Interaction of metal ions with carboxylic and carboxamide groups in protein structures

An analysis of the geometry of metal binding by carboxylic and carboxamide groups in proteins is presented. Most of the ligands are from aspartic and glutamic acid side chains. Water molecules bound to carboxylate anions are known to interact with oxygen lone-pairs. However, metal ions are also found to approach the carboxylate group along the C-O direction. More metal ions are found to be along the syn than the anti lone-pair direction. This seems to be the result of the stability of the five-membered ring that is formed by the carboxylate anion hydrogen bonded to a ligand water molecule and the metal ion in the syn position. Ligand residues are usually from the helix, turn or regions with no regular secondary structure. Because of the steric interactions associated with bringing all the ligands around a metal center, a calcium ion can bind only near the ends of a helix; a metal, like zinc, with a low coordination number, can bind anywhere in the helix. Based on the analysis of the positions of water molecules in the metal coordination sphere, the sequence of the EF hand (a calcium-binding structure) is discussed.