Lecture 5. Carbohydrates PDF

CARBOHYDRATE  INTRODUCTION  Carbohydrates are broadly defined as polyhydroxy aldehydes or ketones and their derivatives or as substances that yields one of these compounds  Composed of carbon, hydrogen, and oxygen general formula Cn (H2O).  Functional groups pre...

CARBOHYDRATE  INTRODUCTION  Carbohydrates are broadly defined as polyhydroxy aldehydes or ketones and their derivatives or as substances that yields one of these compounds  Composed of carbon, hydrogen, and oxygen general formula Cn (H2O).  Functional groups present include hydroxyl groups  -ose indicates sugar  Carbohydrates are one of the three major classes of biological molecules.  Carbohydrates are also the most abundant biological molecules.  DEFINITION Carbohydrates may be defined as polyhydroxyaldehydes or ketones or compounds which produce them on hydrolysis. OR Carbohydrates are carbon compounds that contain large quantities of hydroxyl groups. Carbohydrates are chemically characterized as:  Poly hydroxy aldehydes or  Poly hydroxy ketones.  Sugars that contain an aldehyde group are called Aldoses.  Sugarsthat contain a keto group are called Ketoses. FUNCTIONS OF CARBOHYDRATES  About 65% of the foods in our diet consist of carbohydrates.  Other carbohydrates called disaccharides include sucrose (table sugar) and lactose in milk.  During digestion and cellular metabolism, carbohydrates are converted into glucose,  Major source of energy for the cell  Major structural component of plant cell  Immediate energy in the form of GLUCOSE  Reserve or stored energy in the form of GLYCOGEN in animals and Starch in plants  FUNCTIONS OF CARBOHYDRATES  Nutritional (energy storage, fuels, metabolic intermediates)  Structural (components of nucleotides, plant and bacterial cell walls, arthropod exoskeletons, animal connective tissue)  informational (cell surface of eukaryotes -- molecular recognition, cell-cell communication)  Osmotic pressure regulation (bacteria)  RBCs and Brain cells have an absolute requirement of carbohydrates.  Excess carbohydrate is converted to fat. Glycoproteins and glycolipids are components of cell membranes and receptors.  Oxidized further in our cells to provide our bodies with energy and to provide the cells with carbon atoms for building molecules of protein, lipids, and nucleic acids.  ALDOSES Aldoses are monosaccharides O  with an aldehyde group ║  with many hydroxyl (-OH) C─H aldose │ groups. H─ C─OH triose (3C atoms) │ tetrose (4C atoms) H─ C─OH pentose (5 C atoms) │ hexose (6 C atoms) CH2OH Erythose, an aldotetrose  KETOSES CH2OH │ Ketoses are monosaccharides C=O ketose  with a ketone group │  with many hydroxyl (-OH) H─ C─OH │ groups. H─ C─OH │ H─C─OH │ CH2OH Fructose, a ketohexose  CLASSIFICATION All carbohydrates can be classified as either:  Monosaccharides  Disaccharides  Oligosaccharides  Polysaccharides.  1. MONOSACCHARIDES  Monosaccharides- (from Greek monos: single, sacchar: sugar) also called simple sugars, are the most basic units of carbohydrates.  They are fundamental units of carbohydrates and cannot be further hydrolyzed to simpler compounds.  The general formula is CnH2nOn.  Some monosaccharides have a sweet taste.  They are the simplest form of sugar and are usually colorless, water soluble, and crystalline solids.  Monosaccharides contain a single carbon chain and are classified on the basis of number of carbon atoms they possess, and as aldoses or ketoses depending upon their groups. Monosaccharide (simple sugar)  Important monosaccharides, examples  2. DISACCHARIDES 2. Disaccharides: (also called a double sugar or biose) are condensation products of two monosaccharide units. is the sugar formed when two monosaccharides (simple sugars) are joined ??????????. Like monosaccharides, disaccharides are soluble in water. Three common examples are sucrose, lactose, and maltose.  maltose =glucose + glucose  sucrose = fructose + glucose  lactose = galactose + glucose Through glycosidic bond or glycosidic linkage which is a type of covalent bond that joins a carbohydrate (sugar) molecule to another group, which may or may not be another carbohydrate. DISACCHARIDES  OLIGOSACCHARIDES  Oligosaccharides : (from the Greek olígos, "a few", and sácchar, "sugar") They consist of 3-10 monosaccharide units.  Is a saccharide polymer containing a small number (typically three to ten) of simple sugars (monosaccharides). Oligosaccharides can have many functions including cell recognition and cell binding. For example, glycolipids have an important role in the immune response.  3. POLYSACCHARIDES Condensation products of more than ten monosaccharide units  Simply known as glycans.  Consists of repeating units of monosaccharides(hundreds and thousands) held together by glycosidic bonds.  Linear and branched polymers. They are either linear α(1-4) linkage or branched α (1-6) linkage polymers.  They are primarily concerned with two important functions- structural and storage of energy.  