Conformational adaptation of RTEM beta-lactamase to cefoxitin

A dynamic structure for the acyl-enzyme species of the antibiotic aztreonam with the Citrobacter freundii beta-lactamase revealed by infrared spectroscopy and molecular dynamics simulations

Infrared difference spectra show that at least 4 conformations coexist for the ester carbonyl group of the stable acyl-enzyme species formed between the antibiotic aztreonam and the class C beta-lactamase from Citrobacter freundii. A novel method for the assignment of the bands that arise from the ester carbonyl group has been employed. This has made use of the finding that the infrared absorption intensity of aliphatic esters is surprisingly constant, so a direct comparison with simple model esters has been possible. This has allowed a clear distinction to be made between ester and amide (protein) absorptions. The polarity of the conformer environment varies from hexane-like to strongly hydrogen-bonded. We assume that the conformer with the lowest frequency (1,690 cm(-)(1)) and hence the strongest hydrogen-bonding is the singular conformer observed in the X-ray crystallographic structure, since a good interaction via two hydrogen bonds with the oxyanion hole is seen. Molecular dynamics simulation by the method of locally enhanced sampling revealed that the motion of the ester carbonyl of the acyl-enzyme species in and out of the oxyanion hole is facile. The simulation revealed two pathways for this motion that would go through intermediates that first break one or the other of the two hydrogen bonds to the oxyanion hole, prior to departure of the carbonyl moiety out of the active site. It is likely that such motion for the acyl-enzyme species might also occur with more typical beta-lactam substrates for beta-lactamases, but their detection in the more rapid time scale may prove a challenge.

Crystal structures of the Bacillus licheniformis BS3 class A beta-lactamase and of the acyl-enzyme adduct formed with cefoxitin

The Bacillus licheniformis BS3 beta-lactamase catalyzes the hydrolysis of the beta-lactam ring of penicillins, cephalosporins, and related compounds. The production of beta-lactamases is the most common and thoroughly studied cause of antibiotic resistance. Although they escape the hydrolytic activity of the prototypical Staphylococcus aureus beta-lactamase, many cephems are good substrates for a large number of beta-lactamases. However, the introduction of a 7alpha-methoxy substituent, as in cefoxitin, extends their antibacterial spectrum to many cephalosporin-resistant Gram-negative bacteria. The 7alpha-methoxy group selectively reduces the hydrolytic action of many beta-lactamases without having a significant effect on the affinity for the target enzymes, the membrane penicillin-binding proteins. We report here the crystallographic structures of the BS3 enzyme and its acyl-enzyme adduct with cefoxitin at 1.7 A resolution. The comparison of the two structures reveals a covalent acyl-enzyme adduct with perturbed active site geometry, involving a different conformation of the omega-loop that bears the essential catalytic Glu166 residue. This deformation is induced by the cefoxitin side chain whose position is constrained by the presence of the alpha-methoxy group. The hydrolytic water molecule is also removed from the active site by the 7beta-carbonyl of the acyl intermediate. In light of the interactions and steric hindrances in the active site of the structure of the BS3-cefoxitin acyl-enzyme adduct, the crucial role of the conserved Asn132 residue is confirmed and a better understanding of the kinetic results emerges.

Direct n.m.r. evidence for substrate-induced conformational changes in a beta-lactamase

Cefoxitin and other beta-lactam antibiotics with a methoxy group on the alpha-face behave as very poor substrates of the Bacillus licheniformis beta-lactamase. The kinetic properties of the enzyme-cefoxitin system made it theoretically suitable for a detailed structural study of the acyl-enzyme. Unfortunately, soaking the crystals in cefoxitin solution did not allow detection of a crystalline acyl-enzyme complex. In contrast, direct observation by n.m.r. of the stable acyl-enzyme formed with cefoxitin and moxalactam indicated clear modifications of the enzyme structure, which were reflected in the aromatic and high-field methyl regions of the spectrum. The return to the initial free enzyme spectrum was concomitant with the hydrolysis of the acyl-enzyme, the process being slow enough to allow multidimensional n.m.r. experiments.

