Enzymatic Formation, Stability, and Spontaneous Reactions of 4-fluoromuconolactone, a Metabolite of the Bacterial Degradation of 4-fluorobenzoate

Enzymatic conversion of 4-fluorocatechol in the simultaneous presence of partially purified preparations of catechol 1,2-dioxygenase from Pseudomonas cepacia and muconate cycloisomerase from Alcaligenes eutrophus 335 yielded a product that was unambiguously identified as (+)-4-fluoromuconolactone [(+)-4-carboxymethyl-4-fluoro-but-2-en-4-olide]. This compound was shown to be the only major product formed from 3-fluoro-cis,cis-muconate by the action of muconate cycloisomerases from A. eutrophus 335, A. eutrophus JMP134, and P. cepacia as well as by the action of dichloromuconate cycloisomerase from A. eutrophus JMP134. This finding implies that dichloromuconate cycloisomerase, like the muconate cycloisomerases, catalyzes primarily a cycloisomerization reaction, which only in the case of chloro- and bromo-substituted substrates is connected to a dehalogenation. 4-Fluoromuconolactone at pH 7 decomposes by spontaneous reactions mainly to maleylacetate, which then decarboxylates to give cis-acetylacrylate. Although significant amounts of an unidentified compound are also formed from the fluorolactone, HF elimination to the two isomeric dienelactones (4-carboxymethylenebut-2-en-4-olides) is negligible. However, all spontaneous reactions proceed so slowly that an enzymatic conversion of 4-fluoromuconolactone must be assumed. Participation of dienelactone hydrolases in this reaction is indicated by their induction during growth of various strains with 4-fluorobenzoate. However, experiments with cell extracts of P. putida A3.12 suggest that at least one other hydrolytic enzyme is able to contribute to 4-fluoromuconolactone conversion. In light of these observations, earlier proposals for a 4-fluorobenzoate degradative pathway are discussed.