Bài giảng Biochemistry 2/e - Chapter 7: Carbohydrates

Chapter 7Carbohydratesto accompanyBiochemistry, 2/ebyReginald Garrett and Charles GrishamAll rights reserved. Requests for permission to make copies of any part of the work should be mailed to: Permissions Department, Harcourt Brace & Company, 6277 Sea Harbor Drive, Orlando, Florida 32887-6777 Outline7.1 Carbohydrate Nomenclature 7.2 Monosaccharides 7.3 Oligosaccharides 7.4 Polysaccharides7.1 NomenclatureCarbohydrates are hydrates of carbon Monosaccharides (simple sugars) cannot be broken down into simpler sugars under mild conditions Oligo = "a few" - usually 2 to 10 Polysaccharides are polymers of the simple sugars7.2 MonsaccharidesAn organic chemistry review Aldoses and ketoses contain aldehyde and ketone functions, respectively Triose, tetrose, etc. denotes number of carbons Aldoses with 3C or more and ketoses with 4C or more are chiral Review Fischer projections and D,L system Stereochemistry ReviewRead text on p. 210-213 carefully! D,L designation refers to the configuration of the highest-numbered asymmetric center D,L only refers the stereocenter of interest back to D- and L-glyceraldehyde! D,L do not specify the sign of rotation of plane-polarized light! All structures in Figures 7.2 and 7.3 are D D-sugars predominate in nature More StereochemistryKnow these definitionsStereoisomers that are mirror images of each other are enantiomers Pairs of isomers that have opposite configurations at one or more chiral centers but are NOT mirror images are diastereomers Any 2 sugars in a row in 10.2 and 10.3 are diastereomers Two sugars that differ in configuration at only one chiral center are epimersCyclic monsaccharide structures and anomeric formsGlucose (an aldose) can cyclize to form a cyclic hemiacetal Fructose (a ketose) can cyclize to form a cyclic hemiketal Cyclic form of glucose is a pyranose Cyclic form of fructose is a furanose Cyclic monsaccharide structures and anomeric formsCyclic forms possess anomeric carbons For D-sugars, alpha has OH down, beta up For L-sugars, the reverse is trueMonosaccharide DerivativesReducing sugars: 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 More Monosaccharide DerivativesSugar alcohols: mild reduction of sugars Deoxy sugars: constituents of DNA, etc. Sugar esters: phosphate esters like ATP are important Amino sugars contain an amino group in place of a hydroxyl group Acetals, ketals and glycosides: basis for oligo- and poly-saccharides 7.3 OligosaccharidesDon't memorize structures, but know the important features Be able to identify anomeric carbons and reducing and nonreducing ends Sucrose is NOT a reducing sugar Browse the structures in Fig. 7.19 and Figure 7.20 Note carefully the nomenclature of links! Be able to recognize alpha(1,4), beta(1,4), etc7.4 PolysaccharidesFunctions: storage, structure, recognition Nomenclature: homopolysaccharide vs. heteropolysaccharide Starch and glycogen are storage molecules Chitin and cellulose are structural molecules Cell surface polysaccharides are recognition molecules Starch A plant storage polysaccharide Two forms: amylose and amylopectin Most starch is 10-30% amylose and 70-90% amylopectin Branches in amylopectin every 12-30 residues Amylose has alpha(1,4) links, one reducing end Starch A plant storage polysaccharide Amylose is poorly soluble in water, but forms micellar suspensions In these suspensions, amylose is helicaliodine fits into the helices to produce a blue colorWhy branching in Starch?Consider the phosphorylase reaction... Phosphorylase releases glucose-1-P products from the amylose or amylopectin chains The more branches, the more sites for phosphorylase attack Branches provide a mechanism for quickly releasing (or storing) glucose units for (or from) metabolism GlycogenThe glucose storage device in animals Glycogen constitutes up to 10% of liver mass and 1-2% of muscle mass Glycogen is stored energy for the organism Only difference from starch: number of branches Alpha(1,6) branches every 8-12 residues Like amylopectin, glycogen gives a red-violet color with iodineDextrans A small but significant difference from starch and glycogen If you change the main linkages between glucose from alpha(1,4) to alpha(1,6), you get a new family of polysaccharides - dextrans Branches can be (1,2), (1,3), or (1,4) Dextrans A small but significant difference from starch and glycogen Dextrans formed by bacteria are components of dental plaque Cross-linked dextrans are used as "Sephadex" gels in column chromatography These gels are up to 98% water! Structural Polysaccharides Composition similar to storage polysaccharides, but small structural differences greatly influence properties Cellulose is the most abundant natural polymer on earth Cellulose is the principal strength and support of trees and plants Cellulose can also be soft and fuzzy - in cotton Structural Polysaccharides Composition similar to storage polysaccharides, but small structural differences greatly influence properties Beta(1,4) linkages make all the difference! Strands of cellulose form extended ribbonsOther Structural PolysaccharidesFurther reading - pages 232-235 Chitin - exoskeletons of crustaceans, insects and spiders, and cell walls of fungi similar to cellulose, but C-2s are N-acetyl cellulose strands are parallel, chitins can be parallell or antiparallel Other Structural PolysaccharidesAlginates - Ca-binding polymers in algae Agarose and agaropectin - galactose polymers Glycosaminoglycans - repeating disaccharides with amino sugars and negative charges