Structure of form III crystals of bovine pancreatic trypsin inhibitor.

The structure of bovine pancreatic trypsin inhibitor has been solved in a new crystal form III. The crystals belong to space group P2(1)2(1)2 with a = 55.2 A, b = 38.2 A, c = 24.05 A. The structure was solved on the basis of co-ordinates of forms I and II of the inhibitor by molecular replacement, and the X-ray data extending to 1.7 A were used in a restrained least-squares refinement. The final R factor was 0.16, and the deviation of bonded distances from ideality was 0.020 A. Root-mean-square discrepancy between C alpha co-ordinates of forms III and I are 0.47 A, whilst between forms II and III the discrepancy is 0.39 A. These deviations are about a factor of 3 larger than the expected experimental errors, showing that true differences exist between the three crystal forms. Two residues (Arg39 and Asp50) were modeled with two positions for their side-chains. The final model includes 73 water molecules and one phosphate group bound to the protein. Sixteen water molecules occupy approximately the same positions in all three crystal forms studied to date, indicating their close association with the protein molecule. Temperature factors also show a high degree of correlation between the three crystal forms.