Carbohydrates PDF

Chemistry of Carbohydrates Gandham.Rajeev Each year, 100 metric tons of CO2 is converted to Carbohydrates by plants  Carbohydrates Carbohydrates are the most abundant organic molecules in nature. Hydrates of carbon Carbohydrates are defined as polyhydroxyalde...

Chemistry of Carbohydrates Gandham.Rajeev Each year, 100 metric tons of CO2 is converted to Carbohydrates by plants  Carbohydrates Carbohydrates are the most abundant organic molecules in nature. Hydrates of carbon Carbohydrates are defined as polyhydroxyaldehydes or ketones or compounds which produce them on hydrolysis. Composed of carbon, hydrogen and oxygen  General molecular formula Cn (H2O)n Some carbohydrates contain Sulphur, Nitrogen or Phosphorus Exceptions are acetic acid C2H4O2 and lactic acid C3H6O3.  Functions of Carbohydrates Main sources of ENERGY in body (4kcal/g) – RBCs and Brain cells have an absolute requirement of carbohydrates. Storage form of energy (starch and glycogen) Excess carbohydrate is converted to fat. Glycoproteins and glycolipids are components of cell membranes and receptors. Structural basis of many organisms.e.g. Cellulose in plants, exoskeleton of insects, cell wall of microbes, mucopolysaccharides and ground substance in higher organisms.  Glycobiology & Sugar Code of Life Oligosaccharide Chains Encode Biologic Information An enormous number of glycosidic linkages can be generated between sugars. For example, three different hexoses may be linked to each other to form over 1000 different trisaccharides. Oligosaccharide chains encode biologic information and this depends upon their constituent sugars, their sequences, and their linkages.  The biologic information that sugars contain is expressed via interactions between specific sugars, either free or in glycoconjugates, and proteins (such as lectins) or other molecules. These interactions lead to changes of cellular activity.  Thus, deciphering the so-called ‘sugar code of life’ (one of the principal aims of glycomics) entails elucidating all of the interactions that sugars and sugar-containing molecules participate in, and also the results of these interactions on cellular behavior. This will not be an easy task, considering the diversity of glycans found in cells.  Classification of Carbohydrates Carbohydrates Monosaccharides Disaccharides Oligosaccharides Polysaccharides 1 sugar unit 2 sugar units 3-9 units - >10 e.g.Sucrose e.g. Maltotriose e.g.Glucose, fructose etc Homoglycans - - Heteroglycans e.g. starch, e.g. GAGs or glycogen glycosaminoglycans  Monosaccharides Molecules having only one actual or potential sugar group O ↓ O R H No. of Generic name carbon Aldoses Ketoses * R RI Aldotriose e.g. Ketotriose e.g. Dihydroxyacetone 3 Trioses glyceraldehyde Aldotetrose e.g. Ketotetrose e.g. 4 Tetroses Erythrose Erythrulose Aldopentoses e.g Ketopentoses e.g. 5 Pentoses Arabinose, Xylose, Ribose Xylulose, Ribulose Aldohexose e.g. Glucose, 6 Hexoses Galactose, Mannose Ketohexose e.g. Fructose Aldoheptose: Glucoheptose Ketoheptose e.g Sedoheptulose 7 Heptoses Pentoses of Physiological importance Hexoses of Physiological Importance  Stereoisomers Compounds having same structural formula, but differing in spatial configuration as known as stereoisomers. Asymmetric carbon: Four different groups are attached to the same carbon. The reference molecule is glyceraldehyde. All monosaccharides can be considered as molecules derived from glyceraldehyde by successive addition of carbon atoms.  Penultimate Carbon Stereoisomers  D and L Isomers of glucose D and L Isomers are mirror images of each other. The spatial orientation of H & OH groups on the C-atom (C5 for glucose), adjacent to the terminal primary alcohol carbon determines whether the sugar is D or L Isomer. If the OH group is on the right side, the sugar is of D-Isomer. If the OH group is on the left side, the sugar is of L-Isomer. Mammalian tissues have D- sugars. Configuration of D-aldoses Aldoses Configuration of D-ketoses Ketoses  Optical activity Optical activity is a characteristic feature of compounds with asymmetric carbon atom. When a beam of polarized light is passes through a solution of an optical isomer, it will be rotated either to the right or left. Depending on the rotation, molecules are called dextrorotatory (+) or levorotatory (-). Racemic mixture: If D & L isomers are present in equal concentration, it is known as racemic mixture. NOTE: Racemic mixture does not exhibit any optical activity.  Epimers If two monosaccharides differ from each other in their configuration around a single specific carbon atom, they are referred as epimers to each other. Glucose & galactose are C4-epimers Glucose & mannose are C2-epimers Inter-conversion of epimers is known as epimerization, enzyme epimerases catalyzes this reaction.  