title: Mechanism of the severe inhibition of tetrachlorohydroquinone dehalogenase by its aromatic substrates.
abstract: Tetrachlorohydroquinone (TCHQ) dehalogenase catalyzes the conversion of TCHQ to 2,6-dichlorohydroquinone during degradation of pentachlorophenol by Sphingobium chlorophenolicum. TCHQ dehalogenase is a member of the glutathione S-transferase superfamily. Members of this superfamily typically catalyze nucleophilic attack of glutathione upon an electrophilic substrate to form a glutathione conjugate and contain a single glutathione binding site in each monomer of the typically dimeric enzyme. TCHQ dehalogenase, in contrast to most members of the superfamily, is a monomer and uses 2 equiv of glutathione to catalyze a more complex reaction. The first glutathione is involved in formation of a glutathione conjugate, while the second is involved in the final step of the reaction, a thiol-disulfide exchange reaction that regenerates the free enzyme and forms GSSG. TCHQ dehalogenase is severely inhibited by its aromatic substrates, TCHQ and trichlorohydroquinone (TriCHQ). TriCHQ acts as a noncompetitive inhibitor of the thiol-disulfide exchange reaction required to regenerate the free form of the enzyme. In addition, dissociation of the GSSG product is inhibited by TriCHQ. The thiol-disulfide exchange reaction is the rate-limiting step in the reductive dehalogenation reaction under physiological conditions.
authors.affiliation: Department of Chemistry and Biochemistry, University of Colorado at Boulder, Boulder, Colorado 80309, USA.
authors.forename: Joseph R
authors.initials: JR
authors.lastname: Warner
authors.forename: Shelley D
authors.initials: SD
authors.lastname: Copley
mesh_terms: D001665:Binding Sites; D002021:Buffers; D002384:Catalysis; D004305:Dose-Response Relationship, Drug; D005978:Glutathione / Q000031:analogs & derivatives / Q000378:metabolism* / Q000494:pharmacology; D019803:Glutathione Disulfide / Q000378:metabolism*; D006863:Hydrogen-Ion Concentration; D006867:Hydrolases / Q000037:antagonists & inhibitors* / Q000737:chemistry / Q000302:isolation & purification / Q000378:metabolism; D007700:Kinetics; D008954:Models, Biological; D011485:Protein Binding; D013056:Spectrophotometry, Ultraviolet; D013379:Substrate Specificity
chemical_list: D002021:Buffers; D006867:Hydrolases; C078610:2,3,5,6-tetrachloro-4-hydroquinone reductive dehalogenase; D005978:Glutathione; D019803:Glutathione Disulfide