Mechanism of the severe inhibition of tetrachlorohydroquinone dehalogenase by its aromatic substrates.

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.