Ligand docking to proteins with discrete side-chain flexibility

An algorithm is described that explores the conformational degrees of freedom of the amino acid side-chains and of the ligand when docking a putative ligand into a receptor site. For a given orientation of the ligand relative to the protein, the method can find the lowest energy combination of amino acid side-chains and ligand conformations as well as all other combinations in order of increasing energy within a specified energy cutoff. The amino acid side-chains and the ligand are restricted to discrete low-energy conformations, determined for the former by analysing high-resolution protein structures and in the latter case from a conformational analysis. Coupled to an algorithm for exploring the six degrees of orientational freedom of the ligand with respect to the receptor, the method can be used to perform conformationally flexible ligand docking. A combination of two search methods is employed to explore the conformational degrees of freedom: Dead End Elimination and the A* algorithm. When no ligand is present the approach can be used to predict the lowest energy combinations of side-chain conformations for a given protein backbone structure. The approach is employed to illustrate how such a procedure can be used to estimate the conformational entropy change that accompanies the formation of an intermolecular complex between a protein and a ligand and to demonstrate that the protein's conformational entropy may in some cases increase on binding the ligand. This is due to a modification of the protein's energy hypersurface that makes more conformational states accessible. Our results highlight the need for appropriate methods to estimate the strength of binding.