Enantiomers Enantiomers are a special type of stereoisomers, that are mirror images of each other. Majority of sugars in higher animals are of D-type.  Diastereoisomers The term diastereomers is used to represent the stereoisomers that are not mirror images of one another. Configurational changes with regard to C2 , C3 and C4 will produce eight different monosaccharides. Total D + L forms = 16 isomers of glucose  Physical Properties Reference Carbon atom of Sugars:- Penultimate carbon atom is the reference carbon atom for naming sugars. All monosaccharides can be considered as molecules derived from glyceraldehyde by successive addition of C units. Penultimate carbon  Glycosides When the hemi-acetal group (hydroxyl group of the anomeric carbon) of a monosaccharide is condensed with an alcohol or phenol group, it is called a glycoside. The non-carbohydrate group is called aglycone. Formation of hemiacetals and hemiketals  Between The common C1 -CHO and C5 -OH monosaccharides have cyclic structures : 8 O  Anomerism Anomers have same composition but differ in the orientation of groups around anomeric carbon atom. Anomeric carbon is a carbonyl carbon atom, e.g. 1st carbon atom in glucose is anomeric carbon atom. Carbonyl carbon atom becomes asymetric because of ring structures of monosaccharides in solution thus anomers are encountered in cyclic structures of monosaccaharides. The alpha & beta cyclic forms of D-glucose are known as anomers. They differ from each other in the configuration only around C1 known as anomeric carbon. The hemiacetal (or carbonyl) carbon atom is called the anomeric carbon. In case of alpha anomer, the OH group held by anomeric carbon is on the opposite side of the group CH2OH of sugar ring.  Expression Anomers are expressed as α and β forms. In α form “OH” group is below the plane (OH group is oriented away from the oxygen atom) In β form “OH” group is above the plane (OH group is oriented towards the oxygen atom)  Mutation: When D-glucose is crystallized at room temperature and a fresh solution is prepared, its specific rotation of polarized light is 112o; but after 12- 18 hrs it changes to +52.5 o This change in rotation with time is called as mutarotation. Glucose has two anomers α and β. Traces of linear forms, intermediate forms Chemical Properties of Carbohydrates  Acid-Catalyzed Mutarotation Step 1: – Protonation of the oxygen of the pyranose ring by the acid catalyst. In aqueous solution, the acid catalyst is the hydronium ion. Step 2: – The pyranose ring opens by cleaving the bond between the anomeric carbon and the positively charged oxygen. The ring opening is facilitated by electron release from the OH group at the anomeric carbon and gives the conjugated acid of the open-chain form of D-glucose. Step 3: – The species formed in the preceding step cyclizes to give the conjugate acid of β formation. This cyclization is analogous to the acid-catalyzed nucleophilic additions of aldehydes and ketones. Step 4: – The product of step 3 transfers a proton to water to regenerate the acid and yield the product. Pyranoses and Furanoses d  Reactions of monosaccharides Tautomerization: The process of shifting a hydrogen atom from one carbon atom to another to produce enediols is known as tautomerization. Reducing properties: In mild alkaline solutions, carbohydrates containing a free sugar group (aldehyde or ketone) will tautomerise to form enediols , where two hydroxyl groups are attached to the double-bonded carbon atoms. Since enediols are powerful reducing agents in alkaline medium. When oxidizing agents like cupric ions are present , sugars form a mixture of carboxylic acids by breaking at the double bonds.  Benedict’s test Procedure: 0.5 (8 drops) ml urine + 5ml Benedict’s reagent & boil for 2 mins. Interpretation: Drawback – test is not specific for glucose Fehling’s test: No intermediate colors are formed as because over there a powerful reducing agent KOH is used. Observation Inference No change in colour No sugar Green colour 0-0.5mg% + Yellow 0.5-1.0mg% ++ Orange 1.0-1.5mg% +++ Brick red 1.5-2mg% ++++  Osazone formation All reducing sugars will form osazones with excess of phenylhydrazine when kept at boiling temperature. Osazones are insoluble. Osazones of individual sugars have characteristic crystal from The differences in glucose, fructose and mannose are dependent on 1st and 2nd C & this difference is masked when Phenyhydrazine reacts with these two carbons. So, Glucose, Fructose and Mannose give broom shaped osazones.  