Code and Title of the Paper: F10NB Nutritional Biochemistry Code and Title of the Module: F10NB02 Different Classes of Carbohydrates and their Structures Name of the Content Writer: Dr. S. Sumathi Quadrant – I 10.2.1. Introduction Carbohydrates are the major constituents in most fruits, vegetables, legumes, and cereal grains, and do provide texture and flavor in many processed foods. Carbohydrates are the major energy source for humans by digestion and absorption in the small intestine and, to a lesser extent, by microbial fermentation in the large intestine. Available and unavailable carbohydrates are the two classifications of food carbohydrates. Available carbohydrates are those that are hydrolyzed by enzymes of the human gastrointestinal tract to monosaccharides. These monosaccharides are absorbed in the small intestine and enter the pathways of carbohydrate metabolism. Unavailable carbohydrates are not hydrolyzed by human digestive enzymes, but they may be partially or totally fermented by bacteria in the large intestine, forming short-chain fatty acids that may be absorbed and contribute to the body’s energy needs. The carbohydrates are hydrophilic in nature and forms various sugar derivatives.. Carbohydrates are also covalently bound to many proteins and lipids to form a class called as glycoconjugates that includes glycoproteins, proteoglycans, and glycolipids . OBJECTIVES 1.To outline the classification of different classes of carbohydrates 2.To outline the structure of various types of carbohydrates 3. The enumerate the role of these classes in the microbes, plants and animals 10.3 MONOSACCHARIDES – STRUCTURAL ASPECTS Monosaccharides or simple sugars include the aldoses such as glucose and ketoses such as fructose. Aldoses contain one aldehyde group and hence are polyhydroxy aldehydes. Ketoses contain one ketone group and hence are polyhydoxy ketones. Both have the empirical chemical formula (CH 2 O) n. Important character of monosaccharides is Stereoisomerism. Stereoisomers are the compounds that have the same structural formulae but differ in their spatial configuration. An asymmetric carbon atom (chiral carbon) is a carbon atom that is attached to four different types of atoms or Code and Title of the Paper: F10NB Nutritional Biochemistry Code and Title of the Module: F10NB02 Different Classes of Carbohydrates and their Structures Name of the Content Writer: Dr. S. Sumathi four different groups of atoms. Stereoisomers can be further separated into the two categories of diastereomers and enantiomers. One type of diastereomers (or geometric stereoisomers) differ by "cis" and "trans" orientations. Enantiomers are a class of stereoisomers related like an object and its mirror image. Enantiomers differ in their "handedness" as the left hand and right hand are related. They are a pair of mirror image molecules that cannot be superimposed on each other. The number of possible stereoisomers depends upon the number of chiral centers in the molecule. Van'tHoffs rule states: number of stereoisomers = 2n , where n = number of chiral centers. For example, a molecule with 2 chiral centers can have 4 stereoisomers. Glyceraldehyde –is the reference carbohydrate. Glyceraldehyde is the simplest monosaccharide with one asymmetric carbon atom. There are two forms- The d / l system is commonly used for designating the chirality of sugars and amino acids and has been chosen as the reference carbohydrate to represent the structure of all other classes of carbohydrates. Thus, the three- carbon glyceraldehyde exists as both d -and l -enantiomers. The two-carbon aldehyde, glycolaldehyde does not have a chiral carbon atom and hence has no optical activity. All d - aldoses are related to d - glyceraldehyde, and l -aldoses are similarly related to l -glyceraldehyde. The relation of d -aldoses to d -glyceraldehyde can be seen clearly by looking at a scheme for the chemical synthesis of the series of d -aldoses from d -glyceraldehyde. Code and Title of the Paper: F10NB Nutritional Biochemistry Code and Title of the Module: F10NB02 Different Classes of Carbohydrates and their Structures Name of the Content Writer: Dr. S. Sumathi D and L isomers For sugars with more than one chiral center, the D or L designation refers to the asymmetric carbon farthest from the aldehyde or keto group. Most naturally occurring sugars are D isomers. D& L sugars are mirror images of one another. They have the same name. For example, D- glucose and L-glucose are shown at right. Other stereoisomers have unique names, e.g., glucose, mannose, galactose, etc. The number of Code and Title of the Paper: F10NB Nutritional Biochemistry Code and Title of the Module: F10NB02 Different Classes of Carbohydrates and their Structures Name of the Content Writer: Dr. S. Sumathi stereoisomer is 2 n, where n is the number of asymmetric centers. Thus the six-carbon aldoses that have 4 asymmetric centers, have 16 stereoisomers (8 D-sugars and 8 L-sugars). Optical activity of sugars Optical activity is a characteristic feature of compounds with asymmetric carbon atom. When a beam of polarized