Carbohydrate Chemistry | Monosaccharides, Disaccharides, Examples - PDF

Chemistry of Carbohydrates Definition of carbohydrates Old Definition Carbohydrates are “Hydrates of Carbon”  Definition of Carbohydrates Carbohydrates are organic substances containing C, H and O. Empirical formula/General formu...

Chemistry of Carbohydrates Definition of carbohydrates Old Definition Carbohydrates are “Hydrates of Carbon”  Definition of Carbohydrates Carbohydrates are organic substances containing C, H and O. Empirical formula/General formula for simple carbohydrates : Cn (H2O)n Where n = number of carbon atom present in carbohydrate structure. They are defined as polyhydroxy aldehyde or ketone derivatives or as compounds that yield these derivatives on hydrolysis. Simple Carbohydrates has many Hydroxyl groups (Polyhydroxy). Simple Carbohydrates has carbonyl/ functional groups as Aldehyde or Ketone. Functions of Carbohydrates Carbohydrates serve as chief source of energy/Fuel of body, e.g. glucose (4 Cal/g) Structural role. e.g. glycosaminoglycans in humans, cellulose in plants and chitin in insects. Precursors for many organic compounds (fats, amino acids). Certain carbohydrate derivatives are used as drugs like cardiac glycosides, antibiotics. Storage form of energy, e.g. glycogen in animal tissue and starch in plants, to meet the immediate energy demands of the body. Non-digestible carbohydrates like cellulose, serve as dietary fibers. Constituent of nucleic acids RNA and DNA, e.g. ribose and deoxyribose sugar. Play a role in lubrication, cellular intercommunication and immunity. Carbohydrates are also involved in detoxification, e.g. glucuronic acid.  Classification Of Carbohydrates Carbohydrates are divided into 4 major groups depending upon number of sugar units. Carbohydrates Monosaccharides Disaccharides Oligosaccharides Polysaccharides More One sugar unit Glucose, Two sugar units 3-10 sugar units than 10 sugar units Starch, fructose Maltose, sucrose Raffinose cellulose Monosaccharides Monosaccharides (Greek : mono-one , saccharides (Greek: sakcharon–sugar) The simplest group of carbohydrates. Often referred to as simple sugars. They cannot be further hydrolysed All Monosaccharides are reducing sugars. (strong reducing agents) They have the general formula Cn(H2O)n.  Monosaccharides Sub Classification They can be subdivided further: Depending upon the number of carbon atoms they possess, as trioses, tetroses, pentoses, hexoses, etc. Depending upon whether aldehyde (– CHO) or ketone(-C=O) groups are present as aldoses or ketoses.  Glyceraldehyde Dihydroxyace tone H C O CH 2O H H C O H C O C H2OH CH2 OH 3 C atoms= Tr iose Ke t one gr oup= Ke tos e All togat he r = Ke totr io se Simplest Carbohydrate (Reference sugar) G luco s Fr uc t os e e C O H CH2OH H C OH HO C C O H HO C H H C OH H C OH H C OH CH2 OH H C OH 6 C a t o ms = Hex o se A ld eh y d e g ro u p = A ld CH2OH o se A ll t o g a t h er= A ld o h 6 C atoms = Hexos e Ketone group= Ketos e ex o se Al l togat her= Ketohexos e  Number of Aldoses Ketoses Carbon Atoms (Aldehyde-CHO) (Ketone -C=O) 3 Aldo Triose Keto Triose Triose Glyceraldehyde DiHydroxyAcetone (DHA) 4 Aldo Tetrose Keto Tetrulose Tetrose Erythrose Erythrulose 5 Aldo Pentose Keto Pentulose Pentose Ribose, Xylose, Arabinose Ribulose, Xylulose 6 Aldo Hexose Keto Hexose Hexose Glucose, Galactose ,Mannose Fructose 7 Aldo Heptose Keto Heptulose Heptose Sedoheptose Sedoheptulose 2.Disaccharides They contain two molecules of same or different monosaccharide units. On hydrolysis, they yield two monosaccharide units. Two monosaccharide units are joined by glycosidic bond. Lactulose: is a synthetic disaccharide containing galactose and fructose. It is neither digested nor absorbed in the intestine.  