ThemechanismofenzymeactionFoodBiochemistry食品生物化學ThemechanismofenzymeactionFoodBiochemistry食品生物化學目錄CONTENTS123ThemechanismofenzymeactionThespeedofenzymaticreactionisaffectedFactorofIsolation,purificationandactivitydeterminationofenzymesThemechanismofenzymeaction01Themechanismofenzymeaction6.2CatalysisandactivationenergyofenzymesThecatalyticactionofenzymesandcatalystsismainlytoreducetheactivationenergyrequiredforthereaction,sothatthesameenergycanactivatemoremolecules,therebyspeedingupthereaction.FoodbiochemistryThemechanismofenzymeaction6.2IntermediateproducttheoryThistheoryholdsthatwhenanenzymecatalyzesachemicalreaction,theenzyme(E)combineswiththesubstrate(S)toformanunstableintermediate(ES),whichisthenbrokendownintoaproduct(P)andreleasestheenzyme.FoodbiochemistryThemechanismofenzymeaction6.2InducedfittheoryThistheoryholdsthatwhentheenzymemoleculeisclosetothesubstrate,theenzymeisinducedbythesubstratetoundergoconformationalchangesinfavorofbindingthesubstrate,andthesubstrateconformationisalsochanged,andtheenzymeandthesubstratearecomplementaryandfittoreact.FoodbiochemistryThemechanismofenzymeaction6.2TheactivecenteroftheenzymeTheenzymeactivecenteroractivesitereferstoaspecificregionthatbindsdirectlytothesubstrateandtransformsitintoaproduct.Theactivecenterofenzymeusuallyhasabindingsiteandacatalyticsite.FoodbiochemistryThemechanismofenzymeaction6.2ZymogenandzymogenactivationTheprecursorofanenzymethatisnotbiologicallyactiveiscalledzymogen.Afterthespecificactionofproteolyticenzyme,theconformationchangestoformtheactivesiteoftheenzyme,sothattheenzymehasactivity,thisactivationprocessiscalledtheactivationoftheenzyme.FoodbiochemistryFactorsaffectingtherateofenzymaticreaction02Themechanismofenzymeaction6.2EnzymaticreactionratedeterminationTherateofenzymaticreactionisexpressedastheamountofsubstratereductionorproductproductionperunittime.Ingeneral,whendeterminingthespeedofenzymaticreaction,itisbesttochoosewhentheinitialsubstrateisconsumedwithin5%todeterminetheamountofproductgeneration,thatis,theinitialspeedofthereaction.FoodbiochemistryThemechanismofenzymeaction6.2TheeffectofsubstrateconcentrationWhenthesubstrateconcentrationislow,itisafirst-orderreaction.Asthesubstrateconcentrationcontinuestoincrease,itpresentsamixed-gradereaction.Whenthesubstrateconcentrationcontinuestoincreaseandreachesacertainvalue,theenzymehasbeensaturatedbythesubstrate,andtheenzymaticreactionspeednolongerincreases,reachingthemaximumvalue(Vmax),showingazero-orderreaction.FoodbiochemistryThemechanismofenzymeaction6.2EnzymeconcentrationInfluenceofUndercertaintemperatureandpHconditions,whentheconcentrationofsubstrategreatlyexceedstheconcentrationofenzyme,thatis,theconcentrationofsubstrateinthereactionsolutionisenoughtosaturatetheenzyme,increasingtheconcentrationofenzymecanimprovethereactionspeed,andtheconcentrationofenzymeispositivelyproportionaltothereactionspeed.FoodbiochemistryThemechanismofenzymeaction6.2pHinfluenceTheenzymeisonlyactiveinacertainpHrange.Inthisrange,theenzymaticreactionrateincreaseswithpHuntilitreachesamaximum,andthenbeginstodecrease.ThepHofthesolutionatwhichthereactionratereachesitsmaximumiscalledtheoptimalpHoftheenzyme.TheenzymeactivitydecreasedwhenpHwaslowerorhigherthantheoptimalpH.FoodbiochemistryThemechanismofenzymeaction6.2EffectoftemperatureInacertaintemperaturerange,thespeedofenzymaticreactionisacceleratedwiththeincreaseoftemperature,andthemaximumvalueisreachedwhenthetemperatureisincreasedtoacertainextent.Asthetemperatureincreasesfurther,thespeeddecreasessharply.Undercertainconditions,thetemperatureatwhichtheenzymaticreactionrateismaximumiscalledtheoptimumtemperatureoftheenzyme.FoodbiochemistryThemechanismofenzymeaction6.2TheeffectsofactivatorsSubstancesthatcanincreasetheactivityofenzymesarecalledactivators.Theactivatorsofsomeenzymesaremetalions,whichcanmaintaintheconformationoftheenzymemoleculesandstabilizetheactivecenter.Theactivatorsofsomeenzymesaresmallorganiccompounds,whichcanreducethedisulfidebondsintheenzymemoleculestoactivesulfhydrylgroups.FoodbiochemistryThemechanismofenzymeaction6.2EffectofinhibitorsInhibitorintheformofcovalentbondwiththeenzymemoleculeessentialgroups,therebyinhibitingenzymeactivity,withdialysis,ultrafiltrationandotherphysicalmethodscannotremovetheinhibitorandtheenzymeactivityrecovery,calledirreversibleinhibition,thisinhibitoriscalledirreversibleinhibitor.FoodbiochemistryThemechanismofenzymeaction6.2EffectofinhibitorsInhibitorsbindtoenzymesintheformofnon-covalentbonds,andcanberemovedbyphysicalmethodssuchasdialysisorfiltrationtorestoreenzymeactivity,whichiscalledreversibleinhibition.Accordingtotherelationshipbetweeninhibitorandsubstrate,itcanbedividedintocompetitiveinhibition,non-competitiveinhibitionandanti-competitiveinhibition.FoodbiochemistryIsolation,purificationandactivitydeterminationofenzymes03Themechanismofenzymeaction6.2Generalprocess:Materialselection→Brokencells→extraction→removalofnucleicacidandpolysaccharide→purification→preservationDuringthepurificationprocess,attentionshouldbepaidto:lowtemperature,noseverestirring,addingprotectiveagents,constantlymeasuringspecificvitality,andcalculatingtotalvitality.IsolationandpurificationofenzymesFoodbiochemistryThemechanismofenzymeaction6.2Enzymeactivityreferstotheabilityofanenzymetocatalyzeacertainchemicalreaction,usuallymeasuredbythespeedofthechemicalreactioncatalyzedbytheenzymeunderoptimalconditions.Thefastertheenzymaticreaction,thehighertheactivityoftheenzyme.EnzymeactivityFoodbiochemistryThemechanismofenzymeaction6.2(1)IU:Theamountofenzymerequiredbytheenzymetoconvert1umolsubstratein1minuteunderstandardconditions.(2)Catalyticunit(katal):Theamountofenzymerequiredbytheenzymetoconvert1molofsubstratein1secondundertheoptimalconditions.1Kat=60×106IU.(3)Customvitalityunit.ExpressionofenzymeactivityFoodbiochemistryThemechanismo