Systematic mutagenesis of the active site omega loop of TEM-1 beta-lactamase.

Beta-Lactamase is a bacterial protein that provides resistance against beta-lactam antibiotics. TEM-1 beta-lactamase is the most prevalent plasmid-mediated beta-lactamase in gram-negative bacteria. Normally, this enzyme has high levels of hydrolytic activity for penicillins, but mutant beta-lactamases have evolved with activity toward a variety of beta-lactam antibiotics. It has been shown that active site substitutions are responsible for changes in the substrate specificity. Since mutant beta-lactamases pose a serious threat to antimicrobial therapy, the mechanisms by which mutations can alter the substrate specificity of TEM-1 beta-lactamase are of interest. Previously, screens of random libraries encompassing 31 of 55 active site amino acid positions enabled the identification of the residues responsible for maintaining the substrate specificity of TEM-1 beta-lactamase. In addition to substitutions found in clinical isolates, many other specificity-altering mutations were also identified. Interestingly, many nonspecific substitutions in the N-terminal half of the active site omega loop were found to increase ceftazidime hydrolytic activity and decrease ampicillin hydrolytic activity. To complete the active sight study, eight additional random libraries were constructed and screened for specificity-altering mutations. All additional substitutions found to alter the substrate specificity were located in the C-terminal half of the active site loop. These mutants, much like the N-terminal omega loop mutants, appear to be less stable than the wild-type enzyme. Further analysis of a 165-YYG-167 triple mutant, selected for high levels of ceftazidime hydrolytic activity, provides an example of the correlation which exists between enzyme instability and increased ceftazidime hydrolytic activity in the ceftazidime-selected omega loop mutants.     

NMR investigation of Tyr105 mutants in TEM-1 beta-lactamase: dynamics are correlated with function

The existence of coupled residue motions on various time scales in enzymes is now well accepted, and their detailed characterization has become an essential element in understanding the role of dynamics in catalysis. To this day, a handful of enzyme systems has been shown to rely on essential residue motions for catalysis, but the generality of such phenomena remains to be elucidated. Using NMR spectroscopy, we investigated the electronic and dynamic effects of several mutations at position 105 in TEM-1 beta-lactamase, an enzyme responsible for antibiotic resistance. Even in absence of substrate, our results show that the number and magnitude of short and long range effects on (1)H-(15)N chemical shifts are correlated with the catalytic efficiencies of the various Y105X mutants investigated. In addition, (15)N relaxation experiments on mutant Y105D show that several active-site residues of TEM-1 display significantly altered motions on both picosecond-nanosecond and microsecond-millisecond time scales despite many being far away from the site of mutation. The altered motions among various active-site residues in mutant Y105D may account for the observed decrease in catalytic efficiency, therefore suggesting that short and long range residue motions could play an important catalytic role in TEM-1 beta-lactamase. These results support previous observations suggesting that internal motions play a role in promoting protein function.     

Identification of amino acid substitutions that alter the substrate specificity of TEM-1 beta-lactamase.

TEM-1 beta-lactamase is the most prevalent plasmid-mediated beta-lactamase in gram-negative bacteria. Recently, TEM beta-lactamase variants with amino acid substitutions in the active-site pocket of the enzyme have been identified in natural isolates with increased resistance to extended-spectrum cephalosporins. To identify other amino acid substitutions that alter the activity of TEM-1 towards extended-spectrum cephalosporins, we probed regions around the active-site pocket by random-replacement mutagenesis. This mutagenesis technique involves randomizing the DNA sequence of three to six codons in the blaTEM-1 gene to form a library containing all or nearly all of the possible substitutions for the region randomized. In total, 20 different residue positions that had been randomized were screened for amino acid substitutions that increased enzyme activity towards the extended-spectrum cephalosporin cefotaxime. Substitutions at positions 104, 168, and 238 in the TEM-1 beta-lactamase that resulted in increased enzyme activity towards extended-spectrum cephalosporins were found. In addition, small deletions in the loop containing residues 166 to 170 drastically altered the substrate specificity of the enzyme by increasing activity towards extended-spectrum cephalosporins while virtually eliminating activity towards ampicillin.     

