glycogen between polysaccharides; o not available ▪ As xylose (monosaccharide), lactulose (see lactose), and raffinose (respectively di- and trisaccharide), fiber (cellulose, hemicellulose, lignin, pectins, etc.) and resistant or not digestible starch(polysaccharides). ▪ The members of this class, also if ingested, are not digestible nor absorbable and will be fermented by intestinal flora with the release of short-chain fatty acids and so yielding some energy. CLASSIFICATION AND NOMENCLATURE OF CARBOHYDRATES • Empiric formula for many simpler carbohydrates is (CH2O)n , hence the name “hydrate of carbon”. CLASSES OF CARBOHYDRATES • • • • Monosaccharides - sugars that cannot be hydrolyzed into simpler carbohydrates Disaccharides - are condensation products of two monosaccharide units Oligosaccharides - are condensation products of three to ten monosaccharides Polysaccharides - are condensation products of more than ten monosaccharide units. MONOSACCHARIDES • Galactose and glucose - composed of sixmembered rings, fructose has only five carbon atoms bonded to each other in ring form. • Simple Sugars o Cannot be hydrolyzed into simpler carbohydrates o Classification is based on: ▪ the number of carbon atoms ▪ whether they have an aldehyde or ketone group (as aldoses and ketoses) Aldoses - have one aldehyde group ( -CHO ) and several OH groups as functional groups Ketoses - have one carbonyl or keto group ( -CO- ) and several OH groups • • • Polyhydric Alcohols o sugar alcohols or polyols o the aldehyde or ketone group has been reduced to an alcohol group o occur naturally in foods o are synthesized by reduction of monosaccharides for use in the manufacture of foods for weight reduction and for diabetics. o are poorly absorbed, and have about half the energy yield of sugars DISACCHARIDES • • • • • Meaning "two sugars“ Commonly found in nature as sucrose, lactose, maltose Formed by a condensation reaction where one molecule of water condenses or is released during the joining of two monosaccharides The type of bond that is formed between the two sugars is called a glycosidic bond Maltose o Disaccharide consists of two glucose molecules joined by an alpha glycosidic bond. o Use in alcohol production o Although maltose is uncommon in nature, it can be formed through the breakdown of starch by the enzymes of the mouth • Sucrose o Found in common table sugar and composed of glucose & fructose linked via a 1-2 alpha glycosidic bond o An excellent preservative because it has no "reducing end" or reactive group like the other sugars o Other natural sources of sucrose are found in plants such as sugar cane, sugar beets, and maple syrup o Hydrolysis of sucrose yields mixtures of glucose and fructose called “invert sugars” • Lactose o A disaccharide formed through the condensation of glucose and galactose forming a beta glycosidic bond o Unlike alpha glycosidic bonds, betaglycosidic bonds are unable to be digested by some people o Many people are lactose intolerant and suffer from intestinal cramping and bloating due to the incomplete digestion of the substance POLYSACCHARIDES • • • • LESSON 3 - ISOMERISM: FISCHER STRUCTURE AND HAWORTH STRUCTURE STRUCTURAL OR CONSTITUTIONAL ISOMERISM 1. 2. 3. OLIGOSACCHARIDES • • • • Condensation products of three to ten monosaccharides Important oligosaccharides are raffinose and stachyose (found in beans and legumes) Raffinose and stachyose, because of their unique glycosidic bonds, cannot be broken down into their simple sugars. Therefore, they cannot be absorbed by the small intestine and are often metabolized by bacteria in the large intestine to form unwanted gaseous byproducts. Condensation products of more than ten monosaccharide units Examples are starches and dextrins Starch is the major storage form of carbohydrates in plants and of two different kinds: amylose & amylopectin Non starch polysaccharides are not digested by human enzymes, and major component of dietary fibers Cellulose (a glucose polymer) & inulin (a fructose polymer) • Structural or constitutional isomers share the same chemical formulas, but their atoms