Solution structure of Ca ²⁺ ‐free rat α‐parvalbumin

Mammals express two parvalbumins—an α isoform and a β isoform. In rat, the α‐parvalbumin (α‐PV) exhibits superior divalent ion affinity. For example, the standard free energies for Ca²⁺ binding differ by 5.5 kcal/mol in 0.15 M KCl (pH 7.4). High‐resolution structures of the Ca²⁺‐bound proteins provide little insight into this disparity, prompting a structural analysis of the apo‐proteins. A recent analysis of rat β‐PV suggested that Ca²⁺ removal provokes substantial conformational changes—reorientation of the C, D, and E helices; reorganization of the hydrophobic core; reduced interdomain contact; and remodeling of the AB domain. The energetic penalty attendant to reversing these changes, it was suggested, could contribute to the attenuated divalent ion‐binding signature of that protein. That hypothesis is supported by data presented herein, describing the solution structure and peptide backbone dynamics of Ca²⁺‐free rat α‐PV. In marked contrast to rat β‐PV, the apo‐ and Ca²⁺‐loaded forms of the rat α isoform are quite similar. Significant structural differences appear to be confined to the loop regions of the molecule. This finding implies that the α‐PV isoform enjoys elevated divalent ion affinity because the metal ion‐binding events do not require major structural rearrangement and the concomitant sacrifice of binding energy.