Domain movement is sometimes essential for substrate recognition by an enzyme. X-ray crystallography of aminotransferase with a series of aliphatic substrates showed that the domain movement of aspartate aminotransferase was changed dramatically from an open to a closed form by the addition of only one CH(2) to the side chain of the C4 substrate CH(3)(CH(2))C((alpha))H(NH(3)(+))COO(-). These crystallographic results and reaction kinetics (Kawaguchi, S., Nobe, Y., Yasuoka, J., Wakamiya, T., Kusumoto, S., and Kuramitsu, S. (1997) J. Biochem. (Tokyo) 122, 55-63; Kawaguchi, S. and Kuramitsu, S. (1998) J. Biol. Chem. 273, 18353-18364) enabled us to estimate the free energy required for the domain movement.