Carbohydrates - Lecture Notes (PDF)

Carbohydrates Structure & classification Dr.G.Krishnamoorthy,M.Sc.,M.Phil.,Ph.D Associate Professor Department of Medical Biochemistry Dembi Dollo University Definition Carbohydrates are hydrates of Carbon. H2O of C  Most abundant organic molecules in nature.  Empirical formula (...

Carbohydrates Structure & classification Dr.G.Krishnamoorthy,M.Sc.,M.Phil.,Ph.D Associate Professor Department of Medical Biochemistry Dembi Dollo University Definition Carbohydrates are hydrates of Carbon. H2O of C  Most abundant organic molecules in nature.  Empirical formula (C.H2O)n Carbohydrates may be defined as Polyhydroxyaldehydes or ketones or compounds which produce them on hydrolysis.  Importance of Carbohydrates 1. They are the most abundant dietary source of energy (4 Cal/g) for all organisms. 2. Carbohydrates are precursors for many organic compounds (fats, amino acids, kotone bodies, etc.). 3. Carbohydrates (as glycoproteins and glycolipids) participate in the structure of cell membrane and cellular functions such as cell growth, adhesion and fertilization. 4. They are structural components of many organisms. These include the fiber (cellulose) of plants, exoskeleton of some insects and the cell wall of microorganisms. 5. Carbohydrates also serve as the storage form of energy (glycogen) to meet the immediate energy Structure of Carbohydrates H O C CH2OH H C OH C O HO C H HO C H H C OH H C OH H C OH H C OH CH2OH CH2OH D-glucose D-fructose Classification of Carbohydrates Carbohydra tes Monosacchar Disaccharide Polysacchari Based ides Aldoses Ketoses Reducin s Non- desHeteropolysaccha Homopolysaccha on g reducin rides rides Number g Glycogen Hyaluronic acid of C atoms Maltose Sucrose Starch Chondroitin Trioses Glyceraldehyd DHA Lactose Trehalos Sulphate e e Cellulose Heparin Tetroses Erythrose Erythrulose Inulin Dermatan Pentoses Ribose Ribulose Sulphate Chitin Keratan Sulphate Hexoses Glucose, Fructose Galactose Heptose Glucoheptose Sedoheptul s ose Importance of few important monosaccharides Name of the Importance Carbohydrate s Glyceraldehyde Intermediate of glycolysis Erythrose Intermediate of HMP shunt Ribose Intermediate of HMP shunt and essential component of Nucleic acids Glucose Provides energy, produces amino acids, fatty acids, etc Galactose Essential component of Milk to produce lactate GLYCERALDEHYDE STEREOISOMERS  D & L ISOMERISM OF GLUCOSE Note: D sugars are naturally occurring sugars and body can metabolize only D sugars  OPTICAL ACTIVITY Dextrorotatory-d (+) : If the sugar solution turns the plane of polarized light to right. Levorotatory- l (–) : If the sugar solution turns the plane of polarized light to left. Note: These d & l forms should NOT be confused with the D and L (forms of glucose) DIFFERENT REPRESENTATIONS OF GLUCOSE STRUCTURE OPEN CHAIN HAWORTH PROJECTION FISCHER’S FORMULA FORMULA 1 2 3 4 5 6 Formation of ring structure The formation of these ring structures is the result of a general reaction between alcohols and aldehydes or ketones to form derivatives called hemiacetals or hemiketals, which contain an additional asymmetric carbon atom and thus can exist in two stereoisomeric forms. For example, D-glucose exists in solution as an intramolecular hemiacetal in which the free hydroxyl group at C-5 has reacted with the aldehydic C-1, rendering the latter carbon asymmetric and producing two stereoisomers. These six-membered ring compounds are called pyranoses because they resemble the six membered ring compound pyran. Anomers/anomeric carbon Isomeric forms of monosaccharides that differ only in their configuration about the hemiacetal or hemiketal carbon atom are called anomers. The hemiacetal (or carbonyl) carbon atom is called the anomeric carbon. So, for aldoses (like glucose), the 1st carbon is called anomeric carbon and for ketoses (like fructose) the 2nd carbon is called anomeric carbon Glucose is generally represented in the pyranose form α AND ß forms OF D-GLUCOSE α and ß ANOMERS OF D-GLUCOSE (representation in pyranose form) 1 CHO H C OH 2 HO C H D-glucose 3 H C OH (linear form) 4 H C OH 5 CH2OH 6 6 CH2OH 6 CH2OH 5 O 5 O H H H OH H H 4 H 1 4 H 1 OH OH OH OH OH H 3 2 3 2 H OH H OH -D-glucose -D-glucose Fructose is generally represented in the furanose form α and ß ANOMERS OF D-FRUCTOSE (representation in furanose form)  EPIMERISM Sugars are different from one another, only in configuration with regard to a single C atom (other than the reference C atom). Example: D-Glucose and D-Galactose (differ only in 4 th carbon atom) D-Glucose and D-Mannose (differ only in 2 nd carbon atom) Examples for epimers Mannose HO H H OH D-Mannose  Mutarotation When a plane polarized light is passed into an optically active solution , there is a change in specific rotation of this optically active compound in solution with time, to an equilibrium value. This is called