Nonempirical analysis of pKa shifts in mutated subtilisines

One of the experimentally accessible properties determined by intermolec-ular interactions are pKa shifts of ionizable amino acids (most frequently histidine ) induced by neighboring aminoacid residue substitutions 

Therefore, many theoretical studies on enzyme reaction mechanisms or drug binding depend on more or less arbitrary assumptions of aminoacid protonation state. Due to considerable complexity of this problem the available theoretical pKa estimates are typically based on very crude models highly depending on such empirical factors like dielectric constant. To our knowledge the physical nature of pK shifts has not yet been studied on nonempirical level due to considerable size of involved systems. 

As the test ease we selected several mutated subtilisines, where not only experimental pKa shifts of HIS 64 are known, but also local effective dielectric constants between HIS 64 and mutated site are available [36]. This allows to project experimental pKa shifts into vacuum ApK and compare directly with theoretical values. 

Using Differential Product Stabilization approach [7] one may express pKa shift scaled to vacuum in the following way 

Results presented in the Table 6 indicate that the physical nature of pKa shifts is dominated at least in 99.8% by electrostatic term, with very small contribution of delocalization term not exceeding 0.2 %, whereas exchange component is completely negligible. Inexpensive atomic multipole estimates of the electrostatic term ApK may reasonably approximate costly SCF 

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