light is passed through a solution of an optical isomer,it will be rotated either to the right or left. The term detrorotatory(+) and levoraotatory (-) are used to refer to compounds that respectively rotate the plane polarized light to the right or left. An optical isomer may be designated as D(+),D(-),L(+) and L(-) based on its structural relation with glyceraldehyde. Thus the D and L configurations are based on the structure of glyceraldehydes and their optical activities may be different. Racemic mixture: If the D and L isomers are present in equal concentration, it is known as racemic mixture or DL mixture. Racemic mixture does not exhibit any optical activity because the dextro and levorotatory activities cancel each other. Configuration of D aldoses The configuration of the possible D aldoses starting from D glyceraldehydes is by Kiliani fischer synthesis, by increasing the chain length of an aldose, by one carbon at a time. Thus starting with an aldotriose(3C), aldotetrose(4C), aldopentose(5C) and aldohexose (6C) are formed. Among the various aldohexose glucose,mannose and galactose are most common. And among these D glucose is the only aldohexose that predominantly occurs in nature. Code and Title of the Paper: F10NB Nutritional Biochemistry Code and Title of the Module: F10NB02 Different Classes of Carbohydrates and their Structures Name of the Content Writer: Dr. S. Sumathi Configuration of D –Ketoses Dihydroxy acetone is the starting material for the synthesis of ketose sugars. D xylulose, D Ribulose,D Fructose and D- sedoheptulose are physiologically important. Code and Title of the Paper: F10NB Nutritional Biochemistry Code and Title of the Module: F10NB02 Different Classes of Carbohydrates and their Structures Name of the Content Writer: Dr. S. Sumathi ALDOSES D-ribose D-xylose D-arabinose D-glucose D-galactose D-mannose KETOSES Code and Title of the Paper: F10NB Nutritional Biochemistry Code and Title of the Module: F10NB02 Different Classes of Carbohydrates and their Structures Name of the Content Writer: Dr. S. Sumathi D-ribulose D-fructose Epimers Sugars that vary in their configuration at only one carbon are epimers: for example, d -glucose and d -mannose are C2 epimers, whereas d -glucose and d -galactose are C4 epimers. Enzymes that catalyze epimer formation are called epimerases. Enantiomers Enantiomers are special type of stereoisomers that are mirror images of each other. The two members are designated as D and L –sugars. Enantiomers of glucose are L glucose and D- Glucose. Majority of the sugars in the higher animals are of D type. Code and Title of the Paper: F10NB Nutritional Biochemistry Code and Title of the Module: F10NB02 Different Classes of Carbohydrates and their Structures Name of the Content Writer: Dr. S. Sumathi Diastereomers The term diastereomers is used to represent the stereoisomers that are not mirror images of one another. 10.3.2 Structure of Glucose The aldehyde and ketone moieties of the carbohydrates with five and six carbons will spontaneously react with alcohol groups present in neighbouring carbons to produce intramolecular hemiacetals or hemiketals, respectively. This results in the formation of five- or six-membered rings. Because the five-membered ring structure resembles the organic molecule furan, derivatives with this structure are termed furanoses. Those with six-membered rings resemble the organic molecule pyran and are termed pyranoses. Code and Title of the Paper: F10NB Nutritional Biochemistry Code and Title of the Module: F10NB02 Different Classes of Carbohydrates and their Structures Name of the Content Writer: Dr. S. Sumathi Such structures can be depicted by either Fischer or Haworth style diagrams. The numbering of the carbons in carbohydrates proceeds from the carbonyl carbon, for aldoses, or the carbon nearest the carbonyl, for ketoses. Cyclic Fischer Projection of α-D-Glucose Haworth Projection of α-D-Glucose Code and Title of the Paper: F10NB Nutritional Biochemistry Code and Title of the Module: F10NB02 Different Classes of Carbohydrates and their Structures Name of the Content Writer: Dr. S. Sumathi Anomers Anomers are cyclic monosaccharides or glycosides that are epimers, differing from each other in the configuration of C-1 if they are aldoses or in the configuration at C-2 if they are ketoses. The epimeric carbonin anomers are known as anomeric carbon or anomeric center. The alpha and beta cyclic forms of d D Glucose are known as anomers. These anomers differ in certain physical and chemical properties. Mutarotation: Is the process of interconversion of α- and β- anomers The α- and β- anomers of carbohydrates are typically stable solids. However, in aqueous solution, they quickly equilibrate to an equilibrium mixture of the two forms. Code and Title of the Paper: F10NB Nutritional Biochemistry Code and Title of the Module: F10NB02 Different Classes of Carbohydrates and their Structures Name of the Content Writer: Dr. S. Sumathi For example, in aqueous solution, glucose exists as a mixture of 36% α- and 64% β- (>99% of the pyranose forms exist in solution).