Mutations affecting the catalytic activity of Bacillus cereus 5/B/6 β-lactamase II

The biogenesis of β-lactamase enzymes

The discovery of penicillin by Alexander Fleming marked a new era for modern medicine, allowing not only the treatment of infectious diseases, but also the safe performance of life-saving interventions, like surgery and chemotherapy. Unfortunately, resistance against penicillin, as well as more complex β-lactam antibiotics, has rapidly emerged since the introduction of these drugs in the clinic, and is largely driven by a single type of extra-cytoplasmic proteins, hydrolytic enzymes called β-lactamases. While the structures, biochemistry and epidemiology of these resistance determinants have been extensively characterized, their biogenesis, a complex process including multiple steps and involving several fundamental biochemical pathways, is rarely discussed. In this review, we provide a comprehensive overview of the journey of β-lactamases, from the moment they exit the ribosomal channel until they reach their final cellular destination as folded and active enzymes.

Metallo-β-lactamases in the Age of Multidrug Resistance: From Structure and Mechanism to Evolution, Dissemination, and Inhibitor Design

Antimicrobial resistance is one of the major problems in current practical medicine. The spread of genes coding for resistance determinants among bacteria challenges the use of approved antibiotics, narrowing the options for treatment. Resistance to carbapenems, last resort antibiotics, is a major concern. Metallo-β-lactamases (MBLs) hydrolyze carbapenems, penicillins, and cephalosporins, becoming central to this problem. These enzymes diverge with respect to serine-β-lactamases by exhibiting a different fold, active site, and catalytic features. Elucidating their catalytic mechanism has been a big challenge in the field that has limited the development of useful inhibitors. This review covers exhaustively the details of the active-site chemistries, the diversity of MBL alleles, the catalytic mechanism against different substrates, and how this information has helped developing inhibitors. We also discuss here different aspects critical to understand the success of MBLs in conferring resistance: the molecular determinants of their dissemination, their cell physiology, from the biogenesis to the processing involved in the transit to the periplasm, and the uptake of the Zn(II) ions upon metal starvation conditions, such as those encountered during an infection. In this regard, the chemical, biochemical and microbiological aspects provide an integrative view of the current knowledge of MBLs.

Conserved residues Glu37 and Trp229 play an essential role in protein folding of β‐lactamase

Evolutionary robustness requires that the number of highly conserved amino acid residues in proteins is minimized. In enzymes, such conservation is observed for catalytic residues but also for some residues in the second shell or even further from the active site. β‐Lactamases evolve in response to changing antibiotic selection pressures and are thus expected to be evolutionarily robust, with a limited number of highly conserved amino acid residues. As part of the effort to understand the roles of conserved residues in class A β‐lactamases, we investigate the reasons leading to the conservation of two amino acid residues in the β‐lactamase BlaC, Glu37, and Trp229. Using site‐directed mutagenesis, we have generated point mutations of these residues and observed a drastic decrease in the levels of soluble protein produced in Escherichia coli, thus abolishing completely the resistance of bacteria against β‐lactam antibiotics. However, the purified proteins are structurally and kinetically very similar to the wild‐type enzyme, only differing by exhibiting a slightly lower melting temperature. We conclude that conservation of Glu37 and Trp229 is solely caused by an essential role in the folding process, and we propose that during folding Glu37 primes the formation of the central β‐sheet and Trp229 contributes to the hydrophobic collapse into a molten globule.

Functional analysis of the active site of a metallo-beta-lactamase proliferating in Japan

An R-plasmid-mediated metallo-beta-lactamase was found in Klebsiella pneumoniae DK4 isolated in Japan in 1991. The nucleotide sequence of its structural gene revealed that the beta-lactamase termed DK4 was identical to the IMP-1 metallo-beta-lactamase which was mediated by a chromosomal gene of Serratia marcescens TN9106 isolated in Japan in 1991 (E. Osano et al., Antimicrob. Agents Chemother. 38:71-78, 1994). The dose effect of DK4 beta-lactamase production on the resistance levels indicated a significant contribution of the enzyme to bacterial resistance to all the beta-lactams except monobactams. The enzymatic characteristics of the DK4 beta-lactamase and its kinetic parameters for nine beta-lactams were examined. The DK4 beta-lactamase was confirmed to contain 2 mol of zinc per mol of enzyme protein. The apoenzyme that lacked the two zincs was structurally unstable, and the activities of only 30% of the apoenzyme molecules could be restored by the addition of 1 mM zinc sulfate. The substitution of five conserved histidines (His28, His86, His88, His149, His210) and a cysteine (Cys168) for an alanine indicated that His86, His88, and His149 served as ligands to one of the zincs and that Cys168 played a role as a ligand to the second zinc. Both zinc molecules contribute to the enzymatic process. Mutant enzymes that lack only one of these retained some activity. Additionally, a conserved aspartic acid at position 90 was replaced by asparagine. This mutant enzyme showed an approximately 1,000 times lower k(cat) value for cephalothin than that of the wild-type enzyme but retained the two zincs even after dialysis against zinc-free buffer. The observed effect of pH on the activity suggested that Asp90 functions as a general base in the enzymatic process.

