Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Carbohydrates (CHO) Introduction CHO are widely distributed in plants & animals. In plants, glucose is synthesized by photosynthesis. Animals can synthesize CHO from amino acids & other compounds, however, most of animal CHO is derived from plants. The main disease associated with CHO metabolism is diabetes mellitus. Classification of Carbohydrates CHO are classified into: (1) Monosaccharides. (2)-Disaccharides. (3) Oligosaccharides. (4) Polysaccharides. Monosaccharides They are simple CHO (because can not be hydrolyzed to a simplest form), it may contain 3, 4, 5, 6 or more carbon atoms (known respectively as trioses, tetroses, pentoses, hexoses & so on). It may be aldoses or ketoses depending whether they have an aldehyde or ketone group respectively. Most important monosaccharides are hexoses like glucose, galactose & fructose which are reducing substances because it contains aldehyde or ketone groups. Glucose Glucose is the most important monosaccharide because: 1)-It’s major metabolic fuel in human specially CNS, fetus & RBC. 2)- Precursor for synthesis of all other CHO in the body. 3)-Other hexoses like fructose & galactose are converted inside the body into glucose. 5 56-55 / د بسام/كلية طب نينوى Glucose Structure The structure of glucose can be represented in three ways: 1- Straight-chain structural formula (aldohexose) 2-Cyclic structure: Formed by reaction between aldehyde group & hydroxyl group. 3- Chair form: The cyclic structure is actually in the form of a chair. Glucose Isomerism The most important types of isomerism found with glucose are: (1)- D & L isomerism: The orientation of —H & —OH groups around the carbon atom adjacent to the terminal carbon (carbon 5 in glucose) determines whether glucose (or other sugar) belongs to the D or of its mirror image as the L form. When the —OH group on this carbon is on the right, its the D isomer & when on the left, its the L isomer. Most of the monosaccharides occurring in human including glucose are D isomers. 2 56-55 / د بسام/كلية طب نينوى (2) Pyranose & furanose ring structures: The 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. (3) Alpha & beta anomers: In solution the cyclic structure of glucose is retained but isomerism occurs about position 1( carbonyl or anomeric carbon atom) to give a mixture of α-D-Glucopyranose (38%) & β-D-Glucopyranose (62%). Less than 0.3% is represented by α & β anomers of D-Glucofuranose. 3 56-55 / د بسام/كلية طب نينوى (4) Epimers: The isomers differing as a result of variations in configuration of the —OH & —H on carbon atoms 2, 3 & 4 of glucose are known as epimers. Biologically, the most important epimers of glucose are mannose & galactose, formed by epimerization at carbons 2 & 4 respectively. (5) Aldose-ketose isomerism: The main example of aldose-ketose isomerism is fructose which has a potential keto group in position 2, whereas there is a potential aldehyde group in position 1 of glucose . Examples of aldoses of physiological importance are:1-D-Glucose. 2-D-Glycerose (D-Glyceraldehyde) 3-D-Erythrose. 4-D-Xylose. 5-D-Arabinose. 6-D-Ribose. 7-D-Galactose. 8-D-Mannose. Examples of ketoses of physiological importance are:1-Dihydroxyacetone. 2-D-Xylulose. 3-D-Ribulose. 4-D-Fructose. 4 56-55 / د بسام/كلية طب نينوى Disaccharides They are products of chemical reaction between 2 monosaccharides with loss of a molecule of water (can be hydrolyzed), the linkage between two monosaccharides is known as glycosidic link. The physiological important disaccharides are: 1-Maltose :- Link of two molecules of glucose by 1 4 glycosidic link, produced by enzymatic hydrolysis of starch. 2-Lactose:- Link of one molecule of glucose with one molecule of galactose by 1 4 glycosidic link , present in milk. 3-Sucrose:- Link of one molecule of glucose with one molecule of fructose by 1 2 glycosidic link, present in cane & beet sugar ,some fruits & vegetables. 