RESUMO
Three-dimensional 2.8 A resolution x-ray crystallographic studies show that toluenesulfonamide and pipsylamide bind isomorphously in the aromatic specificity binding site of alpha-chymotrypsin. However, their orientation differs by about 90 degrees from that usually associated with substrate-like molecules, suggesting a nonproductive binding mode. A secondary binding site is also operative in one molecule of the dimer of the pipsylamide derivative and it is located some 22 A from the active site; however, this site is not operative in the toluenesulfonamide derivative. Binding of toluenesulfonamide and pipsylamide in the specificity site occurs without inducing any significant changes in the native enzyme structure, in contrast to the behavior observed upon tosylation or upon transition state analogue binding of phenylethaneboronic acid. The structural changes accompanying the formation of the latter derivatives are generally asymmetric with respect to the dimeric structure of alpha-chymotrypsin and are generally confined to the binding domain or cylinder 2 of the enzyme (sequence greater than 122). These changes are displayed in a new way via diagonal distance map representation.
