Protonation-State Dependence of Hydrogen Bond Strengths and Exchange Rates in a Serine Protease Catalytic Triad: Bovine Chymotrypsinogen A

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Abstract
Hydrogen-1 nuclear magnetic resonance spectroscopy was used to measure D/H fractionation factors and the temperature dependence of the rate of hydrogen exchange at two sites in the catalytic triad of chymotrypsinogen (hydrogen bond between aspartate-102 and histidine-57 and hydrogen bond between histidine-57 and serine-195) as a function of the protonation states of the constituent residues. Connectivities in one-dimensional spectra used to assign NMR data were collected at three pH values: pH 9, at which His-57 is neutral and Asp-102 is negatively charged; pH 3.5, at which His-57 is positively charged and Asp-102 is negatively charged; and pH 1, at which His-57 is positively charged and Asp-102 is neutral. The signal from Hε2 of histidine-57 was assigned by reference to 1H−1H NOE connectivities at pH 3.5 to the previously assigned signals from the Hε1 and Hδ2 of the same residue. The D/H fractonation factor, φ, for the hydrogen bond between Asp-102 and His-57 changed from φ = 2 at pH 9 to φ = 0.4 at pH 3.5. From studies of model systems, it may be concluded that a change of φ of this magnitude corresponds to a large increase in hydrogen bond strength. A signal from the hydrogen bond between Ser-195 and His-57 was detected only at the lower pH values studied. The D/H fractionation factor for this hydrogen bond was φ = 0.7 at pH 3.5, indicative of a moderately strong interaction. Data obtained at pH 1 indicate that the hydrogen bond between Asp-102 and His-57 is weakened but that the hydrogen bond between His-57 and Ser-195 persists. The results are consistent with the hypothesis that changes in hydrogen bonding strength serve to lower barriers along the reaction coordinate in the catalytic mechanism. Large pH-dependent changes were found in the activation enthalpy (ΔH⧧) for exchange with protons from the solvent at the hydrogen bond between aspartate-102 and histidine-57: ΔH⧧ was approximately 10−12 kcal·mol-1 higher at pH 3.5 than at pH 1 or 9.