Carbohydrate Chemistry Lecture 1 PDF

# Carbohydrate Chemistry ## By Dr/Walaa Keshk Prof. of medical biochemistry Faculty of medicine, Tanta university ## The carbohydrates speak: "We are polyhydroxyaldehydes or ketones; _classified into mono-, oligo- and polysaccharides;_ Held together by glycosidic bonds; Supply energy and serve a...

# Carbohydrate Chemistry ## By Dr/Walaa Keshk Prof. of medical biochemistry Faculty of medicine, Tanta university ## The carbohydrates speak: "We are polyhydroxyaldehydes or ketones; _classified into mono-, oligo- and polysaccharides;_ Held together by glycosidic bonds; Supply energy and serve as structural constituents." ## Carbohydrates - Carbohydrates: They are poly-hydroxy aldehydes or ketones - Chemically, they contain carbon, hydrogen and oxygen - The general formula of many simple carbohydrates is [CH2O]n - (mostly hydrate of carbon as the ratio of hydrogen to oxygen 2:1) where n (no of carbon atoms)=3 to 8 - They are found in grains, vegetables, fruits and in milk and other dairy products - The term 'sugar' is applied to carbohydrates soluble in water and sweet to taste. ## Biomedical Importance of Carbohydrates - Primary source of energy (4 kcal/g) SO Inhibits the breakdown of proteins for energy as they are the - Storage form of energy: glycogen in animal and starch in plants - Structural component, e.g. glycosaminoglycans in humans, cellulose in plants. - Non-digestable carbohydrates like cellulose, serve as dietary fibers - Constituent of nucleic acids RNA and DNA, e.g. ribose and deoxyribose sugar - Cellular communication and immunity - Detoxification e.g. glucuronic acid. - Basic food groups which play an important role in a healthy life. ## Classification of Carbohydrates Carbohydrates are divided into four major groups based number of sugar units: - Monosaccharides (one sugar unit) - Disaccharides (2 sugar unit) - Oligosaccharides (3-10 sugar unit) - Polysaccharides (more than 10 sugar unit) ## Nutritional Classification of Carbohydrates ### Complex Carbohydrates (polysaccharides) - Molecules are digested and converted slowly compared to simple carbohydrates. - Found in lentils, beans, peanuts, potatoes, peas, corn, whole-grain bread, cereals ### Simple Carbohydrates (rapid rise in the blood sugar level) - Monosaccharides - Disaccharides - Oligosaccharides ## I. Monosaccharides - Sugars which cannot be further hydrolyzed. - They contain one sugar unit - Building blocks of all carbohydrates ### Classification of Monosaccharides #### 1- According to the number of carbon atoms | Number of carbon atoms | Generic name | |---|---| | 3 | Trioses | | 4 | Tetroses | | 5 | Pentoses | | 6 | Hexoses | | 7 | Heptoses | | 9 | Nanoses | #### 2. On the basis of their functional group | Generic name | Aldoses | Ketoses | |---|---|---| | Triose | Glyceraldehyde | Dihydroxyacetone | | Tetrose | Erythrose | Erythrulose | | Pentose | Ribose <br> Xylose (Epimer of Ri-bose) Arabinose | Ribulose <br> Xylulose (Epimer of Ribulose) | | Hexose | Glucose, Galactose, Mannose | Fructose | | Heptose | | Sedoheptulose | ## Biochemical and biological importance of some hexoses ### 1) Glucose - Sugar of grape - Main blood sugar - The principle sugar utilized by tissues. ### 2) Fructose - In fruits, honey, - Sugar of fetal Blood and is the principle sugar of semen. ### 3) Galactose: present in - combination with glucose in milk sugar (Lactose - In tissues as galactolipid and glycoprotein. All Hexoses have a free functional group, so they are reducing sugars. ## Asymmetric carbon atom - It a carbon atom attached to 4 different atoms or groups. - Number of isomers = 2n (n is the number of asymmetric Carbon atom.) - Ketoses have asymmetric carbon atom less than aldoses (2n-1) - Monosaccharide with no Asymmetric Carbon atom: Dihydroxyacetone ## Number