Specific perturbation by Ca2+ of tyrosyl residue 138 of calmodulin.

Calmodulin that was selectively nitrated at tyrosyl residue 99 or 138 was used as a regional spectral probe of the structural changes associated with Ca2+ or Mg2+ binding to the protein. Perturbations of the spectral properties of the nitrotyrosyl residues correlated well with those of the tyrosyl residues in unmodified calmodulin. The spectral properties of tyrosyl residue 99 were sensitive to ionic strength, but not to Ca2+ binding to the protein. The spectrum of tyrosyl residue 138, in contrast, was markedly affected both by ionic strength and Ca2+ binding. Although Mg2+ caused spectral changes, they were clearly different from those produced by Ca2+ and resembled instead the less specific changes elicited by Na+. Conformational changes associated with Ca2+ but not Mg2+ binding to calmodulin seem to affect selectively the microenvironment around tyrosyl residue 138.