Anaerobic metabolism of phenol in proteobacteria and further studies of phenylphosphate carboxylase

Anaerobic phenol metabolism was studied in three facultative aerobic denitrifying bacteria, Thauera aromatica, “Aromatoleum aromaticum” strain EbN1 (Betaproteobacteria), and Magnetospirillum sp. (Alphaproteobacterium). All species formed phenylphosphate and contained phenylphosphate carboxylase but not phenol carboxylase activity. This is in contrast to direct phenol carboxylation by fermenting bacteria. Antisera raised against subunits of the Thauera phenylphosphate synthase and phenylphosphate carboxylase partly cross-reacted with the corresponding proteins in the other species. Some unsolved features of phenylphosphate carboxylase were addressed in T. aromatica. The core sub-complex of this enzyme consists of three different subunits and catalyzes the exchange of ¹⁴CO₂ with the carboxyl group of 4-hydroxybenzoate, but not phenylphosphate carboxylation. It was inactivated by oxygen or by the oxidizing agent thionin and fully reactivated under reducing conditions. The purified recombinant phosphatase subunit alone had only low phenylphosphate phosphatase activity in the absence of the other components. However, activity was strongly enhanced in the presence of the core enzyme resulting in phenylphosphate carboxylation. Hence, a tight interaction of the carboxylase subunits is required for dephosphorylation of phenylphosphate, which is coupled to the concomitant carboxylation of the produced phenolate to 4-hydroxybenzoate, thus preventing a futile cycle.