are arranged differently. The three types of structural isomers are skeletal isomers, positional isomers, and functional group isomers. Structural isomers differ from stereoisomers, which share the same chemical formulas and the same order of atoms, but have different threedimensional configurations. TYPES OF STRUCTURAL ISOMERS o There are three categories of structural isomers: o Skeletal isomerism (also called chain isomerism) structural isomers in which components of the skeleton are arranged in a different order. This is most commonly seen when the skeleton or backbone consists of a carbon chain. o Position isomerism (also called regioisomerism) constitutional isomers in which a functional group or substituent changes position on a parent structure. o • • • • Functional group isomerism - structural isomers with the same molecular formula, but with atoms connected differently so dissimilar functional groups are formed Structural Isomer Examples o Butane and isobutane (C4H10) are structural isomers of each other. o Pentan-1-ol, pentan-2-ol, and pentan-3-ol are structural isomers that exhibit position isomerism. o Cyclohexane and hex-1-ene are examples of functional group structural isomers. Molecules that have multiple stereocenters become difficult to draw quickly. The Fischer projection is a convention for drawing molecules and quickly designating stereochemistry without laboriously trying to use wedges and hashes to specify the 3-D relationships. Developed at a time before it was possible to experimentally prove absolute configuration for any molecule. Emil Fischer, an early carbohydrate chemist, developed it so that it could continue to be used correctly and consistently even after experimental proof of configuration was developed. The definition is that every carbon is specified completely by a cross designating the carbon (at the center) and the four bonds to that carbon. The stereochemistry of the bonds is defined (now) as the horizontal bonds are in front of the plane (coming toward you, the viewer); the vertical bonds are behind the plane (going away from you). Sometimes (often, even), bonds to hydrogen are omitted. o The configuration of the chirality center farthest from the carbonyl group determines whether a monosaccharide is D or L. o All naturally occurring sugars are D sugars. THE CYCLIC FORMS OF MONOSACCHARIDES • When an aldehyde reacts with an alcohol, a hemiacetal is formed: • When the aldehyde and alcohol are on the same molecule, a stable cyclic hemiacetal is formed: • The C atom that is part of the hemiacetal is a new chirality center, called the anomer carbon. • The Cyclic Forms of D-Glucose o To cyclize D-glucose, first we must determine which alcohol to use to make a six-membered ring. MONOSACCHARIDES FISCHER PROJECTION FORMULA • • • • • • • All carbohydrates have 1 or more chirality centers. Glyceraldehyde, the simplest aldose, has one chirality center, and has two possible enantiomers The prefix D is used when the –OH group is drawn on the right side of the carbon chain. The prefix L is used when the –OH group is drawn on the left side of the carbon chain. The wedged and dashed lines can be re-drawn in a Fischer projection formula: Monosaccharides: With More than One Chirality Center 4 o Glucose has four chirality centers and is drawn as: o The first step in cyclization is to rotate glucose 90° o Next, the chain must be twisted around, forming a sixmembered ring: o • o THE CYCLE FORMS OF FRUCTOSE • Ketohexoses like fructose form fivemembered rings with two anomers • Glucose, with four asymmetric carbon atoms, can form 16 isomers. The more important types of isomerism found with glucose are as follows. o D and L Isomers o The presence of asymmetric carbon atoms also confers optical activity on the compound. o Pyranose and furanose ring structure o Alpha and beat anomers o Epimers o Aldose -ketose isomerism As the reaction occurs, there are two cyclic forms of D-glucose, an a anomer and a b anomer. o These rings are called Haworth projections. Mutarotation of D-Glucose • THE CYCLIC FORMS OF MONOSACCHARIDES • Step [3] Add