Osazones Glucosazone Maltosazone (broom shaped) (star shaped) Lactosazone (powder puff shaped)  Oxidation of Sugars 1) Mild Oxidation Conditions: e.g. hypobromous acid, the aldehyde group is oxidized to carboxyl group to produce ALDONIC acids. Glucose Gluconic acid Mannose Mannonic acid Galactose Galactonic acid 2) When Aldehyde group is protected, and the molecule is oxidized, esp. in the body, the last C is oxidized to COOH producing URONIC acids. Imp- Glucuronic acid is used in body to synthesize heteropolysaccharides and also for conjugation of various substances.  Under strong oxidation conditions(e.g strong acids-HNO3) BOTH groups are oxidized to produce dicarboxylic acids called SACCHARIC acids Glucose --> Glucosaccharic acid Mannose --> Mannaric acid Galactose --> Mucic acid Oxidation by glucose oxidase:- GOD-POD method  Furfural formation Monosaccharides when treated with concentrated H2SO4 undergoes dehydration with the removal of 3 molecules of water. Hexoses give hydroxymethyl furfural and pentoses give furfural. Furfurals condense with phenolic compounds to give various colors. E.g. Molisch’s test: General test for carbohydrates (H2SO4 and α-naphthol) Rapid Furfural and Seliwanoff’s test: Tests for presence of keto group  Reduction to form alcohols When treated with reducing agents such as sodium amalgam, hydrogen can reduce sugars. Aldose yields corresponding alcohol. Ketoses form two alcohols because of appearance of new asymmetric carbon in this process. D-Glucose D-Sorbitol D-Fructose D-Mannitol Sorbitol and Mannitol are used to identify bacterial colonies. Mannitol is used to reduce intracranial pressure by forced diuresis. The osmotic effect of sorbitol and dulcitol produces changes in tissues when they accumulate in abnormal amounts. E.g cataract  Glycosides When the hemi-acetal group (hydroxyl group of the anomeric C ) of a monosaccharide is condensed with an alcohol or phenol group, it is called as a glycoside. The non-carbohydrate group is called aglycone. Glycosides are non –reducing (WHY ?) but they may be hydrolyzed by boiling with dilute acids. α- glycosides are hydrolyzed by maltase from yeast, while beta-glycosides are hydrolyzed by Emulsin from almonds. So enzyme hydrolysis affords a method to distinguish b/w two forms.  Important Glycosides Sugar Aglycon Glycoside Source Importance Glucose Phloretin Phlorizin Rose bark Renal damage Galactose Digitogenin Digitonin Leaves of Cardiac Xylose foxglove stimulant Glucose Indoxyl Plant Leaves of indican indigofera Stain  Formation of esters Hydroxyl groups of sugars can be esterified to form acetates, propionates, benzoates, etc Sugar phosphates are of great biological importance. Metabolism of sugars inside the body starts with phosphorylation. e.g Glucose 6-P04  Amino sugars Amino groups may be substituted for hydroxyl groups of sugars to give rise to amino sugar. Generally the group is added to the second C of hexoses. They are non –reducing and do not form osazones They are found in GAGS, glycoproteins, proteoglycans Abbreviations:- GluNac = N-acetyl –glucosamine GalNac =N-acetyl-galactosamine GLUCOSAMINE or 2 amino-D-glucopyranose (α form)  The amino group may be further acetylated to produce N-acetlyated sugars like N-acetly glucosamine (GlcNac) or N-acetyl galactosamine (GalNac) which are important constituents of glycoproteins and MPS  Deoxy Sugars Oxygen of the hydroxyl group may be removed to form deoxy sugars. They are non reducing. Don’t form osazones. Deoxyribose is present in DNA  Disaccharides When two monosaccharides are combined together by glycosidic linkage, a disaccharide is formed. Two types:- Non-reducing Sucrose Cane sugar Trehalose in yeast Reducing Lactose Milk sugar Maltose Malt sugar  Sucrose Cane sugar, table sugar Glu + Fru (1 2) Sweetening agent Non-reducing No osazones Clinical Importance:- -dental caries -Bypasses metabolic check points- OBESITY -“Sucrase deficiency “  Inversion Hydrolysis of sucrose (optical rotation +66.5o) will produce one molecule of glucose (+52.5o) and one molecule of fructose (-92o) Therefore the products will change the dextrorotation to levorotation (INVERSION) Equimolecular mixture of glucose and fructose thus formed is called as Invert Sugar The enzyme producing hydrolysis of sucrose is called INVERTASE  Lactose Milk sugar Galactose + Glucose (β1 4) Reducing disaccharide Beta glycosidic linkage Osazone – Powder Puff or hedgehog shaped  Maltose 2 glucose residues (α1 4 linkage) Reducing disaccharide Malt sugar Osazone:- Star shaped or flower petal shaped  Isomaltose 2 Glucose in α (1 6) linkage Reducing disaccharide Produced during partial hydrolysis