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Lecture 11

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Biological Sciences
Shelley A.Brunt

Lecture 11: Carbohydrates and Glycoproteins  Carbohydrates can be described by the number of monomeric units they contain.  Monosaccharides are the smallest units of carbohydrate structure.  Oligosaccharides are polymers of two to about 20 monosaccharide residues. The most common oligosaccharides are disaccharides, which consist of two linked monosaccharide residues.  Polysaccharides are polymers that contain many (usually more than 20) monosaccharide residues.  Oligosaccharides and polysaccharides do not have the empirical formula (CH2O)n because water is eliminated during polymer formation.  The term glycan is a more general term for carbohydrate polymers. It can refer to a polymer of identical sugars (homoglycan) or of different sugars (heteroglycan). Most Monosaccharides Are Chiral Compounds  Monosaccharides are water-soluble, white, crystalline solids that have a sweet taste. Examples include glucose and fructose.  Chemically, monosaccharides are polyhydroxy aldehydes, or aldoses, or polyhydroxy ketones, or ketoses. They are classified by their type of carbonyl group and their number of carbon atoms. As a rule, the suffix -ose is used in naming carbohydrates, although there are a number of exceptions.  All monosaccharides contain at least three carbon atoms. One of these is the carbonyl carbon, and each of the remaining carbon atoms bears a hydroxyl group. In aldoses, the most oxidized carbon atom is designated C-1 and is drawn at the top of a Fischer projection. In ketoses, the most oxidized carbon atom is usually C-2.  By convention sugars are said to have the D confirmation when the chiral carbon with the highest number (i.e. the chiral carbon most distant from the carbonyl carbon) is the same as that of C-2 of D-glyceraldehyde (i.e. the – OH group attached to this carbon atom is on the right side of the Fisher projection)  It is mostly D enantiomers that are synthesized in living cells  Sugar molecules that differ in configuration at only one of several chiral centers are called epimers, e.g. D-mannose and D-galactose are epimers of D-glucose but not each other Cyclic monosaccharide structures and anomeric forms  The carbonyl carbon of an aldose containing at least five carbon atoms or of a ketose containing at least six carbon atoms can react with an intramolecular hydroxyl group to form a cyclic hemiacetal or cyclic hemiketal, respectively. The oxygen atom from the reacting hydroxyl group becomes a member of the five- or six-membered ring structures.  Glucose (an aldoses) can cyclize to form a cyclic hemiacetal  Fructose (a ketose) can cyclize to form a cyclic hemiketal  Cyclic form of glucose is a pyranose (the six-membered ring of a monosaccharide)  Cyclic form of fructose is a furanose (the five-membered ring of a monosaccharide)  The most oxidized carbon of a cyclized monosaccharide, the one attached to two oxygen atoms, is referred to as the anomeric carbon. In ring structures, the anomeric carbon is chiral. Thus, the cyclized aldose or ketose can adopt either of two configurations (designated α or β), as illustrated for D-glucose in Figure 8.8. The α and β isomers are called anomers.  For D-sugars, the alpha configuration has OH down, beta configuration has OH up  For L-sugars, the reverse is true Derivatives of Monosaccharides  There are many known derivatives of the basic monosaccharides. They include polymerized monosaccharides, such as oligosaccharides and polysaccharides, as well as several classes of nonpolymerized compounds. Sugar Phosphates  Monosaccharides in metabolic pathways are often converted into phosphate esters Sugar esters  Phosphate esters like ATP are important Deoxy Sugars  Constituents of DNA Amino Sugars  In a number of sugars, an amino group replaces one of the hydroxyl groups in the parent monosaccharide.  Sometimes the amino group is acetylated. Sugar Alcohols  In a sugar alcohol, the carbonyl oxygen of the parent monosaccharide has been reduced, producing a polyhydroxy alcohol.  