Eu3+ luminescence studies of oncomodulin. The origin of the pH-dependent behavior

The Eu3+ 7F0----5D0 excitation spectra of parvalbumin and oncomodulin are pH-dependent. Until now, it had been assumed that both the CD and EF ion-binding sites shared this property and that deprotonation of water molecules coordinated to the bound Eu3+ ions might be responsible for the pH dependence. Results obtained with the site-specific variant of oncomodulin known as D59E, in which glutamate replaces the aspartate naturally present at position 59, have necessitated substantial revision of these ideas. It now appears that the pH-dependent behavior is confined to the CD site. Moreover, we observe no corresponding change in the number of O-H oscillators coordinated to the bound Eu3+ ions in the pH range over which we observe the spectroscopic alteration. It is likely that the behavior results from deprotonation of one or more carboxyl groups clustered at the COOH-terminal end of the CD domain.