title: Covalent modification and active site-directed inactivation of a low molecular weight phosphotyrosyl protein phosphatase.
abstract: Covalent modification experiments were conducted in order to identify active site residues of the 18-kDa cytoplasmic phosphotyrosyl protein phosphatases. The enzyme was inactivated by diethyl pyrocarbonate, phenylglyoxal, cyclohexanedione, iodoacetate, iodoacetamide, phenylarsine oxide, and certain epoxides in a manner consistent with the modification of active site residues. Phenylglyoxal and cyclohexanedione both bind to the active site in a rapid preequilibrium process and thus act as active site-directed inhibitors. The pH dependencies of the inactivation by iodoacetate and by iodoacetamide were examined in detail and compared with rate data for the alkylation of glutathione as a model compound. The enzyme inactivation data permitted the determination of pKa values of two reactive cysteines at or near the active site. Although phosphomycin is simply a competitive inhibitor of the enzyme, it was found that 1,2-epoxy-3-(p-nitrophenoxy)propane (EPNP) and (R)- and (S)-benzylglycidol act as irreversible covalent inactivators, consistent with the importance of a hydrophobic moiety on the substrate in controlling substrate specificity. EPNP exhibits characteristics of an active site-directed inactivator, with a preequilibrium binding constant somewhat smaller than that of phosphate ion. The pH dependencies of inactivation of EPNP and (S)-benzylglycidol are identical to that observed for iodoacetamide and similar to that for iodoacetate, suggesting that they modify similar groups. Sequencing of the tryptic digests of the EPNP-labeled enzyme indicates that Cys-62 and Cys-145 are labeled. Phenylarsine oxide acts as a very slow, tight-binding inhibitor of the enzyme. The results are interpreted in terms of an active site model that incorporates a histidine-cysteine ion pair, similar to that present in papain.
authors.affiliation: Department of Chemistry, Purdue University, West Lafayette, Indiana 47907-1393.
authors.forename: Z Y
authors.initials: ZY
authors.lastname: Zhang
authors.forename: J P
authors.initials: JP
authors.lastname: Davis
authors.forename: R L
authors.initials: RL
authors.lastname: Van Etten
mesh_terms: D000818:Animals; D001120:Arginine / Q000737:chemistry; D001152:Arsenicals / Q000494:pharmacology; D001665:Binding Sites; D001667:Binding, Competitive; D002417:Cattle; D003512:Cyclohexanones / Q000494:pharmacology; D003545:Cysteine / Q000737:chemistry; D004852:Epoxy Compounds / Q000494:pharmacology; D005978:Glutathione / Q000737:chemistry; D006639:Histidine / Q000737:chemistry; D006863:Hydrogen-Ion Concentration; D007460:Iodoacetamide / Q000494:pharmacology; D007461:Iodoacetates / Q000494:pharmacology; D019807:Iodoacetic Acid; D007700:Kinetics; D008970:Molecular Weight; D010658:Phenylglyoxal / Q000494:pharmacology; D017027:Protein Tyrosine Phosphatases / Q000037:antagonists & inhibitors* / Q000737:chemistry
chemical_list: D001152:Arsenicals; D003512:Cyclohexanones; D004852:Epoxy Compounds; D007461:Iodoacetates; C029341:oxophenylarsine; D006639:Histidine; C018031:1,2-cyclohexanedione; D001120:Arginine; D017027:Protein Tyrosine Phosphatases; D005978:Glutathione; D003545:Cysteine; D010658:Phenylglyoxal; D019807:Iodoacetic Acid; D007460:Iodoacetamide