Carbohydrates.pdf

Carbohydrates Glucose The most important sugar in the human body. Known as dextrose, grape sugar, blood sugar. 100-120 mg/ 100ml normal blood level It is regulated by insulin and glucagon Hemiacetal (a molecule made up of a core carbon atom connected to four groups: –O...

Carbohydrates Glucose The most important sugar in the human body. Known as dextrose, grape sugar, blood sugar. 100-120 mg/ 100ml normal blood level It is regulated by insulin and glucagon Hemiacetal (a molecule made up of a core carbon atom connected to four groups: –OR, –OH, –R, and –H.) General formula of C6H12O6 Glucose has an aldehyde group and five hydroxyl groups. Glucose can form an intramolecular cyclic hemiacetal. Galactose Found in the disaccharide, lactose. Found in the cellular membranes of the brain and nervous system. Galactose is the C-4 epimer of glucose. Fructose Also known as levulose Found in honey, corn syrup, sweet fruits. Hemiketal (intermediates in the reaction between alcohols and aldehydes or ketones) Ketose or a ketone sugar Ribose Ribose and its related compound, deoxyribose, are the building blocks of the backbone chains in nucleic acids, better known as DNA and RNA. Ribose is used in RNA and deoxyribose is used in DNA. The deoxy-designation refers to the lack of an alcohol, -oh. Makes up RNA and coenzymes. OXIDATION REACTIONS Aldoses may be oxidized to 3 types of acids ▪ Aldonic acids: aldehyde group is converted to a carboxyl group (glucose – gluconic acid) ▪ Uronic acids: aldehyde is left intact and primary alcohol at the other end is oxidized to COOH Glucose ---glucuronic acid Galactose ---galacturonic acid ▪ Saccharic acids (glycaric acids) –oxidation at both ends of monosaccharide) Glucose ----saccharic acid Galactose ---mucic acid Mannose ---mannaric acid DISACCHARIDES Lactose Found in mammalian milk Made up of β-d-galactose and β -d-glucose Has β(1 4) linkage Lactose intolerance means the body cannot easily digest lactose, a type of natural sugar found in milk and dairy products. Maltose Also known as malt sugar A reducing sugar Linked thru α(1 4) linkage Sucrose Known as table sugar, cane sugar Made up of α–glucose and β–fructose Linked thru α1 → β2 glycosidic bond Chitin Chitin is the second most abundant carbohydrate 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) The molecule is derived from the condensation of two glucose molecules linked in a β(1→4) bond. It is derived from cellulose. POLYSACCHARIDES OR GLYCANS Types Homoglycans / homopolysaccharides (starch, cellulose, glycogen, inulin) Heteroglycans / heteropolysaccharides (gums, uronic acid, amino sugars, mucopolysaccharides) Characteristics Polymers (MW from 200,000) White and amorphous products(glossy) Not sweet Form colloidal solutions or suspensions Cellulose Polymer of β-d-glucose attached by β(1-->4)linkages Only digested and utilized by ruminants (cows, deers, giraffes, and 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 Products obtained from cellulose Microcrystalline cellulose: used as binder-disintegrant in tablets Methylcellulose: suspending agent and laxative Oxidized cellulose: hemostat Sodium carboxymethyl cellulose: laxative Cellulose acetate: rayon; photographic film; plastics Cellulose acetate phthalate: enteric coating Nitrocellulose: explosives. 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 A storage polysaccharide Starch is used as an excipient, a binder in medications to aid the formation of tablets. Industrially it has many applications such as in adhesives, paper making, biofuel, textiles Amylose Made of several d-glucose units. Linked by 1,4-glycosidic bonds. When iodine is added to starch, the color changes to dark blue or black due to the presence of amylose present in the starch. Amylose is soluble in water and can be hydrolyzed into glucose units by the enzymesα-amylase and β-amylase. Amylopectin A polymer of several d-glucose molecules. 