Carbohydrates PDF

Carbohydrates Dr: Israa Burhan Carbohydrates Carbohydrates are widely distributed in plants and animals; they have important structural and metabolic roles. In plants, glucose is synthesized from carbon dioxide and water by photosynthesis and stored as starch...

Carbohydrates Dr: Israa Burhan Carbohydrates Carbohydrates are widely distributed in plants and animals; they have important structural and metabolic roles. In plants, glucose is synthesized from carbon dioxide and water by photosynthesis and stored as starch or used to synthesize cellulose of the plant framework. Animals can synthesize carbohydrate from glycerol, fatty acids and amino acids. Glucose is the most important carbohydrate; most dietary carbohydrate is absorbed into the bloodstream as glucose, and other sugars are converted into glucose in the liver. Glucose is the major metabolic fuel of mammals (except ruminants) and a universal fuel of the fetus. Diseases associated with carbohydrate metabolism include diabetes mellitus, galactosemia, glycogen storage diseases, and lactose intolerance. Carbohydrates have a wide range of functions. The following are few of them: Source of energy for living like glucose Storage form of energy like glycogen in animal tissue and starch in plants Serve as structural component like glycosaminoglycans in humans, cellulose in plants and chitin in insects Non-digestible carbohydrates like cellulose serve as dietary fibers Constituent of nucleic acids RNA and DNA like ribose and deoxyribose sugar Play a role in immunity Carbohydrates are also involved in detoxification like glucuronic acid. Carbohydrates classified as: 1-Monosaccharides Most of carbohydrates are present with a cyclic structure in nature, as a consequence of internal linkages between the carbonyl carbone (of the aldehyde or ketone group) with one of the hydroxyl groups in the same molecule. carbohydrates cannot hydrolyzed into simpler carbohydrates: They classified as trioses, tetroses, pentoses, hexoses, or heptoses, depending upon the number of carbon atoms; and as aldoses or ketoses depending upon whether they have an aldehyde or ketone group as shown in figure : 1 Carbohydrates Dr: Israa Burhan Linear and cyclic structure of carbohydrate Classification of carbohydrate (aldosis) 2 Carbohydrates Dr: Israa Burhan Classification of carbohydrate (ketosis) 2-Disaccharides Disaccharides are condensation products of two monosaccharide units. Examples are maltose (glucose+ glucose) and sucrose (glucose+ fructose) and lactose (glucose+ galactose): 3 Carbohydrates Dr: Israa Burhan Maltose Lactose and sucrose 3-Oligosaccharides Oligosaccharides consist of a short chain of monosaccharide units (3 to 10 units), joined together by a characteristic bond called glycosidic bond which, on hydrolysis, gives two to ten molecules of simple sugar maltotriose is an example. 4 Carbohydrates Dr: Israa Burhan 4-Polysaccharides are condensation products of more than ten monosaccharide units; include Homopolysaccharides (Homoglycans) When a polysaccharide is made up of several units of one and the same type of monosaccharide unit, it is called homopolysaccharide.The most common homoglycans are: Starch, Dextrins, Glycogen, Inulin, Cellulose. Some Homopolysaccharides serve as a storage form of monosaccharides used as fuel, e.g. starch and glycogen, while others serve as structural elements in plants, e.g. cellulose. Heteropolysaccharides (Heteroglycans).They contain two or more different types of monosaccharide units or their derivatives. Heteropolysaccharide present in human beings is glycosaminoglycans (mucopolysaccharides), e.g.Heparin, Chondritin sulfate, Hyaluronic acid, Dermatan sulfate, Keratan sulfate, Blood group polysaccharides : a-Starch Starch contains two types of glucose polymer, amylose and amylopectin. The former consists of long, unbranched chains of D-glucose residues connected by (α 1-4) linkages. Such chains vary in molecular weight from a few thousand to more than a million. Amylopectin also has a high molecular weight (up to 100 million) but unlike amylose is highly branched. The glycosidic linkages joining successive glucose residues in amylopectin chains are (α 1-4); the branch points (occurring every 24 to 30 residues) are (α 1-6) linkages that amylase