the other substituents to the remaining carbons, clockwise around the ring. How to Draw a Haworth Projection from an Acyclic Aldohexose o Example: Draw both anomers of Dmannose in a Haworth projection. o Step [1] Place the O atom in the upper right corner of a hexagon, and add the CH2OH group to the right of the O atom. o Step [2] Place the anomeric C on the first C clockwise from the O atom. FORMS OF ISOMERISM (SUGARS) • • Exhibited by sugars Structures of glucose • Isomers of Glucose o Glucose can form 16 isomers o The most important types of isomerism are: ▪ D and L isomerism ▪ Pyranose and furanose ring structures ▪ Alpha and beta anomers ▪ Epimers ▪ Aldose-ketose isomerism • D and L isomerism o Determined by spatial relationship to the parent compound of the carbohydrates ▪ D-isomer = OH group is on the right  ▪ L-isomer = OH group is on the left o Most monosaccharides in mammals are Dsugars o Glucose in solution is dextrorotatory, so its solution is called Dextrose PYRANOSE AND FURANOSE RING STRUCTURES • • • • • Monosaccharides are predominantly found in a ring form Glucose in solution is 99% in pyranose form Hemiacetal or hemiketal ring – formed when an aldehyde (or ketone) group has reacted with an alcohol group on the same sugar Pyranose ring - if the resulting ring has 6 members (5 C and 1 O) Furanose ring – if it is a 5 membered ring (4 C and 1 O) EPIMERS • • • Sugars that differ in configuration at ONLY 1 POSITION Isomers differing as a result of variations in configuration of the –OH and –H on carbon atoms 2, 3, and 4 glucose (epimers of glucose) Mannose and Galactose o The most important epimers of glucose o Formed by epimerization at carbon 2 and 4, respectively ALDOSE-KETONE ISOMERISM ALPHA AND BETA ANOMERS • • Two possible structures for the pyranose and furanose forms of a monosaccharide Anomers, and (differing only in configuration at anomeric C) o Anomeric C – C1, the original carbonyl C o -anomer - all of the bulky -OH or - CH2OH substituents lie more or less within the plane of the six-membered ring o  -anomer - one of the -OH groups is perpendicular to the plane of the sixmembered ring o  -anomer is slightly more stable than the  -anomer o • Fructose has the same molecular formula as glucose but they differ in their structural formula o Keto-group at position 2 (anomeric carbon of fructose) o Aldehyde group at position 1 (anomeric carbon of glucose) • LESSON 4 - CHEMICAL REACTIONS OF MONOSACCHARIDES effects alpha-carbon oxidation with the formation of a bisphenylhydrazone, known as an osazone. Application of the osazone reaction to Dglucose and D-mannose demonstrates that these compounds differ in a configuration only at C-2. Fischer's train of logic in assigning the configuration of Dglucose 1. Ribose and arabinose (two well-known pentoses) both gave erythrose on Ruff degradation. As expected, Kiliani-Fischer synthesis applied to erythrose gave a mixture of ribose and arabinose. 2. Oxidation of erythrose gave an achiral (optically inactive) aldaric acid. This defines the configuration of erythrose. 3. Oxidation of ribose gave an achiral (optically inactive) aldaric acid. This defines the configuration of both ribose and arabinose. 4. Ruff shortening of glucose gave arabinose, and Kiliani-Fischer synthesis applied to arabinose gave a mixture of glucose and mannose. 5. Glucose and mannose are therefore epimers at C2, a fact confirmed by the common product from their osazone reactions. 6. A pair of structures for these epimers can be written, but which is glucose and which is mannose? INTRODUCTION • • • • As noted above, sugars may be classified as reducing or non-reducing based on their reactivitywith Tollens', Benedict's, or Fehling's reage nts. If sugar is oxidized by these reagents it is called reducing, since the oxidant (Ag(+) or Cu(+2)) is reduced in the reaction, as evidenced by the formation of a silver mirror or precipitation of cuprous oxide. The Tollens' test is commonly used to detect aldehyde functions; and because of the facile interconversion of ketoses and aldoses under the basic conditions of this test, ketoses