80% of amylopectin is present in starch. Linked byα-1,4-glycosidic bonds andα-1,6-glycosidic bonds. When iodine is added to starch, it gives reddish-brown appearance due to the presence of amylopectin. It readily dissolves in hot water. On cooling, it forms a starch paste or starch gel. Glycogen Also known as animal starch Stored in muscle and liver(mostly) Present in cells as granules (high mw) Contains bothα(1,4)links and α(1,6)branchesatevery8to12glucoseunit(more frequent than in starch) Complete hydrolysis yields glucose Hydrolyzed by both α and β-amylases and by glycogen phosphorylase Glycoproteins and proteoglycans Glycoproteins are proteins conjugated to saccharides lacking a serial repeating unit In glycoprotein the protein>>>carbohydrate Examples include enzymes, immunoglobulins or antibodies, certain hormones In proteoglycans proteins are conjugated to polysaccharides with serial repeating units Here carbohydrate>>>protein Proteoglycans modulate cell processes and make cartilage flexible and resilient Glycosaminoglycans They are the polysaccharide chains of proteoglycans They are linked to the protein core via a serine or threonine(o-linked) The chains are linear(unbranched) The glycosaminoglycan chains are long (over 100 monosaccharides) They are composed of repeating disaccharides Heparin I and II A highly sulfated glycosaminoglycan is widely used as an injectable anticoagulant and has the highest negative charge density of any known biological molecule. The most common disaccharide unit is composed of a 2-o-sulfated iduronic acid and 6-o-sulfated, n-sulfated glucosamine, IdoA(2S)-GlcNS(6S) Uronic acid –sugar acids -α-d-glucuronic acid –found in liver cells and is bonded to hydrophobic steroid increasing solubility of steroids Amino sugars – N-acetylglucosamine –an important component of cell wall. N-acetyl galactosamine-component in blood antigens Chondroitin sulfate -component of cartilage and heparin Hyaluronic acid –abundant in the vitreous humour of the eye and joints. Carbohydrate reaction Carbohydrates are classified based on reducing property. Reducing sugars Carbohydrates that can reduce Tollens, Benedict’s or Fehling’s reagents are called reducing sugars (sugar with free aldehyde or ketone group). All monosaccharides and most of the disaccharides are reducing sugars. Some examples are maltose and lactose. Non-reducing sugars Carbohydrates that cannot reduce Tollens’, Benedict’s or Fehling’s reagents are called non-reducing sugars. Sucrose is a non-reducing sugar. Molisch’s test Monosaccharides when treated with concentrated sulfuric acid undergo dehydration with the removal of 3 molecules of water. Therefore, hexoses give hydroxymethyl furfural and pentoses give furfural. The furfural derivative can condense with phenolic compounds to give colored products. This forms the basis of Molisch’s test. It is a general test for carbohydrates. Molisch’s reagent is 10% alcoholic solution of α-naphthol. This is a common chemical test to detect the presence of carbohydrates. Carbohydrates undergo dehydration by sulfuric acid to form furfural (furfuraldehyde) that reacts with α- naphthol to form a violet-colored product. Fehling's Test This is an important test to detect the presence of reducing sugars. Fehling’s solution a is copper sulphate solution and Fehling’s solution b is potassium sodium tartrate. On heating, carbohydrate reduces deep blue solution of copper (ii) ions to red precipitate of insoluble copper oxide. Benedict's Reaction Benedict's reagent is very commonly employed to detect the presence of glucose in urine(glucosuria). It is a standard laboratory test employed for follow-up of diabetes mellitus. Benedict's reagent contains sodium carbonate, copper sulfate and sodium citrate. In alkaline medium, sugars form enediol, cupric ions are reduced, Correspondingly sugar is oxidized. Glucose is a reducing sugar. Any sugar with free aldehyde/keto group will reduce the Benedict’s Reagent. Therefore, this is not specific for glucose. Tollens’ Test Tollens’ Reagent is ammoniacal silver nitrate solution. On reacting with carbohydrate elemental silver is precipitating out of the solution, occasionally onto the inner surface of the reaction vessel. This produces silver mirror on the inner wall of the reaction vessel. Iodine Test Iodine test is used to detect the presence of starch. Iodine is not much soluble in water, so iodine solution is prepared by dissolving iodine in water in presence of potassium iodide. Iodine dissolved in an aqueous solution of potassium iodide reacts with starch to form a starch/iodine complex which gives characteristics blue black color to the reaction mixture.