A Weak Calcium Binding Site in Subtilisin BPN‘ Has a Dramatic Effect on Protein Stability

The crystal structures of both subtilisin 8397 and a thermostable variant (Lys 256 Tyr) have been determined to 2.2 and 1.8 Å resolution. The thermostable variant (8397+1) was previously shown to exhibit enhanced thermostability over 8397 in both aqueous solutions and the polar organic solvent dimethylformamide (Sears, P.; et al . J . Am . Chem . Soc . 1994, 116, 6521−6530). The single substitution did not induce major changes in the protein structure (total rms deviation is 0.41 Å); however, changes in calcium binding were detected. The strong calcium binding site was occupied in both structures as has been seen in other subtilisins (Pantoliano, M.; et al . Biochemistry 1988, 27, 8311−8317). Unexpectedly, the weak calcium binding site was occupied in the 8397+1 structure but not in the 8397 structure. The goal of the Lys 256 Tyr mutation was to improve the stability of subtilisin in DMF by removing a surface charge. However, changing this residue altered calcium binding at a site 12 Å away, illustrating the importance of structure determination in understanding stability changes.