Carbohydrates Notes PDF

Carbohydrates: General characteristics Polyhydroxy aldehydes or ketones and their derivatives the term carbohydrate is derived from the french: hydrate de carbone compounds composed of C, H, and O (CH2O)n when n = 5 then C5H10O5 not all carbohydrates have this empirical formula: deox...

Carbohydrates: General characteristics Polyhydroxy aldehydes or ketones and their derivatives the term carbohydrate is derived from the french: hydrate de carbone compounds composed of C, H, and O (CH2O)n when n = 5 then C5H10O5 not all carbohydrates have this empirical formula: deoxysugars, aminosugars carbohydrates are the most abundant compounds found in nature (cellulose: 100 billion tons annually) Functions of Carbohydrates sources of energy intermediates in the biosynthesis of other basic biochemical entities (fats and proteins) associated with other entities such as glycosides, vitamins and antibiotics) form structural tissues in plants and in microorganisms (cellulose, lignin, murein) participate in biological transport, cell-cell recognition, activation of growth factors, modulation of the immune system Carbohydrates (glycans) have the following basic composition: ⬥ Monosaccharides - simple sugars with multiple OH groups. Based on number of carbons (3, 4, 5, 6), a monosaccharide is a triose, tetrose, pentose or hexose. ⬥ Disaccharides - 2 monosaccharides covalently linked. ⬥ Oligosaccharides - a few monosaccharides (up to 9 or 10) covalently linked. Polysaccharides - polymers consisting of chains of monosaccharide or disaccharide units (Homopolysaccharides, Heteropolysaccharides, Complex carbohydrates)  Monosaccharides Empirical formula is Cn(H2O)n Both open chain and ring structures are possible Mulitple structural isomers are possible Multiple chiral carbon atoms lead to optical isomers (2n where n= number of asymmetric or chiral carbon atoms) The 6-C aldoses have 4 asymmetric centers. Thus, there are 16 possible stereoisomers. Monosaccharides generally have between 3 and 6 carbon atoms The most common monosaccharides are: – Five carbons C5H10O5 - called pentoses – Six carbons C6H12O6 - called hexoses Monosaccharide straight chains have at least one carbonyl group C=O. If the carbonyl group is at the end it is an aldose sugar. If it is within the chain it is a ketose sugar Aldose Ketose Aldose Ketose Diagram of Isomeric Forms of Carbohydrates  Stereoisomers: D and L Forms D or dextrorotatory & L or levorotatory are designations for optical isomers that are based on the configuration about the single asymmetric C in glyceraldehyde. The lower representations are Fischer Projections. For sugars with more than one chiral center, D and L refer to the asymmetric C (C5) farthest from the aldehyde or keto group. Most naturally occurring sugars are D isomers.  Optical isomerism A property exhibited by any compound whose mirror images are non-superimposable Asymmetric compounds rotate plane polarized light Measurement uses an instrument called a polarimeter Rotation is either (+) dextrorotatory or (-) levorotatory – An optical isomer may be designated as D(+), D(-), L(+), and L(-) based on its structural relation with glyceraldehyde If D- and L-isomers are present in equal concentration, it is known as Racemic mixture (DL mixture). This mixture does not exhibit optical activity.  Enantiomers and epimers Two monosaccharides differ from Special stereoisomers that are mirror each other in their configuration images of each other. The two around a single specific carbon (other members are designated as D- and than anomeric carbon) atom. Glucose L-sugars. Majority of them are and galactose are epimers with regard D-type. to C4. Glucose and mannose are epimers with regard to C2.  Hemiacetal and hemiketal formation An aldehyde can react with an alcohol to form a hemiacetal. A ketone can react with an alcohol to form a hemiketal. Pentoses and hexoses can cyclize as the ketone or aldehyde reacts with a distal OH. Glucose forms an intra-molecular hemiacetal, as the C1 aldehyde & C5 OH react, to form a 6-member pyranose ring, named after pyran. These representations of the cyclic sugars are called Haworth projections. Cyclization of glucose produces a new asymmetric center at C1. The 2 stereoisomers are called anomers, α & β. Haworth projections represent the cyclic sugars as having essentially planar rings, with the OH at the anomeric C1: ⬥ α (OH below the ring) ⬥ β (OH above the ring).  