生物化学词典

AacetylCoAcarboxylaseAnenzymethatcatalyzestheATP-dependentsynthesisofmalonylCoAfromacetylCoAandcarbondioxide,thecommittedstepinfattyacidsynthesis.acetylcholinereceptorAligand-gatedchannelthatpromotesalargeinwardcurrentofsodiumionsandtriggersanactionpotential;composedofapentameroffourkindsofpolypeptidesubunits,thechannelopenstoallowpassageofsodiumandpotassiumionswhentwoacetylcholinemoleculespromotetransientopeningofthechannel.acetylcholinesteraseAnenzymeinthesynapticcleftthatconvertsacetylcholineintocholineandacetate;thisenzymepromotestheclosingoftheacetylcholine-receptormembranebyrapidlyreducingtheconcentrationofacetylcholineintheregionbetweenthepresynapticandthepostsynapticmembranes.acetyllysine-bindingdomainAdomainconsistingofafour-helixbundlethatbindspeptidescontainingacetyllysine.Alsocalledabromodomain.actinAhighlyconservedproteinfoundinalleukaryotes;instriatedmuscle,itformsthethinfilamentsofthesarcomereandactivatestheATPaseofmyosin.actinomycinApolypeptideantibioticfromStreptomycesthatinhibitstheelongationphaseofRNAsynthesisbybindingtodouble-helicalDNAbyintercalatingwithit,therebypreventingtheDNAfromservingasatemplate.actionpotentialTheincreaseinmembranepotentialandthechangesinsodiumandpotassiumconductancesthatresultfromalterationsinthepermeabilityoftheaxonmembranetothoseions.Alsocallednerveimpulse.activatedcarriersSmallmoleculescarryingactivatedfunctionalgroupsthatcanbedonatedtoothermolecules;forinstance,ATPcarriesactivatedphosphategroupsandCoAcarriesactivatedacylgroups.activatedmethylcycleAseriesofreactionsinwhichS-methylgroupsfrommethionineareconvertedintoabiochemicallyreactiveformthroughinsertionintoanadenosylgroup;anactiveS-methylgroupcanbetransferredfromS-adenosylmethioninetoacceptormoleculessuchasnorepinephrine.TheremainingpartofthecycleincludestheregenerationofmethioninefromhomocysteineandN5-methyltetrahydrofolate.activationdomainThestructuralregionofatranscriptionfactorthatfacilitatestranscriptioninsomemanner.SeealsoDNA-bindingdomain.BacktoTopactivesiteAspecificregionofanenzymethatbindsthesubstrateandcarriesoutcatalysis.activetransportThetransportofanionoramoleculeagainstaconcentrationgradient,whereDGforthetransportedspeciesispositive;theprocessmustbecoupledtoaninputoffreeenergyfromasourcesuchasATP,anelectrochemicalgradientofNa+orK+,orlight.actomyosinAcomplexformedinvitrobetweenmyosinandactinthatdisplayscertainpropertiesofmuscles,suchascontractinginthepresenceofATP.acyladenylateAmixedanhydrideinwhichthecarboxylgroupofamoleculeislinkedtothephosphorylgroupofAMP;theformationofacyladenylatesisameansofactivatingcarboxylgroupsinbiochemicalreactions,suchastheformationoffattyacylCoAmoleculesfromafreefattyacidandcoenzymeA.acylcarrierproteinAbacterialpolypeptidethatislinkedtophosphopantetheineandactsasacarrierofthegrowingfattyacylchainduringfattyacidbiosynthesis.adaptationTheresettingofthesensitivitylevelofreceptorsduetothecontinuedpresenceofligands.Alsocalleddesensitization.additiontoorformationofdoublebondAreactioninwhichafunctionalgroupisaddedtoadoublebondoragroupisremovedfromamoleculetoformadoublebond.adenylatecyclaseAnenzymethatgeneratescAMP,asecondmessenger,fromATP.adenylatecyclasecascadeAsignal-transductionpathwaythatemployscAMPandaseriesofenzymestoconvertanextracellularsignalintoanintracellularsignal.adipocytesMammaliancellsthatarethemajorstoragesitefortriacylglycerols.A-DNAhelixAright-handeddoublehelixmadeupofantiparallelstrandsheldtogetherbyA-TandG-Cbasepairing;itiswiderandshorterthanB-DNAandisseenindehydratedDNAaswellasindoublestrandedregionsofRNAandinRNA-DNAhelices.aerobicInthepresenceofoxygen.affinitychromatographyAprotein-purificationtechniquebasedonthehighaffinityofmanyproteinsforspecificchemicalgroups.Suchgroupsareattachedtoaninertmatrix,andtheproteinsampleisapplied;onlythoseproteinswithanaffinityforthegroupswillbind.affinitylabelingAmeansofmappingtheactivesiteofanenzymebyusingasubstrateanalogthatbindstotheactivesiteandformsacovalentbondwithanearbyaminoacid.BacktoTopagonistsMoleculesthatbindtoreceptorproteinsandtriggersignalingpathways.alcaptonuriaArelativelyharmlesshereditarydisorderresultingfromtheaberrantbreakdownoftyrosineandphenylalanine.alcoholicfermentationTheanaerobicconversionofglucoseintoethanolwiththeconcomitantproductionofATP.aldolcondensationThecombinationoftwocarbonylcompounds(e.g.,analdehydeandaketone)toformaβ-hydroxycarbonylcompound,oraldol.aldoseAmonosaccharidewhoseC-1carbonatomcontainsanaldehydegroup.allelesAlternativeformsofageneataparticularsiteonachromosome.allostericinteractionAninteractionbetweenasmallmolecule(aligand)andasiteonaproteinthatmaybesomedistanceawayfromtheactivesite;theinteractioncausesaconformationalchangeandconsequentalterationinthecatalyticactivityoftheprotein.αhelixAcommonstructuralmotifinproteins,inwhichapolypeptidemainchainformstheinnerpartofaright-handedhelix,withthesidechainsextendingoutward;thehelixisstabilizedbyintrachainhydrogenbondsbetweenNHandCOgroupsofthemainchain.α-ketoglutaratedehydrogenaseAcitricacidcycleenzymethatcatalyzestheoxidativede