Catalytic and conformational properties of cross-linked derivatives of penicillinase

Calorimetric analysis of cephalosporins using an immobilized TEM-1 beta-lactamase on Ni2+ chelating sepharose fast flow

Two beta-lactamases, penicillinase type I from Bacillus cereus and TEM-1 beta-lactamase from Haemophilus ducreyi, were immobilized on a Chelating Sepharose Fast Flow column loaded with Ni2+ in an active form. Flow-injection analysis of beta-lactams was performed by using an enzyme column reactor fitted into the enzyme thermistor. With both enzymes it was possible to monitor both penicillins and cephalosporins. Moreover, Michaelis constants of the TEM-1 beta-lactamase were markedly increased upon immobilization for all substrates, especially carbenicillin, cephaloridine, and cefoperazone.

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.

Site-directed mutagenesis of beta-lactamase I. Single and double mutants of Glu-166 and Lys-73

Two single mutants and the corresponding double mutant of beta-lactamase I from Bacillus cereus 569/H were constructed and their kinetics investigated. The mutants have Lys-73 replaced by arginine (K73R), or Glu-166 replaced by aspartic acid (E166D), or both (K73R + E166D). All four rate constants in the acyl-enzyme mechanism were determined for the E166D mutant by the methods described by Christensen, Martin & Waley [(1990) Biochem. J. 266, 853-861]. Both the rate constants for acylation and deacylation for the hydrolysis of benzylpenicillin were decreased about 2000-fold in this mutant. In the K73R mutant, and in the double mutant, the rate constants for acylation were decreased about 100-fold and 10,000-fold respectively. All three mutants also had lowered values for the rate constants for the formation and dissociation of the non-covalent enzyme-substrate complex. The specificities of the mutants did not differ greatly from those of wild-type beta-lactamase, but the hydrolysis of cephalosporin C by the K73R mutant gave 'burst' kinetics.

Imipenem as substrate and inhibitor of beta-lactamases

The interaction between imipenem, a carbapenem antibiotic, and two representative beta-lactamases has been studied. The first enzyme was beta-lactamase I, a class-A beta-lactamase from Bacillus cereus; imipenem behaved as a slow substrate (kcat. 6.7 min-1, Km 0.4 mM at 30 degrees C and at pH 7) that reacted by a branched pathway. There was transient formation of an altered species formed in a reversible reaction; this species was probably an acyl-enzyme in a slightly altered, but considerably more labile, conformation. The kinetics of the reaction were investigated by measuring both the concentration of the substrate and the activity of the enzyme, which fell and then rose again more slowly. The second enzyme was the chromosomal class-C beta-lactamase from Pseudomonas aeruginosa; imipenem was a substrate with a low kcat. (0.8 min-1) and a low Km (0.7 microM). Possible implications for the clinical use of imipenem are considered.

The Molecular Basis of β-Lactamase Catalysis and Inhibition

The β-lactamases catalyze the hydrolysis of the lactam bond in β-lactams, thus rendering the β-lactam ineffective as an antibiotic. The increasing spread of resistance to β-lactam antibiotics is largely due to this class of enzyme. Mechanistically these enzymes appear to be related to the transpeptidases and carboxypeptidases involved in the synthesis of the bacterial cell wall. Interest in the basic mechanism of action of the β-lactamases has been spurred by the potential for mechanism-based drug design. The past seven years have seen a significant increase in our knowledge of the catalysis and inhibition of the β-lactamases. The presence of an essential, conserved, serine residue which participates in the formation of a covalent acyl-enzyme intermediate in catalysis, inhibition and inactivation by β-lactams has been established. Unfortunately, few additional details regarding the catalytic mechanism are well established. A generalized reaction pathway can be formulated for most β-lactam inhibitors (reversible or irreversible). This scheme involves partitioning of the initially-formed acyl-enzyme by three pathways: 1) hydrolysis leading to turnover, 2) transient inhibition probably involving formation of an imine or enamine acyl-enzyme, or possibly involving a substantial conformational change in some cases, and 3) imine formation followed by additional covalent modification of the enzyme leading to irreversible inactivation. The flux through each of these pathways varies with the nature of the "substrate" and the particular β-lactamase.

Conformational adaptation of RTEM beta-lactamase to cefoxitin

Cefoxitin, a poor substrate of the RTEM beta-lactamase (penicillin amido-beta-lactam hydrolase, EC 3.5.2.6), induces a reversible change in the conformation of the enzyme. The change is manifested in gradual loss of catalytic activity and increased susceptibility to proteolytic inactivation. It is prevented by antibodies, which stabilize the native conformation. By contrast, divalent cations, which have no effect on the native enzyme, delay recovery from the cefoxitin-induced state, presumably by reacting with sites made accessible in the partly unfolded enzyme. Prolonged exposure to excess of cefoxitin causes a similar delay. The kinetic evidence, namely, the initial burst of consumption of cefoxitin and the subsequent gradual recovery of activity with better substrates, appears to be consistent with acylation of the active site by cefoxitin followed by a slower deacylation step [Fisher et al. (1980) Biochemistry 19, 2895-2901]. However, additional evidence leads us to conclude that the kinetics observed reflect deformation of the active site, rather than its blockage, by cefoxitin. Of most significance is the transient change in specificity, i. e. a preferential interaction of the recovering enzyme with substrates which are closest in structure to cefoxitin.

