Lecture: 9/23‐10/10 CHAPTER 10 Carbohydrates Grains or cereal crops are an abundant source of carbohydrates worldwide. Chapter 10 Outline Monosaccharides are aldehydes or ketones that contain two or more alcohol groups. The smallest monosaccharides are composed of three carbons. The smallest monosaccharides have three carbons. Isomeric Forms of Carbohydrates: Definitions • Isomers: Each of two or more compounds with the same formula but a different arrangement of atoms in the molecule and different properties. • Stereoisomers: Atoms in isomers are connected in the same order but differ in spatial arrangement. • Epimers: Each of two isomers with different configurations of atoms around one of several asymmetric carbon atoms present. • Anomers: Isomers differ from each other in a new asymmetric carbon atom (C‐1 if they are aldoses or in the configuration at C‐2 if they are ketoses) in ring or cyclic structure. • Anantiomers: Chiral molecules that are mirror images or nonsuperimposable mirror images of one another. Isomeric forms of carbohydrates Common monosaccharides Numbers represent the position of carbon atom. The asymmetric carbon atom farthest from the aldehyde or ketone (shown in red) designates the structures as being in the D‐configuration ‐ Carbon atom at C‐1 position is called anomeric carbon atom such as in glucose (aldose) ‐ Carbon atom at C‐2 position is called anomeric carbon atom such as in fructose (ketose) Aldoses, such as glucose and Ketoses, such as fructose, contain mannose, contain an aldehyde a ketone (shown in blue). (shown in blue) The chemical basis for ring formation is that an aldehyde can react with an alcohol to form a hemiacetal, while a ketone can react with an alcohol to form a hemiketal. In the case of glucose, the resulting intramolecular hemiacetal, a six‐ carbon ring, is called a pyranose because of its resemblance to pyran. In the case of the ketose fructose, the intramolecular hemiketal, a five‐carbon ring, is called a furanose because of its resemblance to furan. Formation of a cyclic hemiacetal creates another diastereoisomeric form called an anomer. Pyranose formation Formation of a cyclic hemiacetal creates another diastereoisomeric form called an anomer. The α form means the hydroxyl at C‐1 (anomeric carbon atom) is below the plane of the ring The β form means that the hydroxyl at C‐1 is above the plane of the ring The open‐chain form of glucose cyclizes when the C‐5 hydroxyl group attacks carbon atom C‐1 of the aldehyde group to form an intramolecular hemiacetal. Two anomeric forms, designated α and β, can result. Furanose formation The open‐chain form of fructose cyclizes to a five‐membered ring when the C‐5 hydroxyl group attacks carbon C‐2 of the ketone to form an intramolecular hemiketal. Two anomers are possible, but only the α anomer is shown. Haworth Projections: The carbons atom in the ring are not written out Ring structures of fructose No sweet as fructopyronose Found in honey, one of the sweetest chemical known Fructose can form both five‐membered furanose and six‐membered pyranose rings. In each case, both and anomers are possible. Pyranose rings can adopt two types of conformation, called boat and chair. In the chair form, substituents on the carbon ring atoms have two orientations: axial and equatorial. β‐D‐glucopyranose adopts the chair conformation because the axial positions are occupied by hydrogens, reducing steric hindrance. Chair and boat forms of β‐D‐glucopyranose Abbreviations: a, axial; e, equatorial. The chair form is more stable owing to less steric hindrance because the axial positions are occupied by hydrogen atoms. Definitions: O‐glycosidic bond: A bond formed between the anomeric carbon atom and a hydroxyl group of another molecule is called an O‐glycosidic bond, and the product is called a glycoside. N‐glycosidic bond: A bond formed between the anomeric carbon atom and an amine is called an N‐ glycosidic bond. Ester linkages: Carbohydrates also form ester linkages to phosphates. Modified monosaccharides Carbohydrates can be modified by the addition of substituents (shown in red) other than hydroxyl groups. Such modified carbohydrates are often expressed on cell surfaces Glucosinolates are a special class of glycosides that protect some plants from herbivory. When glucosinolates are hydrolyzed, isothiocyanate is produced, which generates a sharp taste that discourages further eating by the insect. Certain glucosinolates may help prevent cancer in humans. As a reducing sugar, glucose can react with hemoglobin, forming glycosylated hemoglobin (hemoglobin A1c), which is fully functional. Determining the amount of HbA1c in the blood allows monitoring the long‐term control of blood glucose levels in diabetics. Reactions between carbohydrates