Maleylacetone cis-trans-isomerase. Mechanism of the interaction of coenzyme glutathione and substrate maleylacetone in the presence and absence of enzyme

Abstract

Previous studies with maleylacetone cis-trans-isomerase suggest that the coenzyme, glutathione (GSH), may act as a nucleophile in the enzyme-catalyzed cis-trans isomerization. In the absence of enzyme, reaction of the coenzyme with maleylacetone or its trans isomer, fumarylacetone, has been found to lead to an adduct, S-2-(4,6-dioxoheptanoic acidjglutathione. The same diastereomer of the adduct is formed from either geometric isomer, suggesting that the reaction of GSH with maleylacetone proceeds by a slow catalyzed cis-trans isomerization followed by a rapid reaction of GSH conjugate addition to the trans isomer. This mechanism is confirmed by an isotope dilution experiment and a kinetic method involving repetitive addition of fractional equivalents of GSH to a solution of maleylacetone. The intermediate formed during GSH-catalyzed cis-trans isomerization is suggested to be the dienediol formed by sulfur attack at C-2, and this intermediate undergoes bond rotation and elimination before protonation at C-3 can take place. By analogy and additional evidence, a similar mechanism and intermediate are suggested for the enzyme-coenzyme-substrate reaction.Aromatic amino acids such as tyrosine and phenylalanine are metabolized in mammalian liver to a common intermediate, homogentisic acid (2,5-dihydroxyphenylacetic acid). Oxidative cleavage of the aromatic ring yields maleylacetoacetate (la), which in turn undergoes enzyme-catalyzed cistrans isomerization to fumarylacetoacetate (lia, eq 1 ).2 Enzymes capable of carrying out these same transformations are also found in bacteria such as Vibrio 01.3 The focus of attention in this paper is concerned with the mechanism of the catalyzed cis-trans isomerization reaction.The mechanistic investigation is simplified to some degree by the finding that maleylacetone (lb) is also rapidly isomerized by both the liver and bacterial cis-trans-isomerase. Action

Maleylacetoacetate Isomerase (MAAI/GSTZ)-Deficient Mice Reveal a Glutathione-Dependent Nonenzymatic Bypass in Tyrosine Catabolism

“…Therefore, a transferase reaction is not needed. After isomerization, free GSH and FAA are the products of the reaction (21,(33)(34)(35). MAA can bind GSH with or without enzyme and it has also been shown that GSH alone is able to catalyze the conversion of MAA to FAA in vitro (35).…”

Gene Structure, Chromosomal Location, and Expression Pattern of Maleylacetoacetate Isomerase

“…However, MAAI is different in that glutathione binds to MAA spontaneously and the bound MAA-GSH is the substrate of the enzyme. After isomerization, free glutathione and FAA are the products of the reaction (Seltzer and Lin, 1979;Lee and Seltzer, 1989). MAAI binds glutathione, but does not transfer the substrate to the glutathione molecule.…”

Mechanism of Action of Glutathione‐Dependent Enzymes

“…There have been extensive studies in model systems closely related to this enzymic class (142,143,149,(153)(154)(155). Indeed, the cistrans isomerization of maleylacetone is catalyzed by the GSH coenzyme even in the absence of enzyme, but at a 106-fold lower rate (155).…”

“…Bond rotation about C-2-C-3 occurs more rapidly than protonation at C-3 (to explain the lack of solvent D uptake at C-2 or C-3 during reaction) (153). Thus the reaction of the cis isomer is isomerization while the trans isomer gives GSH addition (153).…”