Substrate-induced inactivation of the OXA2 beta-lactamase

The hydrolysis time courses of 22 beta-lactam antibiotics by the class D OXA2 beta-lactamase were studied. Among these, only three appeared to correspond to the integrated Henri-Michaelis equation. 'Burst' kinetics, implying branched pathways, were observed with most penicillins, cephalosporins and with flomoxef and imipenem. Kinetic parameters characteristic of the different phases of the hydrolysis were determined for some substrates. Mechanisms generally accepted to explain such reversible partial inactivations involving branches at either the free enzyme or the acyl-enzyme were inadequate to explain the enzyme behaviour. The hydrolysis of imipenem was characterized by the occurrence of two 'bursts', and that of nitrocefin by a partial substrate-induced inactivation complicated by a competitive inhibition by the hydrolysis product.

Interactions between active-site-serine beta-lactamases and compounds bearing a methoxy side chain on the alpha-face of the beta-lactam ring: kinetic and molecular modelling studies

The interactions between three class A beta-lactamases and compounds bearing a methoxy side chain on the alpha-face of the beta-lactam ring (cefoxitin, moxalactam and temocillin) have been studied. When compared with the situation prevailing with good substrates, both acylation and deacylation steps appeared to be severely impaired. Molecular modelling studies of the structures of the Henri-Michaelis complexes and of the acyl-enzymes indicate a major displacement of the crystallographically observed water molecule which connects the glutamate-166 and serine-70 side chains and underline the role of this water molecule in both reaction steps.

Irreversible inactivation of beta-lactamase I from Bacillus cereus by chlorinated 6-spiroepoxypenicillins

On incubation of the chlorinated 6-spiroepoxypenicillin anilides (I) and (II) [formula: see text] with beta-lactamase 1 from Bacillus cereus, three distinct processes are observed. The inhibitors act as (a) substrates, the turnover of which respectively results in a single product, namely 6-substituted 2(H)-3,4-dihydro-1,4-thiazine, (b) a transiently inhibited enzyme complex, and finally (c) an irreversibly inactivated enzyme complex. Although differing only in their stereochemistry at one centre, the anilide (K) is a more potent irreversible inactivator of beta-lactamase I than is compound (II). Analysis of irreversibly inactivated beta-lactamase I by isoelectric focusing and inspection of peptide fragmentation maps indicated that irreversible inactivation appears to be accompanied by covalent modification. These studies reveal that the chlorinated 6-spiroepoxypenicillin anilide (I) is a mechanism-based beta-lactamase inhibitor.

Specific beta-lactam antibiotics inhibit secretion of lipo-beta-lactamase in yeast

The beta-lactam antibiotic cloxacillin can inhibit secretion of prokaryotic lipo-beta-lactamase into the periplasm of yeast. The results indicate that this phenomenon is specific with respect to both the antibiotic and the lipo-beta-lactamase whose secretion is affected, strongly suggesting that this involves an interaction between the enzyme and its substrates. The effect of the antibiotic on secretion is reversible. With different beta-lactam antibiotics, the clearest difference is observed between type A and type S penicillins; the former exert a strong inhibition of secretion whereas the latter exhibit a weak effect or no effect at all. Type A penicillins have been previously shown to cause a conformational change in various beta-lactamases. Mature lipo-beta-lactamase species in yeast were localized either to the periplasmic space or bound to the outer surface of the cytoplasmic membrane and thus exposed to periplasm. The results are consistent with the hypothesis that binding of cloxacillin to lipo-beta-lactamase induces a conformation on the protein that is unfavourable for its release from the membrane.

Selection and application of antibodies modifying the function of beta-lactamase

Nine monoclonal antibodies directed against class A beta-lactamases were detected and selected by a novel screening procedure based on assaying the modifications in the catalytic and stability properties of beta-lactamase in solution. Unlike conventional screening, e.g., ELISA or immunoprecipitation, the present method does not depend on firm binding and thus favors detection of low affinity antibodies. Individual antibodies were found to affect the enzymatic activity in various ways including stimulation, neutralization, protection and stabilization. Class A beta-lactamases show only 20% among members of this class. In contrast, two of our monoclonal antibodies cross-reacted with different beta-lactamases and thus demonstrate the presence of shared structural epitopes in this class of enzymes. One of the cross-reacting antibodies was elicited by sequential immunization with two different beta-lactamases. Taken together, our findings stress the importance of the screening method in antibody selection and illustrate the use of 'functional' monoclonal antibodies in the study of the structure-function relationship in an enzyme.

