A STRUCTURE-ACTIVITY STUDY OF CALCIUM AFFINITY AND SELECTIVITY USING A SYNTHETIC PEPTIDE MODEL OF THE HELIX-LOOP-HELIX CALCIUM-BINDING MOTIF

The acid pair hypothesis predicts the calcium affinity of the helix-lo op-helix calcium-binding motif based on the number and location of aci dic amino acid residues in chelating positions of the calcium-binding loop region. This study investigates the effects of the number and pos ition of acidic residues in the loop region on calcium affinity and se lectivity using 33-residue synthetic models of single helix-loop-helix calcium-binding motifs. Increasing the number of acidic residues in t he octahedrally arranged chelating positions of the loop region from 3 to 4 by replacing an asparagine in the +y position with an aspartic a cid increases the calcium affinity of the models between 2- and 38-fol d. Differences in affinities are more pronounced in the models contain ing an x axis acid pair. The calcium affinities of peptide models cont aining 3 or 4 acidic residues in chelating positions of the loop regio n and an x axis acid pair are reduced when the residue in the +z posit ion is changed from asparagine to serine. A similar reduction in calci um affinity occurs in the z axis acid paired peptides when the -x chel ating residue is changed from serine to asparagine. Models with 3 acid ic residues in chelating positions containing az axis acid pair have g reater calcium affinity than comparable peptide models with an x axis acid pair. The presence of x or z axis acid pairs in comparable peptid es containing 4 acidic residues in chelating positions does not greatl y alter calcium affinity. Calcium selectivity resides in x axis acid p aired peptides, whereas z axis acid paired peptides exhibit both magne sium-and calcium-induced structural changes. This ion selectivity may be explained by postulating that the z axis residue side chains produc e the initial, rate-limiting interactions with the cation, causing hyd ration shell destabilization and initiating the subsequent ligand inte ractions.