Delineation and Decomposition of Energies Involved in Quaternary Ammonium Binding in the Active Site of Acetylcholinesterase
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- 作者：Daniel M. Quinn ; Shawn R. Feaster ; Haridasan K. Nair ; Nathan A. Baker ; Zoran Radi
p ; //pubs.acs.org/images/entities/cacute.gif" border="0"> ; and Palmer Taylor
- 刊名：Journal of the American Chemical Society
- 出版年：2000
- 出版时间：April 5, 2000
- 年：2000
- 卷：122
- 期：13
- 页码：2975 - 2980
- 全文大小：126K
- 年卷期：v.122,no.13(April 5, 2000)
- ISSN：1520-5126
文摘
The quaternary ammonium binding locus in the active site of mammalian acetylcholinesterase issubtended by the side chains of Trp86, Tyr133, Glu202, and Tyr337. Linear free-energy relationships definethe interactions involved in molecular recognition by mouse acetylcholinesterase of the quaternary ammoniummoiety of ligands. For substrates CH3C(=O)XCH2CH2Y [X = O, Y = CHMe2, or CH2CH3; X = S, Y = H,NH+Me2, or N+Me3 ] and trifluoroacetophenone transition state analogue inhibitors m-YC6H4C(=O)CF3 [Y= H, Me, Et, iPr, tBu, CF3, NH2, NO2, NMe2, or N+Me3], log(kcat/Km) and pKi depend linearly on the molarrefractivity, but not the hydrophobicity, 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 × 105)-fold (
GTI = -32.5 kJ mol-1) and the transitionstate by (2 × 104)-fold (G
= -24.5 kJ mol-1). To evaluate the contribution of cation-
pi.gif" BORDER=0 > interactions,Trp86 was converted into Tyr, Phe, and Ala by site-specific mutagenesis. For this set of enzymes, a linearfree-energy relationship is observed between the pKi values for inhibitions by the respective neutral and cationictransition state analogue inhibitors, m-tert-butyltrifluoroacetophenone and m-(N,N,N-trimethylammonio)trifluoroacetophenone, which indicates that the free energy released on interaction of the quaternary ammoniummoiety with Trp86 arises about equally from cation-pi.gif" BORDER=0 > and charge-independent interactions.
GTI = -32.5 kJ mol-1) and the transitionstate by (2 × 104)-fold (G = -24.5 kJ mol-1). To evaluate the contribution of cation-pi.gif" BORDER=0 > interactions,Trp86 was converted into Tyr, Phe, and Ala by site-specific mutagenesis. For this set of enzymes, a linearfree-energy relationship is observed between the pKi values for inhibitions by the respective neutral and cationictransition state analogue inhibitors, m-tert-butyltrifluoroacetophenone and m-(N,N,N-trimethylammonio)trifluoroacetophenone, which indicates that the free energy released on interaction of the quaternary ammoniummoiety with Trp86 arises about equally from cation-pi.gif" BORDER=0 > and charge-independent interactions.