Crystal structure and kinetic analysis of beta-lactamase inhibitor protein-II in complex with TEM-1 beta-lactamase

The structure of the 28 kDa beta-lactamase inhibitor protein-II (BLIP-II) in complex with the TEM-1 beta-lactamase has been determined to 2.3 A resolution. BLIP-II is a secreted protein produced by the soil bacterium Streptomyces exfoliatus SMF19 and is able to bind and inhibit TEM-1 with subnanomolar affinity. BLIP-II is a seven-bladed beta-propeller with a unique blade motif consisting of only three antiparallel beta-strands. The overall fold is highly similar to the core structure of the human regulator of chromosome condensation (RCC1). Although BLIP-II does not share the same fold with BLIP, the first beta-lactamase inhibitor protein for which structural data was available, a comparison of the two complexes reveals a number of similarities and provides further insights into key components of the TEM-1-BLIP and TEM-1-BLIP-II interfaces. Our preliminary results from gene knock-out studies and scanning electron microscopy also reveal a critical role of BLIP-II in sporulation.     

Site-saturation mutagenesis and three-dimensional modelling of ROB-1 define a substrate binding role of Ser130 in class A beta-lactamases.

Site-saturation mutagenesis was performed on the class A ROB-1 beta-lactamase at conserved Ser130, which is centrally located in the antibiotic binding site where it can participate in both protein-protein and protein-substrate hydrogen bonding. Mutation Thr130 gave a beta-lactamase hydrolysing penicillins and cephalosporins but which showed a 3-fold lower affinity (Km) for ampicillin and cephalexin, and a 30-fold lower hydrolytic (Vmax) activity for ampicillin. In contrast, the hydrolytic activity for cephalexin was similar to the wild-type for the Thr130 mutation. Mutation Gly130 gave a beta-lactamase hydrolysing only penicillins with an affinity and hydrolysis activity for these compounds approximately 15-fold lower than the wild-type, but no detectable activity against cephalosporins. Mutation Ala130 produced an enzyme capable of hydrolysing penicillins only at a low rate. Modelling the ROB-1 active site was done from the refined 2 A X-ray structure of the homologous Bacillus licheniformis beta-lactamase. Ampicillin and cephalexin were docked into the active site and were energy minimized with the CVFF empirical force field. Dockings were stable only when Ser70 was made anionic and Glu166 was made neutral. Interaction energies and distances were calculated for fully hydrated pre-acylation complexes with the Ser, Thr, Gly and Ala130 enzymes. The catalytic data from all mutations and the computed interactions from modelling confirmed that the Ser130 has a structural as well as a functional role in binding and hydrolysis of penicillins. This highly conserved residue also plays a substrate specificity role by hydrogen binding the carboxylic acid group of cephalosporins more tightly than penicillins.     

Inactivation of TEM-1 beta-lactamase by 6-acetylmethylenepenicillanic acid

6-Acetylmethylenepenicillanic acid is a new kinetically irreversible inhibitor of various beta-lactamases. Interaction between 6-acetylmethylenepenicillanate and purified TEM-1 beta-lactamase during the inactivation process was investigated. 6-Acetylmethylenepenicillanate inhibited the enzyme in a second-order fashion with a rate constant of 0.61 microM-1 . S-1. The apparent inactivation constant decreased in the presence of increasing concentrations of the substrate benzylpenicillin. Native enzyme (pI 5.4) was converted into two inactive forms with pI 5.25 and 5.15, the latter form being transient and readily converted into the more stable form with pI 5.15. Even a 50-fold excess of inhibitor over enzyme did not produce any other inactivated species of the enzyme. All the results obtained suggest that 6-acetylmethylenepenicillanate is a potent irreversible and active-site-directed inhibitor of TEM-1 beta-lactamase.     

Oligonucleotide probes for the detection of TEM-1 and TEM-2 beta-lactamase genes and their transposons

We describe the use of molecular probes to detect the TEM-type beta-lactamase genes. As a general probe, we prepared a 656 base pair restriction fragment, entirely within the TEM structural gene. This probe was specific for the TEM family, hybridizing only with TEM-1 and TEM-2. The TEM-1 and TEM-2 beta-lactamases differ by only one amino acid. We synthesized two oligonucleotides whose central bases correspond to this difference. The use of these oligonucleotides enables us to discriminate between TEM-1 and TEM-2 genes. Using oligonucleotides homologous to parts of Tn3, we also monitored the presence of TnA-like transposons in bacteria harboring different beta-lactamase genes. Only the TEM-1 and TEM-2 genes were found to be on transposons with terminal sequences identical to those of Tn3. All hybridization experiments were performed with both dot-blot and colony-hybridization techniques, and the suitability of these two methods for epidemiological studies is compared.     

