X-ray Structures of Mn, Cd and Tb Metal Complexes of Troponin C

The crystal structures of three metal complexes of troponin C (TnC) have been determined and refined where the two occupied structural Ca²⁺ sites in the C domain have been substituted by Mn²⁺, Cd²⁺ and Tb³⁺. The X-ray intensity data were collected to 2.1, 1.8 and 1.8 Å resolution, respectively, on the three metal complexes, which are isomorphous with Ca-TnC. The three complexes have r.m.s. deviations of 0.27, 0.25 and 0.35 Å, respectively, for all protein atoms, from Ca-TnC. Irrespective of the charge on the metal (+2 or +3), the occupied sites 3 and 4 exhibit a distorted pentagonal bipyramidal coordination, like Ca-TnC, with seven ligands, six from the 12-residue binding loop and the seventh from a water molecule. Mn²⁺ at site 4 seems to display a longer distance to one of the carboxyl bidentate ligands representing an intermediate coordination simulating the six-coordinate Mg²⁺. The carboxyl O atoms of the bidentate Glu12 are displaced on the side of the equatorial plane passing through the remaining three ligands with one O atom closer to the plane (Δ of 0.11 to 0.76 Å) than the other (Δ of 0.93 to 1.38 Å). The two axial ligands are an aspartic carboxyl O atom and a water molecule. The metal is displaced (0.18 to 0.56 Å) towards the water facing the water channel.