Crystal structures, at 1.7 Å resolution, were solved for complexes between each of two chemically synthesized partially folded analogues of bovine pancreatic trypsin inhibitor (BPTI) with the proteolytically inactive rat trypsin mutant S195A. The BPTI analogue termed [14-38]Abu retains only the disulfide bond between Cys14 and Cys38, while Cys5, Cys30, Cys51, and Cys55 are replaced by isosteric α-amino-n-butyric acid residues. The analogue K26P,A27D[14-38]Abu contains two further replacements, by statistically favored residues, in the type I β-turn that has been suggested to be a main site for initiation of BPTI folding. As a control, the structure of the complex between S195A trypsin and wild-type BPTI was also solved. Despite significant differences in the degree of structure detected among these three BPTIs in solution by several biophysical techniques, their tertiary folds once bound to S195A trypsin in a crystalline lattice are essentially superimposable.
Bibliographical noteFunding Information:
We thank Professor Lizbeth Hedstrom of Brandeis University (Waltham, MA) for providing the trypsin expression system and many useful suggestions and discussions. This work was supported by National Institutes of Health GM 51628 (G.B. and C.W.) and GM 46436 (D.H.O.). Use of the facilities of the Kahlert Structural Biology Laboratory was supported by a Minnesota Partnership for Biotechnology and Medical Genomics Grant SPAP-05-0013-P-FY06. Computational resources were provided by the Basic Sciences Computing Laboratory of the University of Minnesota Supercomputing Institute.
- crystal structure
- ligand-induced folding
- partially folded protein