The Eu(III) 7F0→5D0 excitation spectra of the parvalbumins are highly pH-dependent. Below pH 6.0, they exhibit a sharp, partially resolved doublet centered near 5,795 Å. However, as the pH is raised, the spectrum becomes increasingly dominated by a much broader signal near 5,784 Å. This behavior has been traced to the Eu(III) ion bound at the CD site, but the identity of the moiety undergoing deprotonation remains uncertain. Site-specific mutagenesis studies on the parvalbumin-like protein known as oncomodulin now suggest that the species in question is a liganding serine hydroxyl group. Specifically, replacement of serine-55 by aspartate (the residue present at the corresponding position in the EF site) affords a protein that retains two functional lanthanide binding sites, but fails to undergo the pH-dependent spectral alteration. By contrast, replacement of aspartate-59 by glycine (the corresponding EF site residue) fails to abolish the pH-dependent behavior.