Examples of disaccharides Examples Product Glycosidic Source Maltose Glucose + glucose α1–4 Malt sugar Lactose Galactose + glucose β 1–4 Milk sugar Sucrose Glucose + fructose α 1– β2 Sugarcane Isomaltose Glucose + glucose α 1–6 Digestion of amylopectin Cellobiose Glucose + glucose β 1–4 Hydrolysis of cellulose Maltose Maltose is composed of two α-D-glucoseunits held together by α (1 4) glycosidic bond. The free aldehyde group present on C1 of second glucose answers the reducing reactions. Maltose can be hydrolysed by dilute acid or the enzyme maltase to liberate two molecules of α -D-glucose. Free aldehyde Non reducing end (Reducing end)  Lactose Lactose is more commonly known as milk sugar since it is the disaccharide found in milk. Lactose is composed of β-D-galactose and β-Dglucose held together by β (1 & 4) glycosidic bond. The anomeric carbon of C1 glucose is free, hence lactose exhibits reducing properties. Reducing end Non reducing end Sucrose Sucrose (cane sugar). Sucrose is made up of α -D-glucose and β- D-fructose. The two monosaccharides are held together by a glycosidic bond (α1 & β2), between C1 of α -glucose and C2 of β -fructose. The reducing groups of glucose and fructose are involved in glycosidic bond, hence sucrose is a non-reducing sugar.  Types of Disaccharides Disaccharides Non Reducing Reducing Sugars Sugars Reducing Sugar Sugar structure possessing free or potential(reactive) aldehyde or ketone group is termed as reducing sugar. Reducing sugars show reducing property efficiently in alkaline medium and reduces certain metallic ions as- Cu++;Bi++;Fe+++ Reducing Sugars give tests positive with Benedict’s Test Fehling’s test Form Osazones. Reducing Shows Mutarotation (Change in Optical activity) Example: Lactose, Maltose.  Non Reducing Sugars Sugar structure not possessing free or potential aldehyde or ketone group in its structure is termed as non reducing sugar. Non reducing sugar does not show reducing property and do not reduce metallic ions. Non reducing sugars give following reducing tests negative. Benedict’s Test Fehling’s test Do not form Osazones Non Reducing sugars do not exhibit Mutarotation (Change in Optical activity) Example: sucrose and trehalose 1.Oligosaccharides: They contain 3 to 10 molecules of monosaccharide units. E.g.: Maltotriose (Glucose + Glucose + Glucose). 2.Polysaccharides: They contain more than ten molecules of monosaccharide units. They are further classified into homopolysaccharides and heteropolysaccharides. Homopolysaccharides (Homoglycans): They are polymer of same monosaccharide units. Heteropolysaccharides (Heteroglycans): They are polymer of different monosaccharide units or their derivatives, namely amino sugars and uronic acids.  Polysaccharide Sub Classification Polysaccharides Homopolysacchrides Heteropolysaccharides Contains > 10 same repeating units Contains > 10 different repeating units Examples of homopolysaccharides Examples Monosaccharide unit Source Starch Glucose Plant, rice Dextrin Glucose From starch hydrolysis Glycogen Glucose Liver, muscle Cellulose Glucose Plant fibers Inulin Fructose Dahlia roots Chitin N-acetyl glucosamine Shells of arthropod  Starch Starch is the chief storage form of carbohydrates in plants. Starch is the most important dietary source for higher animals, including man. Starch is homopolysaccharides composed of α-D-glucose units held by (α-1,4 ) and( α-1-6) glycosidic bond. It is known as glucosan or glucan. Starch granules contain two forms, amylose (15- 20%) in the inner part and amylopectin (80-85%) in the outer part. Starches are hydrolysed by α-amylase (pancreatic or salivary) to liberate dextrins, and finally maltose and glucose units. Amylase acts specifically on D (α-1,4) glycosidic bonds. Structure of starch (α-amylose and amylopectin).  