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 Angstrom 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 Angstrom, respectively, for all protein atoms, from Ca-TnC. Irrespective of the charge on the metal (+2 or +3), the occupied sires 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 (Delta of 0.11 to 0.76 Angstrom) than the other (Delta of 0.93 to 1.38 Angstrom). The two axial ligands are an aspartic carboxyl O atom and a water molecule. The metal is displaced (0.18 to 0.56 Angstrom) towards the water facing the water channel.