Structural Basis of Extended Spectrum TEM -Lactamases

Abstract

The E166Y and the E166Y/R164S TEM-1 β-lactamase mutant enzymes display extended spectrum substrate specificities. Electrospray mass spectrometry demonstrates that, with penicillin G as substrate, the rate-limiting step in catalysis is the hydrolysis of the E166Y acyl-enzyme complex. Comparison of the 1.8-Å resolution x-ray structures of the wild-type and of the E166Y mutant enzymes shows that the binding of cephalosporin substrates is improved, in the mutant enzyme, by the enlargement of the substrate binding site. This enlargement is due to the rigid body displacement of 60 residues driven by the movement of the -loop. These structural observations strongly suggest that the link between the position of the -loop and that of helix H5, plays a central role in the structural events leading to extended spectrum TEM-related enzymes. The increased -loop flexibility caused by the R164S mutation, which is found in several natural mutant TEM enzymes, may lead to similar structural effects. Comparison of the kinetic data of the E166Y, E166Y/R164S, and R164S mutant enzymes supports this hypothesis.