A pulsed field gradient NMR study of the aggregation and hydration of parvalbumin

Pulsed field gradient NMR is a convenient alternative to traditional methods for measuring diffusion of biological macromolecules. In the present study, pulsed field gradient NMR was used to study the effects of calcium binding and hydration on carp parvalbumin. Carp parvalbumin is known to undergo large changes in tertiary structure with calcium loading. The diffusion coefficient is a sensitive guide to changes in molecular shape and in the present study the large changes in tertiary structure were clearly reflected in the measured diffusion coefficient upon calcium loading. The (monomeric) calcium-loaded form had a diffusion coefficient of 1.4 x 10^-10 m2 s^-1 at 298 K, which conforms with the structure being a nearly spherical prolate ellipsoid from X-ray studies. The calcium-free form had a significantly lower diffusion coefficient of 1.1 x 10^-10 m² s^-1. The simplest explanation consistent with the change in diffusion coefficient is that the parvalbumin molecules form dimers upon the removal of Ca²⁺ at the protein concentration studied (1 mM).