Perturbations, including ligand binding, can further alter the conformational ensemble with varying entropic consequences . An active site ligand may form van der Waals contacts with the leucine side chain in a way that strongly prefers one of the apo state alternative conformations (Figure ). The transition from two to one rotameric conformation further decreases conformational entropy. Since most structural models from X-ray crystallography or cryoEM would only have one of the two conformations modeled in the apo state, whether the ligand binding induces a "conformational change" depends on which conformation is modeled in the apo state (usually, but not always, the more populated one). But, by considering the underlying ensemble, we can see that ligand binding induces a "conformational redistribution" and can start to think about the entropic consequences on the free energy of binding.