Crystal structure analysis of subtilisin BPN' mutants engineered for studying thermal stability

The high resolution crystal structures of four genetically engineered subtilisin BPN' variants (E.C. 3.4.21.14) which vary dramatically in their stability have been determined. The simplest variant, S3, contains two altered residues, N218S and S221C. The N218S change was incorporated for its stabilizing effects and its influence on crystallization; the S221C change, a modification of the active site serine, was included to reduce autolysis. The second variant, S12, includes the two additional stabilizing mutations M50F and Y217K. S15, the third variant, in addition to the 4 single site mutations, has residues 75-83, the high-affinity calcium-binding site, deleted. The final variant S22 incorporates all of the above changes and two additional site specific mutations, T22C and S87C, which form a stabilizing disulfide bridge. The structural changes and influence on stability of each of these mutations are discussed in the context of supporting biophysical studies.