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Journal of Bacteriology, July 2004, p. 4556-4567, Vol. 186, No. 14
0021-9193/04/$08.00+0     DOI: 10.1128/JB.186.14.4556-4567.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.

Phenylphosphate Carboxylase: a New C-C Lyase Involved in Anaerobic Phenol Metabolism in Thauera aromatica

Mikrobiologie, Institut für Biologie II, Universität Freiburg, D-79104 Freiburg, Germany

Received 27 February 2004/ Accepted 19 April 2004

The anaerobic metabolism of phenol in the beta-proteobacterium Thauera aromatica proceeds via carboxylation to 4-hydroxybenzoate and is initiated by the ATP-dependent conversion of phenol to phenylphosphate. The subsequent para carboxylation of phenylphosphate to 4-hydroxybenzoate is catalyzed by phenylphosphate carboxylase, which was purified and studied. This enzyme consists of four proteins with molecular masses of 54, 53, 18, and 10 kDa, whose genes are located adjacent to each other in the phenol gene cluster which codes for phenol-induced proteins. Three of the subunits (54, 53, and 10 kDa) were sufficient to catalyze the exchange of ¹⁴CO₂ and the carboxyl group of 4-hydroxybenzoate but not phenylphosphate carboxylation. Phenylphosphate carboxylation was restored when the 18-kDa subunit was added. The following reaction model is proposed. The ¹⁴CO₂ exchange reaction catalyzed by the three subunits of the core enzyme requires the fully reversible release of CO₂ from 4-hydroxybenzoate with formation of a tightly enzyme-bound phenolate intermediate. Carboxylation of phenylphosphate requires in addition the 18-kDa subunit, which is thought to form the same enzyme-bound energized phenolate intermediate from phenylphosphate with virtually irreversible release of phosphate. The 54- and 53-kDa subunits show similarity to UbiD of Escherichia coli, which catalyzes the decarboxylation of a 4-hydroxybenzoate derivative in ubiquinone (ubi) biosynthesis. They also show similarity to components of various decarboxylases acting on aromatic carboxylic acids, such as 4-hydroxybenzoate or vanillate, whereas the 10-kDa subunit is unique. The 18-kDa subunit belongs to a hydratase/phosphatase protein family. Phenylphosphate carboxylase is a member of a new family of carboxylases/decarboxylases that act on phenolic compounds, use CO₂ as a substrate, do not contain biotin or thiamine diphosphate, require K⁺ and a divalent metal cation (Mg²⁺or Mn²⁺) for activity, and are strongly inhibited by oxygen.

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