o-Phthalate derived from plastics' plasticizers and a bacterium's solution to its anaerobic degradation

Phthalic acid esters (phthalates) are anthropogenic compounds that are used as plasticizers. Unfortunately, because phthalates are non-covalently intercalated into plastic polymers they leach into the environment, accumulating in anoxic sediments. This has negative consequences for animal and human health. Denitrifying Betaproteobacteria, such as Aromatoleum aromaticum, can use ortho-phthalate, derived by ester hydrolysis, as a carbon and energy source. Mergelsberg et al. () deconstruct the pathway whereby ortho-phthalic acid is converted, via the highly unstable phthaloyl-CoA, to the central intermediate of anaerobic aromatic degradation, benzoyl-CoA. The latter reaction is catalysed by UbiD-like phthaloyl-CoA decarboxylase (PCD). Succinyl-CoA:o-phthalate CoA-transferase (SPT) generates phthaloyl-CoA, which accumulates at only sub-micromolar concentrations, while the K_m of PCD for phthaloyl-CoA is two-orders of magnitude higher. This seemingly insurmountable kinetic barrier is overcome because A. aromatoleum massively over-produces PCD and because the decarboxylation reaction is irreversible. These features of the pathway facilitate capture of phthaloyl-CoA as it is released from SPT without the need for direct substrate-channelling. The authors provide strong evidence from both in vivo and in vitro studies to support their conclusions. This work reveals how these anaerobic bacteria have rapidly evolved a stop-gap measure to allow them to completely degrade an otherwise recalcitrant aromatic xenobiotic.