Calcium ions are
important messenger molecules in cells, which
bind calcium-binding proteins to trigger many biochemical processes.
We constructed four model systems, each containing one EF-hand loop
of calmodulin with one calcium ion bound, and investigated the binding
energy and free energy of Ca2+ by the quantum mechanics
symmetry-adapted perturbation theory (SAPT) method and the molecular
mechanics with the additive CHARMM36m (C36m) and the polarizable Drude
force fields (FFs). Our results show that the explicit introduction
of polarizability in the Drude not only yields considerably improved
agreement with the binding energy calculated from the SAPT method
but is also able to capture each component of the binding energies
including electrostatic, induction, exchange, and dispersion terms.
However, binding free energies computed with the Drude and the C36m
FFs both deviated significantly from the experimental measurements.
Detailed analysis indicated that one of main reasons might be that
the strong interactions between Ca2+ and the side chain
nitrogen of Asn/Gln in the Drude FF caused the distorted coordination
geometries of calcium. Our work illustrated the importance of polarization
in modeling ion–protein interactions and the difficulty in
generating accurate and balanced FF models to represent the polarization
effects.