consist of (15–20 %) of starch where amylopectin consist of (80–85%), Starch 5 Carbohydrates Dr: Israa Burhan b-Glycogen Glycogen is the main storage polysaccharide of animal cells. Like amylopectin, glycogen is a polymer of (α1-4)-linked subunits of glucose, with (α1-6)-linked branches, but glycogen is more extensively branched (on average, every 8 to 12 residues) and more compact than starch. Glycogen is especially abundant in the liver where it may constitute as much as 7% of the wet weight; it is also present in skeletal muscle. In hepatocytes glycogen is found in large granules. Such glycogen granules also contain, in tightly bound form, that enzymes responsible of synthesis and degradation of glycogen. Because each branch in glycogen ends with a non reducing sugar unit, a glycogen molecule has as many non reducing ends as it has branches, but only one reducing end. When glycogen is used as an energy source, glucose units are removed one at a time from the nonreducing ends. Degradative enzymes that act only at nonreducing ends can work simultaneously on the many branches, speeding the conversion of the polymer to monosaccharides. Glycogen c-Inulin Inulin is a polysaccharide of fructose β (2-1) found in tubers and roots of dahlias, artichokes, and dandelions. It is readily soluble in water and used to help measure kidney function by determining the glomerular filtration rate (GFR). In general, plant inulins contain between 20 and several thousand fructose units. 6 Carbohydrates Dr: Israa Burhan d-Dextrins Dextrins are bacterial and yeast polysaccharides made up of (α 1- 6)- linked poly- D-glucose; all have (α 1-3) branches, and some also have (α 1-2) or (α1-4) branches. Dextrins are white, yellow, or brown powders that are partially or fully water-soluble. e-Cellulose is an organic compound consisting of a linear chain of several hundred to many thousands of β (1→4) linked D- glucose units. Cellulose is an important structural component of the primary cell wall of green plants. Cellulose cannot be digested by mammals because of the absence of an enzyme that hydrolyzes the linkage. It is an important source of “bulk” in the diet. Microorganisms in the gut of ruminants and other herbivores can hydrolyze the linkage and ferment the products to short-chain fatty acids as a major energy source. There is limited bacterial metabolism of cellulose in the human colon. f-Chitin It consists of N-acetyl-D-glucosamine units joined by (1 →4)-glycosidic linkages. The only chemical difference from cellulose is the replacement of the hydroxyl group at C-2 with an acetylated amino group responsible for hard structure. Chitin forms extended fibers similar to those of cellulose, and like cellulose cannot be digested by vertebrates. Chitin is the principal component of the hard exoskeletons of nearly a million species of arthropods insects, lobsters, and crabs, for example and is probably the second most abundant polysaccharide, next to cellulose, in nature 7 Carbohydrates Dr: Israa Burhan Chitin Heteropolysaccharides or Heteroglycans Glycoprotein (Mucoprotein) One component of which is always an aminosugar (hence the name glycosaminoglycans), either D-glucosamine or D galactosamine. except in the case of keratan sulfate) is a uronic acid, either L-glucuronic acid or its epimer L-iduronic acid. 8 Carbohydrates Dr: Israa Burhan Many Monosaccharides are Physiologically Important Derivatives of trioses, tetroses, and pentoses and of the seven carbon sugar sedoheptulose, are formed as metabolic intermediates in glycolysis and the pentose phosphate pathway.Pentoses are important in nucleotides, nucleic acids, and several coenzymes. Glucose, galactose, fructose, and mannose are physiologically the most important hexoses.. The physiologically important disaccharides are maltose, sucrose, and lactose as shawn in Tables. Physiologically important of Pentoses Physiologically important of Hexoses 9 Carbohydrates Dr: Israa Burhan Physiologically important of Disaccharides Some characterization of sugar: 1-α and β anomers Reaction between the aldehyde group or ketone groups at C-1 or C-2 and the OH of carbon in open chain forms a hemiacetal or hemiketal linkage, producing either of two stereoisomers, the α and β anomers, which differ only in the stereochemistry around the hemiacetal carbon. The interconversion of α and β anomers is called mutarotation. Oxidation of the anomeric carbon of glucose and other sugars is the basis for Fehling’s reaction called reducing sugar Formation of the two cyclic forms of D-glucose. 