A bacterial esterase is homologous with nonhaem haloperoxidases and displays brominating activity

Screening GenBank indicated that an esterase from Pseudomonas fluorescens had high sequence similarity with bacterial non-haem haloperoxides. However, this homology was limited to two distinct domains of the published esterase sequence. As errors in the published sequence were suspected, the esterase gene was sequenced again. The revised sequence displayed between 40 and 50% identical amino acids with the haloperoxidases, but distributed along the whole sequence. In addition to the structural homologies with haloperoxidases, the esterase also displayed functional homology. The recombinant esterase, purified from Escherichia coli cells, was capable of both ester hydrolysis and halogenation, as detected in situ by the formation of bromophenol blue or spectrophotometrically by the bromination of monochlorodimedon. The esterase is thus a bifunctional enzyme. The sequence analysis and the biochemical investigations show that the esterase belongs to the haloperoxidase family. It also possessed, however, a typical feature of serine-hydrolases, namely the consensus motif Gly-X-Ser-X-Gly around the active serine of the catalytic triad. By alignment of the esterase with different serine-hydrolase sequences, it was possible to identify the other two residues of the triad. The triad comprised the residues Ser95, Asp223 and His252. Interestingly, a structurally equivalent catalytic triad was also identified in the sequences of all bacterial non-haem haloperoxidases, in highly conserved domains. The presence of a catalytic triad in haloperoxidases is expected to be important in the mechanism of halogenation.