of starch and glycogen  Disaccharides of importance Sugar Composition Source Clinical Significance D-glucopyranosyl- Cane and beet sugar, Rare genetic lack of sucrase Sucrose (1-2) D-fructo sorghum and some leads to sucrose furanoside fruits and vegetables intolerance—diarrhea and flatulence Lack of lactase leads to lactose intolerance diarrhea D-galactopyranosyl- Milk and flatulence; may be Lactose (1-4)D-glucopyranose excreted in the urine in pregnancy  Sugar Composition Source Clinical Significance D-glucopyranosyl- Enzymatic hydrolysis of starch Maltose (1-4)-D-glucopyranose (amylase); germinating cereals and malt D-glucopyranosyl- Enzymatic hydrolysis of starch Isomaltose (1-6)-D-glucopyranose (the branch points in amylopectin) D-galactopyranosyl- Heated milk (small amounts), Not hydrolyzed by Lactulose (1-4)-D-fructofuranose mainly synthetic intestinal enzymes, but fermented by intestinal bacteria; used as a mild osmotic laxative Trehalose D-glucopyranosyl- Yeasts and fungi; the main (1-1)-D-glucopyranoside sugar of insect hemolymph  Polysaccharides Homoglycan Heteroglycan Or Or Homopolysaccharide Heteropolysaccharide  Homoglycans Starch Glycogen Cellulose Inulin Dextrans Chitin  Starch Carbohydrates of the plant kingdom Sources: Potatoes, tapioca, cereals (rice, wheat) and other food grains Composed of Amylose & Amylopectin Amylose: When starch is treated with boiling water, 10 -20 % is solubilized. This part is called amylose, contains glucose units with α-1,4 glycosidic linkages. Mol wt =400,000 or more  Amylopectin: The insoluble part absorbs water and forms paste like gel; This is called as amylopectin. Amylopectin is also made up of glucose units, but is highly branched with molecular weight more than 1 million. The branching points are made by α- 1, 6 linkage  Hydrolysis of starch Starch will form a blue coloured complex with iodine; this color disappears on heating and reappears when cooled. This is a sensitive test for starch. When starch is hydrolyzed by mild acid, smaller and smaller fragments are produced. The hydrolysis for a short time produces amylodextrin (violet color with iodine and non-reducing). Further hydrolysis……………. amylodex erythrodex archrodextrin Maltose Violet Red no color no color Non reducing Non reducing Reducing Reducing  Action of amylases on starch Salivary amylases and pancreatic amylases are α amylases, which act randomly on α, 1-4 linkages to split starch into smaller units called dextrins Beta amylases (plant origin – almonds etc) act consecutively from one end. When beta amylases reach a branch point in amylopectin, enzyme is blocked, leaving a large molecule called as LIMIT DEXTRIN  Glycogen Storage form of energy in animal. Stored in liver and muscle Stores more glucose residues per gram than starch. More branched and compact than starch. Less osmotic pressure. More energy in a smaller space. Glycogen in liver (6-8%) is higher than that in the muscles (1-2%). Liver glycogen - first line of defense against declining blood glucose levels especially between meals.  A homopolysaccharide: linear chain of α(1→4) linked glucosyl residues with branches joined by α (1→6) linkages  Cellulose Glucose units combined by β-1,4 linkages. Straight line str. with no branches. Mol wt 2-5 million. This bond is digested by cellobiases an enzyme not present in humans. Herbivores animals have large caecum which harbor bacteria which break cellulose. White ants (termites) and some wood fungi also have cellulase. Commercial applications: nitrocellulose, cellulose acetate membranes for electrophoresis ETC  Inulin D -fructose in β-1,2 linkages. Source: Bulbs and tubers chicory, dahlia, dandelion, onions, garlic. Not metabolized. Not absorbed nor secreted by kidneys. USE – to measure GFR.  Dextrans Highly branched homoglycan containing Glu residues in 1-6, 1-4 and 1-3 linkages. Produced by microbes. Mol. wt:- 1-4 million. As large sized, they will not move out of vascular compartment so used as plasma expanders.  Chitin N-acetyl glucosamine with beta 1,4 glycosidic linkage Exoskeleton of crustacea and insects. Heteroglycans  Agar and Agarose It is made up of D-galactose and an L-galactose derivative ether – linked between C-3 and C-6 It is dissolved in water at 100 o C, which upon cooling sets into a gel. Agar cannot be digested by bacteria so it is widely used as a supporting media to culture bacterial colonies. Also used as a supporting agent for immuno-diffusion and immuno-electrophoresis. Two components : Agarose (unbranched) Agaropectin (branched) Agarose is made up of D- galactose combined with 3,6-anhydro L-galactose units and is used as a matrix for electrophoresis.  