Microbial degradation of phthalates : biochemistry and environmental implications

Abstract

The environmentally relevant xenobiotic esters of phthalic acid (PA), isophthalic acid (IPA) and terephthalic acid (TPA) are produced on a million ton scale annually and are predominantly used as plastic polymers or plasticizers. Degradation by microorganisms is considered as the most effective means of their elimination from the environment and proceeds via hydrolysis to the corresponding phthalic acid isomers and alcohols under oxic and anoxic conditions. Further degradation of PA, IPA and TPA differs fundamentally between anaerobic and aerobic microorganisms. The latter introduce hydroxyl functionalities by dioxygenases to facilitate subsequent decarboxylation by either aromatizing dehydrogenases or cofactor-free decarboxylases. In contrast, anaerobic bacteria activate the phthalic acids isomers to the respective thioesters using CoA ligases or CoA transferases followed by decarboxylation to the central intermediate benzoyl-CoA. Decarboxylases acting on the three phthalic acid CoA thioesters belong to the UbiD enzyme family that harbor a prenylated FMN cofactor to achieve the mechanistically challenging decarboxylation. Capture of the extremely instable PA-CoA intermediate is accomplished by a massive overproduction of PCD and a balanced production of PA-CoA forming/decarboxylating enzymes. The strategy of anaerobic phthalate degradation probably represents a snapshot of an ongoing evolution of a xenobiotic degradation pathway via a short-lived reaction intermediate.