Mechanism-Based Enzyme Inactivation Using an Allyl Sulfoxide-Allyl Sulfenate Ester Rearrangement1.

Abstract

2-Amino-4-chloro-5-(p-nitrophenylsulfinyl)pentanoic acid (1) has been synthesized and shown to induce mechanism-based inactivation of two pyridoxal phosphate dependent enzymes: (1) cystathionine γ-synthetase, which catalyzes a γ-replacement reaction in bacterial methionine biosynthesis; and (2) methionine γ-lyase, which catalyzes a γ-elimination reaction in bacterial methionine breakdown. The inactivations are irreversible and display saturation kinetics. Each enzyme incorporates roughly 1 mol of tritium per mol of enzyme monomer when inactivated by 2-amino-4-chloro-5-(p-nitro[3H]phenyl-sulfinyl)pentanoic acid (la), confirming that the modification of each protein is covalent and stoichiometric. Substoichiometric labeling (0.12 mol of tritium per mol of enzyme monomer) is given when methionine γ-lyase is fully inactivated by 2-amino-4-chloro-5-[3H]-5-p-nitrophenylsulfinyl)pentanoic acid (lb). Both enzymes, inactivated by 1, are susceptible to reactivation by thiols. Inactivated cystathionine γ-synthetase recovers 25% of its catalytic activity upon incubation with excess dithiothreitol, while methinonine γ-lyase is 100% reactivated by dithiothreitol, mercaptoethanol, and mercaptopropionate. Reactivation generates p-nitrophenylthiolate anion, which forms, in the case of methionine γ-lyase, stoichiometrically with enzyme reactivated. Both enzymes are "protected" from inactivation by 1 in the presence of thiols, which simultaneously generates p-nitrophenylthiol. In the presence of dithiothreitol, the protection reaction gives p-nitrophenylthiol production with pseudo-first-order kinetics. 2-Amino-4-chloro-5-(p-tolylsulfinyl)pentanoic acid (2) and 2-amino-4-(p-nitrophenylsulfinyl)-5-chloropentanoic acid (3), the reverse regioisomer of 1, have also been prepared and give no evidence of inactivation of either enzyme. The data are taken to indicate a novel form of suicide inactivation (Scheme II) wherein β-carbanion-assisted γ-halide elimination generates an allyl sulfoxide-enzyme-pyridoxal adduct (4) which undergoes spontaneous 2,3-sigmatropic rearrangement to an electrophilic allyl sulfenate ester (5). The latter is then captured by an enzymic nucleophile to give an inactive enzyme 6, which may be a mixed disulfide or, less likely, a sulfenamide.