Carbohydrates - Part II Lecture Notes PDF

Lecture NotesMC2 - Biochemistry CARBOHYDRATES PART II BIOLOGICALLY IMPORTANT MONOSACCHARIDES The monosaccharides a...

Lecture NotesMC2 - Biochemistry CARBOHYDRATES PART II BIOLOGICALLY IMPORTANT MONOSACCHARIDES The monosaccharides are carbohydrates which cannot be broken down by hydrolysis into simple sugars. They are derivatives of polyhydric alcohols. If one alcohol group in a polyhydric alcohol is oxidized to either an aldehyde or ketone, the result is simple sugar. 1. GLUCOSE is also called dextrose because it is dextrorotatory it is referred to as grape sugar because it is present in high concentration in grapes also known as "blood sugar", the immediate source of energy for cellular respiration Normally human blood contains 100mg of D-glucose per 100mL. In the disease known as diabetes the concentration of glucose in blood is increased. Since D-glucose is the sugar of the blood, it requires no digestive changes in order to be utilized by the body. 2. FRUCTOSE is a ketose sugar is considered as the sweetest sugar and the sweetness of many fruits is due to the presence of D-fructose and for this reason it is sometimes called fruit sugar it is also called levulose because of the fact that in solution it is strongly levorotatory 3. GALACTOSE is found in biological systems as component of the disaccharide lactose or milk sugar. This is the principal sugar found in the milk of most mammals. it is also found in glycolipids, fat-like substances that are components of the brain and nervous system BIOLOGICALLY IMPORTANT DISACCHARIDES The disaccharides are group of compound sugars composed of two monosaccharides linked together through the glycosidic bond with the loss of water. Glycosidic bond formation is nonspecific, that is, it can occur between a hemiacetal and hemiketal and any of the hydroxyl groups on the second monosaccharide. 1. MALTOSE French word for "malt"; a disaccharide containing two units of glucose; found in germinating grains, used to make beer is the most common reducing disaccharide it is found as constituent of starch and glycogen Page 1  Lecture NotesMC2 - Biochemistry β– maltose 2. LACTOSE Latin word for milk--"lact"; a disaccharide found in milk containing glucose and galactose it is the least sweet common sugars, only 1/6 as sweet as sucrose is used in high calcium diets and in infant foods it is found in urine of pregnant women β- lactose 3. SUCROSE is commonly called cane sugar or beet sugar since it is commercially prepared from sugar cane and sugar beets is composed of glucose and fructose through an -(1,2) -glycosidic bond it can be hydrolyzed by the enzyme invertase. The mixture of two sugars is called invert sugar is it dextrorotatory whereas the product is levorotatory Sucrose Page 2 POLYSACCHARIDES The polysaccharides are complex carbohydrates of high molecular weight composed of many monosaccharide units combined through the loss of molecules of water. The monomeric building blocks used to generate polysaccharides can be varied; in all cases, however, the predominant monosaccharide found in polysaccharides is D-glucose. When polysaccharides are composed of a single monosaccharide building block, they are termed homopolysaccharides. Polysaccharides composed of more than one type of monosaccharide are termed heteropolysaccharides. 1. STARCH composed of 80% – 85% amylopectin and 15% - 20% amylose which is sometimes called amylum, is the most important source of carbohydrates in the diet is the reserve foo in plants is a heterogeneous material composed of the glucose polymers amylase and amylopectin a complex carbohydrate without taste or odor, which is granular or powdery in physical form the main reserve material of plants. It provides almost 25% of total daily energy. Nourishment containing starch gives feeling of satiety and for long reduces hunger. The build of starch grains is diverse and characteristic for particular plants Starches are polymers of glucose. Two types are found: amylose consists of linear, unbranched chains of several hundred glucose residues (units). The glucose residues are linked by a glycosidic bond between their #1 and #4 carbon atoms. amylopectin differs from amylose in being highly branched. At approximately every thirtieth residue along the chain, a short side chain is attached by a glycosidic bond to the #6 carbon atom (the carbon above the ring). The total number of glucose residues in a molecule of amylopectin is several thousand. Page 3  2. GLYCOGEN is often called animal starch, is a glucose polysaccharide found in animal tissue and it is the storage of polysaccharide of animals it is stored in the in the liver and skeletal muscle reserve material of animal organisms and yeast. Muscle glycogen is mainly used to provide energy used for running of muscles. Glycogen in liver is used directly as a source of glucose for brain and erythrocytes. Liver cannot synthetize it. It is decomposed to glucose. 