Biochemistry of Dietary, Functional, and Structural Carbohydrates I, II PDF

Biochemistry of Dietary, Functional and Structural Carbohydrates I, II Objectives Identify different classes of carbohydrates, and list their biological importance. Define chemistry and functions of monosaccharides, and oligosaccharides and their derivatives Define and classify polysac...

Biochemistry of Dietary, Functional and Structural Carbohydrates I, II Objectives Identify different classes of carbohydrates, and list their biological importance. Define chemistry and functions of monosaccharides, and oligosaccharides and their derivatives Define and classify polysaccharides. Discuss structure and function medically important homo and hetero-polysaccharides. Definition of carbohydrates: Carbohydrates are aldehyde (CHO) or ketone (C=O) derivatives of polyhydric alcohols (have more than one OH group) or compounds which yield these derivatives on hydrolysis. Importance of carbohydrates: 1. The chief source of energy and form 60% of human diet. 2. Important structural components in human cells, nucleic acids. 3. Synthesis of antigens e.g. blood group substance. 4. Forms a part of hormones, their receptors & immunoglobulins. 5. Hyalouronic acid (polysaccharide) is important for joint lubrication.  CHO CHO H C OH HO C H HO C H H C OH H C OH HO C H H C OH HO C H CH2OH CH2OH D-Glucose L-Glucose  Classification of carbohydrates According to the number of sugar units in the molecule there are three types: 1.Monosaccharides (simple sugars): They contain one sugar unit, i.e., and the simplest form of sugars. 2.Oligosaccharides: They contain 2 –10 monosaccharide units per molecule. 3. Polysaccharides: They contain more than 10 monosaccharide units per molecule.  I. Monosaccharides - They are classified according to the number of carbon atoms into: Trioses: with three carbons Tetroses: with four carbons Pentoses: with five carbons Hexoses: with six carbons - Each of these groups is subdivided according to the type of functional chemical group into: Aldoses (sugars containing aldehyde group) Ketoses (sugars containing ketone group). Medical significance of some Monosaccharides Ribose: is structural component in nucleic acids & free nucleotides. Glucose (grape sugar, blood sugar or dextrose). Other hexoses have to be converted into glucose to be utilized in the body. It appears in urine in diabetes mellitus. Mannose: is a subunit in glycoproteins and sialic acid that enters in the structure of gangliosides. Galactose: is the subunit of the milk sugar lactose. It enters in structure of glycolipids and glycoproteins. Fructose (semen sugar, fruit sugar, levulose): It is the sweetest sugar known. It is the main sugar in bee’s honey. Its metabolism is not affected in diabetic patients , so it is better than glucose in diabetes mellitus.  Sugar derivatives of monosaccharides with Medical importance: 1.Amino sugars or sugaramines (Replace hydroxyl group at C2 by an amino group). 2.Deoxy sugars (Replace hydroxyl group at C2 or C3 or C6 by hydrogen atom ). 3.Sugar acids (product of oxidation of sugars). 4. Sugar alcohols (product of reduction of sugars). 1.Amino sugars (Sugaramines): - Replace OH group on C2 by an amino group. 1. Glucosamine (Chitosamine): It enters in the structure of some mucopolysaccharides. 2. Galactosamine (Chondrosamine): It enters in the structure of the sulfate-containing mucopolysaccharides. 3. Mannosamine: It enters in the structure of antibiotics and important for their activity e.g., erythromycin. 4. Sialic acid: It enters in structure of glycolipids and Glycoproteins. 2. Deoxysugars: 1. Deoxyribose (Deoxy C2). It enters in structure of DNA. 2. L-Fucose (Deoxy C6 from L-Galactose) & L-Rhamnose (Deoxy C6 from L-Mannose). They enter in the structure of glycoproteins of blood group substance. 3. Sialic acid (Deoxy C3). It is an amino sugar since it contains NH2 group, a deoxy sugar since it contains two hydrogen atoms at C3 a sugar acid since it contains COOH group It enters in structure of glycolipids and glycoproteins. 3. Sugar acids 1. Gluconic acid used as Ca2+ gluconate for intravenous supplementation of calcium for slow dissociation. 2. L-ascorbic acid (vitamin C) is synthesized from glucose in plants and animals except human, primates and guinea pig. It is an antioxidant. 3. Sialic acid 4. Glucuronic acid: It is synthesized in the liver and important in. 1. Glucuronic acid enters in the structure of mucopolysaccharides e.g. Hyalouronic acid. 2. Detoxication by conjugation of certain drugs and toxins and help excretion of bilirubin and steroid hormones. 