The high resolution crystal structure for class A beta-lactamase PER-1 reveals the bases for its increase in breadth of activity

The treatment of infectious diseases by beta-lactam antibiotics is continuo usly challenged by the emergence and dissemination of new beta-lactamases. In most cases, the cephalosporinase activity of class A enzymes results fro m a few mutations in the TEM and SHV penicillinases, The PER-1 beta-lactama se Nas characterized as a class A enzyme displaying a cephalosporinase acti vity. This activity was, however, insensitive to the mutations of residues known to be critical for providing extended substrate profiles to TEM and S HV. The x-ray structure of the protein, solved at 1.9-Angstrom resolution, reveals that two of the most conserved features in class A beta-lactamases are not present in this enzyme: the fold of the Omega-loop and the cis conf ormation of the peptide bond between residues 166 and 167. The new fold of the Omega-loop and the insertion of four residues at the edge of strand S3 generate a broad cavity that may easily accommodate the bulky substituents of cephalosporin substrates. The trans conformation of the 166-167 bond is related to the presence of an aspartic acid at position 136, Selection of c lass A enzymes based on the occurrence of both Asp(136) and Asn(179) identi fies a subgroup of enzymes with high sequence homology.