Isolation of a Staphylococcus aureus beta-lactamase-dicloxacillin complex and kinetic studies on the reactivation of the enzyme

Exposure of the beta-lactamase from Staphylococcus aureus to the slowly reacting substrates cloxacillin or dicloxacillin results in time-dependent inactivation of the enzyme. Methods for the rapid separation of a beta-lactamase-dicloxacillin complex from excess inhibitor, using centrifuged columns of Sephadex G-25 or DEAE-Sephadex G-25, are described. The enzyme-dicloxacillin complex releases active enzyme, with specific activity identical to that of untreated enzyme, after storage at pH 7.5 at 15 degrees C. Full reactivation was accompanied by the release of 0.8 eq of hydrolyzed dicloxacillin. The complex is stable for up to 40 h when stored at pH 3 at 4 degrees C. The reactivation process, which occurs with first-order kinetics at 15 degrees C and pH values between 4 and 8, displays a pH dependence with apparent pKa's of 4.6 and 8.5, and a limiting value of the reactivation rate constant of 0.022 min-1. Deviation from first-order kinetics at pH 9 is consistent with a competing irreversible inactivation of the enzyme at that pH. This behavior differs substantially from that of the similarly inactivated beta-lactamase I from Bacillus cereus, whose rate of reactivation is independent of pH, but which undergoes irreversible denaturation at acidic pH [A. L. Fink, K. M. Behner, and A. K. Tan (1987) Biochemistry 26, 4248-4258]. Addition of hydroxylamine to the S. aureus beta-lactamase-dicloxacillin, complex stimulates the rate of reactivation by a maximum of 35%. This effect is hyperbolically dependent on the concentration of hydroxylamine with half-maximal stimulation at 2.8 mM. The Km for ampicillin hydrolysis catalyzed by the partially reactivated enzyme is identical to that measured for catalysis by the untreated enzyme. We discuss our observations in relation to models for the transient inhibition process.

Induction of SOS functions by alkaline intracellular pH in Escherichia coli

Alkalinization of intracellular pH (pHi) causes an increase in UV resistance in wild-type and pH-sensitive mutant (DZ3) cells of Escherichia coli. Utilizing cells transformed with a plasmid (pA7) which bears the uvrA promoter fused to galK galactokinase structural gene, it was shown that alkaline pHi leads to an increase in the specific activity of galactokinase. This effect was not displayed in a mutant bearing a recA-insensitive lexA gene, nor in cells harboring a plasmid (pA8) in which the galK is fused to a lexA-insensitive uvrA promoter. Hence, the effects of pHi on cells functions may involve the lexA product of the SOS system.

Mechanism of inhibition of RTEM-2 beta-lactamase by cephamycins: relative importance of the 7 alpha-methoxy group and the 3' leaving group

Cefoxitin is a poor substrate of many beta-lactamases, including the RTEM-2 enzyme. Fisher and co-workers [Fisher, J., Belasco, J. G., Khosla, S., & Knowles, J. R. (1980) Biochemistry 19, 2895-2901] showed that the reaction between cefoxitin and RTEM-2 beta-lactamase yielded a moderately stable acyl-enzyme whose hydrolysis was rate-determining to turnover at saturation. The present work shows first that the covalently bound substrate in this acyl-enzyme has a 5-exo-methylene-1,3-thiazine structure, i.e., that the good (carbamoyloxy) 3' leaving group of cefoxitin has been eliminated in formation of the acyl-enzyme. Such an elimination has recently been shown in another case to yield an acyl-beta-lactamase inert to hydrolysis [Faraci, W. S., & Pratt, R. F. (1985) Biochemistry 24, 903-910]. Thus the cefoxitin molecule has two potential sources of beta-lactamase resistance, the 7 alpha-methoxy group and the good 3' leaving group. That the latter is important in the present example is shown by the fact that with analogous substrates where no elimination occurs at the enzyme active site, such as 3'-de(carbamoyloxy)cefoxitin and 3'-decarbamoylcefoxitin, no inert acyl-enzyme accumulates. An analysis of the relevant rate constants shows that the 7 alpha-methoxy group weakens noncovalent binding and slows down both acylation and deacylation rates, but with major effect in the acylation rate, while elimination of the 3' leaving group affects deacylation only.(ABSTRACT TRUNCATED AT 250 WORDS)