Crystal structures of the cadmium‐ and mercury‐substituted metallo‐β‐lactamase from Bacteroides fragilis

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Abstract
The metallo-β-lactamases require zinc or cadmium for hydrolyzing β-lactam antibiotics and are inhibited by mercurial compounds. To date, there are no clinically useful inhibitors of this class of enzymes. The crystal structure of the Zn2+-bound enzyme from Bacteroides fragilis contains a binuclear zinc center in the active site. A hydroxide, coordinated to both zinc atoms, is proposed as the moiety that mounts the nucleophilic attack on the carbonyl carbon atom of the β-lactam ring. To study the metal coordination further, the crystal structures of a Cd2+-bound enzyme and of an Hg2+-soaked zinc-containing enzyme have been determined at 2.I Å and 2.7 Å, respectively. Given the diffraction resolution, the Cd2+-bound enzyme exhibits the same active-site architecture as that of the Zn2+-bound enzyme, consistent with the fact that both forms are enzymatically active. The 10-fold reduction in activity of the Cd2+-bound molecule compared with the Zn2+-bound enzyme is attributed to fine differences in the charge distribution due to the difference in the ionic radii of the two metals. In contrast, in the Hg2+-bound structure, one of the zinc ions, Zn2, was ejected, and the other zinc ion, Zn1, remained in the same site as in the 2-Zn2+-bound structure. Instead of the ejected zinc, a mercury ion binds between Cys 104 and Cys 181, 4.8 Å away from Zn1 and 3.9 Å away from the site where Zn2 is located in the 2-Zn2+-bound molecule. The perturbed binuclear metal cluster explains the inactivation of the enzyme by mercury compounds.