Exceptional active site H-bonding in enzymes? Significance of the ‘oxyanion hole’ in the serine proteases from a model study

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Abstract
For a series of secondary (N-butyl) and corresponding tertiary (N,N-tetramethylene) 5-substituted salicylamides, phenolic pKa values have been measured in order to assess the energetic dependence of intramolecular carboxamide-NH hydrogen bonding upon the basicity of an acceptor oxyanion. The results are summarized in a Brønsted-type α coefficient of 0.12 (slope for ΔpKa, tertiary minus secondary, versus phenolic pKa of tertiary amide). Relative acidities of the same salicylamides in dimethylacetamide solution indicate that offsetting differences in anion hydration are not hidden in this coefficient. It is concluded that rather less transition-state stabilization from hydrogen bonding may be available within the active site of serine proteases than has previously been inferred from directed point mutations involving the enzymes.