Carbohydrates碳水化合物【清华大学出品】

BiochemistryLiuDongOffice:人环楼414Tel:[phone]Email:[e-mail]iologyCarbohydratesCarbohydratesisoneofmajorclassofbiomolecules.Mostcarbohydratesandtheirderivativeshavetheformulaof(CH2O)n.TheyarealsocalledSugarorSaccharide(FromGreeksakcharon,meaning“sugar”)Carbohydratesarethemostabundantbiomoleculesontheearth.Morethanhalfoftheorganiccarbonontheearthisstoredinjusttwotypeofcarbohydratemolecules—StarchandCellulose.Whatarecarbohydrates?Carbohydrateisacollectivenameofpolyhydroxyaldehydesorketones,andtheircondensationalproductsorderivatives.C=OC=OCCHOAldehydegroupKetonegroupHydroxygroup-OHCarbonylgroupCClassificationofCarbohydratesMonosaccharides:Theyconsistofasinglepolyhydroxyaldehydeorketounit.Theycannotbefurtherbrokendownbyhydrolysis.Oligosacchrides:Theyconsistofshortchainsofmono-sacchridesunits,orresidues,joinedbyglycosidicbonds.Amongthem,themostabundantoneisthedisacchrides.Polysacchrides:Theycontainmorethan20monosacchridesunits,andsomehavehundredsandthousandunits.Carbohydratescanbeclassifiedasmonosaccharides,oligosaccharides,andpolysaccharides.MonosaccharidesThesmallestsugar—trioseWhenthecarbonylgroupisatanendofthecarbonchain,orinanaldehydegroup,themonosaccharideisanaldose;anywhereelse,aketose.Monosacchridesisthesimplestformofcarbohydrate.Theycanbedividedintotwofamilies-AldosesandKetoses.TheNamingofMonosacchridesThenumberofcarbonSugarnameatominbackbones3Triose(aldotriose/ketotriose)4Tetrose(aldotetrose/ketotetrose)5Pentose(aldopentose/ketopentose)6Hexose(aldohexose/ketohexose)7Heptose(aldoheptose/ketoheptose)ThestructureoftwopentosesD-riboseisacomponentofRNAand2-deoxy-D-riboseisacomponentofDNAThestructureoftwohexosesLightbeamDetectorDextrorotatoryLevorotatoryChiralityChirality:Astructuralcharacteristicofamoleculethatmakesitimpossibletosuperimposeitonitsmirrorimage.Thechiralityiscausedbyasymmetriccarbonatom(Onecarboncenterwithfourdifferentsidegroups).ChiralityofMonosaccharidesAllthemonosaccharides,exceptdihydroxyacetone,containsoneormoreasymmetric(chiral)carbonatoms.ThestructureofglyceraldehydeThechiralityofmonosaccharidesiscausedbytheirasymmetriccarbonsD-andL-formofmonosaccharideThetwostereoisomericformsofthemonosaccharidemoleculesaredesignatedasD-(Dextrorotary)andL-(Levorotatory)form.Byconvention,theglyceraldehydethathasitshydroxygroupontherightofthechrialcarbonatomisdesignatedtheD-form.AllthemonosaccharidesderivedfromD-glyceraldehydeareD-isomers.Accordingly,allthemonosaccharidesderivedfromL-glyceraldehydeareL-isomers.ThestereoisomersofsugarmoleculesIngeneral,amoleculewithnchiralcenterscanhave2nstereoisomers.Athree-carbonsugar(aldose)hasonechiralcenter,thushas21=2isomers;ThestereoisomersofaldotroseThealdotrosehastwochiralcenters,thushas22=4isomers.ThestereoisomersofaldopentoseThealdopentosehasthreechiralcenters,thushas23=8isomers.ThestereoisomersofaldohexosesThealdohexosehasfourchiralcenters,thushas24=16isomers.ThestereoisomersofketosesKetosewiththreeandfourcarbonsKetoseshavelessstereoisomericformscomparedtoaldoseswithsamenumberofcarbonatoms.ThestereoisomersofketohexosesThealdohexosehasthreechiralcenters,thushas23=8isomers.AninterestingphenomenaforglucoseForanewlypreparedglucosesolution,ithasadetrorotataryvalueof+112º.Withthetimegoes,itdecreasesto+52.7º.Ifglucoseiscrystallizedat+110ºC,andisredissolvedinwater,thedetrorotataryvalueobservedatbeginningis+19º.Afterawhile,Itchangesto+52ºagain.Howtoexplainthis?FormationofhemiacetalandhemiketalsFormationofthetwocyclicformsofD-glucoseCyclicstructuresofmonosaccharidesInaqueous,themonosaccharideswithfiveormorecarbonsoccurpredominantlyincyclicform.Formationofcyclicglucosecreatesanadditionalchiralcarbon,thusproducethetwostereoisomers,aandb.Thesetwoisomericformsarecalledanomerswhichdifferonlyintheirconfigurationaroundthehemiacetalorhemiketalcarbon.PyranoseandFuranosePyranose:Sugarwithsix-memberedringstructureFuranose:Sugarwithfive-memberedringstructurePyranosecanexistintwochairformsSomeimportanthexosederivativesinbiologyOrganismscontainsavarietyofhexosederivatives.Inthesehexosemolecules,ahydroxygroupcanbephosphorylated,orreplacedwithanothersubstitute,oracarbonatomisoxidizedtoacarboxylgroup.Phosphorylationofβ-D-glucosecanpreventitsdiffusiontooutsideofcellsSomeimportanthexosederivativesinbiologyAminosugarsandtheirderivatives(lacticacid)Thesesugarderivativesarecomponentsofbacterialcellwallsandexoskeletonofarthropods.SomeimportanthexosederivativesinbiologyAcidicsugarsUronicAcidAldonicacidLactoneThesearecomponentsofmanyglycoproteinsandglycolipidsDisaccharidesDisaccharideconsistsoftwomonosaccharidesjoinedcovalentlybyanO-glycosidicbond.FormationofmaltoseO-glycosidicbondO-glycosidicbondThestructureofsucroseO-glycosidicbondThestructureoflactoseGlycosidicbondscanbereadilyhydrolyzedbyacidbutresistcleavagebybase.MonosaccharidesarereducingagentsWheninitsopenchain,thecarbonylgroupofmonosaccharidescanbeoxidizedtoacarboxylgroupAnomericcarbonReducingvsnonreducingsugarsSucroseReducingsugarNon-reducingsugarMaltoseFehling’stestTheproductionofaredprecipitate