Crystal structure of IMP-2 metallo-ß-lactamase from Acinetobacter spp.: comparison of active-site loop structures between IMP-1 and IMP-2.

IMP-2, a subclass B1 metallo-ß-lactamase (MBL), is a Zn(II)-containing hydrolase. This hydrolase, involved in antibiotic resistance, catalyzes the hydrolysis of the C-N bond of the ß-lactam ring in ß-lactam antibiotics such as benzylpenicillin and imipenem. The crystal structure of IMP-2 MBL from Acinetobacter spp. was determined at 2.3 Å resolution. This structure is analogous to that of subclass B1 MBLs such as IMP-1 and VIM-2. Comparison of the structures of IMP-1 and IMP-2, which have an 85% amino acid identity, suggests that the amino acid substitution at position 68 on a ß-strand (ß3) (Pro in IMP-1 versus Ser in IMP-2) may be a staple factor affecting the flexibility of loop 1 (comprising residues at positions 60-66; EVNGWGV). In the IMP-1 structure, loop 1 adopts an open, disordered conformation. On the other hand, loop 1 of IMP-2 forms a closed conformation in which the side chain of Trp64, involved in substrate binding, is oriented so as to cover the active site, even though there is an acetate ion in the active site of both IMP-1 and IMP-2. Loop 1 of IMP-2 has a more flexible structure in comparison to IMP-1 due to having a Ser residue instead of the Pro residue at position 68, indicating that this difference in sequence may be a trigger to induce a more flexible conformation in loop 1.

Structure of metallo-beta-lactamase IND-7 from a Chryseobacterium indologenes clinical isolate at 1.65-A resolution.

The X-ray crystal structure of metallo-beta-lactamase from Chryseobacterium indologenes IND-7 was determined at a resolution of 1.65 A. The overall structure adopted a four-layered alphabeta/betaalpha sandwich structure with a dinuclear zinc(II) active site, in which the zinc(II) ions were denoted as Zn1 and Zn2. The overall structure of IND-7 is analogous to those of subclass B1 metallo-beta-lactamases, as determined by X-ray crystallography. A significant structural difference, however, was observed in the dinuclear zinc(II) active site: the coordination geometry around Zn1 changed from tetrahedral, found in other subclass B1 metallo-beta-lactamases, to distorted trigonal bipyramidal, whereas that of Zn2 changed from trigonal bipyramidal to tetrahedral. Arg121(101), which is located in the vicinity of the dinuclear zinc(II) active site, may affect the binding affinity of Zn2 due to an electronic repulsion between the zinc(II) ion(s) and a positively charged guanidyl group of Arg121(101). Moreover, the hydrogen-bonding interaction of Arg121 with Ser71(53), which is conserved in IND-1, IND-3 and IND-5-IND-7, appeared to have important consequences for the binding affinity of Zn2 in conjunction with the above electrostatic effect.