IMP-2, a subclass B1 metallo-β-lactamase (MBL), is a Zn(II)-: Classes A, C, and D are serine enzymes that use a serine residue as a nucleophile, whereas class B consists of metallo enzymes whose active sites contain one or two Zn(II) ion(s) and are referred to as metallo-β-lactamases (MBLs). MBLs are divided into three subclasses (B1, B2, B3) based on the sequence of the Zn(II) ligands.3) MBLs hydrolyze most β-lactams used currently, such as cephems and carbapenems, but not monobactam such as aztreonam. MBLs are hardly blocked by the inhibitors for serine β-lactamases, including clavulanate, sulbactam and, tazobactam.In 1994, IMP-1 MBL, belonging to subclass B1, was first identified from Serratia marcescens and Pseudomonas aeruginosa in Japan. 4,5) Its gene, bla IMP , encodes the IMP-1 enzyme and is integrated as a gene cassette into integrons carried by transferable plasmids. 6) Therefore, the bla IMP gene can spread among different nosocomial pathogens horizontally. To date, at least 48 variants of IMP-type MBLs have been deposited (http://www.lahey.org/Studies) by the end of July 2014.In 1997, an IMP-2 MBL was identified from an Acinetobacter baumannii clinical isolate AC-54/97 in Italy, 7) followed by the isolation of IMP-2-producing A. baumannii, A. lowffii, and P. aeruginosa in Japan.8) The IMP-2 gene (bla IMP-2 ) is also carried as an integron-bone gene cassette, similar to the IMP-1 gene (bla IMP ). 6,7) IMP-2 possesses approximately an 85% amino acid identity with IMP-1, and differs in 36 amino acids from IMP-1: 10 amino acid residues are clustered within the signal peptide region and the remaining 26 amino acid residues are found in the mature protein 7) (Fig. 2C). The structure of IMP-1 suggests that 4 of 26 amino acid residues predicted to be involved in substrate recognition in IMP-2 (Ser68, Gln198, Asp227, and Ser261; the amino acid residues of IMP-1 and IMP-2 are designated by their BBL number 3) ) are located in the neighborhood at its active site within a distance of ca. 9 Å (Fig. 1). The remaining 22 amino acid residues are located at the protein surface or are far from the active site.The kinetic parameters of the hydrolysis of several β-lactams by IMP-2 are overall similar to those by IMP-1, but the catalytic efficiency values of the two enzymes (k cat /K m ) for ampicillin are different for IMP-1 and 0.21 µM −1 s −1 for IMP-2.7) The k cat /K m value of IMP-1 to IMP-2 increases 23-fold, so IMP-1 hydrolyses ampicillin more efficiently than IMP-2. These differences in kinetic parameters might be related to the subtle structural changes arising from the different amino acid sequences of the enzymes, even though the 6 amino acid residues (His116, His118, Asp120, His196, Cys221, and His263) which construct the active site of the enzyme are conserved between IMP-1 and IMP-2.Therefore, determination of the fine three-dimensional