A tale of two enzymes: tetrachlorohydroquinone dehalogenase and maleylacetoacetate isomerase

Tetrachlorohydroquinone (TCHQ) dehalogenase catalyzes two successive reduct ive dehalogenation reactions in the biodegradation of pentachlorophenol by Sphingomonas chlorophenolica. The sequence of the active site region is ver y similar to those of maleylacetoacetate and maleylpyruvate isomerases, and the dehalogenase has isomerase activity nearly equal to that of a maleylac etoacetate isomerase from Vibrio. The roles of several conserved residues i n the dehalogenase and isomerase reactions are discussed. The dehalogenase mechanism includes a thiol-disulfide exchange reaction that regenerates the free enzyme after the reduced aromatic product is released. This reaction does not appear to be catalyzed by the enzyme. (C) 2001 Elsevier Science Ir eland Ltd. All rights reserved.