Specific and non-specific effects of inert anions and cations on the dimerization of alpha-chymotrypsin and the catalytic activities of monomeric and dimeric alpha-chymotrypsins.

The effect of salts on the dimerization and the catalytic activity of dimeric alpha-chymotrypsin (alpha-CT) was investigated. The observed effect was mainly related to the anionic constituent of the salt, and its order depended on the salt concentration. The order of effect of anions on several parameters at [salt] less than 0.02 M was SO4 2-, ClO4-, NO3-, Br-, Cl-, which reflected the electrostatic interaction between the anion and the positively charged surface of alpha-CT. On the other hand, the anion dependence at moderate salt concentrations (0.1-0.2 M) followed the Hofmeister series, SO4 2-, Cl-, Br-, NO3-, ClO4-, which was related to the direct and indirect interactions between the anion and non-charged groups of the protein. The anion order for the dimerization constant of this enzyme, however, was the complete reverse of that generally observed in the aggregation of proteins at high salt concentration (greater than 1 M). This result was accountable for in terms of a specific interaction in the formation of the dimeric enzyme (probably that between the active site of one monomer and Tyr-146 of the other).