蛋白質(zhì)的結(jié)構(gòu)和生物學(xué)功能(Structure-and-biological-function-of-protein)

蛋白質(zhì)的結(jié)構(gòu)和生物學(xué)功能（Structureandbiologicalfunctionofprotein）StructureandbiologicalfunctionofproteinProteinisthebasicsubstancethatmakesupcellsandorganisms,accountingforhalfofthedryweightofcells.Thecontentofproteininbiologicalmembranesaccountsfor60%to70%,andtheproteinoccupies80%oftheorganiccomponentsofprotoplasm.Alltheelementsoftheproteingroup,Chengdu,aresimilar.Theycontainfourelements,C,H,OandN.Theaveragenitrogencontentisabout16%,whichisacharacteristicofproteininthecompositionofelements.Proteinisakindofverycomplexnitrogen-containingmacromolecularcompound,anditsbasiccomponentisaminoacid.1.thebasicunitofproteinaminoacidsThereare20kindsofaminoacidsthatmakeupproteins,19ofwhichcanberepresentedbyageneralformula.Theotherisproline,whichalsohasasimilarstructure,butthesidechainsarelinkedwithnitrogenatomstoformaminoacids.Exceptglycine,aminoacidsintheproteinareasymmetriccarbonatoms,areLtypeandDtype,thedifferencebetweentwoconfigurations,comparedwithglyceraldehydeconfiguration,aLtypeartificialregulation,anotherisatypeofD.Whenwriting,theNH2iswrittentotheleft,typeL,andNH2totheright,andD.KnownaminoacidsinnaturalproteinsareoftypeL.Ofthe20basicaminoacids,manyaresynthesizedfromothercompoundsinlivingorganisms.Butthereare8kindsofaminoacidswhicharenotsynthesizedinhumanbody.Theyarecalledessentialaminoacids.Theyarethreonine(Thr),leucine(Leu),isoleucine(Ile),methylsulfate(Met),phenylalanine(Phe),tryptophan(Try),lysine(Lys)andvaline(Val).Theclassificationof20aminoacids,mainlybasedontheRgroup.InearlieryearsaccordingtothestructureofRbasedaminoacidintoaliphatic,aromaticandheterocyclicthreecategories,whicharedividedintoneutralaliphatic(monoaminodicarboxylicacid(two),acarboxylamino)andalkaline(twoaminoacidswithacarboxylgroup).Inrecentyears,thepolarityofRgrouphasbeenusedtodistinguishthekindsofaminoacids.Foralpha-aminoacidscontaininganaminogroupandacarboxylgroup,dotheyexistinneutralorsolidstatesinneutralorzwitterionicforms?Manyexperimentshaveprovedtoexistmainlyintheformofzwitterionicions.Neutralformsofmolecules;zwitterionicformsAnaminoacid,suchasanaminoacidandacarboxylgroup,thatischemicallycharacterizedasanamphotericcompoundwithweakbasesandweakacids.Thechargedstateofaminoacidinthesolution,andchangeswiththepHvalueofthesolution,ifthenetchargeofaminoacidsisequaltozero,andinmovingtothepositiveornegativephenomenondoesnotoccurintheelectricfield,thesolutioninthisstatepHvalueiscalleditsisoelectricpoint,pIsaid.Sinceallkindsofaminoacidshavespecificisoelectricpoint,whenthepHvalueofthesolutionislowerthantheisoelectricpointofanaminoacid,theaminoacidispositivelychargedandmovestowardthecathodeintheelectricfield.IfthepHvalueofthesolutionishigherthantheisoelectricpointofanaminoacid,theaminoacidhasanetnegativechargeandmovestowardtheanodeintheelectricfield.Thecalculationmethodoftheisoelectricpointofaminoacidforaminoacidaminosinglecarboxylgroup,itsisoelectricpointispK1andPk2arithmeticaverage,frompI=1/2(pK1+pK2)intheformulaisobtained;forcontaining3ionizablegroupofaminoacids,aslongasinordertowriteitfromneutraltoalkalineafteracidhissolutionstovariouskindsofionstoformhighprocess,andthentakethezwitterionicpKvalueonbothsidesofthearithmeticaverage,cangetthevalueofpI.Forexample,whenAspisdissociated,thereare3pKvalues,andunderdifferentpHconditionstherecanbe4ionicforms,asshowninthefollowingfigure.Attheisoelectricpoint,thezwitterionicformismainlyAsp+,sotheAspofpI=1/2(pK1+pK2)=1/2(2.09+3.86)=2.98.Inthesameway,theisoelectricpointofotheraminoacidswith3pKvaluescanbeobtained.Variousaminoacidshavenolightabsorptioninthevisibleregion,whileintheultravioletregiononlytryptophan,tyrosineandphenylalanineareabsorbed.Themaximumabsorptionwavelengthoftryptophanis279nm,themaximumabsorptionwavelengthoftyrosineis278nm,andphenylalanineis259nm.Thecontentoftheseaminoacidscanbedeterminedbyultravioletlight.MostoftheUVabsorptionofproteinsin280nmiscausedbytryptophanandtyrosine.Therefore,itisasimpleandrapidmethodtodeterminethelightabsorptionvalueofproteinsin280nmbyUVspectrophotometrywhentheproteincontentismeasured.2.thechemicalstructureandspatialstructureofproteinsTheaminoacidcompositionoftheprotein,withthepeptidebondtogether.Thepeptidebondisanotheraminoacidandalphaaminoadjacentanaminoacidmoleculeinthealphacarboxylgroupsformedbydehydrationcondensationpolymersofaminoacidstogethersocalledpeptides.TheaminoacidsonthepeptidechainareknownasaminoacidresiduesbecauseofthelossofHonthealphaammoniaandOHonthealphacarboxylgroupintheprocessofmutualconnection.Yetthereactioncontainingafreeaminogrouponthepolypeptidechainaminoacid(NH2),iscalledpeptideaminoterminalaminoacidsoraminoacidsNterminalaminoacid;theotherendcontainsanonreactingfreecarboxylgroup(COOH),calledpeptidecarboxylterminalaminoacidorCterminalaminoacid.Ingeneral,whenapolypeptidechainisexpressed,theNendisalwayswrittentotheleft,andtheCendiswrittentotheright.Whenthepeptidechainissynthesized,thesyntheticdirectionbeginsattheendofNandextendsgraduallytowardtheendofC.Avarietyofproteinmoleculeshaveaspecificspatialstructure,thatis,conformation.Theprimarystructureoftheproteinisalsocalledtheprimarystructureorchemicalstructure,referstothenumberofaminoacid,polypeptidechaincomposedofproteinmoleculesinthetypeandsequencenumberofthepolypeptidechain,butalsoincludetwodisulfidebondswithinthechainorthenumberandthelocationof.Theprimarystructureofproteinmoleculesisformedbycovalentbonds,peptidebondsandtwodisulfidebondsarecovalentbond.Thepeptidebondisthebasicwayofaminoacidsintheproteinmolecule,formingacovalentbackbone.Twosulfurbonds(S-S)arelinkedbydehydrogenationoftwocysteines(residues),andarethemajorchemicalbondsbetweenthepeptidechainsandthepeptidechains.Thetwosulfurbondplaysastabilizingroleinproteinmolecules,andisoftenrelatedtobiologicalactivity.Whenthetwosulfurbondisdestroyed,thebiologicalactivityoftheproteinorpolypeptidewillbelost.Inproteinstructure,thenumberoftwosulfurbondsismore,andthestabilityofproteinstructureisstronger.Amongtheproteinsintheskin,hornandhairthatprotecttheorganism,thetwosulfurbondisthemost.Thetwostagestructureofaproteinisthefoldingandcoilingofthepolypeptidechainitself,whichisaperiodic,repetitivestructureorconformationofapolypeptidechaininaproteinmoleculethatisrolledinasingledirection.Thisperiodicstructureismaintainedbyhydrogenbondsbetweenthepeptidechainsorbetweenthepeptidechains.Thecommontwostagestructuresarealphahelix,betafold,betacorner,etc..Forexample,variousfiberproteinsofanimals,whosemoleculesrevolvearoundalongitudinalaxis,arecalledspirals.Adjacenthelicesarelinkedbyhydrogenbondstomaintainconformationalstability.Nails,hair,andhoofsofhooves,horns,andwoolareallalpha-Helicalfibrin,alsocalledalphakeratin.Beta-sheetismorethanthealphahelixextensionpeptide,connectedtoeachotherbyhydrogencastingbecomelamellar,suchassilk,silkinbetakeratin.Thethree-stagestructureofproteins:referstofurtherfoldingandfoldingonthebasisofthetwo-levelstructuretoformaveryirregularproteinmoleculewithaspecificconformation.Tomaintaintheforceofthreelevelstructureismainlysomeso-calledweakinteractions,i.e.secondarybondingornoncovalentbonds,includinghydrogenbonds,saltbond,hydrophobicandVanderEdLeyetc..Saltbondalsocalledionicbond,sidechaingroupsintheproteinmoleculeispositiveandnegativechargesareclosetoeachother,theelectrostaticattractionformed,suchastheinteractionbetweencarboxylandaminoandguanidino,imidazolegroupsetc..Ishydrophobichydrophobicorhydrophobicsidechainpolypeptidechainofsomeaminoacids(nonpolarsidechains)duetoavoidboilingwatercausedbyclosetoeachotherandadhesiontogether.Itoccupiesaprominentpositioninthemaintenanceofthethreestructureofprotein.FanDehuagravitationistheattractionbetweenmolecules.Inaddition,thetwosulfurbondalsostabilizestheconformationofthethreestructure.Aglobularproteinwithathree-stagestructureisamorecomplexproteininconformationthanfibrin.Thepeptidechainsalsohaveconformationalunitssuchasalphahelix,betafold,etc.theseconformationalunitsareconnectedbyanirregularlycoiledpeptidesegment,sothattheentirepeptideiscastintoanalmostsphericalirregularshape.Enzymes,avarietyofproteins,hormones,antibodies,andproteinsinthecytoplasmandcellmembranesareglobulins.Unlikefibrin,thesurfaceoftheglobulinishydrophilicandthereforesolubleinwater.Thefour-levelstructureofaprotein:aproteinmoleculethatispolymerizedintotwoormorethantwopolypeptideswithathree-classstructure.Thepolypeptidechainsthatconstitutefunctionalunitsarecalledsubunits,andingeneral,thesubunitsalonedonothavebiologicalactivity,andonlywhenaggregatedintofourstructuresdotheyhavecompletebiologicalactivity.Forexample,phosphorylaseiscomposedof2subunits,glutamatedehydrogenaseiscomposedof6identicalsubunits,hemoglobiniscomposedof4differentsubunits(2alphapolypeptide,2betachain)form,eachchainisathreelevelstructureofimmunoglobulin.Thesubunitsynthesisofthefour-orderstructureislinkedbysomesecondarybondsofthemolecularsurface,mainlysaltbondsandhydrogenbonds.Someproteinmoleculeshaveonlyoneortwoorthreestructures,andnofourstructures,suchasmyoglobin,cytochromeC,ribonuclease,lysozyme,etc..Otherproteins,oneortwoandthreeorfour,existsimultaneously,suchashemoglobin,catalase,glutamatedehydrogenase,andsoon.3.propertiesandbiologicalfunctionsofproteinsProteinsaremadeupofmanyaminoacidmolecules,Largemolecularweight.Therefore,someofitspropertiesarethesameasthoseofaminoacids,someofwhicharedifferentfromthoseofaminoacids,suchascolloidalproperties,allostericprocesses,anddenaturation.(1)colloidalproperties:proteinmolecularweightislarge,easytoformcolloidalparticlesinwater,withcolloidalproperties.Inaqueoussolutions,proteinsformhydrophiliccolloids,i.e.,theouterlayerofcolloidalparticlescontainsalayerofwaterfilm.Thewaterfilmseparatestheparticlesfromeachother,sotheparticlesdonotcondenseintopiecesandsink.(2)allostericeffect:proteinmoleculescontainingmorethan2subunits,ifoneofthesubunitswithsmallmolecules,itisnotonlythespatialstructureofthesubunittochangetheconformationofothersubunitswillalsochange,theconformationoftheentireproteinmoleculeandtheactivitywillchangethisphenomenon,calledallostericorallostericeffect.Forexample,certainenzymemoleculescanbindtothefinalproducttheycatalyze,creatingallostericeffectsthatreducetheactivityoftheenzymeandthusactasafeedbackinhibitingagent.(3):proteindenaturationinheavymetalsalts(mercurysalt,silver,coppersalts),acid,alkali,acetaldehydeandureainthepresenceof,orisheatedto70to100DEGC