Glycogen Glycogen is the storage form of energy in animals (animal starch). It is mainly present in skeletal muscles and liver. The main glycosidic bond is α 1-4- linkage. Only at the branching point, the chain is attached by α 1-6 linkage. It is similar to the amylopectin component of starch. But it has more branches than starch. There are 11 to 18 glucose residues between any branch points.  Cellulose Cellulose occurs exclusively in plants and it is the most abundant organic substance in plant kingdom. It is a predominant constituent of plant cell wall. Cellulose is totally absent in animal body. Cellulose is composed of β-D glucose units linked by β (1 - 4) glycosidic bonds. Cellulose cannot be digested by mammals— including man—due to lack of the enzyme that cleaves β -glycosidic bonds (α amylase breaks α bonds only). Biomedical Importance of cellulose Cellulose, though not digested, has great importance in human nutrition. It is a major constituent of fiber, the non-digestable carbohydrate. The functions of dietary fiber include decreasing the absorption of glucose and cholesterol from the intestine, besides increasing the bulk of feces, acts as a stool softener to avoid constipation. Reduced incidence of a number of diseases like: Cardiovascular disease, Colon cancer, Diabetes, Diverticulosis.  Heteropolysaccharides ( More than 10 Different Repeating Units ) Mucopolysaccharides (MPS) or Glycosaminoglycans (GAGs) Types and examples of mucopolysaccharides Acidic Non Sulfated MPS: Hyaluronic Acid Acidic Sulfated MPS: Heparin Heparan Sulfate Chondritin Sulfate Dermatan Sulfate Keratan Sulfate  Hyaluronic acid Hyaluronic acid is an important GAG found in the ground substance of synovial fluid of joints and vitreous humor of eyes. It is also present as a ground substance in connective tissues, and forms a gel around the ovum. Hyaluronic acid serves as a lubricant and shock absorbant in joints.  Chondroitin sulfates Chondroitin 4-sulfate is a major constituent of various mammalian tissues (bone, cartilage, tendons, heart, valves, skin, cornea etc.). Chondroitin 4-sulfate composed of D-glucuronic acid and N-acetyl D- galactosamine 4-sulfate. Heparin Heparin is an anticoagulant (prevents blood clotting) that occurs in blood, lung, liver, kidney, spleen etc. Heparin helps in the release of the enzyme lipoprotein lipase which helps in clearing the turbidity of lipemic plasma. Heparin is composed of alternating units of N-sulfo D-glucosamine 6-sulfate and glucoronate 2-sulfate.  Dermatan sulfate Mostly found in skin, dermatan sulfate is structurally related to chondroitin 4-sulfate. The only difference is that there is an inversion in the configuration around C5 of D-glucuronic acid to form L-iduronic acid. Keratan sulfate It is a heterogeneous GAG with a variable sulfate content, besides small amounts of mannose, fructose, sialic acid etc. Keratan sulfate essentially consists of alternating units of D-galactosamine and N-acetylglucosamine - 6-sulfate.  Blood group substances The blood groups (A, B, AB, and O) antigens (of erythrocyte membrane) contain carbohydrates as glycoproteins or glycolipids. N-Acetylgalactosamine, galactose, fucose, sialic acid etc. are found in the blood group substances. The carbohydrate content also plays a determinant role in blood grouping. STRUCTURE OF GLUCOSE: The structure of glucose can be represented in the following ways: 1. The straight chain structural formula (Fisher projection). 2. Cyclic formula (Ring structure or Haworth projection). Monosaccharide in solution is mainly present in ring form. In solution, aldehyde (CHO) or ketone (C=O) group of monosaccharide react with a hydroxy (OH) group of the same molecule forming a bond hemiacetal or hemiketal respectively.  