β-Substituted β-Phenylpropionyl Chymotrypsins. Structural and Stereochemical Features in Stable Acyl Enzymes

In order to develop effective alternate substrate inhibitors for serine proteases, we have prepared a series of β-substituted β-phenylpropionic acid esters related to some systems known to form stable acyl enzymes with α-chymotrypsin. Some of these compounds were prepared in enantiomerically pure form by asymmetric synthesis. Acyl enzyme species were generated from chymotrypsin by reaction with the active esters, and the progress of deacylation was monitored by the proflavin displacement assay. In some cases, it was possible to distinguish two different deacylation rates that correspond to the two enantiomers. β-Phenylpropionic acyl enzymes with β-substituents that are nonpolar were not especially stable, but a number of the polar derivatives and particularly the acylamino derivatives showed slow rates of deacylation (k_d less than 0.005 min^-1), with three systems showing deacylation enantioselectivities in the range of 500-1500. These results are consistent with a model in which additional stabilization of the acyl enzyme and enantioselectivity in the deacylation process derives from an additional hydrogen bond between the acyl enzyme species (as an acceptor) and the enzyme (as a donor). A number of active site residues that might be involved in this hydrogen bond are discussed.