Electron-bifurcation and fluoride efflux systems inAcetobacteriumspp. drive defluorination of perfluorinated unsaturated carboxylic acids

Abstract

Enzymatic cleavage of C–F bonds in per- and polyfluoroalkyl substances (PFAS) is largely unknown but avidly sought to promote systems biology for PFAS bioremediation. Here, we report the reductive defluorination of α, β-unsaturated per- and polyfluorocarboxylic acids byAcetobacteriumspp. Two critical molecular features inAcetobacteriumspecies enabling reductive defluorination are (i) a functional fluoride efflux transporter (CrcB) and (ii) an electron-bifurcating caffeate reduction pathway (CarABCDE). The fluoride transporter was required for detoxification of released fluoride. Car enzymes were implicated in defluorination by the following evidence: (i) onlyAcetobacteriumspp. withcargenes catalyzed defluorination; (ii) caffeate and PFAS competedin vivo; (iii) models from the X-ray structure of the electron-bifurcating reductase (CarC) positioned the PFAS substrate optimally for reductive defluorination; (iv) products identified by19F-NMR and high-resolution mass spectrometry were consistent with the model. Defluorination biomarkers identified here were found in wastewater treatment plant metagenomes on six continents.

Evolutionary obstacles and not C–F bond strength make PFAS persistent

“…Electron bifurcation enzymes separate electron pairs into single electrons with high and low redox potentials, respectively, and achieve the lowest potentials known to biology (Buckel & Thauer, 2018;Müller et al, 2018). The reduction of perfluorinated acids was catalysed by CarCDE (Yu et al, 2023), and this enzyme system is produced by multiple Acetobacterium spp, a widespread genus found in anaerobic environments. CarCDE has been demonstrated to participate in the reduction of the lignin breakdown product caffeic acid, which is linked to ATP production, and its X-ray structure has been solved (Demmer et al, 2018;Hess et al, 2013).…”

“…More recently, perfluorinated alkenoic acids were shown to undergo reductive defluorination catalysed by an electron bifurcating flavin-iron-sulfur enzyme system (Yu et al, 2023). Electron bifurcation enzymes separate electron pairs into single electrons with high and low redox potentials, respectively, and achieve the lowest potentials known to biology (Buckel & Thauer, 2018;Müller et al, 2018).…”

“…CarCDE has been demonstrated to participate in the reduction of the lignin breakdown product caffeic acid, which is linked to ATP production, and its X-ray structure has been solved (Demmer et al, 2018;Hess et al, 2013). Yu et al (2023) presented evidence that perfluorinated alkenoic acids react with CarCDE and undergo defluorination. Computational docking showed a precise fit of E-perfluoro-4-methylpent-2-enoic acid, (CF 3 ) 2 CFCF=CFCOOH, in a reactive configuration positioned at the CarC enzyme flavin cofactor that participates directly in the reduction.…”

“…Another recent study also illustrated the importance of fluoride toxicity management in supporting microbial PFAS defluorination. In Yu et al (2023), partial defluorination of a perfluoroalkenoic acid was demonstrated to only occur in cells containing the defluorinating enzymes AND a functioning fluoride export system. One bacterium had the defluorinating enzymes but did not catalyse defluorination in vivo.…”