The aldehyde group of glucose at C-1 reacts with alcohol (OH) group of C-5 or C-4 to form either six membered rings called glucopyranose or five membered rings called glucofuranose, respectively. However, in case of glucose, the six membered glucopyranose is much more stable than the glucofuranose ring. In the case of fructose, the more stable form is fructofuranose. Fischer projections  Haworth projection Haworth projection formulae are depicted by a six-membered ring pyranose (based on pyran) or a five-membered ring furanose (based on furan). The cyclic forms of glucose are known as α-D-glucopyranose and α -D- glucofuranose. Blood Glucose is more thermodynamically stable in β-D Glucopyranose form.  Optical Activity & Stereoisomerism All Carbohydrates except Dihydroxyacetone(DHA) possess asymmetric carbon atoms in their structure. Presence of Asymmetric carbon atoms confer two properties: 1. Optical Activity 2. Stereoisomerism (Optical isomers). Asymmetric carbon (Chiral): A carbon atom to which four different atoms or groups of atoms are attached is said to be asymmetric. C C Asymmetric carbon Symmetric carbon Chiral carbon Achiral carbon  Optical Activity Optically active is ability of substance to rotate the plane polarized light to rightor to left. solutions when placed in the tube of Polarimeter. If moves the plane of polarized light toward right are dextrorotatory (d/+). If moves the plane of polarized light toward left are laevorotatory (l/-). Stereoisomerism Stereoisomerism is due to presence of chiral carbon atoms Stereoisomers Such compounds have same chemical and molecular formula but differs only in spatial configuration. Type of Stereoisomerism D and L isomer (Enantiomer). Epimers. Anomers.  D-and L-isomers The orientation of the H and OH groups around the carbon atom just adjacent to the terminal primary alcohol carbon, e.g. C-atom 5 in glucose determines the series. When the – OH group on this carbon is on the right, it belongs to D- series, when the – OH group is on the left, it is a member of L-series H C O H C O H C O H HO C H C H2O H CH2OH D-Glyceraldehyde L-Glyceraldehyde Enantiomers M I R R O R Enantiomers (mirror images) of glucose.  n Number of isomers= 2 (Van’t Hoff’s rule) Where n= number of chiral atoms Glucose contains 4 asymmetric carbons, and thus has 16 isomers. Only D-glucose or D-sugars are utilized by humans. The enzyme machinery of cells is specific to metabolize D-series of monosaccharaides. Anomers (cyclic structure of monosaccharides) The hydroxyl group of monosaccharides can react with its own aldehyde or keto functional group to form hemiacetal and hemiketal. Thus, the aldehyde group of glucose at C1 reacts with alcohol group at C5 to form two types of cyclic hemiacetals namely alpha ( α ) and beta (β ). Carbon 1, after ring formation becomes asymmetric and it is called as anomeric carbon atom. The configuration of glucose is conveniently represented either by Fischer formulae or by Haworth projection formulae.  Anomers The designation α means that the OH-group attached to C-1 is below the plane of the ring, β means that it is above the plane of the ring. Epimers If two monosaccharides differ from each other in their configuration around a single asymmetryic carbon (other than anomeric) atom, they are referred to as epimers to each other. Glucose and Galactose H C O H C O are epimers H C O H H C OH H O C H H C H (C4-epimers) O H O C H C O H HH C O H H C OH C H2 O CH 2O H H D-G a l a c t o s e D -G l uco  Glucose and mannose are epimers with regard to carbon 2 (C2-epimers). H C O H C O H C O H O C H H H C H HO C H O H C O H C OH H H C O H C OH H CH 2O CH2OH H D - M anno D -G lu c se ose Racemic mixture: If D- and L-isomers are present in equal concentration, it is known as racemic mixture or DL mixture. Racemic mixture does not exhibit any optical activity, since the dextro- and levorotatory activities cancel each other.  