Carbohydrates: Structure, Derivatives, and Classification PDF

Carbohydrates: Structure of Monosaccharides and Carbohydrate Derivatives Dr. Burak DURMAZ [email protected] Near East University, Faculty of Medicine, Department of Medical Biochemistry  carbohydrates How doe...

Carbohydrates: Structure of Monosaccharides and Carbohydrate Derivatives Dr. Burak DURMAZ [email protected] Near East University, Faculty of Medicine, Department of Medical Biochemistry  carbohydrates How does the structure of the carbohydrates we consume affect the speed at which our body converts them into energy? For example, when you consume a food rich in simple sugars versus one rich in complex carbohydrates, how and how quickly does your body access that energy?  CARBOHYDRATES Carbohydrates are polyhydroxy aldehydes or ketones, or substances that yield such compounds on hydrolysis. Many, but not all, carbohydrates have the empirical formula (CH2O)n, some also contain nitrogen, phosphorus, or sulfur. Carbohydrates are the most abundant biomolecules on the Earth. Each year, photosynthesis converts more than 100 billion tons of CO2 and H2O into cellulose and other plant products.  FUNCTIONS of CARBOHYDRATES 1. Certain carbohydrates, eg. sugar and starch, are important for diet, and the oxidation of carbohydrates is the central energy-yielding pathway in non-photosynthetic cells. 2. Insoluble carbohydrate polymers (glycans) serve as structural and protective elements in the cell walls of bacteria and plants, and in the connective tissue of animals.  FUNCTIONS of CARBOHYDRATES 3. Some carbohydrate polymers lubricate skeletal joints and participate in recognition and adhesion between cells. 4. Complex carbohydrate polymers covalently bound to proteins or lipids act as signals that determine intracellular location or metabolic fate of these molecules. CLASSIFICATION of CARBOHYDRATES  CLASSIFICATION of CARBOHYDRATES 1. Monosaccharides, or simple sugars, consist of a single polyhydroxy aldehyde or ketone unit. The most abundant monosaccharide in nature is the six-carbon sugar D-glucose, sometimes referred to as dextrose. Monosaccharides of more than four carbons tend to have cyclic structures. 2. Disaccharides (such as maltose, lactose, and sucrose) consist of two monosaccharides joined covalently by an O-glycosidic bond, which is formed when a hydroxyl group of one sugar reacts with the anomeric carbon of the other. This reaction represents the formation of an acetal from a hemiacetal (such as glucopyranose) and an alcohol (a hydroxyl group of the second sugar molecule).  CLASSIFICATION of CARBOHYDRATES 3. Oligosaccharides consist of short chains of monosaccharide units, or residues, joined by characteristic linkages called glycosidic bonds. In cells, most oligosaccharides consisting of three or more units do not occur as free entities but are joined to nonsugar molecules (lipids or proteins) in glycoconjugates. 4. Polysaccharides are sugar polymers containing more than 20 or so monosaccharide units, and some have hundreds or thousands of units. Some polysaccharides, such as cellulose, are linear chains; others, such as glycogen, are branched. Both glycogen and cellulose consist of recurring units of D- glucose, but they differ in the type of glycosidic linkage and consequently have strikingly different properties and biological roles. CLASSIFICATION of CARBOHYDRATES Classification Based on Functional Group 1. Aldoses contain aldehyde group 2. Ketoses contain keto group Classification Based on Carbon Skeleton 1. Trioses contain 3 carbon atoms 2. Tetroses contain 4 carbon atoms 3. Pentoses contain 5 carbon atoms 4. Hexoses contain 6 carbon atoms 5. Heptoses contain 7 carbon atoms  Monosaccharides are either aldose or ketose If the carbonyl group is at an end of the carbon chain (that is, in an aldehyde group) the monosaccharide is an aldose. If the carbonyl group is at any other position (in a keto group) the monosaccharide is a ketose. The simplest monosaccharides are the two three-carbon trioses: glyceraldehyde, an aldotriose, and dihydroxyacetone, a ketotriose. CLASSIFICATION of CARBOHYDRATES Monosaccharides are Asymmetric Compounds All the monosaccharides except dihydroxyacetone contain one or more asymmetric (chiral) carbon atoms and thus occur in optically active isomeric forms. The simplest aldose, glyceraldehyde, contains one chiral center (the middle carbon atom) and therefore has two different optical isomers, stereoisomers or enantiomers. Monosaccharides are Asymmetric Compounds By convention, one of these two forms is designated the D isomer, the other the L isomer. To represent three- dimensional sugar structures on paper, we often use Fischer projection formulas. Compounds containing chiral center are optically active; they rotate the plane of the polarized light (are either dextrorotatory, d or levorotatory, l).  SUGARS HAVE OPTICAL ACTIVITY All monosaccharides except dihydroxyacetonephosphate have at least one assymetric (chiral) carbon. They are optically active. They rotate the plane of polarized light.  