Difference in the binding mechanisms of ABT-263/43b with Bcl-xL/Bcl-2: computational perspective on the accurate binding free energy analysis.

B-cell lymphoma/leukemia gene-2(Bcl-2) protein family known for regulating cell cycle arrest and subsequent cell death is highly expressed in a variety of cancers. Among them, the Bcl-xL and Bcl-2 are two essential proteins in the Bcl-2 family. In the present work, the differences in binding modes as between the two proteins and two ligands ABT-263/43b were investigated and compared. And the computational alanine scanning combined with the recently developed interaction entropy (AS-IE) method was employed for predicting their binding free energies and finding those amino acids that were more critical during the binding process. The result showed that the binding free energy calculated by the AS-IE method was more in line with experimental values than the molecular mechanics/Poisson-Boltzmann surface area (MM/PBSA) method. Besides, no significant difference was found between Bcl-xL and ABT-263/43b in the binding free energy, which Bcl-xL showed slightly weaker binding free energy to 43b because of the fewer number of key residues with interactions. Nonetheless, compared with the Bcl-2 and 43b complex, the Bcl-2 and ABT-263 system had greater number of key residues interacting with ABT-263, in particular, contribute favorably, resulting in a stronger binding ability for the Bcl-2 and ABT-263 systems. The van der Waals and hydrogen bond contributions were significant in the four protein-ligand complexes. Overall, Tyr108 was found to be the common key residues in the Bcl-xL-ligand complex, while Tyr105, Glu100, and Glu143 were established as the common key residue in the Bcl-2-ligand systems. We hope that the predicted hot spot residues and their energy distributions can guide the design of peptide and small-molecule drugs targeting Bcl-xL and Bcl-2.