3. L-iduronic acid is an isomer of D-glucuronic acid and it enters in structure of mucopolysaccharides e.g. Heparin 4. Sugar alcohols 1-Glyceraldehyde reduction gives glycerol that enters in structure of lipids, creams and explosives. 2- Ribose reduction gives Ribitol that is a part of the structure of vitamin B2 (Riboflavin). 3- Glucose reduction gives sorbitol that enters in medical industries. In diabetes, as a result of uncontrolled diabetes, it is formed insides some cells and it can not diffuse out of cells and its accumulates in cells, causes osmotic damage to cells as in cataracts and neuropathy. 4- Mannose reduction gives Mannitol which is injected intravenously to reduce intracranial hypertension. 5- Inositol: - It is a sugar alcohol of 6 OH groups - It presents in high concentration in bran and it combines with 6 molecules of phosphoric acid to give phytic acid. Phytic acid forms insoluble iron, calcium and magnesium salts (phytate) which hinder absorption of Ca2+, Mg2+ and iron in the intestine. - It presents in high concentration in heart and muscles tissues, so it is called muscle sugar - It is considered a member of vitamin B complex because it is essential for synthesis of phospholipids, phosphatidylinositol.  II. Oligosaccharides A. Disaccharides 1- Reducing Disaccharides: It has a free aldehyde group (anomeric carbon)  Maltose  Lactose 2. Non-reducing Disaccharides: It has no free aldehyde group (anomeric carbon)  Sucrose 1- Reducing Disaccharides: A) Maltose (malt sugar): It consists of 2 -glucose units linked by -1,4-glucosidic linkage, It is a reducing disaccharide. It gives osazone called maltosazone (Rosette shaped). It is produced during digestion of starch by amylase enzyme. It is hydrolyzed in human intestine by maltase enzyme.  B) Lactose (milk sugar) : - It is formed of -galactose and -glucose linked by -1,4-glucosidic linkage. - it a reducing disaccharide, forms lactosazone that has sun ray appearance. - It is digestible by lactase into glucose and galactose. - It is excreted physiologically in urine of pregnant and lactating females (lactosuria) and must be differentiated from pathological glucosuria caused by diabetes mellitus by: 1. Osazone test. 2. Barfoed's test. 3. Specific enzymatic reaction such as glucose oxidase.  It is the most suitable sugar for baby feeding as a sweetener for milk because: 1. It is the least sweet sugar so that the baby can nurse a large amount of mother’s milk without getting his appetite lost. 2. It is non-fermentable sugar, so it does not form gases and not cause colic to the infant. 3. It has a laxative effect and prevents constipation. 4. Unabsorbed sugar is used as a food for large intestinal bacteria that form a number of vitamins that benefits the baby. 5. It is easily digested and helps absorption of the minerals of milk. 2. Non-reducing Disaccharides: Sucrose (Cane or Table sugar): It is table sugar and sugar of cane and molasses. It is formed of -glucose linked to -fructose by --1,2-linkage. The 2 anomeric carbons (C1 of glucose and C2 of fructose) are involved in the linkage, therefore it is: 1. Non-reducing sugar. 2. Non osazone forming. It is hydrolyzed by sucrase enzyme into equal amounts of glucose and fructose.  III- Polysaccharides: Definition: They contain more than 10 monosaccharide units per molecule. Classification: - They are classified into: 1-Homopolysaccharides They produce only one type of monosaccharides on hydrolysis 2-Heteropolysaccharides They produce several types of sugars on hydrolysis. 1. Homopolysaccharides - They are named according to the type of that monosaccharide - Pentosans  Pentoses - Hexosans  Hexoses Hexosans 1. Glucosans: Starch Dextrins Dextran Glycogen Cellulose 2.Fructosans: Inulin 3. Galactosans: Agar-Agar 4. N-acetyl-glucosans: Chitin  Comparison between Starch, Glycogen and Cellulose: Starch Glycogen Cellulose 1. Nature: Stored form of carbohydrate in Stored form of Structural form of plants. The core is amylose carbohydrates in animals. carbohydrate in plant cells. (20%) and the shell is amylopectin (80%). 2. Source: Cereals, and tubers, e.g., potatoes. Muscles and liver Linen and cotton 3. Solubility: Amylose is water soluble Water soluble. Water insoluble. amylopectin is insoluble. 4. Nature of the chains: Amylose is helical straight chain (- Branched chain similar Straight chain (large number glucose units linked by -1,4- to amylopectin but its of -glucose units linked by glucosidic bonds). trees are shorter and have -1,4- glucosidic bonds). Amylopectin is branched chain (- more branches than glucose units linked by -1,4- and - amylopectin tree. 1,6-glucosidic bonds). 