and proteins often impair protein function. Such modifications, called advanced glycation products, have been implicated in a number of pathological conditions. Disaccharides and polysaccharides are complex carbohydrates Oligosaccharides contain two or more monosaccharides linked by O‐glycosidic bonds. A large class of enzymes, glycosyltransferases, catalyze the formation of glycosidic bonds. The monosaccharide substrates for glycosyltransferases are activated by attachment to uridine diphosphate (UDP). The acceptor, designated X in this illustration, can be a simple monosaccharide, a complex polysaccharide, or a serine or threonine residue belonging to a protein. Disaccarides Enzymes on the outer surface of intestinal epithelium cleave common disaccharides. Sucrase cleaves sucrose (table sugar), lactase cleaves lactose (milk sugar), and maltase cleaves maltose. Sucrose, lactose, and maltose are common dietary components. The angles in the bonds to the central oxygen atoms do not denote carbon atoms. Electron micrograph of microvilli Lactase and other enzymes that hydrolyze carbohydrates are present on microvilli that project from the outer face of the plasma membrane of intestinal epithelial cells. Definitions: Polycacharides: Large polymeric oligosaccharides are called polysaccharides. Homoplymer: If all of the monosaccharides in the polysaccharide are the same, the polysaccharide is called a homopolymer. Glycogen: The polysaccharide glycogen is the storage form of glucose in animal cells. Most glucose units in glycogen are lined by α‐1, 4‐glycosidic bonds, with branches formed by α‐1, 6‐glycosidic bonds every 10 glucose units. Branch point in glycogen. In this figure, two chains of glucose molecules joined by α‐1,4‐glycosidic bonds are linked by an α‐1,6‐glycosidic bond to create a branch point. Such an α‐1,6‐glycosidic bond forms at approximately every 10 glucose units, making glycogen a highly branched molecule. Glucose is stored as starch in plant. There are two forms of starch: Amylose is a linear polymer of glucose units linked by α‐1, 4‐glycosidic bonds. Amylopectin is a branched polymer, with an α‐1, 6‐glycosidic bond for every 30 α‐1, 4‐glycosidic bonds. Pasta is a common source of starch Cellulose is a homopolymer of glucose units linked by β‐1, 4‐glycocidic bonds. The β linkage yields a straight chain capable of interacting with other cellulose molecules to from strong fibrils. The α linkages of starch and glycogen form compact hollow cylinders suitable for accessible storage. Although mammals cannot digest cellulose and other plant fibers, soluble fibers, such as polygalacturonic acid, aid in digestion. Glycosidic bonds determine polysaccharide structure The β‐1,4 linkages favor straight chains, which are optimal for structural purposes. The α‐1,4 linkages favor bent structures, which are more suitable for storage Glycoproteins: Proteins with carbohydrates attached are called glycoproteins. There are three main classes of glycoproteins: 1. Glycoproteins: The protein is the largest component by weight. Glycoproteins play a variety of roles including as membrane proteins. 2. Proteoglycans: The protein is attached to a particular type of polysaccharide called a glycosaminoglycan. By weight, proteoglycans are mainly carbohydrate. Proteoglycans play structural roles or act as lubricants. 3. Mucins or mucoproteins: Like proteoglycans, mucins are predominantly carbohydrate. The protein is characteristically attached to the carbohydrate by N‐ acetylgalactosamine. Mucins are often lubricants. In all classes of glycoproteins, carbohydrates are attached to the nitrogen atom in the side chain of asparagine (N‐linkage) or to the oxygen atom of the side chain of serine or threonine (O‐linkage). All N‐linked polysaccharides consist of a common pentasaccharide core, consisting of three mannoses, a six‐carbon sugar, and two N‐acetylgalactosamine units. Additional monosaccharides may be attached to the core. Glycosidic bonds between proteins and carbohydrates A glycosidic bond links a carbohydrate to the side chain of asparagine (N‐linked) or to the side chain of serine or threonine (O‐linked). The glycosidic bonds are shown in red. Abbreviations: GlcNAc, N‐ acetylglucosamine; GalNAc, N‐ acetylgalactosamine. N‐linked oligosaccharides (A) high‐mannose type B) Complex type A pentasaccharide core (shaded gray) is common to all N‐linked oligosaccharides and serves as the foundation for a wide variety of N‐linked oligosaccharides. Abbreviations for sugars: Fuc, fucose; Gal, galactose; GalNAc, N‐acetylgalactosamine; GlcNAc, N‐acetylglucosamine; Man, mannose; Sia, sialic acid. Oligosaccharides attached to erythropoietin The hormone Erythropoietin is A glycoprotein Clinical Insight: Erythropoietin is a glycoprotein secreted