3. CELLULOSE is probably the single most abundant organic molecule in the biosphere. It is the major structural material of which plants are made. a major constituent of plant cell walls, consists of long linear chains of glucose, with ß (1-4) is not digested by a human. It belongs to composition of cell walls of plants. It is built of glucose bonded with 1,4-ß-glycosidal bonds. It removes constipation, counteracts formation of digestive tract tumors, lowers glucose level in blood, slows down body mass increment. IMPORTANT REACTIONS OF CARBOHYDRATES 1. OXIDATION Sugars may be classified as reducing or non-reducing based on their reactivity with Tollens', Benedict's or Fehling's reagents. If a sugar is oxidized by these reagents it is called reducing, since the oxidant (Ag(+) or Cu(+2)) is reduced in the reaction, as evidenced by formation of a silver mirror or precipitation of cuprous oxide. The Tollens' test is commonly used to detect aldehyde functions; and because of the facil interconversion of ketoses and aldoses under the basic conditions of this test, ketoses such as fructose also react and are classified as reducing sugars. Page 4  When the aldehyde function of an aldose is oxidized to a carboxylic acid the product is called an aldonic acid. Because of the 2º hydroxyl functions that are also present in these compounds, a mild oxidizing agent such as hypobromite must be used for this conversion (equation 1). If both ends of an aldose chain are oxidized to carboxylic acids the product is called an aldaric acid. By converting an aldose to its corresponding aldaric acid derivative, the ends of the chain become identical (this could also be accomplished by reducing the aldehyde to CH2OH, as noted below). Such an operation will disclose any latent symmetry in the remaining molecule. Thus, ribose, xylose, allose and galactose yield achiral aldaric acids which are, of course, not optically active. The ribose oxidation is shown in equation 2 below. 2. REDUCTION Sodium borohydride reduction of an aldose makes the ends of the resulting alditol chain identical, HOCH2(CHOH)nCH2OH, thereby accomplishing the same configurational change produced by oxidation to an aldaric acid. Thus, allitol and galactitol from reduction of allose and galactose are achiral, and altrose and talose are reduced to the same chiral alditol. A summary of these redox reactions and derivative nomenclature is given in the following table.  Derivatives of HOCH2(CHOH)nCHO HOBr Oxidation ——> HOCH2(CHOH)nCO2H an Aldonic Acid HNO3 Oxidation ——> H2OC(CHOH)nCO2H an Aldaric Acid NaBH4 Reduction ——> HOCH2(CHOH)nCH2OH an Alditol 3. OSAZONE FORMATION The osazone reaction was developed and used by Emil Fischer to identify aldose sugars differing in configuration only at the alpha-carbon. The upper equation shows the general form of the osazone reaction, which affects an alpha-carbon oxidation with formation of a bis-phenylhydrazone, known as an osazone. Application of the osazone reaction to D-glucose and D-mannose demonstrates that these compounds differ in configuration only at C-2. COMMON DISORDERS OF CARBOHYDRATES 1. DIABETES MELLITUS named because of its “sweet urine” Types of Diabetes: A. Type 1 diabetes (insulin-dependent diabetes) It occurs when the pancreas cannot produce insulin, a hormone essential for moving glucose from the blood into cells. Type 1 diabetes is an autoimmune disorder, in which the body makes antibodies that attack the insulin-producing cells in the pancreas. The cause is complex and unclear, but may involve genetics, viruses, diet,  chemicals and environmental factors. Type 1 diabetes was once called juvenile diabetes because it is usually diagnosed in childhood or early adulthood. People with type 1 diabetes must supply insulin by injection, pump or other methods. Possible treatments include transplant of a pancreas or beta cells. B. Type 2 diabetes (noninsulin dependent) It is the most common type of diabetes. It occurs when glucose builds up in the blood due to the body's inability to use insulin effectively. Insulin is a hormone that is essential to transport glucose, a sugar that is the body's primary fuel, into the cells. Type 2 diabetes was once called adult onset diabetes because it is usually diagnosed in adulthood. Excess fat, inactivity and poor eating habits contribute to type 2 diabetes. Sometimes the disease can be prevented or controlled through diet and exercise, but some patients need insulin or other medications. 2. LACTOSE INTOLERANCE The biochemical defect is characterized by a marked decrease in the level of lactase in the small intestine. Because this enzyme is at very low levels in the lactase- deficient patient, whatever lactose enters the system cannot be readily broken down into glucose and galactose. The disaccharide accumulates in the intestine and is metabolized by bacteria, causing a great deal of gastrointestinal discomfort, cramping, and diarrhea. Elimination of lactose from the diet by discontinuing or greatly decreasing the consumption of dairy products usually diminishes the symptoms significantly. 3. GALACTOSEMIA This disease is the result of the lack of an enzyme necessary for the conversion of galactose to glucose after monosaccharides have been absorbed into the bloodstream and have entered the cells. Because galactose accumulates in this disease, the patient develops cataracts and mental retardation at a very early age. 4. HYPOGLYCEMIA It is a condition characterized by low levels of blood glucose. In rare instances, this disorder is caused by a tumor of the pancreas which synthesizes high amounts of insulin and is not under feedback regulation. More frequently, the disorder is secondary to other diseases states or is a result of excessive insulin administration.