Mutarotation Unlike the other stereoisomeric forms, α and β anomers freely interconvert in solution via open chain form of sugar (reversible formation of internal hemiacetal or hemiketal linkage)  Reactions of Monosaccharides 1. Action of base on sugars Sugars are weak acids and can form salts at high pH A 1,2-enediol salt is formed as the result This allows the interconversion of D-mannose, D-fructose and D-glucose The reaction is known as the Lobry de Bruyn-Alberta von Eckenstein reaction - reversible isomerization of monosaccharides. The enediols are highly reactive, hence sugars in alkaline solution are powerful reducing agents 1. Remove proton from C2 2. Reprotonates C1- O 3. Rearrangement at C2 to give ketose.  2. Action of base on sugars Sugars that react with oxidizing agents are called reducing sugars, whereas those that do not are called nonreducing sugars enediols obtained by the action of base are quite susceptible to oxidation when heated in the presence of an oxidizing agent copper sulfate is frequently used as the oxidizing agent and a red preciptate of Cu2O is obtained- (Cupric ions-Cu2+ -- Cuprous ions(Cu+)---(Cuprous oxide) reduction of the metal ion accompanies oxidation of the aldehyde group. sugars which give this reaction are known as reducing sugars Fehling’s solution : KOH or NaOH and CuSO4 Benedict’s solution: Na2CO3 and CuSO4 Barfoed’s reagent: Cupric acetate and acetic acid Widely used test is Benedict’s test  3. Oxidation reactions Aldoses may be oxidized to 3 types of acids 1. Aldonic acids: aldehyde group is converted to a carboxyl group ( glucose – gluconic acid) 2. Uronic acids: aldehyde is left intact and primary alcohol at the other end is oxidized to COOH Glucose --- glucuronic acid Galactose --- galacturonic acid 3. Saccharic acids : Oxidation at both ends of monosaccharides yields glucosaccharic acid. As a reducing sugar, glucose can react with hemoglobin, glucose oxidase converts glucose to forming glycosylated hemoglobin (hemoglobin A1c), which gluconic acid and hydrogen peroxide is fully functional. o-Toluidine or 2-methylaniline which reacts with the hydrogen peroxide using Determining the amount of HbA1c in the blood allows monitoring the long-term control of blood glucose levels in an enzyme called peroxidase to produce a diabetics. color forming chemical. Intensity of the color is directly Reactions between carbohydrates and proteins often impair protein function. proportional to the concentration of glucose Such modifications, called advanced glycation end products, have been implicated in a number of pathological conditions.  4. Reduction either done catalytically (hydrogen and a catalyst) or enzymatically the resultant product is a polyol or sugar alcohol (alditol) glucose form sorbitol (glucitol) mannose forms mannitol fructose forms a mixture of mannitol and sorbitol - Carbonyl group (-CHO or >C=O) of sugar- reduced to an alcohol group- product known as alditol glyceraldehyde gives glycerol Uses of Sugar alcohols as intermediates Mannitol is used as an osmotic diuretic Glycerol is used as a humectant and can be nitrated to nitroglycerin Sorbitol can be dehydrated to tetrahydropyrans and tetrahydrofuran compounds (sorbitans) Sorbitans are converted to detergents known as spans and tweens (used in emulsification procedures) Sorbitol can also be dehydrated to 1,4,3,6-dianhydro-D-sorbitol (isosorbide) which is nitrated to ISDN and ISMN (both used in treatment of angina)(mono and dinitrates)  5. Dehydration: action of strong acids on monosaccharides monosaccharides are normally stable to dilute acids, but are dehydrated by strong acids D-ribose when heated with Furfural concentrated H2SO4 yields furfural (commercial route for the production of THF (tetrahydrofuran) D-glucose under the same conditions yields 5-hydroxymethyl furfural – Basis for the Molisch test – sensitive test Hydroxymethylfurfural for the detection of carbohydrates The furfurals further react with - + -naphthol present in the test reagent to produce a purple product. ve ve  6. Formation of osazones carbohydrates with free aldehyde or ketone groups react with phenylhydrazine to form Monosaccharide osazone. once used for the identification of sugars consists of reacting the monosaccharide with phenylhydrazine in acetic acid a crystalline compound with a sharp Maltose melting point will be obtained D-fructose and D-mannose give the same osazone as D-glucose: needle-shaped osazones Disaccharides-: – Maltose: sunflower-shaped Lactose – Lactose: powder puff-shaped seldom used for identification; we now use HPLC or mass spectrometry  Osazone is the derivative of carbohydrates that are formed by the reaction of carbohydrates with phenyl hydrazine. Aldehyde of aldoses react with phenyl hydrazine to form phenylhydrazones or osazones, which contain two phenyl hydrazine residues per molecule, and a third molecule of the reagent is turned into aniline and ammonia.  