THE 3-D STRUCTURE OF A ZINC METALLO-BETA-LACTAMASE FROM BACILLUS-CEREUS REVEALS A NEW-TYPE OF PROTEIN FOLD

The 3-D structure of Bacillus cereus (569/H/9) beta-lactamase (EC 3.5. 2.6), which catalyses the hydrolysis of nearly all beta-lactams, has b een solved at 2.5 Angstrom resolution by the multiple isomorphous repl acement method, with density modification and phase combination, from crystals of the native protein and of a specially designed mutant (T97 C). The current model includes 212 of the 227 amino acid residues, the zinc ion and 10 water molecules. The protein is folded into a beta be ta sandwich with helices on each external face. To our knowledge, this fold has never been observed. An approximate internal molecular symme try is found, with a 2-fold axis passing roughly through the zinc ion and suggesting a possible gene duplication. The active site is located at one edge of the beta beta sandwich and near the N-terminal end of a helix. The zinc ion is coordinated by three histidine residues (86, 88 and 149) and a water molecule. A sequence comparison of the relevan t metallo-beta-lactamases, based on this protein structure, highlights a few well-conserved amino acid residues. The structure shows that mo st of these residues are in the active site. Among these, aspartic aci d 90 and histidine 210 participate in a proposed catalytic mechanism f or beta-lactam hydrolysis.