The quaternary ammonium binding locus in the active site of mammalian acetylcholinesterase issubtended by the side chains of Tr
p86, Tyr133, Glu202, and Tyr337. Linear free-energy relationshi
ps definethe interactions involved in molecular recognition by mouse acetylcholinesterase of the quaternary ammoniummoiety of ligands. For substrates CH
3C(=O)XCH
2CH
2Y [X = O, Y = CHMe
2, or CH
2CH
3; X = S, Y = H,NH
+Me
2, or N
+Me
3 ] and trifluoroaceto
phenone transition state analogue inhibitors
m-YC
6H
4C(=O)CF
3 [Y= H, Me, Et,
iPr,
tBu, CF
3, NH
2, NO
2, NMe
2, or N
+Me
3], log(
kcat/
Km) and
pKi de
pend linearly on the molarrefractivity, but not the hydro
phobicity, of the substituents Y. These correlations indicate that, in the acylationstage of catalysis, interactions in the quaternary ammonium binding locus stabilize the tetrahedral intermediate(as modeled by transition state analogue affinity) by (5 × 10
5)-fold (
![](/images/gifchars/Delta.gif)
GTI = -32.5 kJ mol
-1) and the transitionstate by (2 × 10
4)-fold (
![](/images/gifchars/Delta.gif)
G![](/images/entities/Dagger.gif)
= -24.5 kJ mol
-1). To evaluate the contribution of cation-
![](/images/gifchars/<font color=)
pi.gif" BORDER=0 > interactions,Tr
p86 was converted into Tyr, Phe, and Ala by site-s
pecific mutagenesis. For this set of enzymes, a linearfree-energy relationshi
p is observed between the
pKi values for inhibitions by the res
pective neutral and cationictransition state analogue inhibitors,
m-
tert-butyltrifluoroaceto
phenone and
m-(
N,N,N-trimethylammonio)trifluoroaceto
phenone, which indicates that the free energy released on interaction of the quaternary ammoniummoiety with Tr
p86 arises about equally from cation-
![](/images/gifchars/<font color=)
pi.gif" BORDER=0 > and charge-inde
pendent interactions.