of Carbon Atoms | Number of carbon atoms | Aldose/Ketose | Number of asymmetric carbon atoms | Number of optical isomers | |---|---|---|---| | 3 | Aldose | 1 | 2 | | 4 | Aldose | 2 | 4 | | 5 | Aldose | 3 | 8 | | 6 | Aldose | 4 | 16 | | 3 | Ketose | 0 | - | | 4 | Ketose | 1 | 2 | | 5 | Ketose | 2 | 4 | | 6 | Ketose | 3 | 8 | ## Monosaccharide in solution - Monosaccharide in solution is mainly present in cyclic structure creating new asymmetric carbon (anomeric carbon). - Glucose, the more stable form is the six membered ring - Fructose, the more stable form is five membered ring. ## Isomers Compounds have the same molecular formula but with different structures ### Isomers #### Constitutional (structural) isomers #### Stereoisomers (spatial isomers) - Ketose-aldose isomerism - Diastereomers - Epimerism - Enantiomers - Optical isomerism - Anomerism. ## Constitutional isomer Isomers differ in functional group ### Ketose-Aldose isomerism #### As Glucose and fructose | Formula | Group | Structure | |---|---|---| | Glucose (C6H12O6) | Aldehyde | CHO <br> H-C-OH <br> OH-C-H <br> H-C-OH <br> H-C-OH <br> CH2OH | | Fructose (C6H12O6) | Keto | CH2OH <br> C=O <br> OH-C-H <br> H-C-OH <br> H-C-OH <br> CH2OH | ## Stereoisomerism Differ in special configuration (arrangement) of H and OH in the space. ### 1. Optical isomerism (D and Lisomerism) (Enantiomers) Non-superimposible mirror images resulting from change in the position of groups (H and OH) around all asymmetric carbon atoms (i.e total reversion of groups). ### Classified based on the orientation of groups around penultimate carbon atom (reference carbon) into: - **D isomer:** OH on the right side _Majority of the sugars in humans_ - **L isomer:** OH on the left side - **D-Glyceraldehyde:** CHO <br> H-C-OH <br> CH2OH - **L —Glyceraldehyde:** CHO <br> OH-C-H <br> CH2OH - **D (+) Glucose:** 1CHO <br> 2 H-C - OH <br> 3 OH-C - H <br> 4 H - C - OH <br> 5 H - C - OH <br> 6 CH2OH - **L (-) Glucose:** 1 CHO <br> 2 OH-C-H <br> 3 H-C-OH <br> 4 OH-C-H <br> 5 H-C-OH <br> 6 CH2OH ### 2. Diasteromer (non mirror image isomer) #### 2A. Epimerism - Carbohydrate isomers differ in configuration around only one carbon with the exception of the carbonyl carbon - C2 Epimer of Glucose is Mannose - C4Epimer of Glucose is Galactose. #### 2B. Anomerism - Ring structure results in creation of an additional asymmetric carbon called anomeric Carbon - Def.: Isomers differ in configuration around the carbonyl carbon (anomeric carbon, (C1 (aldoses),C2 (in ketoses ) after cyclization) - a-isomer: OH on the right side (upper side of the plane of ring). - ẞ-isomer: OH on the left side (lower side of the plane of ring). ## Physical properties of Monosaccharides - All monosaccharides are soluble in water. - All monosaccharides have asymmetric carbon atom except di-hydroxyacetone so exhibit Optical activity, and have optical isomer - Mutarotation ## Optical activity - Optical activity: Capacity to rotate the plane polarized light either to the right (dextrorotatory, +, d) or to the left (levorotatory, -, 1), due to the presence of asymmetric carbon atoms ## Racemic (dl, ±) mixture - Mixture of equal amount of optical isomers (two enantiomer) - Has no optical activity. Since the activity of each isomer cancel one another. ## Mutarotation - Is the spontaneous change of the specific angle of rotation of optically active substance of freshly prepared sugar solution due to the interconversion of a and ß anomers. - a -Glucose freshly dissolved in water, has specific rotation of+112. - ẞ - Glucose freshly dissolved in water, has specific rotation of+19. ## Some chemical properties of monosaccharides ### 1. Reactions with Concentrated acids as H2SO4 Base of molish test, a general test for all carbohydrate. ### 2. Alcoholic Fermentation - By the action of