such as fructose also react and are classified as reducing sugars. When the aldehyde function of an aldose is oxidized to a carboxylic acid the product is called an aldonic acid. Because of the 2º hydroxyl functions that are also present in these compounds, a mild oxidizing agent such as hypobromite must be used for this conversion (equation 1). If both ends of an aldose chain are oxidized to carboxylic acids the product is called an aldaric acid. By converting an aldose to its corresponding aldaric acid derivative, the ends of the chain become identical (this could also be accomplished by reducing the aldehyde to CH2OH, as noted below). Such an operation will disclose any latent symmetry in the remaining molecule. Thus, ribose, xylose, allose, and galactose yield achiral aldaric acids which are, of course, not optically active. The ribose oxidation is shown in equation 2 below. Sodium borohydride reduction of an aldose makes the ends of the resulting alditol chain identical, HOCH2(CHOH)nCH2OH, thereby accomplishing the same configurational change produced by oxidation to an aldaric acid. Thus, allitol and galactitol from the reduction of allose and galactose are achiral, and altrose and talose are reduced to the same chiral alditol. The osazone reaction was developed and used by Emil Fischer to identify aldose sugars differing in a configuration only at the alpha-carbon. The upper equation shows the general form of the osazone reaction, which REACTIONS OF SUGARS • Reduction and Oxidation o Reduction of the Aldehyde Carbonyl Group 2 ▪ The carbonyl group of an aldose is reduced to a 1o alcohol using H2 with Pd. o Oxidation of the Aldehyde Carbonyl Group 3 ▪ The aldehyde group is easily oxidized to a carboxylic acid using Benedict’s reagent. o Aldoses can be oxized, while ketones cannot. POLYSACCHARIDES • Cellulose - makes up the in our diets, which is important in adding bulk to waste to help eliminate it more easily. • Starch - a polymer made up of repeating glucose units joined by alpha glycosodic linkages. o Starch is present in corn, rice, wheat, and potatoes. o The first main type of starch is amylose: LESSON 5 - MECHANISM OF THE FORMATION OF THE BLUE IODO-STARCH COMPLEX AND THE SWELLING OF CELLULOSE IN WATER INTRODUCTION • o The second type of starch is amylopectin. Starch being a very important polysaccharide obtained from plant sources is the primary source of glucose in our food. The breakdown of starch to a single molecule of glucose via hydrolysis should be understood very well. As you may already know, glucose is the main sugar that is utilized by the cell to produce ATP (Adenosine triphosphate). Watch the video link below to understand how starch is broken down into smaller sugar units. HYDROLYSIS OF POLYSACCHARIDE • • FOCUS ON THE HUMAN BODY • Blood Type o There are four blood types—A, B, AB, and O. o Blood type is based on 3 or 4 monosaccharides attached to a membrane protein of red blood cells. o Each blood type has the monosaccharides below: • • o Type A blood contains a fourth monosaccharide: o o Type B contains an additional D-galactose unit. Type AB has both type A and type B carbohydrates. o o The blood of one individual may contain antibodies to another type. Those with type O blood are called universal donors, because people with any other blood type have no antibodies to type O. Those w/ type AB blood are universal recipients because their blood contains no antibodies to A, B, O. Dehydration synthesis - process of forming disaccharides from monosaccharide Many disaccharides or monosaccharide units can also be joined together to form a polysaccharide also by the process of dehydration synthesis. • Polysaccharides can also breakdown into disaccharide and this is done using the process of hydrolysis or adding martyr disaccharide and polysaccharide can also be broken down into monosaccharide using the process of hydrolysis. • Dehydration synthesis - used to build larger molecules from smaller ones Hydrolysis - used to breakdown larger molecules into smaller one • o Hydrolysis - process of adding water to a polysaccharide