Geometry of interaction of metal ions with histidine residues in protein structures.

An analysis of the geometry and the orientation of metal ions bound to histidine residues in proteins is presented. Cations are found to lie in the imidazole plane along the lone pair on the nitrogen atom. Out of the two tautomeric forms of the imidazole ring, the NE2-protonated form is normally preferred. However, when bound to a metal ion the ND1-protonated form is predominant and NE2 is the ligand atom. When the metal coordination is through ND1, steric interactions shift the side chain torsional angle, chi 2 from its preferred value of 90 or 270 degrees. The orientation of histidine residues is usually stabilized through hydrogen bonding; ND1-protonated form of a helical residue can form a hydrogen bond with the carbonyl oxygen atom in the preceding turn of the helix. A considerable number of ligands are found in helices and beta-sheets. A helical residue bound to a heme group is usually found near the C-terminus of the helix. Two ligand groups four residues apart in a helix, or two residues apart in a beta-strand are used in many proteins to bind metal ions.