Examples starch, glycogen, cellulose, dextran and inulin and chitin. Proteoglycans and glycosaminoglycans. STRUCTURE  TYPES OF POLYSACCHARIDES  Polysaccharides are often quite heterogeneous, containing slight modifications of the repeating unit. Simply known as glycans. Depending on the structure, these macromolecules can have distinctproperties from their monosaccharide building blocks. They may be amorphous or even insoluble in water.  (a) Homopolysaccharides : They are entirely made up of one type of monosaccharides. On hydrolysis, they yield only one kind of monosaccharide  E.g. starch, glycogen, cellulose, dextran and inulin and chitin.  (b) Heteropolysaccharides : They are made up of more than one type of monosaccharides. On hydrolysis they yield more than one type of monosaccharides.  E.g. Proteoglycans and glycosaminoglycans.  Two Multiple monomer monomer Unbranched Branched types, types, unbranche branched d  HOMOPOLYSACCHARIDE 1. STARCH. Starch is a polymer consisting of D glucose units. It contains two polysaccharide units, amylose and amylopectin. Amylose consists of long unbranched chains of Alpha D- glucose(50-5000 glucose units) connected by α(1-4) glycosidic linkages. Amylopectin consists of long chains of glucose up to 1 million glucose units connected by linear α(1-4) linkage and branched α (1-6) linkage glycosidic linkages, with branches every 24 to 30 glucose units along the chain. 2. DEXTRINS These are immediate products of hydrolysis of starch by acids or by the enzyme amylase. Consists of complex mixture of molecules of different sizes and structures. Present in the leaves of all starch producing plants. Sweet in taste.  HOMOPOLYSACCHARIDE 3. GLYCOGEN Also known as animal starch. Stored in muscle and liver (mostly). Present in cells as granules (high MW). Contains both linear α(1-4) linkage and branched α (1-6) linkage 4. CELLULOSE AND HEMICELLULOSE A polymer consisting of long, unbranched chains of D-glucose connected by β(1-4) glycosidic linkages, may contain from 15000 glucose units in one molecule. at every 12- 14glucose unit (more frequent than in starch).  HOMOPOLYSACCHARIDE 5. CHITIN Chitinis 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 Consists of N-acetyl-D-glucosamine units 6. INULIN β-(1,2)linked fructofuranoses linear only; no branching Lower molecular weight than starch and on hydrolysis yields fructose, sources include onions, garlic etc. Used as diagnostic agent for the evaluation of glomerular filtration rate (renal function test)  HETEROPOLYSACCHARIDES It is composed of a mixture of monosaccharides. On hydrolysis, they yield a mixture of monosaccharides. HETEROPOLYSACCHARIDES - are further classified into two types: A. NEUTRAL SUGARS. B. MUCOPOLYSACCHARIDES. A. NEUTRAL SUGARS. This group Includes some hemicellulose, some gums, mucilages & pectic substances. Give more than one type of sugar units on hydrolysis & sometimes non-sugar components. B. MUCOPOLYSACCHARIDES. Mucopolysaccharides are heteroglycans made up of polysaccharides made up of repeating units of sugar derivatives, namely amino sugars & uronic sugar. Commonly known as Glycosaminoglycans (GAG). Mucopolysaccharides are essential components of tissue structure. Some Mucopolysaccharides are found in combination with proteins to form Mucoproteins or mucoids or proteoglycans. Mucopolysaccharides include hyaluronic acid ,heparin, dermatan sulfate, keratan sulfate. 1.HYALURONATE (Hyaluronan) or Hyluronic acid. It is a glycosaminoglycan with a repeating disaccharide consisting of 2 glucose derivatives, glucuronate (glucuronic acid) & N-acetyl-glucosamine. The glycosidic linkages are ß (1-3) & ß (1-4). Hyaluronidase is an enzyme present in high concentration in testes, seminal fluid, in certain snake & insects that breaks ß(1-4) linkages. Present in skin, synovial fluid, seminal fluid it promotes fertilization of ovum. 2. KERATAN SULFATE It is a heterogeneous GAG with a variable sulfate content, besides small amounts of mannose, fructose, sialic acid etc. It is essentially consists of alternating units of D- galactosamine and N-acetylglucasamine. They are present in cartilage, bone, cornea, nail, hoofs, claws. 3. HEPARIN  Heparin is an anticoagulant (prevents blood clotting) that occurs in blood, lung, liver, kidney, spleen etc.  Commercial preparation of heparin is from animal lung tissues.  Made in mast cell & released into the blood.  Heparin is composed of alternating units of N-sulfo D- glucosamine 6-sulfate & glucoronate 2-sulfate, 4.CHONDROITIN SULFATE  Chondroitin sulfate is a sulfated glycosaminoglycan composed of a chain of alternating sugars. It is usually found attached to proteins as part of a proteoglycan.  It is present in ground substance of connective tissues widely distributed in cartilage, bone, tendons, cornea and skin.  ISOMERS /STEREOISOMERISM  The molecules that have the same molecular formula, but have a different arrangement of the atoms in space (different structures) Isomers. OR  Isomers are compounds with different physical and chemical properties but the same molecular formula.  