molecule in order to break it down into smaller units. We can take simple sugar or monosaccharides and join two of these together to form double sugar or disaccharide. MOLECULE OF POLYSACCHARIDE STARCH • The body does not possess the enzymes necessary to break cellulose down into glucose IODINE • • The first part of the chain is consisting of four subunits. • • With the appropriate enzyme present we can add a molecule of water and using the process of hydrolysis, the starch molecule breaksapart and water is added. • • • • A maltose molecule has broken off of the large starch molecule In the laboratory, Iodine is used to detect starch present in the food that you eat. Swelling refers to the amount of water that a food source, like plants containing cellulose, can absorb. Water together with heat helps breakdown the hydrogen bonds between polysaccharide molecules allowing gelatinization. Iodine can be used to detect for the presence of starch. A solution of iodine and potassium iodide is orange-brown if its added to a sample that contains starch, the color changes into deep blue. Color is caused by charge transfer complexes. In such complexes, light can easily excite electrons to a higher energy level each complex absorbs light of particular wavelength. Wavelength – determines the color of complex There are two different charge transfer complexes involved in the iodine test: o Complex 1: Together, iodine and iodide form polyiodide ions. For example, with 3,5,7 iodide atoms this polyiodides are charge transfer complexes with an overall brownish color. o Complex 2: Starch contains amylose which is a linear chain of glucose units. This chain forms a helix shape. This complex absorbs light of a different wavelength and the color turns dark blue. LESSON 6 - PHYSIOLOGICALLY AND MEDICINALLY IMPORTANT CARBOHYDRATES INTRODUCTION • The breakdown of starch into maltose using hydrolysis takes place in the mouth with the help of enzyme salivary amylase. • Glucose, as the most important carbohydrate, is the dietary carbohydrate absorbed into the bloodstream. Glucose is the major metabolic fuel of mammals (except ruminants) and a universal fuel of the fetus. It is the precursor for synthesis of all the other carbohydrates in the body, including glycogen for storage; ribose and deoxyribose in nucleic acids; galactose for synthesis of lactose in milk, in glycolipids, and in combination with protein in glycoproteins and proteoglycans. PHYSIOLOGICALLY IMPORTANT MONOSACCHARIDES • • • With proper enzyme present maltose can also undergo hydrolysis and as water added break up into two molecules of gulcose. The breakdown of maltose to simple glucose molecules takes place into small intestine As well as breaking down starch, the process of hydrolysis is also used to break glycogen down into glucose when a quick source of energy is needed for muscles. o Derivatives of trioses, tetroses, and pentoses and of seven-carbon sugar (sedoheptulose) ▪ Formed as metabolic intermediates in glycolysis and pentose phosphate pathway ▪ Pentoses – are important in nucleotides, nucleic acids and several coenzymes (table 15-2) ▪ Hexoses - glucose, galactose, fructose, and mannose (table 15-3) ▪ Ketoses & Aldoses – fig.15-6 and 15-7 15-2 Pentoses of Physiological Importance Biochemical and Clinical Sugar Source Importance D-Ribose Nucleic acids and Structural component of metabolic nucleic acids and coenzyme, intermediate including ATP, NAD(P), and flavin coenzymes D-Ribulose Metabolic Intermediate in the pentose intermediate phosphate pathway D-Arabinose Plant gums, Constituent of glycoproteins • • • If hemiacetal (aldehyde + -OH) portion is glucose, the resulting compound is Glucoside, if galactose, Galactoside The aglycone may be methanol, glycerol, a sterol, a phenol, or a base such as adenine MEDICINALLY IMPORTANT GLYCOSIDES o Cardiac glycosides • Contain steroids as aglycone  Include derivatives of digitalis (digoxin, digitoxin) and strophantus (ouabin) o Aminoglycoside Antibiotics ▪ Streptomycin DEOXYSUGARS • Lack an oxygen atom  one hydroxyl group has been replaced by hydrogen o Examples: o deoxyribose in DNA ▪ L-fucose - deoxysugar in glycoproteins ▪ 2-deoxyglucose – used experimentally as inhibitor of glucose metabolism AMINOSUGARS • • D-Xylose L-Xylulose Plant gums, proteoglycans, glycosaminoglycans Metabolic intermediate Constituent of glycoproteins Excreted in the urine in essent o o Carboxylic acid derivatives of glucose ▪ D-glucuronate – for glucuronide formation and in glycosaminoglycans ▪ L-iduronate (in glycosaminoglycans) and Lgulonate (an intermediate in the uronic acid pathway) – metabolic derivatives of Dglucuronate GLYCOSIDES • • • Are formed by the condensation between the hydroxyl group of the anomeric carbon of monosaccharide (glycone), and a second compound that may not be another monosaccharide (aglycone) O-glycosidic bond – if the second group is a hydroxyl o Acetal link (formed from an aldehyde and–OH) N-glycosidic bond – if the second group is amine o Ex.: adenine and ribose in ATP Hexosamines o components of glycoproteins, gangliosides, & glucosaminoglycosides Include: o D-glucosamine – constituent of hyaluronic acid o D-galactosamine (chondrosamine) – constituent of chondroitin o D-mannosamine o Aminoglycosides antibiotics (streptomycin) and macrolides (erythromycin) DISACCHARIDES OF PHYSIOLOGIC IMPORTANCE POLYSACCHARIDES GLYCOGEN • • • • Starch o Homopolymer of glucose forming an - glucosidic chain (called glucosan or glucan) o Most important dietary source of carbohydrates in cereals, potatoes, legumes and other vegetables • Two main constituents o Amylose (1320%) - nonbranching (linear) helical structure o Amylopectin (80-85%) INULIN • • • • • A polysaccharide of fructose found in tubers and roots of dahlias, artichokes, and dandelions Readily soluble in water Use to determine glomerular filtration Not hydrolyzed by intestinal enzymes DEXTRINS • branched chains composed of 24-30 glucose residues united by 1-4 linkages and by 1-6 linkage The extent of starch hydrolysis by amylase enzyme is determined by: o Its structure o Degree of crystallization or hydration (the result of cooking) o Whether it is enclosed in intact (and indigestible) plant cell walls Storage polysaccharide in animals and is sometimes called the animal starch Two main constituents Glycogen is stored in the liver and muscle where it is synthesized and degraded depending upon the energy requirements of the body. Highly branched 12-14 -D-glucopyranose o Each residue is linked by 1→4 linkages with branching by 1→6 linkage). Intermediates in the hydrolysis of starch CELLULOSE • • Chief constituent of plant cell wall Insoluble and is consist of -D glucopyranose linked by 1→4 bonds • Mammals cannot digest cellulose because of the absence of enzyme that hydrolyzes the 1→4 bonds It is an important source of “bulk” in the diet, and the major component of dietary fiber (insoluble fiber) GLYCEMIC INDEX • • Measure of digestibility of starchy food Based on the extent to which it raises the blood concentration of glucose compared with an equivalent amount of glucose or a reference food such as white bread or boiled rice. • CHITIN • • Structural polysaccharide in the exoskeleton of crustaceans and insects and also in mushroom Consists of N-acetyl-D-glucosamine units joined by 1→4 glycosidic bonds GLYCOSAMINOGLYCANS (MUCOPOLYSACCHARIDES) • • • • Complex carbohydrates containing amino sugars and uronic acid May be attached in protein molecule to form proteoglycan o Proteoglycans provide the ground or packing substance of connective tissues o Hold large quantities & water supply, occupies space lubricating other structures Large number of -OH and negative charges (repulsion) keep carbohydrate chains apart E.g. Hyaluronic acid, chondroitin sulfate and heparin GLYCOPROTEINS (MUCOPROTEINS) • • • • • Proteins containing branched & unbranched oligosaccharide chains (table 14-5) They occur in the cell membranes and many other situations Serum albumin is an example Neuraminic Acid – nine carbon sugar derived from mannosamine and pyruvate Sialic Acid –N- or O-acyl derivatives of neuraminic acid o Constituents of both glycoproteins & gangliosides (glycolipids)