mutarotation. Mutarotation (Example) When a plane polarized light is passed in to a fresh solution of alpha D glucose, its initial specific rotation of the plane polarized light is +112o But after 12-18 hours, it changes to +52.5o Similarly if the plane polarized light is passed into a fresh solution of beta D glucose, its initial specific rotation of the plane polarized light is 19o. But after sometime, it changes to a value of +52.5o. At equilibrium there was 36% of alpha D glucose available and 64% of beta D glucose available This change in rotation with time is called mutarotation. This is explained by the fact that D-glucose has two anomers, namely alpha and beta varieties. The anomers are produced by the spatial configuration with reference to the first carbon atom in glucose REACTIONS OF MONOSACCHARIDES Oxidation. Reduction. Dehydration. Formation of Esters Glycoside formation, etc. REDUCING PROPERTIES If a sugar has a free aldehyde or keto group at its anomeric carbon, then it is said to be a reducing sugar. Tests done to identify the reducing action of sugars include : ◦ Benedict’s test. ◦ Fehlings ◦ Barfoed’s test. ◦ Osazone test (confirmatory test to identify monosaccharides) Reduction is more efficient in alkaline medium than in acidic medium.  BENEDICT’S TEST: PRINCIPLE REAGENT: Na2CO3, CuSO4, Na citrate SUGAR ENEDIOL Cu++ CuSO4 OXIDISED REDUCED SUGAR ACID Cu+ Cu2O 2Cu(OH)2 Benedict’s test It is a simple test to find out the presence of reducing substances (glucose) in the urine. PROCEDURE : Take 2.5 ml of Benedict’s reagent and add 4 drops of urine heat it for few seconds One can see change of color from green to brick red which depends on the amount of glucose present in it. BENEDICT’S TEST water Benedict’s 0.5-1% 1- 1.5% 1.5– 2% Clinical applications of Benedict’s test  Benedict’s test is called semi- quantitative test because based on the concentration of the sugar in the given sample (urine), the colour produced varies  No sugar in urine (0%) – blue colour  Mild (0.5-1%) – green  Moderate (1-1.5%) – yellow  Severe (1.5 – 2%) – brick red precipitate Used to diagnose Diabetes mellitus Importance of Disaccharides Name of the Importance Carbohydrate s Maltose Component of malt Lactose Component of milk, provides glucose Sucrose (Non- Component of sugarcane reducing) Trehalose Present in plants and fungi MALTOSE Malt sugar. Two α-D-glucose units held together by α (14) glycosidic bond.  Also produced in the body during the course of digestion of starch by the enzyme pancreatic alpha amylase. LACTOSE β 1-4 linkage Present in milk. β-D-galactose & alpha-D- glucose units held together by β (14) glycosidic bond. Clinical aspect – Lactose intolerance Lactose is hydrolyzed in the body by the intestinal enzyme LACTASE to form glucose and galactose Defect in the enzyme Lactase leads to Lactose intolerance Lactose accumulates in the gut and takes more water into bowels by osmotic effect leading to DIARRHOEA  SUCROSE Cane sugar. α-D-glucose & β-D-fructose units held together by (α1 β2) glycosidic bond. Both the anomeric carbons of glucose & fructose are involved in glycosidic bond formation Hence Sucrose is called as non- reducing sugar  TREHALOSE  Trehalose is a disaccharide with two glucose units linked by alpha 1, 1 linkage  It is found in plants, fungi (Mushrooms) and some insects  It is a non-reducing sugar, as both the anomeric carbons of glucose are involved in glycosidic linkage  POLYSACCHARIDE Repeat units of monosaccharides or their derivatives held together by glycosidic bonds. Importance of some important Homopolysaccharides Name of the Importance Carbohydrate s Starch Stored carbohydrate nutrient in Rice (plant) Glycogen Stored carbohydrate nutrient in liver and muscle Cellulose Component of plant stem and other parts of plants, hence its considered as dietary fiber to humans not to animals  HOMOPOLYSACCHARIDES or HOMOGLYCANS ◦ Starch ◦ Glycogen ◦ Cellulose ◦ Inulin ◦ Dextrans ◦ Chitin STARCH Carbohydrate reserve of plants. Present in Cereals, Roots, Tuber, Vegetables. Consists of Amylose (water soluble) & Amylopectin (water insoluble). Starch Starch is the reserve carbohydrate of plants Sources: Potatoes, Tapioca, Rice, Wheat etc., Made of glucose units Starch consists of - Amylose - Amylopectin  AMYLOSE Long unbranched chain. 200 – 20,000 D-glucose units held together by α (14) glycosidic linkages. AMYLOSE  AMYLOPECTIN Branched chain. (α 16 glycosidic bonds at branches). 