We determined the crystal structure of the liganded form of alpha-aminotransferase from a hyperthermophile, Pyrococcus horikoshii. This hyperthermophilic enzyme did not show domain movement upon binding of an acidic substrate, glutamate, except for a small movement of the alpha-helix from Glu16 to Ala25. The omega-carboxyl group of the acidic substrate was recognized by Tyr70* without its side-chain movement, but not by positively charged Arg or Lys. Compared with the homologous enzymes from Thermus thermophilus HB8 and Escherichia coli, it was suggested that the more thermophilic the enzyme is, the smaller the domain movement is. This rule seems to be applicable to many other enzymes already reported.