Low pH dimerization of chymotrypsin in solution

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Abstract
The mechanism of the acid dimerization of .alpha.-chymotrypsin in solution was reexamined using a number of chemical derivatives. Blocking of the carboxyl of Tyr-146, while that of Asp-64 remained free, eliminated completely the ability of .alpha.-chymotrypsin to dimerize, as did methylation of His-57. O-Acetylation of Tyr-146 reduced the dimerization constant of that of .gamma.-chymotrypsin, and deacetylation did not reverse it to the value of .alpha.-chymotrypsin. Acetylation and deacetylation of the other accessible tyrosines did not affect the dimerization. The mechanism proposed by Aune and Timasheff for the solution dimerization which involves the electrostatic interaction between the His-57 imidazolium ring and the terminal carboxyl of Tyr-146 is still most consistent with all the experimental observations. The interactions in dilute solution may differ somewhat from those observed in crystals. In particular, the 2 intermolecular bridges formed by sulfate ions in crystals cannot be present in solution.