Adaptive Partitioning QM/MM for Molecular Dynamics Simulations: 6. Proton Transport through a Biological Channel.

Adaptive quantum-mechanics/molecular-mechanics (QM/MM) dynamics simulations feature on-the-fly reclassification of atoms as QM or MM continuously and smoothly as trajectories are propagated. This allows one to use small, mobile QM subsystems, the contents of which are dynamically updated as needed. In this work, we report the first adaptive QM/MM simulations of H+ transfer through a biological channel, in particular, the protein EcCLC, a chloride channel (CLC) Cl-/H+ antiporter derived from E. coli. To this end, the H+ indicator previously formulated for approximating the location of an excess H+ in bulk water was extended to include Cl- ions and carboxyl groups as H+ donors/acceptors. Furthermore, when setting up buffer groups, a new "sushi-roll" scheme was employed to group multiple water molecules, ions, and titratable residues along the one-dimensional channel for adaptive partitions. Our simulations reveal that the H+ relay path, which consists of water molecules in the pore, a bound Cl- ion at the central binding site (Cl-cen) of the protein, and the external gating residue E148, exhibits certain mobility within the channel. A two-stage journey of H+ migration was observed: the H+ moves toward Cl-cen and is then shared between Cl-cen and nearby water molecules in the first stage and departs from Cl-cen via nearly concerted transfer to protonate E148 in the second stage. Most of the simulated trajectories show the bound Cl- ion in the channel to be transiently protonated, a possibility that was previously suggested by experiments and computations. Comparisons with conventional QM/MM simulations revealed that both adaptive and conventional treatments yield similar qualitative pictures. This work demonstrates the feasibility of adaptive QM/MM in the simulations of H+ migration through biological channels.

Chloride Ion Transport by the E. coli CLC Cl-/H+ Antiporter: A Combined Quantum-Mechanical and Molecular-Mechanical Study.

We performed steered molecular dynamics (SMD) and umbrella sampling simulations of Cl- ion migration through the transmembrane domain of a prototypical E. coli CLC Cl-/H+ antiporter by employing combined quantum-mechanical (QM) and molecular-mechanical (MM) calculations. The SMD simulations revealed interesting conformational changes of the protein. While no large-amplitude motions of the protein were observed during pore opening, the side chain rotation of the protonated external gating residue Glu148 was found to be critical for full access of the channel entrance by Cl-. Moving the anion into the external binding site (Sext) induced small-amplitude shifting of the protein backbone at the N-terminal end of helix F. As Cl- traveled through the pore, rigid-body swinging motions of helix R separated it from helix D. Helix R returned to its original position once Cl- exited the channel. Population analysis based on polarized wavefunction from QM/MM calculations discovered significant (up to 20%) charge loss for Cl- along the ion translocation pathway inside the pore. The delocalized charge was redistributed onto the pore residues, especially the functional groups containing π bonds (e.g., the Tyr445 side chain), while the charges of the H atoms coordinating Cl- changed almost negligibly. Potentials of mean force computed from umbrella sampling at the QM/MM and MM levels both displayed barriers at the same locations near the pore entrance and exit. However, the QM/MM PMF showed higher barriers (~10 kcal/mol) than the MM PMF (~2 kcal/mol). Binding energy calculations indicated that the interactions between Cl- and certain pore residues were overestimated by the semi-empirical PM3 Hamiltonian and underestimated by the CHARMM36 force fields, both of which were employed in the umbrella sampling simulations. In particular, CHARMM36 underestimated binding interactions for the functional groups containing π bonds, missing the stabilizations of the Cl- ion due to electron delocalization. The results suggested that it is important to explore these quantum effects for accurate descriptions of the Cl- transport.