X-RAY STRUCTURE OF THE ZN-II BETA-LACTAMASE FROM BACTEROIDES-FRAGILISIN AN ORTHORHOMBIC CRYSTAL FORM

beta-Lactamases are extracellular or periplasmic bacterial enzymes whi ch confer resistance to beta-lactam antibiotics. On the basis of their catalytic mechanisms, they can be divided into two major groups: acti ve-site serine enzymes (classes A, C and D) and the Zn-II enzymes (cla ss B). The first crystal structure of a class B enzyme, the metaIlo-be ta-lactamase from Bacillus ceveus, has been solved at 2.5 Angstrom res olution [Carfi, Fares, Duee, Galleni, Duet, Frere & Dideberg (1995). E MBO J. 14, 4914-4921]. Recently, the crystal structure of the metallo- beta-lactamase from Bacteroides fragilis has been determined in a tetr agonal space group [Concha, Rasmussen, Bush & Herzberg (1996). Structu re, 4, 823-836]. The structure of the metallo-beta-lactamase from B. f ragilis in an orthorhombic crystal form at 2.0 Angstrom resolution is reported here. The final crystallographic R is 0.196 for all the 32501 observed reflections in the range 10-2.0 Angstrom. The refined model includes 458 residues, 437 water molecules, four zinc and two sodium i ons. These structures are discussed with reference to Zn binding and a ctivity. A catalytic mechanism is proposed which is coherent with meta llo-beta-lactamases being active with either one Zn ion (as in Aeromon as hydrophila) or two Zn ions (as in B. fragilis) bound to the protein .