Increased folding stability of TEM-1 beta-lactamase by in vitro selection

In vitro selections of stabilized proteins lead to more robust enzymes and, at the same time, yield novel insights into the principles of protein stability. We employed Proside, a method of in vitro selection, to find stabilized variants of TEM-1 β-lactamase from Escherichia coli. Proside links the increased protease resistance of stabilized proteins to the infectivity of a filamentous phage. Several libraries of TEM-1 β-lactamase variants were generated by error-prone PCR, and variants with increased protease resistance were obtained by raising temperature or guanidinium chloride concentration during proteolytic selections. Despite the small size of phage libraries, several strongly stabilizing mutations could be obtained, and a manual combination of the best shifted the profiles for thermal unfolding and temperature-dependent inactivation of β-lactamase by almost 20 °C to a higher temperature. The wild-type protein unfolds in two stages: from the native state via an intermediate of the molten-globule type to the unfolded form. In the course of the selections, the native protein was stabilized by 27 kJ mol^− 1 relative to the intermediate and the cooperativity of unfolding was strongly increased. Three of our stabilizing replacements (M182T, A224V, and R275L) had been identified independently in naturally occurring β-lactamase variants with extended substrate spectrum. In these variants, they acted as global suppressors of destabilizations caused by the mutations in the active site. The comparison between the crystal structure of our best variant and the crystal structure of the wild-type protein indicates that most of the selected mutations optimize helices and their packing. The stabilization by the E147G substitution is remarkable. It removes steric strain that originates from an overly tight packing of two helices in the wild-type protein. Such unfavorable van der Waals repulsions are not easily identified in crystal structures or by computational approaches, but they strongly reduce the conformational stability of a protein.     

Noncovalent interaction energies in covalent complexes: TEM-1 beta-lactamase and beta-lactams

The class A beta-lactamase TEM-1 is a key bacterial resistance enzyme against beta-lactam antibiotics, but little is known about the energetic bases for complementarity between TEM-1 and its inhibitors. Most inhibitors form a covalent adduct with the catalytic Ser70, making the measurement of equilibrium constants, and hence interaction energies, technically difficult. This study evaluates noncovalent interactions within covalent complexes by examining the differential stability of TEM-1 and its inhibitor adducts. The thermal denaturation of TEM-1 follows a two-state, reversible model with a melting temperature (T(m)) of 51.6C and a van't Hoff enthalpy of unfolding (DeltaH(VH)) of 146.2 kcal/mol at pH 7.0. The stability of the enzyme changes on forming an inhibitor adduct. As expected, some inhibitors stabilize TEM-1; transition-state analogues increase the T(m) by up to 3.7C (1.7 kcal/mol). Surprisingly, all beta-lactam covalent acyl--enzyme complexes tested destabilize TEM-1 significantly relative to the apo-enzyme. For instance, the clinically used inhibitor clavulanic acid and the beta-lactamase-resistant beta-lactams moxalactam and imipenem destabilize TEM-1 by over 2.6C (1.2 kcal/mol) in their covalent adducts. Based on the structure of the TEM-1/imipenem complex (Maveyraud et al., J Am Chem Soc 1998;120:9748--52), destabilization by moxalactam and imipenem is thought to be caused by a steric clash between the side-chain of Asn132 and the 6(7)-alpha group of these beta-lactams. To test this hypothesis, the mutant enzyme N132A was made. In contrast with wild-type, the covalent complexes between N132A and both imipenem and moxalactam stabilize the enzyme, consistent with the hypothesis. To investigate the structural bases of this dramatic change in stability, the structure of N132A/imipenem was determined by X-ray crystallography. In the complex with N132A, imipenem adopts a very different conformation from that observed in the wild-type complex, and the putative destabilizing interaction with residue 132 is relieved. Studies of several enzymes suggest that beta-lactams, and covalent inhibitors in general, can have either net favorable or net unfavorable noncovalent interaction energies within the covalent complex. In the case of TEM-1, such unfavorable interactions convert substrate analogues into very effective inhibitors.     

