The Structure and Function of Enzymes.docx

The Structure and Function of Enzymes.docx

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TheStructureandFunctionofEnzymes

TheStructureandFunctionofEnzymes

Chemicalreactionsinbiologicalsystemsrarelyoccurintheabsenceofacatalyst.Thesecatalystsarespecificproteinscalledenzymes.Thestrikingcharacteristicsofallenzymesaretheircatalyticpowerandspecificity.Furthermore,theactivityofmanyenzymesisregulated.Inaddition,someenzymesareintimatelyinvolvedinthetransformationofdifferentformsofenergy.Letusexaminethesehighlydistinctiveandbiologicallycrucialpropertiesofenzymes.

EnzymesHaveEnormousCatalyticPower

Enzymesacceleratereactionsbyfactorsofatleastamillion.Indeed,mostreactionsinbiologicalsystemsdonotoccuratperceptibleratesintheabsenceofenzymes.Evenareactionassimpleasthehydrationofcarbondioxideiscatalyzedbyanenzyme.

Otherwise,thetransferofCO2fromthetissuesintothebloodandthentothealveolarairwouldbeincomplete.Carbonicanhydrase,theenzymethatcatalyzesthisreaction,isoneofthefastestknown.Eachenzymemoleculecanhydrate105moleculesofCO2inonesecond.Thiscatalyzedreactionis107timesfasterthantheuncatalyzedreaction.

EnzymesareHighlySpecific

Enzymesarehighlyspecificbothinthereactioncatalyzedandintheirchoiceofreactants,whicharecalledsubstrates.Anenzymeusuallycatalyzesasinglechemicalreactionorasetofcloselyrelatedreactions.Thedegreeofspecificityforsubstrateisusuallyhighandsometimesvirtuallyabsolute.

Letusconsiderproteolyticenzymesasanexample.Thereactioncatalyzedbytheseenzymesisthehydrolysisofapeptidebond.

Mostproteolyticenzymesalsocatalyzeadifferentbutrelatedreaction,namelythehydrolysisofanesterbond.

Proteolyticenzymesvarymarkedlyintheirdegreeofsubstratespecificity.Subtilisin,whichcomesfromcertainbacteria,isquiteundiscriminatingaboutthenatureofthesidechainsadjacenttothepeptidebondtobecleaved.Trypsinisquitespecificinthatitsplitspeptidebondsonthecarboxylsideoflysineandargentineresiduesonly.Thrombin,anenzymeparticipatinginbloodclotting,isevenmorespecificthantrypsin.Thesidechainonthecarboxylsideofthesusceptiblepeptidebondmustbearginine,whereastheoneontheaminosidemustbeglycine.

AnotherexampleofthehighdegreeofspecificityofenzymesisprovidedbyDNApolymeraseI.ThisenzymesynthesizesDNAbylinkingtogetherfourkindsofnucleotidebuildingblocks.ThesequenceofnucleotidesintheDNAstrandthatisbeingsynthesizedisdeterminedbythesequenceofnucleotidesinanotherDNAstrandthatservesasatemplate.DNApolymeraseIisremarkablypreciseincarryingouttheinstructionsgivenbythetemplate.ThewrongnucleotideisinsertedintoanewDANstrandlessthanonceinamilliontimes.

TheActivitiesofSomeEnzymesAreRegulated

Someenzymesaresynthesizedinaninactiveprecursorformandareactivatedataphysiologicallyappropriatetimeandplace.Thedigestiveenzymesexemplifythiskindofcontrol.Forexample,trypsinogenissynthesizedinthepancreasandisactivatedbypeptide-bondcleavageinthesmallintestinetoformtheactiveenzymetrypsin.Thistypeofcontrolisalsorepeatedlyusedinthesequenceofenzymaticreactionsleadingtotheclottingofblood.Theenzymaticallyinactiveprecursorsofproteolyticenzymesarecalledzymogens.

Anothermechanismthatcontrolsactivityisthecovalentinsertionofasmallgrouponanenzyme.Thiscontrolmechanismiscalledcovalentmodification.Forexample,theactivitiesoftheenzymesthatsynthesizeanddegradeglycogenareregulatedbytheattachmentofaphosphorylgrouptoaspecificserineresidueontheseenzymes.Thismodificationcanbereversedbyhydrolysis.Specificenzymescatalyzetheinsertionandremovalofphosphorylandothermodifyinggroups.

Adifferentkindofregulatorymechanismaffectsmanyreactionsequencesresultinginthesynthesisofsmallmoleculessuchasaminoacids.Theenzymethatcatalyzesthefirststepinsuchabiosyntheticpathwayisinhibitedbytheultimateproduct.Thebiosynthesisofisoleucineinbacteriaillustratesthistypeofcontrol,whichiscalledfeedbackinhibition.Threonineisconvertedintoisoleucineinfivesteps,thefirstofwhichiscatalyzedbythreoninedeaminase.Thisenzymeisinhibitedwhentheconcentrationofisoleucinereachesasufficientlyhighlevel.Isoleucinebindstoaregulatorysiteontheenzyme,whichisdistinctfromitscatalyticsite.Theinhibitionofthreoninedeasminaseismediatedbyanallostericinteraction,whichisreversible.Whenthelevelofisoleucinedropssufficiently,threoninedeaminasebecomesactiveagain,andconsequentlyisoleucineisagainsynthesized.

