Absorption and fluorescence spectroscopic studies of the Ca2(+)-dependent lipid binding protein p36: the annexin repeat as the Ca2+ binding site.

The existence of a single tryptophan residue in the protein p36, a member of a recently characterized family of Ca2+ binding proteins called annexins, is exploited to provide unique spectroscopic information on the annexin repeat motif and its role in Ca2+ binding. The differences in ultraviolet absorption and fluorescence excitation upon Ca2+ binding are interpreted solely in terms of this tryptophan, which, in view of the pronounced blue-shifts and the presence of vibronic structure, seems to reside in a highly nonpolar environment. The fluorescence emission from the protein is correspondingly blue-shifted, and it is found to transfer energy in resonance with Tb3+ absorption lines in the near-ultraviolet. This effect allows us to locate the Tb3+ and, by implication, the Ca2+ binding site to within ca. 8 A of the tryptophan residue.