The decameric structure of bovine pancreatic trypsin inhibitor (BPTI) crystallized from thiocyanate at 2.7Å resolution

The structure of a monoclinic form of bovine pancreatic trypsin inhibitor (BPTI) crystallized from a thiocyanate solution has been determined and refined at 2.7 Angstrom resolution. The space group is P2(1) with a = 71.56, b = 73.83, c = 64.47 Angstrom,beta = 93.9 degrees and Z = 20. The ten independent molecules were located by a multi-body molecular-replacement search as developed in the AMoRe program, starting from a single monomer model (PDB code: 6PTI). The molecular arrangement of the subunits is a decamer resulting from the combination of two orthogonal fivefold and twofold noncrystallographic axes. This builds a globular micelle-like particle which minimizes hydrophobic interactions with the solvent. The refinement was conducted with non-crystallographic symmetry constraints up to a final residual of R = 0.20 (R-free = 0.26). The cost-mean-square deviations from ideal geometry were 0.015 Angstrom and 1.6 degrees on bond distances and bond angles, respectively. Several sites for thiocyanate ions were analyzed.