Thespecificityofsomeenzymesisunderphysiologicalcontrol.Thesynthesisoflactosebythemammaryglandisaparticularlystrikingexample.Lactosesynthetase,theenzymethatcatalyzesthesynthesisoflactose,consistsofacatalyticsubunitandamodifiersubunit.Thecatalyticsubunitbyitselfcannotsynthesizelactose.Ithasadifferentrole,whichistocatalyzetheattachmentofgalactosetoaproteinthatcontainsacovalentlylinkedcarbohydratechain.Themodifiersubunitaltersthespecificityofthecatalyticsubunitsothatitlinksgalactosetoglucosetoformlactose.Thelevelofthemodifiersubunitisunderhormonalcontrol.Duringpregnancy,thecatalyticsubunitisformedinthemammarygland,butlittlemodifiersubunitisformed.Atthetimeofbirth,hormonallevelschangedrastically,andthemodifiersubunitissynthesizedinlargeamounts.Themodifiersubunitthenbindstothecatalyticsubunittoformanactivelactosesynthetasecomplexthatproduceslargeamountsoflactose.Thissystemclearlyshowsthathormonescanexerttheirphysiologicaleffectsbyalteringthespecificityofenzymes.

EnzymesTransformDifferentKindsofEnergy

Inmanybiochemicalreactions,theenergyofthereactantsisconvertedintoadifferentformwithhighefficiency.Forexample,inphotosynthesis,lightenergyisconvertedintochemical-bondenergy.Inmitochondria,thefreeenergycontainedinsmallmoleculesderivedfromfoodsisconvertedintoadifferentcurrency,thatofadenosinetriphosphate（ATP）.Thechemical-bondenergyofATPisthemutilizedinmanydifferentways.Inmuscularcontraction,theenergyofATPisconvertedintomechanicalenergy.CellsandorganelleshavepumpsthatutilizeATPtotransportmoleculesandionsagainstchemicalandelectricalgradients.Thesetransformationsofenergyarecarriedoutbyenzymemoleculesthatareintegralpartsofhighlyorganizedassembilies.

EnzymesDoNotAlterReactionEquilibria

Anenzymeisacatalystandconsequentlyitcannotaltertheequilibriumofachemicalreaction.Thismeansthatanenzymeacceleratestheforwardandreversereactionbypreciselythesamefactor.ConsidertheinterconversionofAandB.Supposethatintheabsenceofenzymetheforwardrate（KF）is10-4sec-1andthereverserate（KR）is10-6sec-1.TheequilibriumconstantKisgivenbytheratiooftheserates:

TheequilibriumconcentrationofBis100timesthatofA,whetherornotenzymeispresent.However,itwouldtakeseveralhourstoapproachthisequilibriumwithoutenzyme,whereasequilibriumwouldbeattainedwithinasecondwhenenzymeispresent.Thus,enzymesacceleratetheattainmentofequilibriabutdonotshifttheirpositions.

EnzymesDecreasetheActivationEnergiesofReactionsCatalyzedbyThem

Achemicalreaction,A→B,goesthroughatransitionstatethathasahigherenergythaneitherAorB.TherateoftheforwardreactiondependsonthetemperatureandonthedifferenceinfreeenergybetweenthatofAandthetransitionstate,whichiscalledtheGibbsfreeenergyofaactivationandsymbolizedby△G.

Thereactionrateisproportionaltothefractionofmoleculesthathaveafreeenergyequaltoorgreaterthan△G≠.Theproportionofmoleculesthathaveanenergyequaltoorgreaterthan△G≠increaseswithtemperature.

Enzymesacceleratereactionsbydecreasing△G≠,theactivationbarrier.Thecombinationofsubstrateandenzymecreatesanewreactionpathwaywhosetransition-stateenergyislowerthanitwouldbeifthereactionweretakingplaceintheabsenceofenzyme.

FormationofanEnzyme-SubstrateComplexIstheFirstStepinEnzymaticCatalysis

Themakingandbreakingofchemicalbondsbyanenzymeareprecededbytheformationofanenzyme-substrate（ES）complex.Thesubstrateisboundtoaspecificregionoftheenzymecalledtheactivesite.Mostenzymesarehighlyselectiveintheirbindingofsubstrates.Indeed,thecatalyticspecificityofenzymesdependsinlargepartonthespecificityofthebindingprocess.Furthermore,thecontrolofenzymaticactivitymayalsotakeplaceatthisstage.

TheexistenceofEScomplexeshasbeenshowninavarietyofways:

EScomplexeshavebeendirectlyvisualizedbyelectronmicroscopyandX-raycrystallography.Complexesofnucleicacidsandtheirpolymeraseenzymesareevidentinelectronmicrographs.Detailedinformationconcerningthelocationandinteractionsofglycyl-L-tyrosine,asubstrateofcarboxypeptidaseA,hasbeenobtainedfromX-raystudiesofthatEScomplex.

Thephysicalpropertiesofanenzyme,suchasitssolubilityorheatstability,frequentlychangeuponformationofanEScomplex.

TheSpectroscopiccharacteristicsofmanyenzymesandsubstrateschangeuponformationofanEScomplexjustastheabsorptionspectrumofdeoxyhemoglobinchangesmarkedlywhenitbindsoxygenorwhenitisoxidizedtotheferricstate,asdescribedpreviously.Thesechangesareparticularlystrikingiftheenzymecontainsacoloredprostheticgroup.Tryptophansynthetase,abacterial