Stereoisomers In general, a molecule with n chiral centers can have 2n stereoisomers. Glyceraldehyde has 21 = 2; the aldohexoses, with four chiral centers, have 24 =16 stereoisomers. The stereoisomers of monosaccharides of each carbon-chain length can be divided into two groups that differ in the configuration about the chiral center most distant from the carbonyl carbon. Those in which the configuration at this reference carbon is the same as that of D-glyceraldehyde are designated D isomers, and those with the same configuration as L-glyceraldehyde are L isomers. When the hydroxyl group on the reference carbon is on the right in the projection formula, the sugar is the D isomer; when on the left, it is the L isomer. Of the 16 possible aldohexoses, eight are D forms and eight are L. Most of the hexoses of living organisms are D isomers. Stereoisomers The stereoisomers of monosaccharides of each carbon-chain length can be divided into two groups that differ in the configuration about the chiral center most distant from the carbonyl carbon. Those in which the configuration at this reference carbon is the same as that of D-glyceraldehyde are designated D isomers, and those with the same configuration as L-glyceraldehyde are L isomers. When the hydroxyl group on the reference carbon is on the right in the projection formula, the sugar is the D isomer; when on the left, it is the L isomer. Of the 16 possible aldohexoses, eight are D forms and eight are L. Most of the hexoses of living organisms are D isomers. Stereoisomers Stereoisomers Stereoisomers Stereoisomers  EPIMERS The carbons of a sugar are numbered beginning at the end of the chain nearest the carbonyl group. Each of the eight D-aldohexoses, which differ in the stereochemistry at C-2, C-3, or C-4, has its own name: D-glucose, D- galactose, D-mannose, and so forth. Two sugars that differ only in the configuration around one carbon atom are called epimers. Some sugars occur naturally in their L form; examples are L-arabinose and the L isomers of some sugar derivatives that are common components of glycoconjugates. Formation of hemiacetals and hemiketals Aldoses and ketoses are not mainly found in straight- chain forms. In aqueous solution, aldotetroses and all monosaccharides with five or more carbon atoms in the backbone occur predominantly as cyclic (ring) structures in which the carbonyl group has formed a covalent bond with the oxygen of a hydroxyl group along the chain. The formation of these ring structures is the result of a general reaction between alcohols and aldehydes or ketones to form derivatives called hemiacetals or hemiketals which contain an additional asymmetric carbon atom and thus can exist in two stereoisomeric forms. Formation of hemiacetals and hemiketals An aldehyde can form a hemiacetal with an alcohol. An additional alcohol is used to form an acetal. An ketone can form a hemiketal with an alcohol. An additional alcohol is used to form a ketal. These reactions are reversible.  Monosaccharides Have Cyclic Structures For example, D-glucose exists in solution as an intra-molecular hemiacetal in which the free hydroxyl group at C-5 has reacted with the aldehydic C-1, rendering the latter carbon asymmetric and producing two stereoisomers, designated a and b. These six-membered ring compounds are called pyranoses because they resemble the six membered ring compound pyran. The systematic names for the two ring forms of D-glucose are a-D- glucopyranose and b-D- glucopyranose. Formation of the two cyclic forms of D-glucose. Solution of a-anomer of glucose rotates the plane of polarized light to right (10% solution at 20°C +112 °). For the b-anomer, this value: +18.7° If freshly prepared a-D-glucose solution is monitored , it is observed that the optical activity decreases and finally reaches +52.7°. The reason of that is conversion of some of the a-D- glucose to b-D-glucose. This rotation is called mutarotation. Mutarotation  Mutarotation a-D-Glucopyranose D-Glucopyranose β-D-Glucopyranose In solution, D-Glucose exists as intra-molecular hemiasetal. Mutarotation causes the formation of an equilibrium mixture consisting of 63.6% of the β anomer and 36.4% of the a anomer (open chain form 0.02%). Enzymes use only one of these forms or derivatives. Mutarotase enzyme catalyzes the convertion between them. Pyranoses and furanoses Conformational formulas of pyranoses Monosaccharide Derivatives In addition to simple hexoses, there are a number of sugar derivatives in which a hydroxyl group in the parent compound is replaced with another substituent, or a carbon atom is oxidized to a carboxyl group. Phosphorylated Sugars Monosaccharide Derivatives Amino Sugars In glucosamine, galactosamine, and mannosamine, the hydroxyl at C-2 of the parent compound is replaced with an amino group. The amino group is nearly always condensed with acetic acid, as in N-acetylglucosamine. This glucosamine derivative is part of many structural polymers, including those of the bacterial cell wall. Monosaccharide Derivatives Bacterial cell walls also contain a derivative of glucosamine, N-acetyl- muramic acid, in which lactic acid (a three-carbon carboxylic acid) is ether-linked to the oxygen at C- 3 of N-acetyl- glucosamine. Monosaccharide Derivatives Deoxy Sugars The substitution of a hydrogen for the group at C-6 of L-galactose or L-mannose produces L-fucose or L-rhamnose, respectively; these deoxy sugars are found in plant polysaccharides and in the complex oligosaccharide components of glycoproteins and glycolipids.  Aldonic Acids and Uronic Acids Oxidation of the carbonyl (aldehyde) carbon of glucose to the carboxyl level produces gluconic acid; other aldoses yield other aldonic acids. Oxidation of the carbon at the other end of the carbon chain—C-6 of glucose, galactose, or mannose—forms the corresponding uronic acid: glucuronic, galacturonic, or mannuronic acid. Both aldonic and uronic acids form stable intramolecular esters called lactones.  SUGAR ACIDS SUGAR ALDONIC A. URONIC A. ALDARIC A. Glucose Gluconic Glucuronic Glucaric Galactose Galactonic Galacturonic Galactaric  Aldonic Acids and Uronic Acids In addition to acidic hexose derivatives, there is a nine-carbon acidic sugar derivative called N-acetylneuraminic acid (sialic acid). This is a derivative of N-acetylmannosamine, and is a com- ponent of many glycoproteins and glycolipids in animals. The carboxylic acid groups of the acidic sugar derivatives are ionized at pH 7. Therefore they carry net negative charge. Reduced Sugars (Poliols) Reduced Sugars Dihydroxyacetone, Glyceraldehyde Glycerol Glucose, Fructose Sorbitol Mannose Mannitol Galactose Galactitol Xylose Xylitol Alcohol derivatives of monosaccharides are formed by the reduction of carbonyl group. Sugars as reducing agents