Mechanism of protein hydrophobic chromatography. Protein unfolding and its contribution to effective hydrophobicity

Several different monomeric proteins with either one or two domains were used to study the effect of protein unfolding on effective hydrophobicity of proteins. Protein unfolding was inhibited by cross-linking with either toluene di-isocyanate or glutaraldehyde. The native enzyme was much more readily bound to the hydrophobic resin than the cross-linked species. The ligand, 3-phosphoglycerate, significantly decreased the amount of phosphoglycerate kinase able to bind to octyl-Sepharose. Sucrose also caused a statistically significant decrease in protein binding to the hydrophobic resin. These studies support the concept that the degree of protein unfolding influences effective hydrophobicity as measured by retention on octyl-Sepharose.

Thermal denaturation of native and cross-linked Bacillus cereus 569/H beta-lactamase I

Thermal denaturation of native and internally cross-linked Bacillus cereus 569/H beta-lactamase I (beta-lactamhydrolase, EC 3.5.2.6) was investigated using differential scanning calorimetry. Application of temperature-scanning kinetics provided an estimate of various activation parameters for the denaturation process. Evidence is presented to indicate that subtle temperature-induced conformational changes preceding gross denaturation are sufficient to cause inactivation of the enzyme.

Modification of the conformational equilibria in the sodium and potassium dependent adenosinetriphosphatase with glutaraldehyde

Glutaraldehyde treatment of electroplax membrane preparations of Na,K-ATPase leads to irreversible changes in the enzymic behavior of the protein, which are not due to modification of the active site. When the glutaraldehyde treatment is carried out in a medium containing K+ and without Na+, the "K+-modified enzyme" so produced shows the following changes in enzymic properties: The steady-state phosphorylation by ATP and the rate of ATP-ADP exchange are decreased to approximately 40% of control, while Na,K-ATPase activity decreases to approximately 15% of control. Phosphatase activity is decreased very little, but the potassium activation parameters of the reaction are changed, from K0.5 approximately equal to 5 mM and nH = 1.9 in control to K0.5 approximately equal to 0.5 mM and nH = 1 in K+-modified enzyme. KI(app) for nucleotide inhibition of phosphatase activity is increased significantly. Changes in the cation dependence of the ATPase reaction are also observed. All of these effects can be explained by assuming that the cross-linking of surface groups in protein subunits when they are in conformation E2 shifts the intrinsic conformational equilibrium of the enzyme toward E2. We considered the simplest mathematical model for the coupling between K+ binding and the conformational equilibrium, with equivalent potassium sites that must be simultaneously in the same state. If one assumes that the potassium activation of phosphatase activity in the K+-modified enzyme reflects the affinity for K+ of E2, the behavior of the phosphatase activity in the native enzyme can be fit if there are only two potassium sites, whose affinity is 80-fold higher in E2 than in E1, and the equilibrium constant for E2 in equilibrium E1 is about 250. The same sites can explain the activation of dephosphorylation during ATP hydrolysis. Independent of the model chosen, potassium ions must be required for the catalytic action of form E2 and cannot be merely "allosteric activators". The enzyme modified with glutaraldehyde in a medium containing Na+ also has interesting properties, but their rationalization is less straightforward. The Na,K-ATPase activity is inhibited more than the "partial reactions", as in the K+-modified enzyme. We suggest that this is a generally expected result of modifications of the enzyme.

Interaction of the pBR 322-coded RTEM beta-lactamase with substrates. Evidence for specific conformational transitions

The rate of inactivation of RTEM-1 beta-lactamase by Pronase is accelerated by class A ('resistant') penicillins. Other substrates (class S penicillin and cephalosporins) protect against the inactivation. Cefoxitin, a semi-synthetic cephamycin, induces a more extensive, hysteretic response. In its presence the enzyme is inactivated by trypsin as well as by Pronase.

Mechanism of substrate-induced inactivation of beta-lactamase I

beta-Lactamase I (from Bacillus cereus 569/H) is inactivated by certain substrates (e.g. methicillin or cloxacillin) but not by others (e.g. benzylpenicillin). Emzyme that had been inactivated was found to be labelled stoichiometrically, as shown by the use of radioactive methicillin. Use of the penamaldate reaction showed the presence of a penicilloyl group in the enzyme inactivated by either methicillin or cloxacillin. In conditions under which enzymic activity was regained the penicilloyl group was shed. When the activity of beta-lactamase I was measured in 0.3-1.2 M guanidinium chloride the rates of hydrolysis of methicillin or cloxacillin (but not benzylpenicillin) were greatly reduced. The unliganded enzyme was stable. The results are explained by supposing that a normal intermediate, the acyl enzyme, is prone to unfold.

Cross-linking preserves conformational changes induced in penicillinase by its substrates

Exopenicillinase of Bacillus cereus 569/H was cross-linked with toluene 2,4-diisocyanate in the presence of cephalothin, cloxacillin or no substrate. The derivatives show significant differences in susceptibility to inactivation by heat, urea, iodination or proteolysis. Such differences can be predicted from the contrasting effects of these substrates on the conformation of the enzyme.