INVESTIGATION OF CA2-BINDING SITES ON THE 33 KDA PROTEIN IN PHOTOSYSTEM-2 USING TB3+ AS A FLUORESCENCE PROBE()

Terbium (Tb3+) was used as a fluorescence probe in the study of calciu m-binding sites on 33 kDa protein of photosystem 2. The fluorescence o f Tb3+ was enhanced markedly when bound to the 33 kDa protein, and the non-radiative energy transfer between tryptophan (Trp) residue and Tb 3+, bound to the calcium-binding sites on the 33 kDa protein, took pla ce. According to the Forster non-radiative energy transfer mechanism, the average distance between the bound Tb3+ and Trp residue was found to be 1.05 nn. The pH titration indicated that major groups in the 33 kDa protein, involved in Ca2+ ions binding, were the carboxylic side g roups of the glutamic acid and/or aspartic acid.