The therapeutic problems posed by class D beta-lactamases, a family of serine 
enzymes that hydrolyse beta-lactam antibiotics following an 
acylation-deacylation mechanism, are increased by the very low level of 
sensitivity of these enzymes to beta-lactamase inhibitors. To gain structural 
and mechanistic insights to aid the design of new inhibitors, we have determined 
the crystal structure of OXA-13 from Pseudomonas aeruginosa in the apo form and 
in complex with the carbapenem meropenem. The native form consisted of a dimer 
displaying an overall organisation similar to that found in the closely related 
enzyme OXA-10. In the acyl-enzyme complex, the positioning of the antibiotic 
appeared to be ensured mainly by (i) the covalent acyl bond and (ii) a strong 
salt-bridge involving the carboxylate moiety of the drug. Comparison of the 
structures of OXA-13 in the apo form and in complex with meropenem revealed an 
unsuspected flexibility in the region of the essential serine 115 residue, with 
possible consequences for the catalytic properties of the enzyme. In the apo 
form, the Ser115 side-chain is oriented outside the active site, whereas the 
general base Lys70 adopts a conformation that seems to be incompatible with the 
activation of the catalytic water molecule required for the deacylation step. In 
the OXA-13:meropenem complex, a 3.5 A movement of the backbone of the 114-116 
loop towards the side-chain of Lys70 was observed, which seems to be driven by a 
displacement of the neighbouring 91-104 loop and which results in the 
repositioning of the side-chain hydroxyl group of Ser115 toward the catalytic 
centre. Concomitantly, the side-chain of Lys70 is forced to curve in the 
direction of the deacylating water molecule, which is then strongly bound and 
activated by this residue. However, a distance of ca 5 A separates the catalytic 
water molecule from the acyl carbonyl group of meropenem, a structural feature 
that accounts for the inhibition of OXA-13 by this drug. Finally, the low level 
of penicillinase activity revealed by the kinetic analysis of OXA-13 could be 
related to the specific presence in position 73 of a serine residue located 
close to the general base Lys70, which results in a decrease of the number of 
hydrogen-bonding interactions stabilising the catalytic water molecule.