enzyme | Definition, Mechanisms, & Nomenclature | Britannica

An enzyme is a substance that acts as a catalyst in living organisms, regulating the rate at which chemical reactions proceed without itself ... enzyme TableofContents Introduction&TopQuestionsChemicalnatureNomenclatureMechanismofenzymeactionFactorsaffectingenzymeactivity FastFacts 2-MinSummary TopQuestions Whatisanenzyme? Whatareenzymescomposedof? Whatareexamplesofenzymes? Whatfactorsaffectenzymeactivity? RelatedContent Media Videos Images More MoreArticlesOnThisTopic Contributors ArticleHistory Home Health&Medicine Anatomy&Physiology enzyme biochemistry Print print Print Pleaseselectwhichsectionsyouwouldliketoprint: TableOfContents Cite verifiedCite Whileeveryefforthasbeenmadetofollowcitationstylerules,theremaybesomediscrepancies. Pleaserefertotheappropriatestylemanualorothersourcesifyouhaveanyquestions. 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ExternalWebsites NationalLibraryofMedicine-Enzymes:principlesandbiotechnologicalapplications BritannicaWebsites ArticlesfromBritannicaEncyclopediasforelementaryandhighschoolstudents. enzyme-Children'sEncyclopedia(Ages8-11) enzyme-StudentEncyclopedia(Ages11andup) By TheEditorsofEncyclopaediaBritannica • LastUpdated: Apr10,2022 • EditHistory enzyme Seeallmedia KeyPeople: RichardHenderson EmilFischer MaudLeonoraMenten GünterBlobel AriehWarshel ...(Showmore) RelatedTopics: neuraminidase renin-angiotensinsystem allostericcontrol induction cooperativity ...(Showmore) Seeallrelatedcontent→ TopQuestionsWhatisanenzyme?Anenzymeisasubstancethatactsasacatalystinlivingorganisms,regulatingtherateatwhichchemicalreactionsproceedwithoutitselfbeingalteredintheprocess.Thebiologicalprocessesthatoccurwithinalllivingorganismsarechemicalreactions,andmostareregulatedbyenzymes.Withoutenzymes,manyofthesereactionswouldnottakeplaceataperceptiblerate.Enzymescatalyzeallaspectsofcellmetabolism.Thisincludesthedigestionoffood,inwhichlargenutrientmolecules(suchasproteins,carbohydrates,andfats)arebrokendownintosmallermolecules;theconservationandtransformationofchemicalenergy;andtheconstructionofcellularmacromoleculesfromsmallerprecursors.Manyinheritedhumandiseases,suchasalbinismandphenylketonuria,resultfromadeficiencyofaparticularenzyme.phenylketonuriaReadmoreaboutphenylketonuria,aninabilitytometabolizephenylalanine.Whatareenzymescomposedof?Alargeproteinenzymemoleculeiscomposedofoneormoreaminoacidchainscalledpolypeptidechains.Theaminoacidsequencedeterminesthecharacteristicfoldingpatternsoftheprotein’sstructure,whichisessentialtoenzymespecificity.Iftheenzymeissubjectedtochanges,suchasfluctuationsintemperatureorpH,theproteinstructuremayloseitsintegrity(denature)anditsenzymaticability.Boundtosomeenzymesisanadditionalchemicalcomponentcalledacofactor,whichisadirectparticipantinthecatalyticeventandthusisrequiredforenzymaticactivity.Acofactormaybeeitheracoenzyme—anorganicmolecule,suchasavitamin—oraninorganicmetalion.Someenzymesrequireboth.Allenzymeswereoncethoughttobeproteins,butsincethe1980sthecatalyticabilityofcertainnucleicacids,calledribozymes(orcatalyticRNAs),hasbeendemonstrated,refutingthisaxiom. Readmorebelow: