Anaerobic Degradation of p-Alkylated Benzoates and Toluenes

Abstract

The anaerobic degradation of 4-alkylbenzoates and 4-alkyltoluenes is to date a rarely reported microbial capacity. The newly isolated Alphaproteobacterium <i>Magnetospirillum</i> sp. strain pMbN1 represents the first pure culture demonstrated to degrade 4-methylbenzoate completely to CO₂ in a process coupled to denitrification. Differential proteogenomic studies in conjunction with targeted metabolite analyses and enzyme activity measurements elucidated a specific 4-methylbenzoyl-coenzyme A (CoA) pathway in this bacterium alongside the classical central benzoyl-CoA pathway. Whilst these two pathways are analogous, in the former the <i>p</i>-methyl group is retained and its 4-methylbenzoyl-CoA reductase (MbrCBAD) is phylogenetically distinct from the archetypical class I benzoyl-CoA reductase (BcrCBAD). Subsequent global regulatory studies on strain pMbN1 grown with binary or ternary substrate mixtures revealed benzoate to repress the anaerobic utilization of 4-methylbenzoate and succinate. The shared nutritional property of betaproteobacterial ‘<i>Aromatoleum aromaticum'</i> pCyN1 and <i>Thauera</i> sp. strain pCyN2 is the anaerobic degradation of the plant-derived hydrocarbon <i>p</i>-cymene (4-isopropyltoluene) coupled to denitrification. Notably, the two strains employ two different peripheral pathways for the conversion of <i>p</i>-cymene to 4-isopropylbenzoyl-CoA as the possible first common intermediate. In ‘<i>A. aromaticum'</i> pCyN1 a putative <i>p</i>-cymene dehydrogenase (CmdABC) is proposed to hydroxylate the benzylic methyl group, which is subsequently further oxidized to the CoA-thioester. In contrast, <i>Thauera</i> sp. strain pCyN2 employs a reaction sequence analogous to the known anaerobic toluene pathway, involving a distinct branching (4-isopropylbenzyl)succinate synthase (IbsABCDEF).