CRYSTAL STRUCTURE OF A METALLO [[beta]]-LACTAMASE II FROM B. CEREUS AT 2.5Å. Stella Maris Fabiane^[[section]], M. Sohi^[[section]], T. Wan^[[section]], D. J. Payne^{[[paragraph]]}, J. H. Bateson^{[[daggerdbl]]}, T. Mitchell^{[[daggerdbl]]}, B. J. Sutton^[[section]], ^[[section]]King's College London, 26-29 Drury Lane, London WC2B 5RL, UK, ^{[[paragraph]]}SmithKline Beecham Pharmaceuticals, 1250 South Collegeville Rd, P. O. Box 5089, Collegeville, PA 19426-0989, USA, ^{[[daggerdbl]]}SmithKline Beecham Pharmaceuticals, Brockham Park, Betchworth, Surrey RH3 7AJ, UK

Crystals of the Zn-dependent [[beta]]-lactamase ('cephalosporinase') from Bacillus cereus 569/H were grown using the vapour diffusion technique. They belong to space group P3₁21, with cell dimensions a=b=68.14Å, c=180.68A, with two enzyme molecules per asymmetric unit. The structure of the enzyme was solved by multiple isomorphous replacement and solvent flattening techniques and refined to 2.5Å. The final crystallographic R factor is 19.2% (R_free = 26.8%).

The structure reveals an arrangement of secondary structural elements very similar to the N-terminal nucleophile amidohydrolases. The intermolecular interactions between the non-crystallographically related molecules involve head-to-tail packing of [[alpha]]-helices, and a continuous [[beta]]-sheet extending across the interface. The active site is flanked by a disordered loop that may be important for the mechanism; cryocrystallographic studies are in progress to investigate its role in substrate binding.