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Carbohydrate Chemistry Carmen Sato­Bigbee, Ph.D

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Carbohydrate Chemistry Carmen Sato­Bigbee, Ph.D Carbohydrate Chemistry Carmen Sato­Bigbee, Ph.D. Objectives: 1) To be familiar with the general properties and chemistry of carbohydrates. 2) To be familiar with the structure of typical carbohydrate families and the biological substances in which they occur. 3) To introduce the basic mechanisms of carbohydrate digestion and absorption Resources: Lehninger et al., Principles of Biochemistry 2005; Marks et al., Basic Medical Biochemistry 2005. Carbohydrates play a wide variety of roles functioning as energy stores (starch, glycogen), fuels (glucose, fructose), metabolic intermediates (fructose, glyceraldehyde), recognition molecules (cell receptors, blood group molecules, interaction sperm­egg), extracellular matrix components (glycosaminoglycans), and cell adhesion molecules (carbohydrates in endothelial cells allowing interaction with lymphocytes, carbohydrate epitopes in nerve cell membranes involved in cell migration and maturation). Carbohydrates can be classified by the number of sugar units into monosaccharides (1 unit), disaccharides (2), oligosaccharides (3 to about 10), and polysaccharides (tens to thousands). a) Monosaccharides: Carbohydrates that cannot be hydrolyzed into simpler sugars. Examples: glucose, galactose, fructose, ribose. Monosaccharides are used as fuels and metabolic intermediates in numerous biological pathways that will be discussed in the next classes. In general they are either an aldehyde or a ketone, with multiple hydroxyl groups. A sugar with an aldehyde group is an aldose, with a ketone group is a ketose. Monosaccharides with 3, 4, 5, 6, 7 carbons are called trioses, tetroses, pentoses, hexoses and heptoses, respectively. Glucose is both an aldose and a hexose. Fructose is both a ketose and a hexose. Monosaccharides can be D­ or L­ stereoisomers: Glyceraldehyde (a triose) has one asymmetric carbon (C with four different substituted groups). This determines the existence of two stereoisomers which are enantiomeres (mirror images): D­ glyceraldehyde and L­glyceraldehyde. Sugars with more than one asymmetric carbon will be D or L depending on the configuration of the asymmetric carbon farthest from the aldehyde or ketone group, this is C5 in the case of glucose and fructose. C5 with the same configuration as asymmetric carbon in D­ glyceraldehyde: D­glucose, D­fructose. Epimers are stereoisomers that differ at only one asymmetric carbon (D­glucose and D­ galactose). Note that the designation D or L merely relates to the conformation of a given molecule to that of glyceraldehyde and does not specify the sign of rotation of plane­polarized light. D­glucose may also be called D(+)­glucose because is dextrorotatory, whereas D­fructose may also be called D(­)­fructose because it is levorotatory. D­series monosaccharides predominate in nature, just as L­amino acids do. These conformations are highly conserved trough out evolution due to the stereospecificity of the enzymes that synthesize and metabolize these molecules. Cyclic structures Monosaccharides exist in cyclic structures. Why? Alcoholes react with aldehydes to form hemiacetals. A similar reaction occurs between the aldehyde group of an aldose and a hydroxyl group in the same sugar molecule. The new cyclic structure of glucose is an internal hemiacetal that is in equilibrium with the open chain form which is an aldehyde. The resulting six carbon cyclic sugar is also called a pyranose (glucopyranose) because it is similar to pyran. A ketone reacts with an alcohol to form a hemiketal. When the ketone group in a ketose reacts with a hydroxyl group in the same sugar, this results in a cyclic form. This form is a hemiketal that is in equilibrium with the open form which is a ketone. In this case, the five carbon ring is also called a furanose (fructofuranose) because it resembles furan. Figure 1­ Modified from Garret and Grisham, Biochemistry, 2005 The formation of the cyclic structure creates a new asymmetric center in which the carbon of the initial aldehyde of ketone group is now bound to the oxygen within the ring. This carbon is called the anomeric carbon. Notice that the generation of this anomeric carbon determines the existence of two new configurations or anomers a and b. In the Haworth projections, a indicates that the hydroxyl group linked to the anomeric carbon is below the plane of the ring. b indicates that the hydroxyl group is above the plane of the ring. We will see later that this is particularly important in determining the characteristics of more complex carbohydrates and will explain why humans cannot digest cellulose while cows do! Mutarotation: Process by which the a and b anomers equilibrate in solution with the open form. In solution 