【生物化學(xué)】Chapter1-Amino-a課件

Chapter1

AminoacidsChapter1

AminoacidsOutlineStructureandclassificationofAAPropertiesandfunctionofAASeparationandpurificationofAAOutlineStructureandclassific2AminoacidstructureDifferentside-chain(Rgroup)DifferentchemicalandphysicalpropertiesAminogroupCarboxylicacidgroupAminoacidstructureDifferentProteinogenicAA&non-protenogenicAAProteinogenicAAorcanonicalAA.EncodedbygeneticcodesanddirectlyintroducedintoproteinduringtranslationDifferinsidechain(Rgroup)20commonlyfoundAllorganismshavesamesetof202rarelyfound(selenocysteineandpyrrolysine)Non-proteinogenicAAornon-canonicalAANeverdirectlyintroducedintoproteinsduringtranslationCanbenaturally-occurringorchemicalmodificationsofproteinogenicAAProteinogenicAA&non-proteno4AminoAcidClassificationAliphaticAromaticSulfurcontainingPolar/unchargedbasic/acidicHydrophobic:waterfearing.non-polarsidechainsHydrophillic:waterloving.polar,neutralchains,negativelycharged,positivelychargedAminoAcidClassificationAliph6Non-polarAminoAcids6Non-polarAminoAcids6Polar,UnchargedAminoAcids6Polar,UnchargedAminoAcids3AromaticAminoAcids3AromaticAminoAcids86Polar,ChargedAminoAcids6Polar,ChargedAminoAcids21st&22ndAAs21st&22ndAAsEssentialAminoAcidsAll20commonaminoacidsarerequiredforproteinsynthesis,butthehumanbodyisabletosynthesizeonly12ofthem.Theother8(theessentialaminoacids)mustbeobtainedfromfood,whichincludethreonine,isoleucine,phenylalanine,methionine,tryptophan,valine,,leucine,andlysine.Histidineandargininearecalledsemi-essentialaminoacidsbecausetheyarealsorequiredtobeobtainedfromfoodundersomeconditions.EssentialAminoAcidsAll20coAbbreviationsfor22aminoacidsAlanine-Ala-AArginine-Arg-RAsparagine-Asn-NAsparticAcid-Asp-DCysteine-Cys-CGlutamine-Gln-QGlutamicAcid-Glu-EGlycine-Gly-GHistidine-His-HIsoleucine-Ile-ILeucine-Leu-LLysine-Lys-KMethionine-Met-MPhenylalanine-Phe-FProline-Pro-PSerine-Ser-SThreonine-Thr-TTryptophan-Trp-WTyrosine-Tyr-YValine-Val-VAsparagine/AsparticAcid-Asx-BGlutamine/GlutamicAcid-Glx-ZSelenocysteine-sec-uPyrrolysine-pyl-oAbbreviationsfor22aminoaciDiagrammaticrepresentationofAAclassificationDiagrammaticrepresentationof13Propertiesof20CommonAminoAcidsPropertiesof20CommonAminoAminoAcidsnotfoundinProteinsAminoAcidsnotfoundinProtePropertiesofAminoAcidsCapacitytopolymerizeChiralityNovelacid-basepropertiesVariedstructureandchemicalfunctionalityPropertiesofAminoAcidsCapacAACondensationandPeptideBondFormationAPeptideBondisanamidebondlinkingtogetheraminoacidsformingpeptidesandproteins.PeptidebondAACondensationandPeptideBo【生物化學(xué)】Chapter1-Amino-a課件L-AminoAcidsSinceallaminoAcidsbutglycinearechiralmolecules,it’spossibletohaveenantiomers.However,natureusesonlyoneenantiomer,theL-aminoacid.L-glyceraldehydeL-aminoacidL-AminoAcidsSinceallaminoAZwitterionsandpIAZwitterionisadipolarion.Sinceaminoacidscontainbothanacidandabase,aninternalacid-basereactionformsazwitterion.AminoacidZwitterionAminoacidsexistprimarilyaszwitterions.ZwitterionsandpIAZwitterionZwitterionsAminoacidzwitterionsareamphoteric.Theycanreactaseitheracidsorbases.InacidsolutionInbasesolutionzwitterionprotonatedzwitteriondeprotonatedZwitterionsAminoacidzwitteriIsoelectricPointsTheisoelectricpointofanaminoacidoccursatthepHwheretheaminoacidexistsasthezwitterion.deprotonatedbasesolutionhighpHprotonatedacidsolutionlowpHzwitterionisoelectricpointIsoelectricPointsTheisoelectTitrationCurveforAlaninepI(isoelectricpoint)=thepHatwhichthenumberofpositiveandnegativechargesonapopulationofmoleculesisequal(i.e.nonetcharge).pK1carboxylicacid=2pK2aminogroup=10pI=(pK1+pK2)/2TitrationCurveforAlaninepITitrationCurveforGlutamicAcidpK1carboxylicacid=2.2pK2Rgroup=4.3pK3aminogroup=9.7pI=(pK1+pK2)/2pI=(2.2+4.3)/2pI=3.25TitrationCurveforGlutamicApK1carboxylicacid=2.2pK2aminogroup=9.0pK3Rgroup=10.5pI=(pK2+pK3)/2pI=(9+10.5)/2pI=9.75TitrationCurveforLysinepK1carboxylicacid=2.2TitraNonetchargeMinimumsolubilityinwaterproteinwillprecipitateoutatitsisoelectricpointcanseparateaminoacidsandpeptidesbasedinelectrophoresis:+chargedaminoacidsmoveto–electrode-chargedaminoacidsmoveto+electrode0aminoacidsattheirisoelectricpointsdonotmoveIsoelectricPointsNonetchargeIsoelectricPointReactionwithNinhydrinUsedtovisualizespotsorbandsofaminoacidsseparatedbychromatographyorelectrophoresis.DeeppurplecolorformedwithtracesofanyaminoacidexceptPro(yellow).ReactionwithNinhydrinUsedtReactionwithHNO2

