Transcript PPT
Chapter 17: Carbohydrates 1 IMPORTANT FUNCTIONS OF CARBOHYDRATES • • • • To provide energy through their oxidation To supply carbon for the synthesis of cell components To serve as a stored form of chemical energy To form a part of the structural elements of some cells and tissues • Biomolecule – a general term referring to organic compounds essential to life • Biochemistry – a study of the compounds and processes associated with living organisms 2 CARBOHYDRATES • Carbohydrates are polyhydroxy aldehydes or ketones, or substances that yield such compounds upon hydrolysis. Example: 3 CLASSIFICATION OF CARBOHYDRATES • Carbohydrates are classified according to size: Monosaccharide – a single polyhydroxy aldehyde or ketone unit Disaccharide – composed of two monosaccharide units Polysaccharide – very long chains of linked monosaccharide units 4 STEREOCHEMISTRY • Many carbohydrates exist as enantiomers – stereoisomers that are mirror images. 5 • A chiral object cannot be superimposed on its mirror image. • A chiral carbon is one that has four different groups attached to it. 6 • The presence of a single chiral carbon gives rise to stereoisomerism. If a carbon atom is attached to four different groups, it is chiral. If any two groups are identical, it is not chiral. 7 • Compounds can have more than one chiral carbon: • The maximum number of stereoisomers is 2n where n= number of chiral carbon atoms. • Therefore, this compound with two chiral carbon atoms has 22 or 4 stereoisomers. • The compound on the previous slide with four chiral carbon atoms has 24 or 16 stereoisomers. 8 FISCHER PROJECTIONS • Fischer projections depict three-dimensional shapes for chiral molecules, with the chiral carbon represented by the intersection of two lines. 9 • Fischer projections of carbohydrates have the carbonyl (C=O) at the top. It is projecting away from the viewer behind the plane in which it is drawn. • The hydroxyl group on the chiral carbon farthest from the C=O group determines whether the carbohydrate is D (OH on right) or L (OH on left). The two horizontal bonds are coming toward the viewer out of the plane in which they are drawn. 10 • D and L enantiomers rotate polarized light in opposite directions. 11 • The enantiomer that rotates polarized light to the left is the levorotatory or (-) enantiomer. • The enantiomer that rotates it to the right is the dextrorotatory or (+) enantiomer. • The D and L designations do not represent dextrorotatory and levorotatory. • In some instances only the D or L enantiomers are found in nature. They are rarely found together in the same biological system. • For example, humans can only metabolize the D-isomers of monosaccharides. 12 MONOSACCHARIDE CLASSIFICATION • Is the monosaccharide an aldehyde (aldose) or ketone (ketose)? • How many carbon atoms are in the monosaccharide? 13 COMBINING MONOSACCHARIDE CLASSIFICATIONS 14 PHYSICAL PROPERTIES OF MONOSACCHARIDES • Most are called sugars because they taste sweet. • Because of the many –OH groups, they form hydrogen bonds with water molecules and are extremely water soluble. 15 MONOSACCHARIDE REACTIONS • All monosaccharides with at least five carbon atoms exist predominantly as cyclic hemiacetals and hemiketals. • A Haworth structure can be used to depict the (-OH on the anomeric carbon pointing down) and (-OH on the anomeric carbon pointing up) anomers of a monosaccharide. • Anomers are stereoisomers that differ in the 3-D arrangement of groups at the anomeric carbon of an acetal, ketal, hemiacetal, or hemiketal group. 16 SIX-MEMBERED PYRANOSE RING SYSTEM 17 FIVE-MEMBERED FURANOSE RING SYSTEM 18 MONOSACCHARIDE REACTIONS, cont. • The –OH groups of monosaccharides can behave as alcohols and react with acids (especially phosphoric acid) to form esters. 19 MONOSACCHARIDE REACTIONS, cont. • Cyclic monosaccharide hemiacetals and hemiketals react with alcohols to form acetals and ketals, referred to as glycosides. 20 IMPORTANT MONOSACCHARIDES • Ribose and Deoxyribose Used in the synthesis of DNA and RNA • Glucose Most nutritionally important monosaccharide Sometimes called dextrose or blood sugar 21 IMPORTANT MONOSACCHARIDES, cont. • Galactose A component of lactose (milk sugar) • Fructose The sweetest monosaccharide Sometimes called levulose or fruit sugar 22 DISACCHARIDES • Two monosaccharide units linked together by acetal or ketal glycosidic linkages • A glycosidic linkage is identified by: • the numbers associated with the carbon atoms joined ﾊ together by the linkage • the configuration of the linkage for any anomeric carbon atom joined by the linkage 23 IMPORTANT DISACCHARIDES • Maltose Two glucose units linked (14) Formed during the digestion of starch to glucose Found in germinating grain Hemiacetal means maltose is a reducing sugar 24 IMPORTANT DISACCHARIDES, cont. • Lactose Galactose and glucose units linked (14) Found in milk Hemiacetal means lactose is a reducing sugar • Sucrose Fructose and glucose units Found in many plants (especially sugar cane, sugar beets) Not a reducing sugar 25 POLYSACCHARIDES • Starch A polymer consisting of glucose units Has two forms •Unbranched amylose (10-20%) •Branched amylopectin (80-90%) Amylose complexes with iodine to form a dark blue color 26 THE STRUCTURE OF AMYLOSE The molecular conformation of starch and the starch-iodine complex: (a) the helical conformation of the amylose chain and (b) the starch-iodine complex. 27 AMYLOPECTIN STRUCTURE 28 POLYSACCHARIDES, cont. • Glycogen (animal starch) A polymer of glucose units Used to store glucose, especially in the liver and muscles Structurally similar to amylopectin with (14) and (16) linkages, but more highly branched 29 POLYSACCHARIDES, cont. • Cellulose A polymer of glucose units The most important structural polysaccharide Found in plant cell walls Linear polymer like amylose, but has (14) glycosidic linkages Not easily digested, a constituent of dietary fiber 30 CELLULOSE STRUCTURE 31