1Chapter 8EnzymesYou are responsible for the entire chapter, but you will not be tested on details of different types of enzyme inhibitors (pp. 266-268).Chymotrypsin with bound substrateSubstrate(s) Product(s)EnzymeS PBiochemical free energy change atpH 7.0Activation energiesE + S ES EP E + P2[P][S]K'eq=∆G'º = - R T ln K'eqS PHow do enzymes accelerate reactions?1. Chemical reactions between the substrate and functional groups on the enzyme can provide alternative, lower-energy reaction pathways.(Example:group transfer through an intermediate with the group transiently covalently attached to the enzyme)2. Binding energy, ∆ GB, is a major source of free energy used by enzymes to lower the activation energies of reactions.Dihydrofolate reductase binding its substrate NADP+The other bound substrate is tetrahydrofolate.Example of “Induced Fit”3The Role of Binding Energy in Catalysis4Rate enhancement by entropy reductionGeneral acid-base catalysisAmino Acids in General Acid-Base CatalysisCovalent and General Acid-Base CatalysisH+(more later about chymotrypsin)5Enzyme Kinetics: Michaelis-Menton EquationVo =Vmax[S]_________Km + [S]E + S ES E + Pk1k-1k2Vo = k2[ES]Rate of ES formation = k1[E][S] = k1([Etotal] - [ES]) [S]Rate of ES breakdown = k-1[ES ] + k2[ES]k1([Etotal] - [ES ]) [S] = k-1[ES ] + k2[ES](steady state assumption)k1[Etotal][S] - k1[ES][S] = ( k-1+ k2)[ES]k1[Etotal][S] = (k1[S] + k-1+ k2)[ES][ES] = [Etotal][S]________________________[S] + (k2 + k-1)___________k1= [Etotal][S]____________KM+ [S]Vo = k2[ES]Vo =k2[Etotal][S]____________KM+ [S]Vo= Vmaxwhen [Etotal] = [ES] (at saturation)Therefore Vmax = k2[Etotal]Vo = Vmax[S]____________KM+ [S]When Vo = Vmax_____2Vo = Vmax[S]____________KM+ [S]Vmax_____ =2Vmax[S]____________KM+ [S]1_____ =2[S]____________KM+ [S]KM = [S]when Vo = Vmax_____26Double-Reciprocal or Lineweaver-Burk Plot1 KM 1 ______ = _______ +______Vo Vmax[S] VmaxVo =Vmax[S]_________Km + [S]=kcat[Etotal][S]_________________Km + [S]Catalytic efficiency, kcat/KmAn estimate of "how perfect" the enzyme i Measures how the enzyme performs when S is lowThe upper limit for kcat/Km is the diffusion limit - the rate at which E and S diffuse together7Many enzymes catalyze reactions with two or more
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