Charge is the major discriminating factor for glutathione reductase versus trypanothione reductase inhibitors

Benson et al. (Biochem. J. 1992, 286, 9) reported three novel competitive inhibitors of trypanothione reductase (TR), which were selected to complement a hydrophobic region identified on the TR structure which was not present on human glutathione reductase (hGR). Benson et al. also noted that chlorpromazine, a tricyclic antidepressant known to have trypanocidal activity, was an inhibitor of TR. Here we show that chlorpromazine is a competitive inhibitor of TRs from Crithidia fasciculata (Ki = 14 microM) and Trypanosoma cruzi (Ki = 10 microM), but the drug binds > 50-fold more weakly (Ki = 762 microM) to hGR. Analogues of chlorpromazine differing in the length of the side chain carrying the positively charged R-group are also selective TR inhibitors whereas, a tricyclic structure carrying a negatively charged side chain is a competitive inhibitor with selectivity for hGR (K(hGR)i = 165 microM vs. K(TR)i = 1400 microM). This finding suggests that simple charge characteristics, rather than differences in hydrophobicity, may account for a significant portion of the selectivity of this series of inhibitors for these two enzymes. Electrostatic analysis of the structures of TR and hGR thus provides a rationale for these results, and offers a new principle for inhibitor design. The principle gains further support from the observation that all known tricyclic competitive inhibitors of TR are positively charged. In order to investigate the in vivo relevance of our findings we have examined the effect of chlorpromazine and its negatively charged analogue on the growth of C. fasciculata parasites. Consistent with our kinetics, chlorpromazine (50 microM) inhibited the growth of parasites by 50%, while no measurable decrease in parasite growth rate was noted in the presence of the negatively charged inhibitor (400 microM). Furthermore, the highly similar inhibitory profiles of C. fasciculata TR and T. cruzi TR suggest that drug-design studies using the structurally better-studied C. fasciculata TR are also relevant to the human pathogen T. cruzi.

Hyperexpression in Escherichia coli, purification, and characterization of the metallo-beta-lactamase of Bacillus cereus 5/B/6

We used site-directed mutagenesis to introduce both a NdeI restriction endonuclease site and an initiator codon at the junction of the leader and structural gene sequences of the metallo-beta-lactamase of Bacillus cereus 5/B/6. This construct allowed us to clone just the beta-lactamase structural gene sequence into an Escherichia coli expression vector. E. coli cells were transformed with the recombinant plasmid, the B. cereus beta-lactamase was expressed, and these E. coli cells were disrupted by sonic oscillation. When the resultant suspensions were clarified by ultracentrifugation, the B. cereus beta-lactamase represented 15% of the total protein in the supernatant. Subsequent gel filtration and ion-exchange chromatography allowed the first reported purification to homogeneity of the B. cereus beta-lactamase from E. coli with an 87% recovery and an overall yield of 17 mg of enzyme per liter of cell culture. The electrophoretic mobilities of the enzyme expressed in and purified from E. coli and the enzyme purified directly from B. cereus were identical in both native and sodium dodecyl sulfate gel electrophoreses. As with the B. cereus enzyme, Km and Vmax (using cephalosporin C as substrate) for the enzyme purified from E. coli were 0.39 mM and 1333 units/mg protein, respectively. Likewise, the Co(II)-reconstituted enzyme purified from E. coli, which retained 29% of the activity of the Zn(II) enzyme, had electronic absorption spectra with maxima at 347, 551, 617, and 646 nm with extinction coefficients of 900, 250, 173, and 150 M-1 cm-1, respectively.