7. Formation of esters Several sugar esters important in metabolism The alcoholic groups of monosaccharides may be esterified by non-enzymatic or enzymatic reactions. ATP donates the phosphate moiety in ester formation In alcohol phosphorylations, ATP is almost always the phosphate donor  8. Fermentation A fermentation is defined as an energy-yielding metabolic pathway with no net change in the oxidation state of products as compared to substrates Yeast can ferment glucose, fructose, maltose and sucrose Ultimately, glucose is converted to pyruvate through glycolysis, and the pyruvate is then converted to CO2 and ethanol by a two-step enzymatic process The net reaction is: C6H12O6 → 2C2H5OH + 2CO2  Derivatives of Monosaccharides 1. Sugar Acids: Oxidation of aldehyde or primary alcohol group in monosaccharide yields sugar acids. E.g., Gluconic acid and glucuronic acid. 2. Sugar Alcohols: Reduction of aldoses and ketoses. E.g., Sorbitol, Mannitol. 3. Alditols: Polyhydroxy alcohols are produced on reduction of monosaccharides. E.g., Ribitol in flavin coenzymes, glycerol and myo-inositol of lipids, Xylitol is a sweetener in sugarless gums and candies. 4. Amino Sugars: One or more hydroxyl groups are replaced by amino groups, the products are amino sugars. E.g., glucosamine and galactosamine (heteropolysaccharides) 5. Deoxysugars: These are the sugars that contain one oxygen less than the parent molecule. E.g., D-2-Deoxyribose is the constituent of DNA. 6. L-Ascorbic acid (vitamin C): Water-soluble vitamin that resembles that of a monosaccharide.  Glycosidic Bonds The anomeric hydroxyl groups of two sugars can join together, splitting out water to form a glycosidic bond. Two glucose molecules combine to form a disaccharide known as maltose. Examples of glycosides: Glucovanillin, Cardiac glycosides, Streptomycin, Ouabain  Disaccharides A disaccharide is formed when a hydroxyl group on one monosaccharide reacts with the anomeric carbon of another monosaccharide to form a glycosidic bond Each disaccharide has a specific glycosidic linkage (depending on which hydroxyl reacts with which anomer) The three most common disaccharides are maltose, lactose and sucrose When hydrolyzed using acid or an enzyme, the following monosaccharides are produced:  Maltose 2 α-D-glucose molecules joined via α(1→4) linkage known as malt sugar produced by the partial hydrolysis of starch (either salivary amylase or pancreatic amylase) used as a nutrient (malt extract; Hordeum vulgare); as a sweetener and as a fermentative reagent isomaltose, held together by α(1→6) glycosidic linkage  Lactose β-D-galactose joined to α−D-glucose via β (1→4) linkage milk contains the alpha and beta-anomers in a 2:3 ratio β-lactose is sweeter and more soluble than ordinary α- lactose used in infant formulations, medium for penicillin production and as a diluent in pharmaceuticals Many humans cannot digest milk because they lack intestinal lactase. Such individuals are lactose intolerant.  Sucrose Sugarcane α-D-glucopyranosido- β-D-fructofuranoside – (α1→β2) glycosidic bond, C1 of α-glucose and C2 of β-fructose. also known as table sugar commercially obtained from sugar cane or sugar beet used pharmaceutically to make syrups, troches Sucrose is dextrorotatory but when hydrolysed becomes levorotatory. – Inversion: Enzyme sucrase (invertase) hydrolyses sucrose initially into α-D-glucopyranose and β-D-fructofuranose. But β-D-fructofuranose is less stable and immediately is converted to β-D-fructopyranose. Sucrose is a non-reducing sugar and it cannot form osazones.  Cellobiose Cellobiose consists of 2 molecules of glucose linked by a β(1→4) glycosidic bond It is usually obtained by the partial hydrolysis of cellulose  Polysaccharides homoglycans (starch, cellulose, glycogen, inulin) heteroglycans (gums, mucopolysaccharides) characteristics: polymers (MW from 200,000) White and amorphous products (glassy) not sweet not reducing; do not give the typical aldose or ketose reactions) form colloidal solutions or suspensions  Homopolysaccharides Starch most common storage polysaccharide in plants composed of 10 – 30% α−amylose and 70-90% amylopectin depending on the source the chains are of varying length, having molecular weights from several thousands to half a million Main sources of starch are rice, corn, wheat, potatoes and cassava A storage polysaccharide Starch is used as a binder in medications to aid the formation of tablets Industrially it has many applications: adhesives, paper making, biofuels, textiles, etc  Amylose Amylose and amylopectin are the 2 forms of starch. Amylopectin is a highly branched structure, with branches occurring every 12 to 30 Amylopectin residues