Ionizable P1 residues in serine proteinase inhibitors undergo large pK shifts on complex formation.

The burial of charged residues in proteins is rare as it is thermodynamically strongly disfavored. However, in "standard mechanism" protein inhibitors of serine proteinases, the P1 residue, which is highly exposed, becomes buried in the S1 specificity pocket of the enzyme. In many enzymes, such as Streptomyces griseus proteinase B (SGPB) the S1 pocket is hydrophobic. We measured the pH dependence of the association equilibrium constant for the interaction of SGPB with turkey ovomucoid third domain P1 mutants, Glu18 OMTKY3 and His18 OMTKY3. In order to eliminate the effects of other ionizable groups on the enzyme and the inhibitor, we divided these pH dependences by the pH dependence of the association equilibrium constant for the Gln18 OMTKY3 mutant. This yielded for Glu18, pKf (free inhibitor) of 4.46 +/- 0.05 and pKc (complex) of 8.74 +/- 0.06. For His18 the values are pKf 6.63 +/- 0.08 and pKc 4.31 +/- 0.07. At low pH values Glu18 variant is a relatively good inhibitor for SGPB. This may be biologically relevant.