Structural basis for the ß-lactamase activity of EstU1, a family VIII carboxylesterase.

ABSTRACT

EstU1 is a unique family VIII carboxylesterase that displays hydrolytic activity toward the amide bond of clinically used ß-lactam antibiotics as well as the ester bond of p-nitrophenyl esters. EstU1 assumes a ß-lactamase-like modular architecture and contains the residues Ser100, Lys103, and Tyr218, which correspond to the three catalytic residues (Ser64, Lys67, and Tyr150, respectively) of class C ß-lactamases. The structure of the EstU1/cephalothin complex demonstrates that the active site of EstU1 is not ideally tailored to perform an efficient deacylation reaction during the hydrolysis of ß-lactam antibiotics. This result explains the weak ß-lactamase activity of EstU1 compared with class C ß-lactamases. Finally, structural and sequential comparison of EstU1 with other family VIII carboxylesterases elucidates an operative molecular strategy used by family VIII carboxylesterases to extend their substrate spectrum.