Self-association of diisopropylphosphoryl-alpha-chymotrypsin: the involvement of active site of the enzyme in its self-association behaviour

The self-association of diisopropylphosphoryl(DIP)-alpha-chymotrypsin is studied in order to find out whether the active site of the enzyme is involved in its self-association behaviour or not. Sedimentation coefficient as well as the weight-average (Archibald) molecular weight data are obtained as a function of concentration using an analytical ultracentrifugation technique. The analysis indicated that the experimental data fits the model of indefinite self-association. The comparison of the data with earlier data on alpha-chymotrypsin revealed that after the modification at the active site, the association constant for the self-association is reduced by about 47%, and the system deviated from ideality. Results showed further that Ser-195, at the active site, appears to be involved in the self-association behaviour of alpha-chymotrypsin; however, the participation of other groups at the active site is also implicated.