Engineering Magnesium Selectivity in the Helix–Loop–Helix Calcium-Binding Motif

Engineering magnesium selectivity into the helix–loop–helix (hlh) cation binding site is relatively unstudied in the calmodulin superfamily of calcium-regulated proteins, which include parvalbumin, oncomodulin, troponin C, calbindin, and calmodulin. Studies using a 33-residue synthetic peptide model of thehlhcation binding motif have indicated that magnesium will induce structural change in those peptide motifs containing three or four acid residues in chelating positions with a single-acid-pair on the Z-axis. Decreasing the cation binding cavity size in Z-axis acid-paired motifs through replacement of chelating residues in the +Z or −X metal ion coordinating positions in the loop region by glutamic acid has been successful in decreasing the calcium ion affinity. The same changes did not create or enhance magnesium binding in the 33-residue modelhlhcation binding motif.