Effects of volume and surface property in hydrolysis by acetylcholinesterase. The trimethyl site

Publisher Website
Google Scholar
Abstract
.beta.-Substituted ethyl acetates, XCH2CH2OCOCH3, were prepared, and their hydrolysis by acetylcholinesterase was studied. Log of enzymic reactivity, normalized for intrinsic reactivity in hydrolysis by OH-, log (kcat/Km)n, rises linearly with increasing refraction volume, MR (or RD25), for substrates with .beta.-X = H, Cl, Br, CH3CH2, (CH3)2CH, (CH3)2S+, (CH3)3N+ and (CH3)3C. Larger substituents may be accommodated ((CH3)3Si and (CH3CH2)3N+) with no further increase in rate. Substrates with .beta.-substituents CH3S, CH3S(O), (CH3)3N+(OH) and CH3S(O2) are less reactive than consistent with the relation with MR by factors of 5-40, indicating that hydrophobic surface and desolvation of the substrate-enzyme interface may be necessary for maximum reactivity correlated with MR. Values of log (kcat/Km)n for substrates with .beta.-substituents X = CH3S, Cl, Br, CH3CH2, (CH3)2CH, (CH3)3C and (CH3)3Si rise linearly with increasing hydrophobicity, .pi., but reactivity of substrates with X = (CH3)3N+ and (CH3)2S+ are more reactive than consistent with a relation to .pi. by factors of 300 and 40 and with X = CH3S(O2), CH3S(O) and (CH3)2N+(OH), by factors of 7-100. Reactivity appears related to volume of the .beta.-substituent and its fit in its subsite, which is trimethyl rather than anionic, and the hydrophobicity of its surface.