Crystal structures of the cadmium- and mercury-substituted metallo-beta-lactamase from Bacteroides fragilis.
The metallo-beta-lactamases require zinc or cadmium for hydrolyzing beta-lactam antibiotics and are inhibited by mercurial compounds. To data, there are no clinically useful inhibitors of this class of enzymes. The crystal structure of the Zn(2+)-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 beta-lactam ring. To study the metal coordination further, the crystal structures of a Cd(2+)-bound enzyme and of an Hg(2+)-soaked zinc-containing enzyme have been determined at 2.1 A and 2.7 A, respectively. Given the diffraction resolution, the Cd(2+)-bound enzyme exhibits the same active-site architecture as that of the Zn(2+)-bound enzyme, consistent with the fact that both forms are enzymatically active. The 10-fold reduction in activity of the Cd(2+)-bound molecule compared with the Zn(2+)-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 Hg(2+)-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-Zn(2+)-bound structure. Instead of the ejected zinc, a mercury ion binds between Cys 104 and Cys 181, 4.8 A away from Zn1 and 3.9 A away from the site where Zn2 is located in the 2-Zn(2+)-bound molecule. The perturbed binuclear metal cluster explains the inactivation of the enzyme by mercury compounds.
Concha
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Center for Advanced Research in Biotechnology, University of Maryland Biotechnology Institute, Rockville, Maryland 20850, USA.
Rasmussen
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BA
Bush
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K
Herzberg
O
O
D001441:Bacteroides fragilis / Q000201:enzymology*; D001665:Binding Sites; D002104:Cadmium / Q000737:chemistry / Q000378:metabolism*; D002384:Catalysis; D003460:Crystallization; D018360:Crystallography, X-Ray; D008628:Mercury / Q000737:chemistry / Q000378:metabolism*; D015394:Molecular Structure; D010400:Penicillin G / Q000378:metabolism; D013329:Structure-Activity Relationship; D015032:Zinc / Q000737:chemistry / Q000378:metabolism; D001618:beta-Lactamases / Q000737:chemistry* / Q000378:metabolism
D002104:Cadmium; D001618:beta-Lactamases; D008628:Mercury; D015032:Zinc; D010400:Penicillin G