UNANTICIPATED INHIBITION OF THE METALLO-BETA-LACTAMASE FROM BACTEROIDES-FRAGILIS BY 4-MORPHOLINEETHANESULFONIC ACID (MES) - A CRYSTALLOGRAPHIC STUDY AT 1.85-ANGSTROM RESOLUTION

AS part of a structure-aided effort to design clinically useful inhibi tors of metallo-beta-lactamases, the X-ray crystal structure of a comp lex between the metallo-beta-lactamase from Bacteroides fragilis and 4 -morpholinoethanesulfonic acid (MES) has been determined and a model f or the structure has been refined to a crystallographic R-factor of 0. 151 for data between 10.0- and 1.85-Angstrom resolution. Although the binding of MES was an adventitious result of the use of MES as a buffe r in the crystallization mixture, MES was subsequently shown to be a c ompetitive inhibitor of the enzyme, with a K-i of 23 +/- 5 mM. MES bin ds in the same fashion to both of the molecules in the crystallographi c asymmetric unit; both direct and solvent-mediated hydrogen bonds to the protein and to the binuclear zinc cluster are observed, involving the oxygens of the sulfonic acid group and the nitrogen of the morphol ino ring. In addition, there are hydrophobic interactions between the morpholino ring and residues in the flexible beta-strand of the enzyme between residues 26 and 36. Comparison of this structure with the pre viously reported unliganded structures of the same enzyme [Concha, N. O., Rasmussen, B. A., Bush, K., and Herzberg, O. (1996) Structure 4, 8 23-836; Carfi, A., Duee, E., Paul-Soto, R., Galleni, M., Frere, J.-M., and Dideberg, O. (1998) Acta Crystallogr. D54, 47-57] reveals that al though the overall conservation of structure in the three different cr ystal lattices is very high, binding of MES is correlated with a signi ficant change in the conformation of this beta-strand. The flexibility of this beta-strand will be an important consideration in the design of inhibitors of the metallo-beta-lactamases.