1.85 A RESOLUTION STRUCTURE OF THE ZINC(II) BETA-LACTAMASE FROM BACILLUS-CEREUS

Class B beta-lactamases are wide-spectrum enzymes which require bivale nt metal ions for activity. The structure of the class B zinc-ion-depe ndent beta-lactamase from Bacillus cereus (BCII) has been refined at 1 .85 Angstrom resolution using data collected on cryocooled crystals (1 00 K). The enzyme from B. cereus has a molecular mass of 24946Da and i s folded into a beta-sandwich structure with helices on the external f aces. The active site is located in a groove running between the two b eta-sheets [Carfi et al. (1995). EMBO J. 14, 4914-4921]. The 100 K hig h-resolution BCII structure shows one fully and one partially occupied zinc site. The zinc ion in the fully occupied site (the catalytic zin c) is coordinated by three histidines and one water molecule. The seco nd zinc ion is at 3.7 Angstrom from the first one and is coordinated b y one histidine, one cysteine, one aspartate and one unknown molecule (which is most likely to be a carbonate ion). In the B. cereus zinc be ta-lactamase the affinity for the second metal ion is low at the pH of crystallization [K-d = 25mM, 293K; Baldwin ef al. (1978). Biochem. J. 175, 441-447] and the dissociation constant of the second zinc ion th us apparently decreased at the cryogenic temperature. In addition, the structure of the apo enzyme was determined at 2.5 Angstrom resolution . The removal of the zinc ion by chelating agents causes small changes in the active-site environment.