title: Protein engineering of subtilisin BPN': enhanced stabilization through the introduction of two cysteines to form a disulfide bond.
abstract: Introduction of a disulfide bond by site-directed mutagenesis was found to enhance the stability of subtilisin BPN' (EC 3.4.21.14) under a variety of conditions. The location of the new disulfide bond was selected with the aid of a computer program, which scored various sites according to the amount of distortion that an introduced disulfide linkage would create in a 1.3-A X-ray model of native subtilisin BPN'. Of the several amino acid pairs identified by this program as suitable candidates, Thr-22 and Ser-87 were selected by using the additional requirement that the individual cysteine substitutions occur at positions that exhibit some degree of variability in related subtilisin amino acid sequences. A subtilisin variant containing cysteine residues at positions 22 and 87 was created by site-directed mutagenesis and was shown to have an activity essentially equivalent to that of the wild-type enzyme. Differential scanning calorimetry experiments demonstrated the variant protein to have a melting temperature 3.1 degrees C higher than that of the wild-type protein and 5.8 degrees C higher than that of the reduced form (-SH HS-) of the variant protein. Kinetic experiments performed under a variety of conditions, including 8 M urea, showed that the Cys-22/Cys-87 disulfide variant undergoes thermal inactivation at half the rate of that of the wild-type enzyme. The increased thermal stability of this disulfide variant is consistent with a decrease in entropy for the unfolded state relative to the unfolded state that contains no cross-link, as would be predicted from the statistical thermodynamics of polymers.
mesh_terms: D000900:Anti-Bacterial Agents; D001426:Bacterial Proteins; D001430:Bacteriocins; D002152:Calorimetry, Differential Scanning; D003198:Computer Simulation; D003545:Cysteine; D004220:Disulfides; D005818:Genetic Engineering / Q000379:methods; D008958:Models, Molecular; D009154:Mutation; D010455:Peptides; D010456:Peptides, Cyclic / Q000235:genetics / Q000302:isolation & purification / Q000378:metabolism; D011487:Protein Conformation; D014961:X-Ray Diffraction
chemical_list: D000900:Anti-Bacterial Agents; D001426:Bacterial Proteins; D001430:Bacteriocins; D004220:Disulfides; D010455:Peptides; D010456:Peptides, Cyclic; C002836:subtilin; D003545:Cysteine