From the results of molecular orbital study on the effects of substituents on the proton transfer from Ser-195 to His-57 in the "charge relay system"of α-chymotrypsin, the acylation step is thought to be as follows : (1) the carbonyl carbon of the substrate approaches to the oxygen of Ser-195 in the "charge relay system" ; (2) after the proton of Ser-195 is transferred to N■2 of His-57 by the trigger of the interaction between the oxygen of Ser-195 and the carbonyl carbon of the substrate, the oxygen of Ser-195 covalent-bonds with the carbonyl carbon of the substrate ; (3) acyl-Ser-195 rotates by 120 degrees around the serine Cα-Cβ bond. The rotation of acyl-Ser-195 must be accompanied with the deeper movement of the aromatic part of the substrate in the pocket of α-chymotrypsin. Moreover, it was shown that a significant role in accelerating the proton transfer from Ser-195 to His-57 was played by the interaction between the oxygen of Ser-195 and carbonyl carbon of substrate, not by two hydrogen bonds between the carbonyl oxygen and the backbone-NH-groups of Gly-193 and Ser-195.