Functional Analysis of the Active Site of a Metallo-β-Lactamase Proliferating in Japan

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Abstract
An R-plasmid-mediated metallo-β-lactamase was found inKlebsiella pneumoniae DK4 isolated in Japan in 1991. The nucleotide sequence of its structural gene revealed that the β-lactamase termed DK4 was identical to the IMP-1 metallo-β-lactamase which was mediated by a chromosomal gene ofSerratia marcescens TN9106 isolated in Japan in 1991 (E. Osano et al., Antimicrob. Agents Chemother. 38:71–78, 1994). The dose effect of DK4 β-lactamase production on the resistance levels indicated a significant contribution of the enzyme to bacterial resistance to all the β-lactams except monobactams. The enzymatic characteristics of the DK4 β-lactamase and its kinetic parameters for nine β-lactams were examined. The DK4 β-lactamase was confirmed to contain 2 mol of zinc per mol of enzyme protein. The apoenzyme that lacked the two zincs was structurally unstable, and the activities of only 30% of the apoenzyme molecules could be restored by the addition of 1 mM zinc sulfate. The substitution of five conserved histidines (His28, His86, His88, His149, His210) and a cysteine (Cys168) for an alanine indicated that His86, His88, and His149 served as ligands to one of the zincs and that Cys168 played a role as a ligand to the second zinc. Both zinc molecules contribute to the enzymatic process. Mutant enzymes that lack only one of these retained some activity. Additionally, a conserved aspartic acid at position 90 was replaced by asparagine. This mutant enzyme showed an approximately 1,000 times lower k cat value for cephalothin than that of the wild-type enzyme but retained the two zincs even after dialysis against zinc-free buffer. The observed effect of pH on the activity suggested that Asp90 functions as a general base in the enzymatic process.