Proton‐NMR studies of the solution conformations of vitamin‐D‐induced bovine intestinal calcium‐binding protein

Abstract

1H NMR is used to study the solution structure of vitamin-D-induced bovine intestinal Ca-binding protein. The study of the native protein is aided by the recently published crystal structure; it is shown that the conformations of the molecule in the crystal and in solution are very similar. The effect of pH and temperature on the native structure is described. The structure of the apoprotein is described, and the effect of pH and temperature on its fold is outlined. A comparison between apo and native protein folds indicates that the folds are very similar. The 2-folds are related by a Ca titration, which indicates that the protein binds 2 Ca2+ sequentially. Both steps in the Ca2+ titration occur under conditions of slow exchange (kex 80 s-1). The effect of binding Ca2+ ions is to cause twisting motions of helices, with the helices acting as rods, relaying the conformational change induced by Ca2+ binding to the linker regions of the protein.