20 – 30 glucose units per branch. AMYLOPECTIN Test for starch Iodine test: Starch reacts with Iodine solution to form blue colored complex This test is used to identify starch in the laboratory In the body During digestion, Starch is acted upon by the enzyme salivary Alpha amylase (in the mouth and in the intestine) to form dextrins These dextrins are further hydrolysed by pancreatic alpha amylase in the intestine to form maltose, which is further cleaved to glucose units  GLYCOGEN Reserve carbohydrate in animals. Stored in liver and muscle Made of glucose units with both alpha 1, 4 and alpha 1, 6 linkage More branched and compact than amylopectin. Every 11th sugar molecule has a branch. Synthesized in the body by a process called as GLYCOGENESIS GLYCOGEN STRUCTURE  CELLULOSE Chief carbohydrate in plants. Made up of glucose units combined with cellobiose bridges (beta 1, 4 linkage). No branching point. Cannot be digested by human due to absence of Cellobiase. Nutritional importance of cellulose in human beings -Cellulose is a component of FIBRE (non-digestable carbohydrate or unavailable carbohydrate) -These fibre rich foods (vegetables & fruits) can prevent number of diseases like diabetes, cardiovascular diseases etc., -It increases the bulk of stools, and helps to prevent constipation INULIN Found in onion, garlic etc. Inulin is made up of D- fructose units with repeating ß-1,2 linkages. Clinical importance of inulin Inulin acts as a marker for Kidney function (glomerular filtration rate) as it is not significantly metabolized by the body but filtered and excreted completely by glomerulus of the kidney. Hence inulin is used in renal function tests to assess the function of kidney  CHITIN Chitin is found in crustaceans eg.lobsters,crabs,shrimps,insects. Composed of N-acetyl glucosamine units joined by ß-1,4 glycosidic linkages. Importance of Heteropolysaccharides Name of the Importance Carbohydrate s Hyaluronic acid Serves as a lubricant, shock absorber, promotes wound healing Chondroitin Helps to maintain the structure and shapes of Sulphate tissues Heparin Acts as an anticoagulant Dermatan Maintains the shapes of tissues Sulphate Keratan Sulphate Corneal transparency, nervous system development, embryonic development, wound healing, etc.  HETEROPOLYSACCHARI DES Mucopolysaccharides or Glycosaminoglycans (GAGs)  Hyaluronic acid  Heparin  Chondroitin sulphate  Dermatan sulphate  Keratan sulphate MUCOPOLYSACCHARID ES Polysaccharides that are composed not only of a mixture of simple sugars but also of derivatives of sugars such as amino sugars and uronic sugars are called mucopolysaccharides. Also known as glycosaminoglycans (GAGs) Most of these act as structural support material for connective tissue or mucous substances of the body. They serve both as a lubricant and a cementing substance.  HYALURONIC ACID Present in connective tissues, tendons, synovial fluid and vitreous humor of eye. Composed of repeating units of D- glucuronic acid and N-acetyl glucosamine s(beta 1,3-beta 1,4 linkage) Hyaluronidase, an enzyme secreted by some pathogenic bacteria, can hydrolyze the glycosidic linkages of hyaluronate, rendering tissues more susceptible to bacterial invasion. HYALURONIC ACID  Heparin Heparin is a natural anticoagulant made in mast cells (a type of leukocyte) and released into the blood, where it inhibits blood coagulation by binding to the protein antithrombin. Present in lung, spleen and monocytes. Contains repeating units of Glucosamine sulphate and L-Iduronate sulphate (α-1,4 linkage) Purified heparin is routinely added to blood samples obtained for clinical analysis, and to blood donated for transfusion, to prevent clotting. HEPARIN  CHONDROITIN SULPHATE Present in ground substances of connective tissues of cartilages, bones & tendons. Composed of D-Glucuronic acid and N-acetyl-D- galactosamine sulphate (Beta 1,3 , Beta 1,4 linkage) DERMATAN SULPHATE Found in skin, blood vessels & heart vessels. Contains L-iduronic acid and N-acetyl galactosamine sulphate (alpha 1,3 and beta 1,4 linkage) KERATAN SULPHATE Only GAG not having Uronic acid. Found in cornea and tendons. Repeating units are D-Galactose & N-acetyl galactosamine sulphate (beta 1,4 and beta 1,3 linkage) Clinical aspect - Mucopolysaccharidoses  Mucopolysaccharidoses are a group of inborn errors of metabolism characterised by excessive intra lysosomal accumulation of GAGs in various tissues.  This is due to the lack of enzymes in lysosomes that degrade GAGs  Example: Hurler’s syndrome – lack of enzyme α -L-Iduronidase - leads to the accumulation of heparin sulphate and dematan sulphate Proteoglycans & Glycoproteins PROTEOGLYCANS GLYCOPROTEINS 1. Heavily glycosylated 1. Less glycosylated (contains more carbohydrate units attached) 2. Core protein attached to 2. Protein attached to carbohydrate residues oligosaccharide chain, especially glycosaminoglycans glycosaminoglycans are not usually present 3. Found in Extracellular matrix 3.Found on cell membranes attached to integral membrane proteins and in other proteins 4. Eg. Decorin – found in skin & 4. Eg. a.Mucin – secreted in blood vessels, contains mucus of respiratory & dermatan sulphate and digestive tracts chondroitin sulphate residues b. Antibodies or immunoglobulins Structure of a proteoglycan Glycoprotein Thank you