Proteogenomic evidence for beta-oxidation of plant-derived 3-phenylpropanoids in "Aromatoleum aromaticum" EbN1

The betaproteobacterium “Aromatoleum aromaticum” EbN1 utilizes eight different plant‐derived nonhydroxylated (e.g. cinnamate) and hydroxylated (e.g. p‐coumarate) 3‐phenylpropanoids with nitrate as electron acceptor. Differential protein profiling (2D‐DIGE) revealed abundance increases of five proteins (EbA5316 to EbA5320) during anaerobic growth with cinnamate, hydrocinnamate, p‐coumarate, and 3‐(4‐hydroxyphenyl)propanoate, compared to anaerobic benzoate‐adapted cells serving as reference state. The predicted functions of four of these proteins (EbA5317, fatty acid‐coenzyme A (CoA) ligase; EbA5318, enoyl‐CoA hydratase/isomerase; EbA5319, β‐ketothiolase; and EbA5320, 3‐hydroxyacyl‐CoA dehydrogenase) suggest β‐oxidation of the above 3‐phenylpropanoids to benzoyl‐CoA and p‐hydroxybenzoyl‐CoA, respectively. The fifth protein (EbA5316, ABC‐type periplasmic solute‐binding protein) could be involved in 3‐phenylpropanoid uptake. The detection of 3‐hydroxy‐3‐phenylpropanoate during anaerobic growth with cinnamate and hydrocinnamate or 3‐hydroxy‐3‐(4‐hydroxyphenyl)propanoate during anaerobic growth with p‑coumarate and 3‐(4‐hydroxyphenyl)propanoate supports the proteome‐predicted β‐oxidation pathway. Based on the specific formation of EbA5316–20 also during anaerobic growth with further 3‐phenylpropanoid growth substrates including cinnamyl alcohol, m‐coumarate, 3‐(3,4‐dihydroxyphenyl)propanoate and 3,4‐dihydroxycinnamate (caffeate), a common β‐oxidation route is proposed for 3‐phenylpropanoid degradation in strain EbN1. The low amount of metabolites attributable to cometabolic transformation of nongrowth supporting 3‐phenylpropanoids (e.g. o‐coumarate, ferulate) may be indicative for a high substrate specificity of the involved enzymes