andVanSlykeDeterminationProcannotreactwithHNO2ReactionwithHNO2

andVanSReactionwithDNFBandPITCReactionwithDNFBandPITCUV-absorbingPropertiesofAminoAcidsOnlythreeaminoacids,Phe,Tyr,andTrp,absorblightinthenearUVrange(230nm-300nm).TheseaminoacidsdominatetheUVabsorptionspectraofproteins.Thewavelengthmaximafortyrosineandtryptophanarearound280nm.Incontrast,nucleicacidshaveabsorptionmaximumof260nm.ThusasimpleUVscancanallowonetodistinguishbetweenproteinandnucleicacids.UV-absorbingPropertiesofAmiFunctionalSignificanceofAAR-GroupsInsolutionitisthenatureoftheaminoacidR-groupsthatdictatestructure-functionrelationshipsofpeptidesandproteins.Thehydrophobicaminoacidswillgenerallybeencounteredintheinteriorofproteinsshieldedfromdirectcontactwithwater.Conversely,thehydrophilicaminoacidsaregenerallyfoundontheexteriorofproteinsaswellasintheactivecentersofenzymes.TheimidazoleringofhistidineallowsittoactaseitheraprotondonororacceptoratphysiologicalpH.Hence,itisfrequentlyfoundinthereactivecenterofenzymes.Theprimaryalcoholofserineandthreonineaswellasthethiol(-SH)ofcysteineallowtheseaminoacidstoactasnucleophilesduringenzymaticcatalysis.Additionally,thethiolofcysteineisabletoformadisulfidebondwithothercysteines:Cysteine-SH+HS-Cysteine<-------->Cysteine-S-S-CysteineDisulfidebondingbetweencysteinesindifferentpolypeptidechainsofoligomericproteinsplaysacrucialroleinorderingthestructureofcomplexproteins,e.g.theinsulinreceptor.FunctionalSignificanceofAASeparationandAnalysisofAAMixturesSeparatingaminoacidsinmixturesisusuallybasedonrelativedifferencesintheirphysicalandchemicaltraits.Theseareallmediatedbytheir“R”orfunctionalgroups.Thegeneralstrategyistoexploittheabilityofagivenaminoacidtopartitionbetweentwodifferentphases.Itcouldbetwoliquidphases,asolid-liquidphase,oragas-liquidphase.Inparticular,solid-liquidphasemethodsareroutinely