Mucopolysaccharides or GAG [ URONIC ACID + AMINO SUGAR] n Acetylated amino sugars, sulfate and carboxyl groups may be present also  Heteropolysaccharides Polymers made from more than one kind of monosaccharides or monosaccharide derivatives. Eg : Glycosoaminoglycans, Agar Agarose  Long, Unbranched heteropolysaccharide, made of repeating disaccharide units containing uronic acid & amino sugars. Amino sugar – Glucosamine or Galactosamine (Present in there acetylated form) Uronic acid – D-Glucuronic acid or L-Iduronic acid GAGs are the most important group of heteroglycan in humans.  First isolated from mucin so called mucopolysaccharides. Major components of extracellular matrix of connective tissue, including bone and cartilage, synovial fluid, vitreous humor and secretions of mucus producing cells.  Gel forming component of extracellular matrix The anionic groups (carboxy & sulfate groups) being strongly hydrophilic tend to bind large amount of water producing gel like matrix, that forms the bodies ground substance. Heteropolysaccharide chains repel one another and therefore exist in extended conformation in solutions. This produces slippery consistency of mucus secretions and synovial fluid.  Structural support to connective tissue GAGs form matrix or ground substance that stabilizes and supports the cellular and fibrous components of tissues. Other functions: Plays an important role in mediating cell-cell interactions Their slippery consistency makes them suitable for a lubricant action in joints.  Classification GAGS Acidic Neutral Sulfate containing Sulfate free Blood group substances Chondroitin Sulphate Dermatan sulphate Hyaluronic acid keratan sulphate Heparin Heparan Sulphate 82  Hyaluronic acid It is sulfate free GAG. Synovial fluid of joints, vitreous humor, connective tissues and cartilage.  Functions of Hyaluronic acid Serves as a lubricant and shock absorbant in joints. Acts as seives in extracellular matrix. Permits cell migration during morphogenesis & wound repair. Hyaluronidase is an enzyme that breaks β1 – 4 linkages of hyaluronic acid. Present in high concentration in seminal fluid, & in certain snake and insect venoms.  Hyaluronidase enzyme of semen degrades the gel around ovum & allows effective penetration of sperm into ovum, thus helps in fertilization. The invasive power of some pathogenic organism may be increased because they secrete hyaluronidase.  Chondroitin 4-sulfate Most abundant GAG in body.  Widely distributed in bone, cartilage & tendons. Functions: In cartilage, it binds collagen & hold fibers in a tight strong network. Role in Compressibility of cartilage in weight bearing.  Dermatan sulfate Contains repeating units of L-iduronic acid and N-acetyl glucosamine 4 sulfate.  Present in skin, cardiac valves & tendon. Function: Present in sclera of eye where it has important function in maintaining overall shape of eye.  Heparin Only GAG present intracellular: In granules of mast cells and also in lung, liver and skin.  Strongly acidic due to presence of more sulfate group. Functions: It is an anticoagulant (prevents blood clotting) Heparin helps in the release of the enzyme lipoprotein lipase (LPL) which helps to clear the lipidemia after fatty meal – so called clearing factor.  Heparan sulfate Structurally similar to heparin, but has a Lower molecular weight Contains higher acetyl groups & less sulfate group Predominant uronic acid is D-Glucuronic acid It is an extracellular GAG found in basement membrane and is an essential component of cell surfaces. Determines charge selectiveness of renal glomerulus.  Keratan sulfate Only GAG with no uronic acid. Found in cornea & tendons. Function: Maintains the corneal transparency. Proteoglycan aggregate Proteoglycan aggregate  Blood group substances (blood gr Antigens) RBC membrane contains several antigenic substance, based on which classified into different blood groups. They contain carbohydrates as glycoproteins or glycolipids. N-Acetylgalactosamine, galactose, fucose, sialic acid etc are found in blood gr substances. Carbohydrate content plays a determinant role in blood grouping.  Agar Contains galactose, glucose & other sugars. Obtained from sea weeds Functions: Cannot be digested by bacteria. So used as supporting agent to culture bacterial colonies. Also as support medium of immuno diffusion & immuno-electrophoresis.  Agarose Galactose and 3,6 anhydrous galactose units Used as matrix for electrophoresis.  Reference Books Test Book of Biochemistry- Harper Test Book of Biochemistry - Dr. U.Satyanarayana Test Book of Medical Biochemistry-DM.Vasudevan Test Book of Medical Biochemistry – MN Chatterjea Thank you