Mutarotation Mutarotation is defined as the spontaneous gradual change in the specific optical rotation to a constant fixed rotation. Representing the interconversion of α and β forms of D-glucose to an equilibrium mixture. α -D- Equilibrium mixture β -D-Glucose Glucose + 52.7° + 18.7° + 112.2° The equilibrium mixture contains 63% β -anomer and 36% α anomer of glucose with 1% open chain form. Mutarotation of glucose representing α and β anomers Inversion: Sucrose is dextrorotatory in nature (+66.5°). After hydrolysis, it gives mixture of glucose and fructose, The hydrolyzed mixture shows levorotatory activity. This phenomenon is called inversion. This is because optical activity of fructose is –92° and glucose is 52.7°.. The enzyme that digests sucrose is sucrase, it is also known as invertase.  Monosaccharide Derivatives 1- Sugar acids: Oxidation of aldehyde group (C1) to COOH results in the formation of gluconic acid. H C O COOH H C OH H C OH HO C H HO C H O H C OH H C OH H C OH H C OH CH2OH CH2OH D-Glucose D-Gluconic acid Oxidation of terminal alcohol group (CH2OH COOH) to leads to production of glucuronic acid. the H C O H C O H C O H HO H C OH C H HO C H Oxidation H C OH H C OH H C OH H C OH C H2O H COOH D-Glucos D - gl u c u r o n i c a c i e d  2- Sugar Alcohols Reduction of monosaccharides by reducing agent the aldehyde or keto group is reduced to corresponding alcohol. H C O CH2OH H C OH H C OH HO HO C H C H H2 H C OH H C OH H C OH H C OH CH2OH CH2OH D-Glucose D-Sorbitol Reduction of fructose result in formation of two compounds D. sorbitol or D. mannitol. CH2OH CH2OH CH2OH HO C H C O H C OH HO C HH HO C HH HO C HH C H2 H2 C OH C OH OH H C OH H C OH H C OH CH2OH CH2OH CH2OH D-Fructose D-Sorbitol D- Mannitol Sorbitol and dulcitol when accumulate in tissues in large amounts cause strong osmotic effects leading to swelling of cells, and certain pathological conditions. e.g. cataract, peripheral neuropathy, nephropathy. Mannitol is useful to reduce intracranial tension by forced diuresis. 3- Deoxysugars : These are sugars in which the hydroxyl group is replaced by a hydrogen atom. The most important examples are: 2-deoxyribose: It is present in the structure of DNA. O C H 2O H O H H H H H OH H β-D- Deoxyribose  4- Aminosugars : These are sugars in which an amino group (NH2) replaces the hydroxyl group on the second carbon e.g. glucosamine (GluN), galactosamine (GlaN) and mannosamine (ManN). Aminosugars are important constituents of glycosaminoglycans (GAGs) and some types of glycolipids and glycoproteins.. Several antibiotics contain aminosugars which are important for their antibiotic activity. 5- Aminosugar acids : These are formed by the addition of acids to aminosugars. Addition of pyruvic acid to mannosamine gives neuraminic acid. The N-acetyl derivatives of the aminosugar acids are called sialic acids e.g. N-acetyl neuraminic acid (NANA). NANA enters in the structure of glycolipids and glycoproteins. Acetylation Glucosamine Reaction N-Acetylglucosamine (Amino Sugar) (Acetylated AminoSugar)  COOH COOH C O C O CH2 CH2 H C OH H C O O H C OH H2 N C H H 2N C H H3C C HN C H HO C HH O HO C HH HO C H CH3-C-COOH C OH CH3COOH C OH H C OH H C OH Pyruvate acid Acetylation H C OH H C OH CH2OH CH2OH CH2OH D-Mannoseamine Neuraminic acid N-acetyl-D-Neuraminic acid (NANA) Sialic acid 6- Ester formation: The hydroxyl groups of monosaccharides can form esters with Phosphoric acids. Esterification Glucose Glucose -6-p or Glucose-1-P Phosphorylatio n H C H C O O H C OH H C OH O HO C H HO P OH HO C H OH H C OH H C OH H C OH Phosphorylation H C OH O CH2O P OH CH2OH OH D-Glucose D-Glucose-6-phosphatate THANK YOU