Ascorbic acid-dependent turnover and reactivation of 2,4-dichlorophenoxyacetic acid/alpha-ketoglutarate dioxygenase using thiophenoxyacetic acid

The first step in catabolism of the broadleaf herbicide 2,4-dichlorophenoxyacetic acid (2,4-D) is catalyzed by 2,4-D/alpha-ketoglutarate (alpha-KG)-dioxygenase (TfdA) in Ralstonia eutropha (formerly Alcaligenes eutrophus) JMP134. This oxygen- and ferrous-ion-dependent enzyme couples the oxidative decarboxylation of alpha-KG (yielding CO2 and succinate) with the oxidation of 2,4-D to produce 2,4-dichlorophenol and glyoxylate. TfdA was shown to utilize thiophenoxyacetic acid (TPAA) to produce thiophenol, allowing the development of a continuous spectrophotometric assay for the enzyme using the thiol-reactive reagent 4,4'-dithiodipyridine. In contrast to the reaction with 2,4-D, however, the kinetics of TPAA oxidation were nonlinear and ascorbic acid was found to be required for and consumed during TPAA oxidation. The ascorbic acid was needed to reduce a reversibly oxidized inactive state that was formed by reaction of the ferrous enzyme with oxygen, either in the absence of substrate or in the presence of TPAA. The dependency on this reductant was not due to an uncoupling of alpha-KG decarboxylation from substrate hydroxylation, as has been reported for several other alpha-KG-dependent hydroxylases. Significantly, the rate of formation of this reversibly oxidized species was much lower when the enzyme was turning over 2,4-D. Evidence also was obtained for the generation of an inactive enzyme species that could not be reversed by ascorbate. The latter species, not associated with protein fragmentation, arose from an oxidative reaction that is likely to involve hydroxyl radical reactions. On the basis of initial rate studies, the kcat and Km values for TPAA were estimated to be 20-fold lower and 80-fold higher than the corresponding values for 2,4-D. The results are incorporated into a model of TfdA reactivity involving both catalytic and inactivating events.