The Coupling Character of Ca2+ with Glutamic Acid: Implication for the Conformational Behavior and Transformation of Ca2+-ATPase in Transmembrane Ca2+ Channel

The gas-phase chelating structures of Ca2+ with glutamic acid have been investigated by density functional theory calculations. Accurate geometric structures, relative stability (including the vertical and the adiabatic binding energies), charge distributions, bonding analysis, and harmonic vibrational frequencies have been explored. Optimizations revealed a total of sixteen minima on the PES, and the lowest energy form corresponds to a tridentate chelate in which Ca2+ interacts with three oxygen atoms belonging to two carboxyl groups and there is proton transfer from the neighboring −COOH to −NH2. Another tridentate chelating form is only a few kcal/mol higher in energy, where the Ca2+ binds to one amino nitrogen and two carbonyl oxygens. The differences of geometries and energies have been discussed from the viewpoint of binding modes, charge distributions, repulsions of the functional groups, electrostatics, and polarizations. Furthermore, because of the similarity in geometry and energy, the geometric transformation processes, which are important in Ca2+ binding and transformation in the biological system, among different isomers have also been investigated. Some properties to the biological relevance have been discussed.