A BACTERIAL ESTERASE IS HOMOLOGOUS WITH NONHEME HALOPEROXIDASES AND DISPLAYS BROMINATING ACTIVITY

Screening GenBank indicated that an esterase from Pseudomonas fluoresc ens had high sequence similarity with bacterial non-haem haloperoxides . However, this homology was limited to two distinct domains of the pu blished esterase sequence. As errors in the published sequence were su spected, the esterase gene was sequenced again. The revised sequence d isplayed between 40 and 50% identical amino acids with the haloperoxid ases, but distributed along the whole sequence. In addition to the str uctural homologies with haloperoxidases, the esterase also displayed f unctional homology. The recombinant esterase, purified from Escherichi a coil cells, was capable of both ester hydrolysis and halogenation, a s detected in situ by the formation of bromophenol blue or spectrophot ometrical ly 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 fa mily. It also possessed, however, a typical feature of serine-hydrolas es, namely the consensus motif Gly-X-Ser-X-Cly around the active serin e 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 w as also identified in the sequences of all bacterial non-haem halopero xidases, in highly conserved domains. The presence of a catalytic tria d in haloperoxidases is expected to be important in the mechanism of h alogenation.