title: A structure/activity study of calcium affinity and selectivity using a synthetic peptide model of the helix-loop-helix calcium-binding motif.
abstract: The acid pair hypothesis predicts the calcium affinity of the helix-loop-helix calcium-binding motif based on the number and location of acidic amino acid residues in chelating positions of the calcium-binding loop region. This study investigates the effects of the number and position of acidic residues in the loop region on calcium affinity and selectivity using 33-residue synthetic models of single helix-loop-helix calcium-binding motifs. Increasing the number of acidic residues in the octahedrally arranged chelating positions of the loop region from 3 to 4 by replacing an asparagine in the +y position with an aspartic acid increases the calcium affinity of the models between 2- and 38-fold. Differences in affinities are more pronounced in the models containing an x axis acid pair. The calcium affinities of peptide models containing 3 or 4 acidic residues in chelating positions of the loop region and an x axis acid pair are reduced when the residue in the +z position is changed from asparagine to serine. A similar reduction in calcium affinity occurs in the z axis acid paired peptides when the -x chelating residue is changed from serine to asparagine. Models with 3 acidic residues in chelating positions containing a z axis acid pair have greater calcium affinity than comparable peptide models with an x axis acid pair. The presence of x or z axis acid pairs in comparable peptides containing 4 acidic residues in chelating positions does not greatly alter calcium affinity. Calcium selectivity resides in x axis acid paired peptides, whereas z axis acid paired peptides exhibit both magnesium- and calcium-induced structural changes. This ion selectivity may be explained by postulating that the z axis residue side chains produce the initial, rate-limiting interactions with the cation, causing hydration shell destabilization and initiating the subsequent ligand interactions.
authors.affiliation: Faculty of Pharmaceutical Sciences, Division of Pharmaceutical Chemistry, University of British Columbia, Vancouver, Canada.
authors.forename: R M
authors.initials: RM
authors.lastname: Procyshyn
authors.forename: R E
authors.initials: RE
authors.lastname: Reid
mesh_terms: D000595:Amino Acid Sequence; D001216:Asparagine; D001665:Binding Sites; D002118:Calcium / Q000378:metabolism*; D002147:Calmodulin / Q000031:analogs & derivatives* / Q000138:chemical synthesis / Q000737:chemistry / Q000378:metabolism*; D018257:Helix-Loop-Helix Motifs; D007700:Kinetics; D008274:Magnesium / Q000378:metabolism; D008961:Models, Structural; D008969:Molecular Sequence Data; D010455:Peptides / Q000138:chemical synthesis / Q000737:chemistry*; D011487:Protein Conformation; D017433:Protein Structure, Secondary; D012694:Serine; D013329:Structure-Activity Relationship
chemical_list: D002147:Calmodulin; D010455:Peptides; D012694:Serine; D001216:Asparagine; D008274:Magnesium; D002118:Calcium