63% of D­glucose will exist as b­D­glucose, 36% as a­D­glucose and only 1% in the open form. In solution a six carbon sugar in cyclic conformation is not really planar and the preferred conformation is in the chair form. Figure 2­ Taken from Lehninger et al. Functional groups Amino groups: for example glucosamine and galactosamine (OH group in C2 replaced by NH2). Found in many oligo­and polysaccharides. Modified amino sugars (O acetylated or sulfated) are the main components of heparin (polysaccharide found in mast cells near the wall of blood vessels and on endothelial cells) and polysaccharides of connective tissue. Sulfate groups: sulfated sugars are components of polysaccharides like keratin and condroitin sulfate, keratan sulfate and dermatan sulfate (all component of connective tissue). Negative groups on heparin are crucial to its anticoagulant function facilitating the action of clotting inhibitors. Phosphate groups: Phosphate groups in carbohydrates play extremely important and different functions. The addition of a phosphate group (as ester bond) to glucose as it occurs in glucose­6­P makes the molecule more polar so it cannot cross the plasma membrane (lipidic). We will see in a future class that glucose released from glycogen in muscle stays within the cells because it is transformed into glucose­6­P and is then directly used in glycolysis. A phosphate group can also be used to link the sugar to nucleosides creating for example UDP­ glucose, a high­energy compound that we are going to see in detail in the “Bioenergetics” class. We will see later in “Glycolysis” that the first step in this pathway is the addition of a phosphate group to glucose producing glucose­6­P. Several subsequent intermediates in this pathway are also phosphorylated sugars including fructose­1,6­bisphosphate, dihydroxyacetone phosphate and glyceraldehyde­3­phosphate. These phosphate groups are then used to synthesize ATP from ADP. Oxidation and reduction of carbohydrates: The aldehyde group of a sugar can be oxidized to carboxylic acid. Oxidation of glucose at C1: gluconic acid, at C6: glucuronic acid. A sugar that can be oxidized at C1 is called a reducing sugar because it can reduce another compound. This property of glucose is used to determine glucose concentrations in blood and urine. Reduction of the aldehyde group of a sugar results in the formation of a polyalcohol. Glucose can be reduced to sorbitol and galactose to galactitol. Glycosides: The hydroxyl group on the anomeric carbon of a monosaccharide can react with the hydroxyl or amino group of different compounds to form glycosides. When occurs between two monosaccharides, this type of linkage is an O­glycosidic bond. Sugars can be linked by O­glycosidic bonds to produce di­, oligo­, and polysaccharides. The bond can be either a or b depending on the position of the atom linked to the anomeric carbon. The linkage formed between the hydroxyl group of a sugar and the amino group of other molecule is called an N­glycosidic bond. This type of bond links the sugars (ribose or deoxyribose) to the different bases to form nucleotides. N­glycosidic linkages in the majority of biological compounds have b configuration. b) Disaccharides: Two monosaccharides joined by an O­glycosidic linkage. Most common disaccharides: sucrose, lactose and maltose. Before being absorbed, these sugars must be hydrolyzed to monosaccharides by specific enzymes located in the outer surface (brush border) of the absorptive cells lining the small intestine. Sucrose (table sugar): Glucose and fructose joined by their anomeric carbons, therefore it is a non­reducing sugar. The configuration of this glycosidic linkage is a for glucose and b for fructose. Before being absorbed, sucrose is hydrolyzed to glucose and fructose by the enzyme sucrase. Lactose (milk sugar): Galactose linked to glucose by a b­1,4 glycosidic linkage. It is hydrolyzed in the intestine by the enzyme lactase. Many adults are intolerant to milk because they have a deficiency of lactase (late onset lactose deficiency, particularly high in certain populations). Figure 3­ Modified from Marks et al. Maltose (produced by hydrolysis of starch): Two glucose molecules linked by an a­1,4 glycosidic linkage. Before being absorbed, is hydrolyzed to glucose by intestinal maltase. c) Oligosaccharides: Found as carbohydrates linked to glycolipids such as gangliosides and glycoproteins including immunoglobulins, blood clotting proteins, and adhesion molecules. d) Polysaccharides: Many are stores of glucose. Glycogen: It is the store of glucose in animal cells (specially liver and muscle cells). Highly branched and formed by glucose residues (thousands) linked by a­1,4­glycosidic bonds (along the linear chains), and a­1,6­glycosidic bonds (at the branching points). We will see in the “Glycogen” class, that cells store glucose
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