Infrared studies of interaction between metal ions and Ca2+‐binding proteins Marker bands for identifying the types of coordination of the side‐chain COO− groups to metal ions in pike parvalbumin (pI = 4.10)

Metal-ligand interactions in the Ca(2+)-binding sites of pike parvalbumin (pI = 4.10) have been examined by Fourier-transform infrared spectroscopy. The region of the COO- antisymmetric stretch provides useful information on the types of coordination of the COO- groups to the metal ions in the Mg(2+)-, Mn(2+)-, and Ca(2+)-bound forms. In the spectrum of the Ca(2+)-bound form, two bands are observed at 1,582 and 1,553 cm-1, whereas, in the spectra of the Mg(2+)- and Mn(2+)-bound forms, bands are observed only in the region around 1,582 cm-1 and no band is found in the region around 1,553 cm-1. The 1,553-cm-1 band of the Ca(2+)-bound form reflects the bidentate coordination of the COO- groups of both Glu-62 in the CD site and Glu-101 in the EF site to the Ca2+ ions, which has been made clear by X-ray analysis as a feature of the Ca(2+)-bound form. Absence of such a band in the spectrum of the Mn(2+)-bound form is consistent with the X-ray structure of this form where both of the two COO- groups are unidentate. These unidentate COO- groups of Glu-62 and Glu-101 in the Mn(2+)-bound form seem to give rise to a band at 1,577-1,574 cm-1. The spectrum of the Mg(2+)-bound form is also consistent with the 'pseudo-bridging' coordination of the COO- group of Glu-101 reported in the X-ray structure of a form where the Mg2+ ion occupies only the EF site, and the same spectrum is further indicative of the 'pseudo-bridging' coordination of the COO- group of Glu-62.