BCHM461 Final Exam Textbook Notes Chapters 7 8 and 10 LaRonde Spring 2014 Chapter 7 Carbohydrates and Glycobiology Carbohydrates are polyhydroxy aldehydes or ketones or substances that yield such compounds on hydrolysis There are 3 major size classes of carbohydrates monosaccharides oligosaccharides and polysaccharides Monosaccharides simple sugars that consist of a single polyhydroxyl aldehyde or ketone unit the most abundant is the 6 carbon sugar D glucose dextrose Oligosaccharides consist of short chains of monosaccharide units or residues joined by glycosidic bonds the most abundant are disaccharides with two monosaccharide units i e sucrose consists of glucose and fructose Polysaccharides sugar polymers containing more than 20 or so monosaccharide units i e cellulose and glycogen I 7 1 Monosaccharides and Disaccharides Many of the carbon atoms to which hydroxyl groups are attached are chiral centers this gives rise to the many sugar stereoisomers and is significant because the enzymes that act on sugars are strictly stereospecific The two families of monosaccharides are aldoses and ketoses Monosaccharides are colorless crystalline solids that are freely soluble in water but The backbones are unbranched carbon chains in which all the carbon atoms are linked insoluble in nonpolar solvents by single bonds o One carbon atom is double bonded to an oxygen atom to form a carbonyl group each of the other carbon atoms has a hydroxyl group Aldose carbonyl group is at the end of the carbonyl chain Ketose carbonyl group is at any other position along the carbon chain Monosaccharides with 4 5 6 7 carbon atoms tetroses pentoses hexoses and heptoses respectively Monosaccharides have asymmetrical centers Enantiomers two different optical isomers due to central chiral carbon atom A molecule with n chiral centers can have 2n stereoisomers When the hydroxyl group on the reference carbon is on the right dextro in a projection formula that has the carbonyl carbon at the top the sugar is the D isomer When the hydroxyl group is on the left levo of the reference carbon it is the L isomer Two sugars that differ only in the configuration around one carbon atom are called epimers i e glucose and mannose glucose and galactose The common monosaccharides have cyclic structures 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 When the two molecules that react are both monosaccharides the acetal or ketal formed is a disaccharide Cyclic sugar structures are more accurately represented in Haworth perspective formulas than in the Fischer projections Isomeric forms of monosaccharides that differ only in their configuration about the hemiacetal and hemiketal carbon atom are called anomers and the carbonyl carbon is called the anomeric carbon The alpha and beta anomers of D glucose interconvert in aqueous solution by a process called mutarotation in which one ring form opens briefly into the linear form then closes again to produce the other anomer Monosaccharides are reducing agents The carbonyl carbon is oxidized to a carboxyl group Glucose and other sugars capable of reducing relatively mild oxidizing agents such as cupric Cu2 ion are called reducing sugars maltose and lactose Disaccharides maltose lactose and sucrose consist of two monosaccharides joined covalently by a glycosidic bond which is formed when a hydroxyl group of one sugar molecule reacts with the anomeric carbon of the other Glycosidic bonds are readily hydrolyzed by acid but resist cleavage by base Because the carbonyl carbon can be oxidized only when the sugar is in its linear form formation of a glycosidic bond renders a sugar nonreducing sucrose contains no free anomeric carbon atom because both are involved in the glycosidic bond The end of a chain with a free anomeric carbon is called the reducing end II 7 2 Polysaccharides Polysaccharides also called glycans differ from each other in the identity of their recurring monosaccharide units in the length of their chains in the types of bonds linking the units and the degree of branching Homopolysaccharides contain one single monomeric species o Serve as storage forms of monosaccharides i e starch glycogen cellulose and chitin o Starch contains two types of glucose polymer amylose unbranched glucose residues in alpha configuration and amylopectin highly branched o Glycogen is a polymer of linked subunits of glucose with branches more branches and compact than starch Because each branch in glycogen ends with a nonreducing sugar unit a glycogen molecule with n branches has n 1 nonreducing ends but only one reducing end When glycogen is used as an energy storage glucose units are removed one at a time from the nonreducing ends Why not keep monomeric glucose stored in cells Because the osmolarity would be greatly increased and water will flow into cell to cause it to rupture o Dextrans are bacterial and yeast polysaccharides dental plaques are rich in dextrans provides a source of glucose for bacterial metabolism o Cellulose is a fibrous tough water insoluble substance found in cell walls of plants linear unbranched homopolysaccharide glucose residues in beta configuration Cattle sheep and goats have rumens containing symbiotic microorganisms to help break down cellulose o Chitin is a linear homopolysaccharide composed of N acetylglucosamine residues contained in the exoskeleton of arthropods Heteropolysaccharides contain two or more monomeric species o Provide extracellular support for organisms i e peptidoglycan o Bacterial and algal cell walls contain structural heteropolysaccharides o Peptide cross links weld the polysaccharide chains into a strong sheath peptidoglycan that envelopes the entire cell and prevents cellular swelling and lysis due to the osmotic entry of water Penicillin and related antibiotics kill bacteria by preventing o Algae contains agarose in the cell walls agar is a complex mixture of polysaccharides with the same backbone structure but substituted with sulfate and pyruvate Agarose gels are used as inert support for the electrophoretic separation of nucleic acids for DNA sequencing o Glycosaminoglycans are heteropolysaccharides of the extracellular matrix ECM linear polymers composed of repeating disaccharides not found in plants Sulfated glycosaminoglycans are attached to extracellular proteins to form proteoglycans Hyaluronan forms the viscous fluid