Mild reduction of sugars Sugar Acids (Reducing Sugars)  Sugar acids are carboxylic acids derived from aldoses, either by oxidation of C-1 (the aldehydic carbon) to yield an aldonic acid or by oxidation of the highest-numbered carbon (the carbon bearing the primary alcohol) to yield an alduronic acid.  Sugars with free anomeric carbons – they will reduce oxidizing agents, such as peroxide, ferricyanide and some metals (Cu and Ag)  These redox reactions convert the sugar to a sugar acid  Glucose is a reducing sugar – so these reactions are the basis for diagnostic tests for blood sugar Metabolically important sugar phosphates  The triose phosphates, ribose 5-phosphate, and glucose 6-phosphate are simple alcohol- phosphate esters. Glucose 1-phosphate is a hemiacetal phosphate, which is more reactive than an alcohol phosphate. The ability of UDP-glucose to act as a glucosyl donor (Section 7.3) is evidence of this reactivity. Deoxy Sugars  2-Deoxy-D-ribose is an important building block for DNA. L-Fucose (6-deoxy-L- galactose) is widely distributed in plants, animals, and microorganisms. Despite its unusual L configuration, fucose is derived metabolically from D-mannose. Sugar Alcohols  Glycerol and myo-inositol are important components of lipids (Section 10.4). Ribitol is a component of flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD) (Section 7.4).  In general, sugar alcohols are named by replacing the suffix -ose of the parent monosaccharides with -itol. Amino Sugars  Amino sugars formed from glucose and galactose commonly occur in glycoconjugates. N-Acetylneuraminic acid (NeuNAc) is an acid formed from N-acetylmannosamine and pyruvate. When this compound cyclizes to form a pyranose, the carbonyl group at C-2 (from the pyruvate moiety) reacts with the hydroxyl group of C-6. NeuNAc is an important constituent of many glycoproteins and of a family of lipids called gangliosides (Section 9.5). Neuraminic acid and its derivatives, including NeuNAc, are collectively known as sialic acids. Disaccharides  The glycosidic bond is the primary structural linkage in all polymers of monosaccharides.  A glycosidic bond is an acetal linkage in which the anomeric carbon of a sugar is condensed with an alcohol, an amine, or a thiol.  Compounds containing glycosidic bonds are called glycosides; if glucose supplies the anomeric carbon, they are specifically termed glucosides.  The glycosides include disaccharides, polysaccharides, and some carbohydrate derivatives.  The end containing the free anomeric carbon atom is called the reducing sugar  The other end is the nonreducing end Polysaccharides  Polysaccharides are frequently divided into two broad classes.  Homoglycans, or homopolysaccharides, are polymers containing residues of only one type of monosaccharide.  Heteroglycans, or heteropolysaccharides, are polymers containing residues of more than one type of monosaccharide.  Polysaccharides are created without a template by the addition of particular monosaccharide and oligosaccharide residues. As a result, the lengths and compositions of polysaccharide molecules may vary within a population of these molecules.  Most polysaccharides can also be classified according to their biological roles. For example, starch and glycogen are storage polysaccharides while cellulose and chitin are structural polysaccharides. Starch and Glycogen  D-Glucose is synthesized in all species. Excess glucose can be broken down to produce metabolic energy.  Glucose residues are stored as polysaccharides until they are needed for energy production.  The most common storage homoglycan of glucose in plants and fungi is starch and in animals it is glycogen. Both types of polysaccharides occur in bacteria.  Starch is present in plant cells as a mixture of amylose and amylopectin and is stored in granules whose diameters range from 3 to 100 μm.  Most starch is 10-30% amylose and 70-90% amylopectin  Amylose is an unbranched polymer of about 100 to 1000 D-glucose residues connected by α-(1→4) glycosidic linkages, specifically termed α-(1→4) glucosidic bonds because the anomeric carbons belong to glucose residues.  Amylopectin is branched every 12-30 residue via α-(1→6) glycosidic linkages Structure of amylose
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