AssociationConstantassociationconstant(Ka)=providesameasureoftheaffinityof the ligandLfortheprotein-higherK,=higheraffinity- equivalenttotheratio oftheratesoftheforward(association)andthereverse(dissociation)reactionsthatformthePLcomplexK.=品-
[L]RemainsConstantK.-P品-·when[L]>>>[ligand-binding sites], thebinding of the ligandbytheproteindoesK,[L] = [PL)][P]notappreciablychange [L]
Dissociation Constantdissociationconstant(K.)=reciprocalofK-equilibriumconstantforthereleaseof ligand-lowerK.=higheraffinityK=PI -K·when[L]=Ka,V2oftheligand-bindingsitesareoccupied[PL] = [PIL] Kd[L]Y=LI+Ka
GraphicalRepresentationsofLigand Binding1.0LY=[L] +[L]K[L]+KdY0.5[L] atwhich/zoftheavailableligand-bindingsitesareoccupied(Y=0.5)correspondsKd510to1/Ka(a)[L] (arbitraryunits)Nelson&Cox,LehningerPninciplesofBicchemistry.Be,o 202 W.H.Freeman andCompany
RepresentativeK.ValuesTABLE5-1ProteinDissociation Constants:Some Examplesand RangeProteinLigandKlyBiotin1×10-15Avidin (egg white)InsulinInsulinreceptor (human)1×10-Anti-HIV immunoglobulin (human)gp11 (HIV-1 surface protein)4×10-10Nickel-binding protein (E.coti)Nt1x10-Calmodultin (raty)Ca23×10-2×10-4Typical receptor-ligand interactionsSequence-specific protein-DNABiotin-avidinAntibody-antigenEnzyme-substrate→10~%101410~1210-810410~210~1010~6highaffinitylowaffinityKgtm)Color bars indicate the range of dissociation constants typical of various elasses of interactionsin biological systems.A few interactions,such as that between the protein avidin and the enzymecofactorbiotin, fall outside the normal ranges, The avidin-biotin interaction is so tight it may beconsidered irreversible. Sequence-specific protein-DNA interactions reflect proteins that bind to aparticular sequence of mucleotides in DNA, as opposed to general binding to any DNA site.ncondtiosundewficdityasnenumd.kliluestbisproteingindintnctioronoddosociaton.contanf.saktolyfosthepartcutarsolutipnCanbe sheted. goietienes lly sevectordens ofmigritode,by changeinthttolutorn sut conoentrahir.pitor sthervaobimdinpatafaneorttodwnpaeaosaaatH.imogotuinstdesortbedlutmintpechopeerTdneeDdautoedinonet