Alternative substrates of 2,4-dichlorophenoxyacetate/α-ketoglutarate dioxygenase

2,4-Dichlorophenoxyacetate/α-ketoglutarate dioxygenase (TfdA), the first enzyme in the catabolic pathway for the herbicide 2,4-dichlorophenoxyacetic acid (2,4-D), oxidizes α-ketoglutarate (α-kG) to CO₂ and succinate while hydroxylating 2,4-D to yield an unstable hemiacetal that decomposes into 2,4-dichlorophenol and glyoxylate. In an effort to extend the potential biotechnological utility of this enzyme, a variety of non-phenoxyacetate compounds were examined as potential substrates. 2-Naphthoxyacetic acid was the best alternative substrate tested, followed by benzofuran-2-carboxylic acid, 2,4-dichlorocinnamic acid, 2-chlorocinnamic acid, 1-naphthoxyacetic acid, and 4-chlorocinnamic acid. TfdA appeared to oxidize the olefin bond of the cinnamic acids and benzofuran-2-carboxylate to form the corresponding epoxides. Whole cells were observed to also catalyze a TfdA-dependent oxidation of 2,4-dichlorocinnamic acid. Based on the ability of TfdA to metabolize chlorinated cinnamic acids, we speculate that tfdA-like sequences present in 2,4-D non-degrading natural isolates may function in metabolism of substituted cinnamic acids. These results support the use of TfdA and related enzymes in the specific oxidation of non-phenoxyacetate substrates.