Biotin carboxylase is a homodimer that utilizes ATP to carboxylate biotin. Studies of the enzyme using X-ray crystallography revealed a prominent conformational change upon binding ATP. To determine the importance of this closing motion, the potential of mean force with the closure angle as a reaction coordinate was calculated using molecular dynamics simulations and umbrella sampling for a monomer of Escherichia coli biotin carboxylase in water with restraints to simulate attachment to a surface. The result suggests that the most stable state for the enzyme is a closed state different from both the ATP-bound and open state X-ray crystallography structures. There is also a significant motion of a region near the dimer interface not predicted by considering only open and closed configurations, which may have implications for the dynamics and activity of the dimer.