Haloenol lactones. Potent enzyme-activated irreversible inhibitors for alpha-chymotrypsin

Abstract

Haloenol lactones can act as enzyme-activated irreversible inhibitors for alpha-chymotrypsin: acyl transfer to the active site serine releases a halomethyl ketone that remains tethered in the active site during the lifetime of the acyl enzyme, poised to alkylate an accessible nucleophilic residue. To investigate the structural determinants for chymotrypsin suicide inactivation with haloenol lactones, we prepared a series of nine lactones, differing in ring size (6-membered valerolactones and 5-membered butyrolactones) and in the nature of the aromatic substituent (phenyl and alpha-naphthyl), and the halogen (bromine and iodine). The inactivating behavior of these lactones is characterized by a binding constant (Ki) and three rate constants, for inactivation (k2), catalytic hydrolysis (kc), and spontaneous hydrolysis (kh). The six-membered valerolactones were much more potent inactivators than the butyrolactones, having both higher affinity and more rapid inactivation; the alpha-naphthyl-substituted lactones were also more effective, but the nature of the halogen had relatively little effect. The spontaneous rate of hydrolysis of all of these lactones is low. The turnovers per inactivation of these lactones vary from 91-1.7, with some of the alpha-naphthyl-substituted lactones approaching ideal behavior (stoichiometric inactivation). These studies indicate that several haloenol lactones are effective enzyme-activated irreversible inhibitors of chymotrypsin, and that their potency and efficiency depends markedly upon certain structural features of the lactone system.