Section 23.4.5.2. Bovine pancreatic trypsin inhibitor

Bovine pancreatic trypsin inhibitor (BPTI) is a protein of 58 amino-acid residues whose X-ray crystal structure was obtained in the original crystal form to 1.5 Å resolution (Deisenhofer & Steigemann, 1975). Subsequently, 1 Å X-ray data were obtained from a different crystal form, and the new model was jointly refined with 1.8 Å neutron diffraction data (Wlodawer et al., 1984). Minor differences in structure between the two crystal forms of BPTI were observed (Wlodawer et al., 1984). The interesting contribution of the 1 Å model to the understanding of solvent structure resulted from the ability to refine occupancy at this resolution. A total of 63 water molecules were placed in the model, 20 of them within 1 Å of a water molecule found in the structure solved in the original crystal form. During refinement against the 1 Å data set, full occupancy was assigned to all protein atoms, and water occupancy was allowed to refine. Of the 63 water-molecule positions, 29 were found to be fully occupied. The remaining 34 had partial occupancies, with 0.4 being the minimum occupancy found. Given that there are very few contacts between protein molecules in the crystal (Wlodawer et al., 1984), it is reasonable to assume that this observation is representative of water occupancies on protein surfaces in general. It is likely that well over half of the water positions found on protein surfaces are less than fully occupied, although there is no definitive proof that this is true.