title: Structure of the extended-spectrum class C beta-lactamase of Enterobacter cloacae GC1, a natural mutant with a tandem tripeptide insertion.
abstract: A class C beta-lactamase from a clinical isolate of Enterobacter cloacae strain GC1 with improved hydrolytic activity for oxyimino beta-lactam antibiotics has been analyzed by X-ray crystallography to 1.8 A resolution. Relative to the wild-type P99 beta-lactamase, this natural mutant contains a highly unique tandem repeat Ala211-Val212-Arg213 [Nugaka et al. (1995) J. Biol. Chem. 270, 5729-5735]. The 39.4 kDa chromosomal beta-lactamase crystallizes from poly(ethylene glycol) 8000 in potassium phosphate in space group P2(1)2(1)2 with cell dimensions a = 78.0 A, b = 69.5 A, and c = 63.1 A. The crystal structure was solved by the molecular replacement method, and the model has been refined to an R-factor of 0.20 for all nonzero data from 8 to 1.8 A. Deviations of model bonds and angles from ideal values are 0.008 A and 1.4 degrees, respectively. Overlay of alpha-carbon atoms in the GC1 and P99 beta-lactamases results in an rms deviation of 0.6 A. Largest deviations occur in a loop containing Gln120 and in the Omega loop region (200-218) where the three residues 213-215 are disordered. Possibly as a result of this disorder, the width of the opening to the substrate binding cavity, as measured from the 318-324 beta-strand to two loops containing Gln120 and Tyr150 on the other side, is 0.6-1.4 A wider than in P99. It is suggested that conformational flexibility in the expanded Omega loop, and its influence on adjacent protein structure, may facilitate hydrolysis of oxyimino beta-lactams by making the acyl intermediate more open to attack by water. Nevertheless, backbone atoms in core catalytic site residues Ser64, Lys67, Tyr150, Asn152, Lys318, and Ser321 deviate only 0.4 A (rmsd) from atoms in P99. A rotation of a potential catalytic base, Tyr150, relative to P99 at pH 8, is consistent with the requirement for a lower than normal pK(a) for this residue.
authors.affiliation: Department of Molecular and Cell Biology, The University of Connecticut, Storrs 06269-3125, USA.
authors.forename: G V
authors.initials: GV
authors.lastname: Crichlow
authors.forename: A P
authors.initials: AP
authors.lastname: Kuzin
authors.forename: M
authors.initials: M
authors.lastname: Nukaga
authors.forename: K
authors.initials: K
authors.lastname: Mayama
authors.forename: T
authors.initials: T
authors.lastname: Sawai
authors.forename: J R
authors.initials: JR
authors.lastname: Knox
mesh_terms: D000595:Amino Acid Sequence; D020134:Catalytic Domain / Q000235:genetics; D002511:Cephalosporins / Q000737:chemistry; D003198:Computer Simulation; D018360:Crystallography, X-Ray; D016972:Enterobacter cloacae / Q000201:enzymology* / Q000235:genetics*; D006801:Humans; D008958:Models, Molecular; D008969:Molecular Sequence Data; D009154:Mutation; D010446:Peptide Fragments / Q000737:chemistry* / Q000235:genetics; D017434:Protein Structure, Tertiary; D020080:Tandem Repeat Sequences / Q000235:genetics; D001618:beta-Lactamases / Q000737:chemistry* / Q000235:genetics
chemical_list: D002511:Cephalosporins; D010446:Peptide Fragments; D001618:beta-Lactamases