4-Isomaltose:- Link of two molecules of glucose by 1 6 glycosidic link, produced by enzymatic hydrolysis of starch. If the glycosidic link between aldehyde or ketone group of one monosaccharide & the hydroxyl group of another monosaccharide the produced disaccharide have reducing property(because contain aldehyde or ketone group) as in maltose & lactose while if the glycosidic link between aldehyde or ketone group of the two molecules of monosaccharide the produced disaccharide have no reducing property (because not contain aldehyde or ketone group) as in sucrose. Oligosaccharides They are products of condensation of 3-10 monosaccharides as in maltotriose. Polysaccharides They are products of condensation of more than 10 monosaccharide units, they can be classified according to their origin or constituent: 5 56-55 / د بسام/كلية طب نينوى *Classification according to origin I-Plant Origin: They include:A-Starch: Polysaccharide of plant origin (homopolymer of glucose forming an α-glucosidic chain, called glucosan or glucan) , it consist of: 1-Amylose (one unbranched chain of glucose molecules linked by α-1 4-glucosidic linkages with only terminal aldehyde is free). 2-Amylopectin :consists of branched chains composed of 24–30 glucose residues united by α1 4 glucosidic linkages in the chains & by α 1 6 linkages at the branch points . B- Nonstarch polysaccharides: they include mainly: 1- Cellulose : It forms the major component of dietary fiber. they are not digested by human enzymes because they linked by β 1 4 link because human intestine lack the enzyme that capable to hydrolyze this link, however, there is some bacterial metabolism of cellulose in the human colon. 2-Inulin: Its a polysaccharide of fructose used to determine the glomerular filtration rate, but its not hydrolyzed by intestinal enzymes. II-Animal Origin: Include mainly glycogen which is the storage polysaccharide in animals including human & sometimes called animal starch. It has structure similar to amylopectin except that branching is more extensive. *Classification according to constituent Polysaccharides are sometimes classified as hexosans or pentosans, depending on the identity of the constituent monosaccharides. Glycosides Glycosides are formed by condensation between the hydroxyl group of the anomeric carbon of a monosaccharide & a second compound that may or may not be another monosaccharide. Deoxy Sugars Deoxy sugars are formed when one hydroxyl group is replaced by hydrogen(lack oxygen atom),e.g. deoxyribose in DNA. Amino Sugars (Hexosamines) They include mainly : 1- D-glucosamine. 6 56-55 / د بسام/كلية طب نينوى 2-D-galactosamine. 3-D-mannosamine. 4-Several antibiotics (e.g., erythromycin) . Glycosaminoglycans (Mucopolysaccharides) They are complex CHO containing amino sugars & uronic acids. Glycoprotein (Mucoprotein) Formed by conjugation of oligosaccharide with protein, if CHO constitute 5-15% it's called glycoprotein while if CHO constitute 1575% it's called mucoprotein. They present mainly in cell membranes. Digestion & Absorption of Carbohydrates Carbohydrates account for a large proportion of daily intake. The main dietary digestible carbohydrates are: 1-starch (60-70%) 2- Sucrose (20-30%) 3-lactose (5%) Absorption of carbohydrates from small intestine occurs only after conversion of dietary carbohydrates into monosaccharide by the following: 1- α-amylase: Present in the mouth (salivary α-amylase) & in the small intestine (pancreatic α-amylase) split α-1,4-glucosidic links of starch converting it into maltose, & isomaltose. 2-Small intestine: In the brush border of small intestinal mucosa, the last stage of digestion occur where monosaccharides produced to be ready for absorption, this occur by the following enzymes: 1-Maltase—hydrolyzes maltose into two glucose molecules. 2-Sucrase—hydrolyzes sucrose into glucose + fructose. 3-Lactase—hydrolyzes lactose into glucose + galactose. 4-Isomaltase—hydrolyze 1, 6 links of isomaltose. The end products of carbohydrates digestion (monosaccharides) are absorbed by small intestinal cells into the liver through the portal vein. The main dietary indigestible carbohydrate is cellulose which contain β-glucosidic links due to lack of β-amylase enzyme in human. 7 56-55 / د بسام/كلية طب نينوى