ChemicalnaturecoenzymeReadmoreaboutcoenzymes.Whatareexamplesofenzymes?Practicallyallofthenumerousandcomplexbiochemicalreactionsthattakeplaceinanimals,plants,andmicroorganismsareregulatedbyenzymes,andsotherearemanyexamples.Amongsomeofthebetter-knownenzymesarethedigestiveenzymesofanimals.Theenzymepepsin,forexample,isacriticalcomponentofgastricjuices,helpingtobreakdownfoodparticlesinthestomach.Likewise,theenzymeamylase,whichispresentinsaliva,convertsstarchintosugar,helpingtoinitiatedigestion.Inmedicine,theenzymethrombinisusedtopromotewoundhealing.Otherenzymesareusedtodiagnosecertaindiseases.Theenzymelysozyme,whichdestroyscellwalls,isusedtokillbacteria.Theenzymecatalasebringsaboutthereactionbywhichhydrogenperoxideisdecomposedtowaterandoxygen.Catalaseprotectscellularorganellesandtissuesfromdamagebyperoxide,whichiscontinuouslyproducedbymetabolicreactions.catalaseReadmoreaboutcatalase.Whatfactorsaffectenzymeactivity?Enzymeactivityisaffectedbyvariousfactors,includingsubstrateconcentrationandthepresenceofinhibitingmolecules.Therateofanenzymaticreactionincreaseswithincreasedsubstrateconcentration,reachingmaximumvelocitywhenallactivesitesoftheenzymemoleculesareengaged.Thus,enzymaticreactionrateisdeterminedbythespeedatwhichtheactivesitesconvertsubstratetoproduct.Inhibitionofenzymeactivityoccursindifferentways.Competitiveinhibitionoccurswhenmoleculessimilartothesubstratemoleculesbindtotheactivesiteandpreventbindingoftheactualsubstrate.Noncompetitiveinhibitionoccurswhenaninhibitorbindstotheenzymeatalocationotherthantheactivesite.Anotherfactoraffectingenzymeactivityisallostericcontrol,whichcaninvolvestimulationofenzymeactionaswellasinhibition.Allostericstimulationandinhibitionallowproductionofenergyandmaterialsbythecellwhentheyareneededandinhibitproductionwhenthesupplyisadequate. Readmorebelow: FactorsaffectingenzymeactivityallostericcontrolReadmoreaboutallostericcontrol.enzyme,asubstancethatactsasacatalystinlivingorganisms,regulatingtherateatwhichchemicalreactionsproceedwithoutitselfbeingalteredintheprocess.Abrieftreatmentofenzymesfollows.Forfulltreatment,seeprotein:Enzymes.Thebiologicalprocessesthatoccurwithinalllivingorganismsarechemicalreactions,andmostareregulatedbyenzymes.Withoutenzymes,manyofthesereactionswouldnottakeplaceataperceptiblerate.Enzymescatalyzeallaspectsofcellmetabolism.Thisincludesthedigestionoffood,inwhichlargenutrientmolecules(suchasproteins,carbohydrates,andfats)arebrokendownintosmallermolecules;theconservationandtransformationofchemicalenergy;andtheconstructionofcellularmacromoleculesfromsmallerprecursors.Manyinheritedhumandiseases,suchasalbinismandphenylketonuria,resultfromadeficiencyofaparticularenzyme.Enzymesalsohavevaluableindustrialandmedicalapplications.Thefermentingofwine,leaveningofbread,curdlingofcheese,andbrewingofbeerhavebeenpracticedfromearliesttimes,butnotuntilthe19thcenturywerethesereactionsunderstoodtobetheresultofthecatalyticactivityofenzymes.Sincethen,enzymeshaveassumedanincreasingimportanceinindustrialprocessesthatinvolveorganicchemicalreactions.Theusesofenzymesinmedicineincludekillingdisease-causingmicroorganisms,promotingwoundhealing,anddiagnosingcertaindiseases.enzyme;cheesemakingRennet,whichcontainstheproteaseenzymechymosin,isaddedtomilkduringcheesemaking.