Chapter 7 Carbohydrates and Glycobiology Glycoconjugates complex carbohydrate polymers covalently attached to proteins or lipids that act as signals that determine their intracellular location or metabolic fate Carbohydrates are polyhydroxy aldehydes or ketones or substances that yield such compounds on hydrolysis o Usually have the empirical formula CH2O n o Some contain nitrogen phosphorus or sulfur Three major size classes o Monosaccharides aldehyde or ketone unit cyclic o Oligosaccharides glycosidic bonds simple sugars that consist of a single polyhydroxy Monosaccharides that have four or more carbons tend to be short chains of monosaccharide units joined by Most abundant are disaccharides which have two monosaccharide units All common mono and oligosaccharides have a name ending in ose Most oligosaccharides consisting of three or more units are glycoconjugates o Polysaccharides monosaccharide units Can be linear or branched sugar polymers containing more than 20 7 1 Monosaccharides and Disaccharides monosaccharides are usually aldehydes or ketones with two or more hydroxyl groups o Many have chiral carbons to which the hydroxyl groups are attached The Two Families of Monosaccharides Are Aldoses and Ketoses monosaccharides are colorless crystalline solids that are freely soluble in water but insoluble in nonpolar solvents o most have a sweet taste o have unbranched carbon chain backbones with single bonds only if a carbonyl group is at the end of the carbon chain in a monosaccharide the monosaccharide is an aldose If the carbonyl group is anywhere else ketone the monosaccharide is a ketose Monosaccharides Have Asymmetric Centers optically active isomers a molecule with n chiral centers can have 2n stereoisomers two groups of stereoisomers that differ in the configuration about the chiral center most distant from the carbonyl carbon o D isomers when the configuration at this carbon is the same as that Hydroxyl group is on the right in a projection formula o L isomers when the configuration at this carbon is the same as that of D glyceraldehyde of L glyceraldehyde Hydroxyl group is on the left in a projection formula two sugars that differ only in the configuration around one carbon Epimers atom The Common Monosaccharides Have Cyclic Structures the cyclic structure is created by a covalent bond between the carbonyl group and the oxygen of a hydroxyl group along the chain o a result of a reaction between alcohols and aldehydes or ketones to form derivatives called hemiacetals or hemiketals means that the hydroxyl group at the anomeric center is in a Fischer projection on the same side as the hydroxyl attached at the farthest chiral center is the opposite Pyranoses Furanoses about the hemiacetal or hemiketal carbon six membered ring compounds five membered aldose ring compounds isomers of monosaccharides that differ only in their configuration Anomers mutarotation o anomeric carbon the hemiacetal or carbonyl carbon atom a process by which the and anomers of D glucose interconvert in aqueous solution Haworth perspective formulas forms of monosaccharides Conformations are interconvertible without the breakage of covalent bonds Configurations can be interconverted only by breaking a covalent bond used to show the stereochemistry of ring Organisms Contain a Variety of Hexose Derivatives Aldonic acids Uronic acid yielded by oxidation of an aldose to the carboxyl level There are a number of sugar derivatives in which a hydroxyl group I replaced with another substituent or the carbon atom is oxidized to a carboxyl group chain than the carbonyl Carbohydrates are often phosphorylated as intermediates in their synthesis and metabolism yielded by oxidation of the carbon at the opposite end of the o Sugar phosphates are stable and bear neutral pH and a negative o Sugar phosphorylation traps sugars in the cell o Phosphorylation also activates sugars for subsequent chemical charge transformation Monosaccharides Are Reducing Agents The carbonyl group of a monosaccharide can be oxidized to a carboxylic acid by relatively mild oxidizing agents Reducing sugars glucose and other sugars capable of reducing cupric ion Disaccharides Contain a Glycosidic Bond O glycosidic bond make a disaccharide the bond that joins two monosaccharides covalently to o A hydroxyl group of one sugar reacts with the anomeric carbon of the o Represents formation of an acetal from a hemiacetal and an alcohol o Resulting compound is a glycoside o Readily hydrolyzed to yield the free monosaccharides by boiling with other dilute acid Oxidation of a sugar by cupric ion occurs only with the linear form Reducing end the end of a chain with a free anomeric carbon 7 2 Polysaccharides Most carbohydrates are polysaccharides also called glycans Polysaccharides differ from each other o In the identity of their recurring monosaccharide units o In the length of their chains o In the types of bonds linking the units o In the degree of branching Homopolysaccharides contain only a single monomer species o Some serve as storage forms of monosaccharides or structural elements Heteropolysaccharides contain two or more different kinds o Provide extracellular support Some Homopolysaccharides Are Stored Forms of Fuel The most important storage polysaccharides o Starch in plant cells o Glycogen in animal cells Starch contains amylose and amylopectin two types of glucose polymer o Amylose consists of long unbranched chains of D glucose residues connected by 1 4 linkages o Amylopectin is highly branched and the chains are joined by the same linkages as amylose Branch points are 1 6 linkages Glycogen the main storage polysaccharide of animal cells o A linked polymer of 1 4 linked subunits of glucose with 1 o More extensively branched and more compact o A glycogen molecule with n branches has n 1 nonreducing ends and 6 linked branches only one reducing end bacterial and yeast polysaccharides made up of 1 6 linked o All have 1 3 branches and some also have 1 2 or 1 4 Dextrans poly D glucose branches Some Homopolysaccharides Serve Structural Roles Cellulose is found in the cell walls of plants and in cotton o Linear unbranched homopolysaccharide o Glucose residues have the configuration unlike amylose o Glucose residues are linked by 1 4 glycosidic bonds Glycogen and starch are hydrolyzed by amylases and glycosidases enzymes in the saliva and intestine that break glycosidic bonds between glucose units o Animals cannot digest cellulose a linear