Proton nuclear magnetic resonance studies of porcine intestinal calcium binding protein.

1H nuclear magnetic resonance has been employed to study the environment of several proton nuclei (primarily those arising from aromatic residues) of the porcine intestinal calcium binding protein. An assignment for the single tyrosine (Tyr-16) residue has been made on the basis of laser photochemically induced dynamic nuclear polarization (CIDNP) and mononuclear decoupling experiments. pH titration studies have shown that the tyrosine pKa is unusually high in the apoprotein and increases even further upon the addition of calcium. However, the observation of a CIDNP effect with this tyrosine in both the presence and absence of calcium indicates that it is solvent accessible and therefore exposed on the surface of the molecule. Under the conditions of these experiments, the protein was observed to bind calcium with a 2:1 stoichiometry, at a rate of exchange slow enough that the NMR spectra are in the slow-exchange limit. The presence of upfield shifted phenylalanine and methyl resonances in the apoprotein indicates that there is a well-defined tertiary structure in the absence of calcium.