Lanthanides as probes for calcium in biological systems

Abstract

Calcium ion plays an essential role in many biological processes. The environment about Ca 2+may be probed by substitution of tripositive lanthanide ions, Ln 3+. Ca 2+proteins fall into two broad classes: those that are inhibited by Ln 3+substitution and those that are not. It is suggested that although Ca 2+undertakes a primarily structural role in the Ln 3+non-inhibited proteins, Ca 2+may be near the active site or participate in the mechanism of action of Ln 3+inhibited proteins. Ca 2+and Ln 3+radii are similar; most Ln 3+are slightly larger than Ca 2+in complexes of the same coordination number, and substitution of Ln 3+for Ca 2+is accommodated by a slight decrease in bond distance or by an increase in coordination number. Luminescence from Tb 3+has been demonstrated to be a sensitive environmental probe of Ca 2+binding sites in proteins.