10 Carbohydrates Dr: Israa Burhan 2-D and L isomerism: D and L designations are based on the configuration the single asymmetric carbon (chiral carbon): is a carbon atom that is attached to four different types of atoms or four different groups of atoms) the farthest from the aldehyde or keto group. The L and D forms of this sugar, and of glucose. The orientation of the H and OH groups around the carbon atom adjacent (carbon 5 in glucose) chiral carbon determines whether the sugar belongs to the D or L series. When the OH group on this carbon is on the right the sugar is the D- isomer; when it is on the left, it is the L-isomer. Most of the monosaccharides occurring in mammals are D sugars, and the enzymes responsible for their metabolism are specific for this configuration. In solution, glucose is dextrorotatory hence the alternative name dextrose, often used in clinical practice that the presence of asymmetric carbon atoms also confers optical activity on the compound. When a beam of plane-polarized light is passed through a solution of an optical isomer, it will be rotated either to the right, dextrorotatory (+); or to the left, levorotatory (−). 11 Carbohydrates Dr: Israa Burhan 3-Pyranose and furanose ring structures: The stable ring structures of monosaccharides are similar to the ring structures of either pyran (a six membered ring) or furan a five-membered ring. For glucose in solution, more than 99% is in the pyranose form. The pyranose forms of D-glucose and the furanose forms of D-fructose are shown here as Haworth perspective formulas. 4-Epimers: Isomers differing as a result of variations in configuration of the OH and H on carbon atoms 2, 3, and 4 of glucose are known as epimers. Biologically, the most important epimers of glucose are mannose and galactose, formed by epimerization at carbons 2 and 4, respectively. 12 Carbohydrates Dr: Israa Burhan Glycosidic bond A glycosidic bond is formed between the hemiacetal or hemiketal group of saccharide (or a molecule derived from a saccharide) and the hydroxyl group of some compound such as an alcohol. Glycosidic bonds of the form discussed above are known as O-glycosidic bonds, If the hemiacetal portion is glucose, the resulting compound is a glucoside; if galactose, a galactoside; and so on. If the second group is an amine, an N-glycosidic bond is formed, for example, between adenine and ribose in nucleotides such as ATP. The glycosides that are important in medicine because of their action on the heart (cardiac glycosides). These include derivatives of digitalis and strophanthus such as ouabain, an inhibitor of the Na+-K+ ATPase of cell membranes. Other glycosides include antibiotics such as streptomycin. Deoxy Sugars Lack an Oxygen Atom Deoxy sugars are those in which a hydroxyl group has been replaced by hydrogen. An example is deoxyribose in DNA. deoxyribose are important in nucleotides 13 Carbohydrates Dr: Israa Burhan Oxidation (Sugar Acid Formation) When aldoses oxidize under proper conditions they may form: –Aldonic acid –Saccharic acids –Uronic acid. Oxidation of an aldose with hypobromous acid (HOBr), which acts as an oxidizing agent gives aldonic acid. Thus, glucose is oxidized to gluconic acid. Oxidation of aldoses with nitric acid under proper conditions convert both aldehyde and terminal primary alcohol groups to carboxyl groups, forming saccharic acid. When an aldose is oxidized in such a way that the terminal primary alcohol group is converted to carboxyl without oxidation of the aldehyde group (usually by specific enzymes), a uronic acid is formed Reduction to Form Sugar Alcohol Both aldoses and ketoses may be reduced by enzymes or non enzymatic to the corresponding polyhydroxy alcohols. The alcohols formed from glucose, mannose, fructose and galactose Manitol, the sugar alcohol derived from mannose, is frequently used medically as an osmotic diuretic to reduce cerebral edema. Sorbitol, the sugar alcohol derived from glucose, often accumulates in the lenses of diabetics and produces cataracts. 14