title: Characterization of the metal-binding sites of the beta-lactamase from Bacteroides fragilis.
abstract: In an effort to better understand the structure and function of the metallo-beta-lactamase from Bacteroides fragilis, spectroscopic and metal-binding studies were performed on the native, metal-substituted, and mutant forms of the enzyme. Atomic absorption studies demonstrate that the native B. fragilis enzyme tightly binds 2 mol of Zn(II) and, along with mutagenesis studies, that the presence of both metal ions is required for full catalytic activity. EPR spectroscopy was used to confirm that the Co(II)-substituted beta-lactamase binds 2 mol of Co(II) per mole of enzyme, that the two Co(II)'s are highspin and probably uncoupled, with apparent g values of 6.5, 4.2, and 2.0, and that the coordination number of the Co(II) is 5 or 6. This number of ligands for the Co(II)-substituted enzyme is confirmed by UV-Vis spectra, which demonstrate the presence of very weak d-d transitions between 550 and 650 nm (epsilon approximately 30 M-1.cm-1) and an intense feature at 320 nm (epsilon approximately 1570 M-1.cm-1). The latter is assigned to a cysteine sulfur to Co(II) ligand-to-metal charge transfer band, and this assignment is confirmed by the disappearance of this band in the UV-Vis spectrum of a Co(II)-substituted C168S mutant. H NMR studies on the Co(II)-substituted enzyme suggest the presence of three histidine ligands bound to Co(II). Taken together, these studies support the sequence comparison study of Rasmussen et al., in which there is a catalytic metal-binding site with three histidines and one cysteine (C168). The remaining ligands are postulated to be water molecules involved in catalysis. Mutagenesis studies, in combination with activity assays and metal-binding studies, have been used to identify Asp61, Asp90, Asp152, and Asp183 as possible ligands to the second metal-binding site, with Asp90 and Asp152 having a pronounced effect on kcat. These results are discussed in light of the recent crystal structure of the metallo-beta-lactamase from B. cereus.
authors.affiliation: Department of Chemistry, Pennsylvania State University, University Park 16802, USA.
authors.forename: M W
authors.initials: MW
authors.lastname: Crowder
authors.forename: Z
authors.initials: Z
authors.lastname: Wang
authors.forename: S L
authors.initials: SL
authors.lastname: Franklin
authors.forename: E P
authors.initials: EP
authors.lastname: Zovinka
authors.forename: S J
authors.initials: SJ
authors.lastname: Benkovic
mesh_terms: D001441:Bacteroides fragilis / Q000201:enzymology*; D001665:Binding Sites; D002413:Cations, Divalent / Q000378:metabolism; D003001:Cloning, Molecular; D017931:DNA Primers; D004578:Electron Spin Resonance Spectroscopy; D004926:Escherichia coli; D007700:Kinetics; D009682:Magnetic Resonance Spectroscopy; D008670:Metals / Q000378:metabolism*; D016297:Mutagenesis, Site-Directed; D017354:Point Mutation; D016133:Polymerase Chain Reaction; D011487:Protein Conformation; D011994:Recombinant Proteins / Q000737:chemistry / Q000302:isolation & purification / Q000378:metabolism; D013053:Spectrophotometry; D013054:Spectrophotometry, Atomic; D013816:Thermodynamics; D001618:beta-Lactamases / Q000737:chemistry* / Q000302:isolation & purification / Q000378:metabolism*
chemical_list: D002413:Cations, Divalent; D017931:DNA Primers; D008670:Metals; D011994:Recombinant Proteins; D001618:beta-Lactamases