Recruitment of a double bond isomerase to serve as a reductive dehalogenase during biodegradation of pentachlorophenol

Tetrachlorohydquinone dehalogenase catalyzes the replacement of chlorine at oms on tetrachlorohydroquinone and trichlorohydroquinone with hydrogen atom s during the biodegradation of pentachlorophenol by Sphingomonas chlorophen olica. The sequence of the active site region of tetrachlorohydroquinone de halogenase is very similar to those of the corresponding regions of maleyla cetoacetate isomerases, enzymes that catalyze the glutathione-dependent iso merization of a cis double bond in maleylacetoacetate to the trans configur ation during the catabolism of phenylalanine and tyrosine. Furthermore, tet rachlorohydroquinone dehalogenase catalyzes the isomerization of maleylacet one (an analogue of maleylacetoacetate) at a rate nearly comparable to that of a bonafide bacterial maleylacetoacetate isomerase. Since maleylacetoace tate isomerase is involved in a common and presumably ancient pathway for c atabolism of tyrosine, while tetrachlorohydroquinone dehalogenase catalyzes a more specialized reaction, it is likely that tetrachlorohydroquinone deh alogenase arose from a maleylacetoacetate isomerase. The substrates and ove rall transformations involved in the dehalogenation and isomerization react ions are strikingly different. This enzyme provides a remarkable example of Nature's ability to recruit an enzyme with a useful structural scaffold an d elaborate upon its basic catalytic capabilities to generate a catalyst fo r a newly needed reaction.