Variability of conformations at crystal contacts in BPTI represent true low-energy structures: correspondence among...

The structures of five basic pancreatic trypsin inhibitor (BPTI) molecules are compared to establish the extent and nature of the conformational variability resulting from crystal packing effects. BPTI is an ideal system to evaluate such factors because of the availability of high resolution X-ray models of five different BPTI structures, each in a different crystal packing environment. Differences observed among the structures are found to be distributed throughout the molecule, although the regions that display most variability are associated with the loop structures (residues 14-17 and 24-29). The regions of structure that show the largest rms deviations from the mean of the five packing motifs correlate well with the presence of intermolecular contacts in the crystal lattice. For most of the molecules there is also a correspondence between a larger number of intermolecular contacts and systematically higher B-factors, although it is not apparent whether this is induced by the crystal contact or results from the fact that the contacts are made predominantly through surface loops. The conformational differences seen among the X-ray models constitute more than local shifts at the lattice contact surfaces, and in fact involve in some cases the making and breaking of intramolecular H-bonds. The magnitudes of the differences among packing models are significantly larger than those usually associated with changes induced by mutagenesis; for instance; the structural differences at the site of mutation observed on removing an internal disulfide from the molecule are significantly less than those associated with lattice contact effects. The crystal packing conformations are compared to representative structures of BPTI generated during a 96-psec molecular dynamics (MD) simulation. This comparison shows a high level of correspondence between the protein flexibility indicated by the X-ray and MD analyses, and specifically between those regions that are most variable. This suggests that the regions that show most variability among the crystal packing models are not artifacts of crystallization, but rather represent true low-energy conformers that have been preferentially selected by crystallization factors.