KM, kcat, and kcat KM

KM,kcat,andkcatKMmax是指最大反应速度。当底物浓度足够大时，体系中酶的活性中心达到饱和状态，其反应速度达到最大。由此可见，最大反应速度max随酶浓度的变化而变化。kcat指反应常数(catalyticconstant)，kcat可以由这个公式计算得到：kcat=max/[E][E]指酶浓度，由此可以说，kcat表示了每单位时间内（秒）每摩尔的酶（或者说每摩尔的活性中心）能够把多少摩尔的底物转化成产物。Km俗称米氏常数，以浓度做单位，米氏常数定义为反应速度达到最大反应速度一半时的底物浓度。Km可以反映出酶与底物的亲和力，Km越低，亲和力越大。kcat/Km称为催化效率，常常以此来比较不同的酶而同一底物，或者不同底物而同一种酶。KM,kcat,andkcat/KMKM-TheMichaelisconstant,KM,isoftenassociatedwiththeaffinityoftheenzymeforsubstrate,butthisisnotalwayscorrect.Amoreaccuratestatementisthat,forreactionsobeyingMichaelis-Mentenkinetics,KMisameasureofthesubstrateconcentrationrequiredforeffectivecatalysistooccur.Thatis,anenzymewithahighKMrequiresahighersubstrateconcentrationtoachieveagivenreactionvelocitythananenzymewithalowKM.Table11.1listsvaluesofKM,kcat,andkcat/KMforselectedenzymes.kcat-TheMichaelis-Mentenequation,V=Vmax[S]/(KM+[S]),canberewrittenasV=kcat[E]t[S]/(KM+[S])whereVmax=kcat[E]t.kcatincorporatestherateconstantsforallthereactionsbetweenESandE+Pinamultistepenzymaticprocess.Foratwo-stepreaction,kcat=k2.Formorecomplexreactions,kcatdependsonwhichstepsintheprocessarerate-limiting.kcatgivesadirectmeasureofthecatalyticproductionofproductunderoptimumconditions(saturatedenzyme).Theunitsofkcatareseconds-1.Thereciprocalofkcatcanbethoughtofasthetimerequiredbyanenzymemoleculeto"turnover"onesubstratemolecule.Alternatively,kcatmeasuresthenumberofsubstratemoleculesturnedoverperenzymemoleculepersecond.Thus,kcatissometimescalledtheturnovernumber.SometypicalturnovernumbersareshowninTable11.1,aswell.kcat/KM-Thisratioisoftenthoughtofasameasureofenzymeefficiency.Eitheralargevalueofkcat(rapidturnover)orasmallvalueofKM(highaffinityforsubstrate)makeskcat/KMlarge.When[S]<