5. Digestibility: Digestible by amylase Non-digestible but HCl Is hydrolyzed by HCl or amylase into into dextrins and hydrolysis gives cellobiose. dextrins and maltose. maltose.  Cellulose It is indigestible in humans because of lack of enyzmes that hydrolyze - glycosidic linkage, but it is very essential in human food because: 1. Prevention of constipation by increasing the bulk of stools. 2. Its fermentation by intestinal bacteria give some water soluble vitamins and volatile fatty acids that is anticancer for colon cells. 3. It adsorbs toxins present in foods and prevents its absorption. 4. It adsorbs cholesterol present in foods and decrease its absorption. 5. It increases gastric emptying time, and so prevents rapid rise of blood glucose level (This is beneficial for dieters and diabetics). 6. It prevents cancer colon (volatile fatty acids produced from its fermentation, decrease contact time by stimulating prestalsis and by adsorbing carcinogens on its surface).  Dextran It is synthesized by certain bacteria. It has a great biochemical importance. 1. It is used as plasma substitute to restore blood pressure in cases of shock. 2. Combination of dextran with iron used for treatment of iron deficiency anemia by injection. 3. Sodium dextran sulfate is an anticoagulant.  2. Heteropolysaccharides They are polysaccharides that on hydrolysis produce several types of sugars. I. Non-nitrogenous heteropolysaccharides: II. Nitrogenous heteropolysaccharides: Contain sugar amines A) Neutral nitrogenous (Glycoproteins): Does not contain uronic acids B) Acidic nitrogenous (Mucopolysaccharides, Glycosaminoglycans & Proteoglycans) Contain uronic acids  Sulfur-free: Hyaluronic acid  Sulfur-containing: Chondroitin sulfate, Heparin.  A- Neutral Nitrogenous Heteropolysaccharides: (Glycoproteins) - They do not contain uronic acids or sulfate groups. - They are formed of a large protein core and smaller branched chains of carbohydrate. B- Acidic Nitrogenous Heteropolysaccharides: (Mucopolysaccharides) conjugated form free soluble form with proteins (hyaluronic acid) (Proteoglycans) Proteoglycans: (mucopolysaccharides + proteins). They consist of very small core protein molecule and a huge carbohydrate tree that is highly complex in the form of repeating disaccharide units. Acidic Nitrogenous Heteropolysaccharides: Sulfur free Mucopolysaccharides Hyaluronic acid It is formed of N-acetyl glucosamine linked to glucuronic acid. It is formed of 400 - 4000 repeating disaccharide units. Hyaluronic acid is unique among the GAGs in that: 1- It is un-sulfated 2- It is not found covalently attached to proteins as a proteoglycan 3- Only GAG present both in animals and bacteria Biochemical importance: 1. The molecule is coiled and entwined making a very firm gel which prevents bacterial invasion of the skin. 2. It is present in connective tissue matrix, vitreous humor of the eye, in the skin, synovial fluid, around the ovum, and in the umbilical cord to preserve the full-form of these structures. 3. Hyaluronic acid imbibes water and forms a incompressible substance due to the presence of several OH groups causing repulsion between carbohydrate units enabling the molecule to perform its function as lubrication in joint synovial fluids. Hyaluronidase Enzyme or Spreading factor: It is the enzyme that hydrolyzes hyaluronic acid. It is present in sperms to help penetration of the ovum and fertilization. It is present in some virulent strains of bacteria that are able spread through infected wounds.. It is present also in snake and scorpion venom. It is used in medicine to treat fibrosis and to dissolve mucus. Acidic Nitrogenous Heteropolysaccharides: Sulfated Mucopolysaccharides Chondroitin Sulfate  It is formed of sulfated N-acetyl-galactosamine and glucuronic acid or iduronic acid. Present in cornea of the eye, tendons, ligaments, bones, cartilage and connective tissue matrix. They absorb water, form incompressible substances by means of their ionizable OH and sulfate groups, creating negative charges leading to repulsion between the molecules. Acidic Nitrogenous Heteropolysaccharides: Sulfated Mucopolysaccharides Heparin It is formed of a long repeat of sulfated -glucosamine and sulfated iduronic acid. Biomedical significance: 1. It is an anticoagulant produced by mast cells and prevents intravascular clotting. Therefore, it is used in cases of increased coagulability, e.g deep venous thrombosis. 2. It binds to and liberates lipoprotein lipase enzyme to clear the turbid plasma from the absorbed lipids after meals. 3. It participates in cell-cell and cell-matrix interaction, cell proliferation, apoptosis and differentiation.