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 St 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, pK(f) (free inhibitor) of 4.46 ± 0.05 and pK(c) (complex) of 8.74 ± 0.06. For His18 the values are pK(f)6.63 ± 0.08 and pK(c) 4.31 ± 0.07. At low pH values Glu18 variant is a relatively good inhibitor for SGPB. This may be biologically relevant.
All Science Journal Classification (ASJC) codes
- Molecular Biology
- Cell Biology