<p> This entry represents a beta-lactamase structural motif, which contins a cluster of alpha-helices and an alpha/beta sandwich. In addition to beta-lactamases, this domain is also found in D-ala carboxypeptidase/transpeptidase, esterase (EstB) [<cite idref="PUB00016301"/>], the penicillin receptor BlaR (C-terminal domain), D-aminopeptidase (N-terminal domain) [<cite idref="PUB00013322"/>], penicillin-biding proteins (e.g. PBP2x, PBP5), and in glutaminase (GlnA). Beta-lactamases are the most common bacterial resistance mechanism against beta-lactam antibiotics [<cite idref="PUB00016300"/>]. Beta-lactamases appear to have evolved from DD-transpeptidases, which are penicillin-binding proteins involved in cell wall biosynthesis, and as such are one of the main targets of beta-lactam antibiotics. </p>