Crystal structure of rat α‐parvalbumin at 1.05 Å resolution

The crystal structure of rat α‐parvalbumin has been determined at 1.05 Å resolution, using synchrotron data collected at Advanced Photon Source beamline 19‐ID. After refinement with SHELX, employing anisotropic displacement parameters and riding hydrogen atoms, R = 0.132 and R_free = 0.162. The average coordinate estimated standard deviations are 0.021 Å and 0.038 Å for backbone atoms and side‐chain atoms, respectively. Besides providing a more precise view of the α‐isoform than previously available, these data permit comparison with the 0.91 Å structure determined for pike β‐parvalbumin. Visualization of the anisotropic displacement parameters as thermal ellipsoids yields insight into the atomic motion within the Ca²⁺‐binding sites. The asymmetric unit includes three parvalbumin (PV) molecules. Interestingly, the EF site in one displays uncharacteristic flexibility. The ellipsoids for Asp‐92 are particularly large and non‐spherical, and the shape of the Ca²⁺ ellipsoid implies significant vibrational motion perpendicular to the plane defined by the four y and z ligands. The relative dearth of crystal‐packing interactions in this site suggests that the heightened flexibility may be the result of diminished intermolecular contacts. The implication is that, by impeding conformational mobility, crystal‐packing forces may cause serious overestimation of EF‐hand rigidity. The high quality of the data permitted 11 residues to be modeled in alternative side‐chain conformations, including the two core residues, Ile‐97 and Leu‐105. The discrete disorder observed for Ile‐97 may have functional ramifications, providing a mechanism for communicating binding status between the CD and EF binding loops and between the PV metal ion‐binding domain and the N‐terminal AB region.