©Fedecandoniphoto/Dreamstime.com Chemicalnature Allenzymeswereoncethoughttobeproteins,butsincethe1980sthecatalyticabilityofcertainnucleicacids,calledribozymes(orcatalyticRNAs),hasbeendemonstrated,refutingthisaxiom.BecausesolittleisyetknownabouttheenzymaticfunctioningofRNA,thisdiscussionwillfocusprimarilyonproteinenzymes. Alargeproteinenzymemoleculeiscomposedofoneormoreaminoacidchainscalledpolypeptidechains.Theaminoacidsequencedeterminesthecharacteristicfoldingpatternsoftheprotein’sstructure,whichisessentialtoenzymespecificity.Iftheenzymeissubjectedtochanges,suchasfluctuationsintemperatureorpH,theproteinstructuremayloseitsintegrity(denature)anditsenzymaticability.Denaturationissometimes,butnotalways,reversible. Boundtosomeenzymesisanadditionalchemicalcomponentcalledacofactor,whichisadirectparticipantinthecatalyticeventandthusisrequiredforenzymaticactivity.Acofactormaybeeitheracoenzyme—anorganicmolecule,suchasavitamin—oraninorganicmetalion;someenzymesrequireboth.Acofactormaybeeithertightlyorlooselyboundtotheenzyme.Iftightlyconnected,thecofactorisreferredtoasaprostheticgroup. Nomenclature Anenzymewillinteractwithonlyonetypeofsubstanceorgroupofsubstances,calledthesubstrate,tocatalyzeacertainkindofreaction.Becauseofthisspecificity,enzymesoftenhavebeennamedbyaddingthesuffix“-ase”tothesubstrate’sname(asinurease,whichcatalyzesthebreakdownofurea).Notallenzymeshavebeennamedinthismanner,however,andtoeasetheconfusionsurroundingenzymenomenclature,aclassificationsystemhasbeendevelopedbasedonthetypeofreactiontheenzymecatalyzes.Therearesixprincipalcategoriesandtheirreactions:(1)oxidoreductases,whichareinvolvedinelectrontransfer;(2)transferases,whichtransferachemicalgroupfromonesubstancetoanother;(3)hydrolases,whichcleavethesubstratebyuptakeofawatermolecule(hydrolysis);(4)lyases,whichformdoublebondsbyaddingorremovingachemicalgroup;(5)isomerases,whichtransferagroupwithinamoleculetoformanisomer;and(6)ligases,orsynthetases,whichcoupletheformationofvariouschemicalbondstothebreakdownofapyrophosphatebondinadenosinetriphosphateorasimilarnucleotide. Mechanismofenzymeaction Inmostchemicalreactions,anenergybarrierexiststhatmustbeovercomeforthereactiontooccur.Thisbarrierpreventscomplexmoleculessuchasproteinsandnucleicacidsfromspontaneouslydegrading,andsoisnecessaryforthepreservationoflife.Whenmetabolicchangesarerequiredinacell,however,certainofthesecomplexmoleculesmustbebrokendown,andthisenergybarriermustbesurmounted.Heatcouldprovidetheadditionalneededenergy(calledactivationenergy),buttheriseintemperaturewouldkillthecell.Thealternativeistolowertheactivationenergylevelthroughtheuseofacatalyst.Thisistherolethatenzymesplay.Theyreactwiththesubstratetoformanintermediatecomplex—a“transitionstate”—thatrequireslessenergyforthereactiontoproceed.Theunstableintermediatecompoundquicklybreaksdowntoformreactionproducts,andtheunchangedenzymeisfreetoreactwithothersubstratemolecules. Onlyacertainregionoftheenzyme,calledtheactivesite,bindstothesubstrate.Theactivesiteisagrooveorpocketformedbythefoldingpatternoftheprotein.Thisthree-dimensionalstructure,togetherwiththechemicalandelectricalpropertiesoftheaminoacidsandcofactorswithintheactivesite,permitsonlyaparticularsubstratetobindtothesite,thusdeterminingtheenzyme’sspecificity.enzyme;activesiteTheactivesiteofanenzymeisagrooveorpocketthatbindsaspecificsubstrate.EncyclopædiaBritannica,Inc. Enzymesynthesisandactivityalsoareinfluencedbygeneticcontrolanddistributioninacell.Someenzymesarenotproducedbycertaincells,andothersareformedonlywhenrequired.Enzymesarenotalwaysfounduniformlywithinacell;oftentheyarecompartmentalizedinthenucleus,onthecellmembrane,orinsubcellularstructures.Theratesofenzymesynthesisandactivityarefurtherinfluencedbyhormones,neurosecretions,andotherchemicalsthataffectthecell’sinternalenvironment. LoadNextPage