Abstract
The static accessibility modified discrete charge model for electrostatic interactions in proteins is extended to the prediction of the pH dependence of hydrogen exchange reactions. The exchange rate profiles of buried amide protons are shown to follow the calculated pH dependence of the electrostatic component of protein stability. Rate profiles are calculated for individual buried amide protons in ribonuclease S and bovine pancreatic trypsin inhibitor. The electrostatic free energy of stabilization of the protein and the energy required to bring the catalytic ion to an exchange site are expressed as an apparent, pH-dependent contribution to the activation energy. Changes in the electrostatic stabilization of the proteins affect the calculated exchange rate for buried amide protons by more than 1000, while local field effects raise or lower the predicted exchange rates by less than 100. The pH dependence of exchangeable protons at the protein surface, such as the C-2 imidazole protons, is shown to follow the estimated energy required to introduce the catalytic ion at the exchange site. These calculations are discussed in terms of current models for proton exchange which incorporate the dynamic nature of the structure to explain exchange data from the interior of a protein.
Highlights
Dependenceofexchangeableprotons at theprotein surface, suchas the C-2 imidazole protons,is shown to follow the estimated energy requiredto introduce the catalytic ion at the exchange site
The pH exchange rate profiles for slowly exchanging amide protons have been determined for individual sites by NMR techniques for bovine pancreatic trypsin inhibitor (Wuthrich and Wagner, 1979; Hilton and Woodward, 1978)
We assume thatthe pH-dependent acceleration, or hindrance, of the proton exchange reaction is the result of a unique set of long range, overlapping coulombic fields associated with the pro
Summary
The electrostatic work factor, W,], the interaction energy between two formal charges, i and j , at a spherical interface between two media of different dielectric constant, was derived in the original work of Tanford and Kirkwood (1957), and is described in detail by Matthew et al (1979) This number depends on the dimension of the sphere, the distance of separation of the charges, rlJ,the external and internal dielectric constants, and the ionic strength of the external medius The W,] values have been modified byinclusion of the term (1 - SA,) to account for the 1 0 4 effects of the protein structure. The overall electrostatic stabilization is calculated by summing over the interactions of all charge pairs
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