microorganisms - Produce ethyl alcohol and carbon dioxide. - Some bacteria and yeast can obtain energy anaerobically (in absence of oxygen) by this method. ### 3. Properties of -OH group #### a. Esterification - The most important esters are esters with phosphoric acid as glucose 6-phosphate. #### b. Oxidation of the last primary alcoholic group Gives uronic acid as Glucose gives glucuronic acid (constituent of mucopolysaccharides + having detoxification function) #### c. Formation of Glycosides - Compounds containing a carbohydrate and another carb or a non-carb. - Glucoside (if 1st carb is glucose) or galactosides (if 1st carb is galactose) ### 3. Properties of the carbonyl group (aldehyde or ketone group) #### 1. Reduction: Gives corresponding alcohol - Accumulation of sorbitol and dulcitol in the tissues may cause cataract, nephropathy. #### 2. Reducing properties of free carbonyl group Can reduce Fehling and Bendict reagent, used for detection of glucose in urine of diabetic patients #### 3. Oxidation of aldo-sugars Gives carboxylic acids: Aldonic or Aldaric acids according to the strength of the oxidizing agent ## Derived sugars - Monosaccharides whose structure cannot be represented by general formula or which have some unusual features. - They are: - Acid Sugars (by oxidation of sugars) - Sugar Alcohols (by reduction of sugars) - Deoxy Sugars as Deoxyribose in DNA - Amino Sugars as Glucosamine, Galactosamine - Glycosides ## Disaccharides [Cn(H2O)n-1] - They are Glycosides formed by condensation of 2 monosaccharides. - They are crystalline, water soluble and sweet to taste. - They are sub-classified on the basis of the presence or absence of free reducing (aldehyde or ketone) group. ## Disaccharides Classification | Reducing (with free aldehyde or ketone group) | Non-reducing (absence of free aldehyde or ketone group) | |---|---| | Example | Constituent | Example | Constituent | | Maltose | Glucose + Glucose | Sucrose | Glucose + Fructose | | Lactose | Galactose + Glucose | Trehalose | Glucose + Glucose | | Isomaltose | Glucose + Glucose | | | ## 1) Sucrose - Cane sugar, beet sugar or table sugar. - It is formed by plant but not by human beings. - a-D-glucose +B-D-fructose linked together by a-1-2 glucosidic linkage - It does not contain a free aldehyde or ketonic groups, Thus - It does not reduce Fehling or Benedict solutions. - Does not show mutarotation. - Hydrolysis by dilute acids or by sucrase (as invertase) enzyme it gives glucose and fructose (hydrolytic products are laevorotatory) . - Honey is largely 'invert sugar' and the presence of fructose accounts for the greater sweetness of honey. ## 2) Lactose (milk sugar) - Disaccharide of ẞ-D galactose and a-D- glucose linked by ẞ-(1,4) galactosidic linkage. - Lactose contains free aldehyde group (from glucose ) thus it reduces Fehling and Benedict solutions. - It is a non-fermentable sugar by yeast because there is no lactase enzyme in yeast. ## Lactose is considered to be a perfect sugar for baby because: - It is not fermented, so do not produce gases that cause disturbances to the baby (presence of lactase enzyme in the intestine). - It is not so sweaty, so does not cause nausea and the baby can take its requirements. - It the only natural source of galactose that enter in structure of galactolipids which are found in great amounts in brain and nerves. - Has a mild laxative effects. ## 3. Maltose - Two molecules of a-D- glucose through a-1-4 glucosidic linkage. - Sources: during digestion of starch by salivary amylase, - Properties: - Has optical activity. - Hydrolyzed by maltase enzyme to Glucose. - Has free carbonyl group so it can: - Show mutarotation. - Reduce Fehling or Benedict. - Isomaltose: linkage is a-1-6 glucosidic linkage. It results from hydrolysis of starch and glycogen. - Thank you