Dynamic and Static Water Molecules Complement the TN16 Conformational Heterogeneity inside the Tubulin Cavity.

TN16 is one of the most promising inhibitors of α, β dimer of tubulin that occupies the cavity in the β-subunit located at the dimeric interface, known as the colchicine binding site. The experimentally determined structure of the complex (Protein Data Bank entry 3HKD) presents the conformation and position of the ligand based on the "best fit", keeping the controversy of other significant binding modes open for further investigation. Computation has already revealed that TN16 experiences fluctuations within the binding pocket, but the insight from that previous report was limited by the shorter windows of sampling and by the approximations on the surrounding environment by implicit solvation. This article reports that in most of the cases straightforward MMGBSA calculations of binding energy revealed a gradual loss of stabilization that was inconsistent with the structural observations, and thus, it indicated the lack of consideration of stabilizing factors with appropriate weightage. Consideration of the structurally packed water molecules in the space between the ligand and receptor successfully eliminated such discrepancies between the structure and stability, serving as the "litmus test" of the importance of explicit consideration of such structurally packed water in the calculations. Such consideration has further evidenced a quasi-degenerate character of the different binding modes of TN16 that has rationalized the observed intrinsic fluctuations of TN16 within the pocket, which is likely to be the most critical insight into its entropy-dominated binding. Quantum mechanical calculations have revealed a relay of electron density from TN16 to the protein via a water molecule in a concerted manner.