Difference in the Inhibitory Effect of Thiol Compounds and Demetallation Rates from the Zn(II) Active Site of Metallo-β-lactamases (IMP-1 and IMP-6) Associated with a Single Amino Acid Substitution

Gram-negative bacteria producing metallo-fi-lacta-mases (MBLs) have become a considerable threat to public health. MBLs including the IMP, VIM, and NDM types are Zn(II) enzymes that hydrolyze the fi-lactam ring present in a broad range of antibiotics, such as N-benzylpenicillin, meropenem, and imipenem. Among IMPs, IMP-1 and IMP-6 differ in a single amino acid substitution at position 262, where serine in IMP-1 is replaced by glycine in IMP-6, conferring a change in substrate specificity. To investigate how this mutation influences enzyme function, we examined lactamase inhibition by thiol compounds. Ethyl 3-mercaptopropionate acted as a competitive inhibitor of IMP-1, but a noncompetitive inhibitor of IMP-6. A comparison of the crystal structures previously reported for IMP-1 (PDB code: 5EV6) and IMP-6 (PDB code: 6LVJ) revealed a hydrogen bond between the side chain of Ser262 and Cys221 in IMP-1 but the absence of hydrogen bond in IMP-6, which affects the Zn2 coordination sphere in its active site. We investigated the demetallation rates of IMP-1 and IMP-6 in the presence of chelating agent ethylenediaminetetraacetic acid (EDTA) and found that the demetallation reactions had fast and slow phases with a first-order rate constant (kfast = 1.76 h-1, kslow = 0.108 h-1 for IMP-1, and kfast = 14.0 h-1 and kslow = 1.66 h-1 for IMP-6). The difference in the flexibility of the Zn2 coordination sphere between IMP-1 and IMP -6 may influence the demetallation rate, the catalytic efficiency against fi-lactam antibiotics, and the inhibitory effect of thiol compounds.