Improving sampling of binding free energy differences between covalently bound ligands in alternate binding pockets using MT-REXEE

Abstract: The primary limitation for the application of alchemical free energy methods to a wider variety of complex molecular systems in achieving reasonable sampling. Flexible binding complexes often have high free energy barriers, which require prohibitively long simulations to sample sufficiently to obtain reliable free energy estimates. An example of such a system is the FabB--acyl chain complex, which features acyl chain substrates from 4 to 16 carbons in length. Previous experimental evidence supports that there are also at least two accessible binding pockets for the acyl chain bound to FabB. Sampling of both pockets would require prohibitively long MD simulations or the creation of complicated and nonobvious collective variables. With the multiple topology replica exchange of expanded ensemble (MT-REXEE) enhanced sampling approach, we can obtain highly efficient sampling between both pockets by adaptively growing and shrinking the chains in the simulation ensemble, allowing each simulation to visit chain lengths where transitions between the pockets do occur. To enable this enhanced sampling for the chain growth problem, we demonstrate a new and improved conformational swapping selection algorithm for MT-REXEE, and validate this algorithm on a previously studied simultaneous calculation of multiple relative binding free energies. Using the new swapping approach gives significantly enhanced sampling even for this simpler problem, as demonstrated by faster convergence of free energy estimates. This case study demonstrates the utility of MT-REXEE and its open-source implementation for systems that feature high free energy barriers for a subset of ligands of interest, demonstrating a valuable addition to the existing stable of enhanced sampling methods.