Spectroscopic studies on cobalt(II)-substituted metallo-beta-lactamase ImiS from Aeromonas veronii bv. sobria

In an effort to probe the structure of a group Bb metallo-beta-lactamase, Co(II)-substituted ImiS was prepared and characterized by electronic absorption, NMR, and EPR spectroscopies. ImiS containing 1 equiv of Co(II) (Co(II)(1)-ImiS) was shown to be catalytically active. Electronic absorption studies of Co(II)(1)-ImiS revealed the presence of two distinct features: (1) an intense sulfur to Co(II) ligand to metal charge transfer band and (2) less intense, Co(II) ligand field transitions that suggest 4-coordinate Co(II) in Co(II)(1)-ImiS. (1)H NMR studies of Co(II)(1)-ImiS suggest that one histidine, one aspartic acid, and one cysteine coordinate the metal ion in Co(II)(1)-ImiS. The addition of a second Co(II) to Co(II)(1)-ImiS did not result in any additional solvent-exchangeable NMR resonances, strongly suggesting that the second Co(II) does not bind to a site with histidine ligands. EPR studies reveal that the metal ion in Co(II)(1)-ImiS is 4-coordinate and that the second Co(II) is 5/6 coordinate. Taken together, these data indicate that the catalytic site in ImiS is the consensus Zn(2) site, in which Co(II) (and by extrapolation Zn(II)) is 4-coordinate and bound by Cys221, His263, Asp120, and probably one solvent water molecule. These studies also show that the second, inhibitory metal ion does not bind to the consensus Zn(1) site and that the metal ion binds at a site significantly removed from the active site. These results give the first structural information on metallo-beta-lactamase ImiS and suggest that the second metal binding site in ImiS may be targeted for inhibitors.