Iodine (I2) can insert in the middle of the amylose helix to give a blue color that is characteristic and diagnostic for starch Digestion of starch a-amylase attacks a(1→4) linkages and cannot attack attack a(1→4) linkage close to a(1→6) branch points  Dextrans products of the reaction of glucose and the enzyme transglucosidase from Leuconostoc mesenteroides contains α (1→4), α (1→6) and α (1→3) linkages MW: 40,000; 70,000; 75,000 used as plasma extenders (treatment of shock) also used as molecular sieves to separate proteins and other large molecules (gel filtration chromatography) components of dental plaques produced by the partial hydrolysis of starch along with maltose and glucose dextrins are often referred to as either amylodextrins, erythrodextrins or achrodextrins used as mucilages (glues) also used in infant formulas (prevent the curdling of milk in baby’s stomach)  Inulin β-(1→2) linked fructofuranoses linear only; no branching lower molecular weight than starch Jerusalem artichokes colors yellow with iodine hydrolysis yields fructose sources include onions, garlic, dandelions and jerusalem artichokes used as diagnostic agent for the evaluation of glomerular filtration rate (renal function test)  Glycogen Animal starch. Storage form of glucose found in the liver and muscle of animals. Composed of many (more than 10) monosaccharide units. Contains many highly branched glucose units. Joined by α(1→4) linkages and branched by α(1→6) linkages. glycogen and iodine gives a red-violet color hydrolyzed by both α and β-amylases and by glycogen phosphorylase  Cellulose Polymer of β-D-glucose attached by β(1,4) linkages Only digested and utilized by ruminants (cows, deers, giraffes, camels) A structural polysaccharide Yields glucose upon complete hydrolysis Partial hydrolysis yields cellobiose Most abundant of all carbohydrates Cotton flax: 97-99% cellulose Wood: ~ 50% cellulose Gives no color with iodine Held together with lignin in woody plant tissues  Products obtained from cellulose Microcrystalline cellulose : used as binder-disintegrant in tablets Methylcellulose: suspending agent and bulk laxative Oxidized cellulose: hemostat Sodium carboxymethyl cellulose: laxative Cellulose acetate: rayon; photographic film; plastics Cellulose acetate phthalate: enteric coating Nitrocellulose: explosives; collodion (pyroxylin)  Chitin Chitin is the second most abundant carbohydrate polymer Like cellulose, chitin is a structural polymer Present in the cell wall of fungi and in the exoskeletons of crustaceans, insects and spiders Chitin is used commercially in coatings (extends the shelf life of fruits and meats) A chitin derivative binds to iron atoms in meat and slows the rancidity process  Heteropolysaccharides Glycoproteins and proteoglycans Proteins with carbohydrates attached are called glycoproteins. There are three main classes of glycoproteins: 1. Glycoproteins: The protein is the largest component by weight. Glycoproteins play a variety of roles, including as membrane proteins. 2. Proteoglycans: The protein is attached to a particular type of polysaccharide called a glycosaminoglycan. By weight, proteoglycans are mainly carbohydrate. Proteoglycans play structural roles or act as lubricants. 3. Mucins or mucoproteins: Like proteoglycans, mucins are predominantly carbohydrate. The protein is characteristically attached to the carbohydrate by N-acetylgalactosamine. Mucins are often lubricants.  Glycosaminoglycans Involved in a variety of extracellular functions; Chondroitin is found in tendons, cartilage and other connective tissues Heparin is a carbohydrate with anticoagulant properties. It is used in blood banks to prevent clotting and in the prevention of blood clots in patients recovering from serious injury or surgery  Lectins Glycan-binding proteins bind to specific oligosaccharides on the cell surface. Lectins are a particular class of glycan-binding protein. Proteins that bind carbohydrates with high specificity and with moderate to high affinity – Found in all organisms (plants, animals, bacteria and viruses – Function in cell-cell recognition, signaling, and adhesion processes – Plant lectins: concanavalin A, wheat germ agglutinin, ricin – Animal lectins: galectin-1, selectins Mannan-binding lectin (MBL) is a glycoprotein found in blood plasma. It binds cell surface carbohydrates of disease-causing microorganisms & promotes phagocytosis of these organisms as part of the immune response. Selectins are integral proteins of mammalian cell plasma membranes with roles in cell-cell recognition & binding – Bacterial lectins: enterotoxin, cholera toxin – Viral lectins: influenza virus hemagglutinin, polyoma virus protein-1