For example, a molecule with the formula AB2C2, has two ways it can be drawn:  Examples of isomers: Glucose, Fructose, Galactose, Mannose Same chemical formula C6 H12 O6  Stereoisomers have identical molecular formulas and arrangements of atoms. They differ from each other only in the spatial orientation of groups in the molecule. The simplest forms of stereoisomers are cis and trans isomers, both of which are created by the restricted rotation about a double bond or ring system. Butene, C 4H 8, exists in both cis and trans forms.  EPIMERS  These are stereoisomer that differ in the position of the hydroxyl group at only one asymmetric carbon. (An asymmetric carbon atom (chiral carbon) is a carbon atom that is attached to four different types of atoms or groups of atoms. OR  EPIMERS are sugars that differ in configuration at ONLY 1 POSITION.  Examples of epimers : 1. D-glucose & D-galactose (epimeric at C4) 2. D-glucose & D-mannose (epimeric at C2) 3. D-idose & L-glucose (epimeric at C5)  ASYMMETRIC CARBON  A carbon linked to four different atoms or groups farthest from the carbonyl carbon  Also called Chiral carbon  The number of asymmetric carbon atoms (n) determines the possible isomers of a given compound which is equal to 2n.  Glucose contains 4 asymmetric carbons, and thus has 16 isomers 1 2  ENANTIOMERS Enantiomers are chiral molecules that are mirror images of one another. Furthermore, the molecules are non-superimposable on one another. This means that the molecules cannot be placed on top of one another and give the same molecule. Non- Superimposable COMPLETE mirror image (differ in configuration at EVERY CHIRAL CENTER.  MUTAROTATION  Unlike the other stereoisomeric forms, α and β anomers spontaneously interconvert in solution.  This is called mutarotation.  CYCLIZATION  Less then 1%of CHO exist in an open chain form.  Predominantly found in ring form.  involving reaction of C-5 OH group with the C-1 aldehyde group or C-2 of keto group.  Six membered ring structures are called Pyranoses.  Five membered ring structures are called Furanoses.  CYCLIC STRUCTURE FOR GLUCOSE STEP 2 Fold into a hexagon.  Bond the C5 –O– to C1.  Place the C6 group above the CH2OH 6 ring. O 5  Write the –OH groups on C2 and C4 below the ring. 4 1  Write the –OH group on C3 OH OH 3 2 OH above the ring.  Write a new –OH on C1. OH CYCLIC STRUCTURE FOR GLUCOSE (CONT) STEP 3 Write the new –OH on C1 down for the  form. up for the  form. CH2OH CH2OH  O O OH OH OH OH OH OH  OH OH -D-Glucose -D-Glucose  FISCHER PROJECTIONS A Fischer projection  is used to represent carbohydrates.  places the most oxidized group at the top.  shows chiral carbons as the intersection of vertical and horizontal lines. Copyright © 2005 by Pearson Education, Inc. Publishing as Benjamin Cummings  D AND L NOTATIONS In a Fischer projection, the −OH group on the  chiral carbon farthest from the carbonyl group determines an L or D isomer.  left is assigned the letter L for the L-form.  right is assigned the letter D for the D-form.  OPTICAL ACTIVITY  When a plane polarized light is passed through a solution containing monosaccharides the light will either be rotated towards right or left.  This rotation is because of the presence of asymmetric carbon atom.  If it is rotated towards left- levorotatory (-)  If it is rotated towards right- dextrorotatory(+)  GLYCOLYSIS Glycolysis is defined as sequence of reactions of glucose to lactate & pyruvate with the production of ATP. It is derived from greek word glycose -sweet or sugar, lysis- dissolution. Site: Cytosolic fraction of cell GYCOGENESIS Glycogenesis is defined as synthesis of glucose from glucose. Site: Cytosol of liver and muscle cells. Its an anabolic process so require ATP, UTP  GLUCONEOGENESIS Gluconeogenesis is a metabolic pathway that results in the generation of glucose from non-carbohydrate carbon substrates such as lactate, glycerol, and glucogenic amino acids. Site: Mainly occurs in Liver & kidney matrix in cytosol. GLYCOGENOLYSIS Glycogenolysis is the biochemical breakdown of glycogen to glucose whereas glycogenesis is the opposite, the formation of glycogen from glucose. Site: Glycogenolysis takes place in the cells of muscle and liver tissues in response to hormonal and neural signals.  SOME CLINICAL CONCEPTS  Dextrose (glucose in solution in dextrorotatory form) is frequently used in medical practice.  Fructose is abundantly found in the semen which is utilized by the sperms for energy. Several diseases are associated with carbohydrate's e.g., diabetes mellitus, glycogen storage diseases galactosemia.  Accumulation of sorbitol and dulcitol in the tissues may cause certain pathological conditions e.g. cataract, nephropathy.  The non-digestible carbohydrate cellulose plays a significant role in human nutrition.  The mucopolysaccharide hyaluronic acid serves as lubricant and shock absorbent in joints.  The mucopolysaccharide heparin is an anticoagulant( prevents blood clotting).  The survival of Antarctic fish below -2oC is attributed to the antifreeze glycoproteins.  Streptomycin is a glycoside employed in the treatment of tuberculosis