TheStructureandFunctionofEnzymesChemicalreactionsinbiologicalsystemsrarelyoccurintheabsenceofacatalyst.Thesecatalystsarespecificproteinscalledenzymes.Thestrikingcharacteristicsofallenzymesaretheircatalyticpowerandspecificity.Furthermore,theactivityofmanyenzymesisregulated.Inaddition,someenzymesareintimatelyinvolvedinthetransformationofdifferentformsofenergy.Letusexaminethesehighlydistinctiveandbiologicallycrucialpropertiesofenzymes.EnzymesHaveEnormousCatalyticPowerEnzymesacceleratereactionsbyfactorsofatleastamillion.Indeed,mostreactionsinbiologicalsystemsdonotoccuratperceptibleratesintheabsenceofenzymes.Evenareactionassimpleasthehydrationofcarbondioxideiscatalyzedbyanenzyme.Otherwise,thetransferofCO2fromthetissuesintothebloodandthentothealveolarairwouldbeincomplete.Carbonicanhydrase,theenzymethatcatalyzesthisreaction,isoneofthefastestknown.Eachenzymemoleculecanhydrate105moleculesofCO2inonesecond.Thiscatalyzedreactionis107timesfasterthantheuncatalyzedreaction.EnzymesareHighlySpecificEnzymesarehighlyspecificbothinthereactioncatalyzedandintheirchoiceofreactants,whicharecalledsubstrates.Anenzymeusuallycatalyzesasinglechemicalreactionorasetofcloselyrelatedreactions.Thedegreeofspecificityforsubstrateisusuallyhighandsometimesvirtuallyabsolute.Letusconsiderproteolyticenzymesasanexample.Thereactioncatalyzedbytheseenzymesisthehydrolysisofapeptidebond.Mostproteolyticenzymesalsocatalyzeadifferentbutrelatedreaction,namelythehydrolysisofanesterbond.Proteolyticenzymesvarymarkedlyintheirdegreeofsubstratespecificity.Subtilisin,whichcomesfromcertainbacteria,isquiteundiscriminatingaboutthenatureofthesidechainsadjacenttothepeptidebondtobecleaved.Trypsinisquitespecificinthatitsplitspeptidebondsonthecarboxylsideoflysineandargentineresiduesonly.Thrombin,anenzymeparticipatinginbloodclotting,isevenmorespecificthantrypsin.Thesidechainonthecarboxylsideofthesusceptiblepeptidebondmustbearginine,whereastheoneontheaminosidemustbeglycine.Anotherexampleofthehighdegreeofspecificityofenzymesisprov...
