Phenylphosphate Synthase: a New Phosphotransferase Catalyzing the First Step in Anaerobic Phenol Metabolism in Thauera aromatica

Abstract

ABSTRACT The anaerobic metabolism of phenol in the beta-proteobacterium Thauera aromatica proceeds via para -carboxylation of phenol (biological Kolbe-Schmitt carboxylation). In the first step, phenol is converted to phenylphosphate which is then carboxylated to 4-hydroxybenzoate in the second step. Phenylphosphate formation is catalyzed by the novel enzyme phenylphosphate synthase, which was studied. Phenylphosphate synthase consists of three proteins whose genes are located adjacent to each other on the phenol operon and were overproduced in Escherichia coli . The promoter region and operon structure of the phenol gene cluster were investigated. Protein 1 (70 kDa) resembles the central part of classical phosphoenolpyruvate synthase which contains a conserved histidine residue. It catalyzes the exchange of free [ 14 C]phenol and the phenol moiety of phenylphosphate but not the phosphorylation of phenol. Phosphorylation of phenol requires protein 1, MgATP, and another protein, protein 2 (40 kDa), which resembles the N-terminal part of phosphoenol pyruvate synthase. Proteins 1 and 2 catalyze the following reaction: phenol + MgATP + H 2 O→phenylphosphate + MgAMP + orthophosphate. The phosphoryl group in phenylphosphate is derived from the β-phosphate group of ATP. The free energy of ATP hydrolysis obviously favors the trapping of phenol ( K m , 0.04 mM), even at a low ambient substrate concentration. The reaction is stimulated severalfold by another protein, protein 3 (24 kDa), which contains two cystathionine-β-synthase domains of unknown function but does not show significant overall similarity to known proteins. The molecular and catalytic features of phenylphosphate synthase resemble those of phosphoenolpyruvate synthase, albeit with interesting modifications.