TEM-1 beta-lactamase as a scaffold for protein recognition and assay

A large number of different proteins or protein domains have been investigated as possible scaffolds to engineer antibody-like molecules. We have previously shown that the TEM-1 beta-lactamase can accommodate insertions of random sequences in two loops surrounding its active site without compromising its activity. From the libraries that were generated, active enzymes binding with high affinities to monoclonal antibodies raised against prostate-specific antigen, a protein unrelated to beta-lactamase, could be isolated. Antibody binding was shown to affect markedly the enzyme activity. As a consequence, these enzymes have the potential to be used as signaling molecules in direct or competitive homogeneous immunoassay. Preliminary results showed that beta-lactamase clones binding to streptavidin could also be isolated, indicating that some enzymes in the libraries have the ability to recognize proteins other than antibodies. In this paper, we show that, in addition to beta-lactamases binding to streptavidin, beta-lactamase clones binding to horse spleen ferritin and beta-galactosidase could be isolated. Affinity maturation of a clone binding to ferritin allowed obtaining beta-lactamases with affinities comprised between 10 and 20 nM (Kd) for the protein. Contrary to what was observed for beta-lactamases issued from selections on antibodies, enzyme complexation induced only a modest effect on enzyme activity, in the three cases studied. This kind of enzyme could prove useful in replacement of enzyme-conjugated antibodies in enzyme-linked immunosorbant assays (ELISA) or in other applications that use antibodies conjugated to an enzyme.     

Determinants of binding affinity and specificity for the interaction of TEM-1 and SME-1 beta-lactamase with beta-lactamase inhibitory protein

The hydrolysis of beta-lactam antibiotics by class A beta-lactamases is a common cause of bacterial resistance to these agents. The beta-lactamase inhibitory protein (BLIP) is able to bind and inhibit several class A beta-lactamases, including TEM-1 beta-lactamase and SME-1 beta-lactamase. Although the TEM-1 and SME-1 enzymes share 33% amino acid sequence identity and a similar fold, they differ substantially in surface electrostatic properties and the conformation of a loop-helix region that BLIP binds. Alanine-scanning mutagenesis was performed to identify the residues on BLIP that contribute to its binding affinity for each of these enzymes. The results indicate that the sequence requirements for binding are similar for both enzymes with most of the binding free energy provided by two patches of aromatic residues on the surface of BLIP. Polar residues such as several serines in the interface do not make significant contributions to affinity for either enzyme. In addition, the specificity of binding is significantly altered by mutation of two charged residues, Glu73 and Lys74, that are buried in the structure of the TEM-1.BLIP complex as well as by residues located on two loops that insert into the active site pocket. Based on the results, a E73A/Y50A double mutant was constructed that exhibited a 220,000-fold change in binding specificity for the TEM-1 versus SME-1 enzymes.     

Mechanism of inactivation of TEM-1 beta-lactamase by 6-acetylmethylenepenicillanic acid.

The interaction between 6-acetylmethylenepenicillanic acid (compound Ro 15-1903; AMPA) and TEM-1 beta-lactamase was investigated in order to elucidate the mechanism of action of AMPA. Formation of the enzyme-inhibitor complex (EA) was accompanied by a shift of the absorbance maximum from 292 nm to 303 nm and an increase in the absorption. Regeneration of activity was very slow and incomplete, reaching about one-third of the initial activity after 48 h at 37 degrees C. This behaviour indicated a branched pathway of the decay of the inactivated enzyme. Kinetic and isoelectric-focusing experiments proved this assumption. The first-order constant of regeneration of active enzyme was 6 X 10(-6)-10 X 10(-6) s-1, whereas the rate constant leading to inactive enzyme (EA') was 10 X 10(-6)-15 X 10(-6) s-1 at pH 7.0. Both constants became larger at higher pH. Inactive enzyme (EA') consisted of two major species, with pI 5.36 (EA'1) and 5.30 (EA'2). The former increased at the beginning of incubation but decreased after prolonged incubation. From consideration of these results and previous data [Arisawa & Then (1983) Biochem. J. 209, 609-615], a likely mechanism of inactivation of TEM-1 beta-lactamase by AMPA is discussed.     

Structure guided mutagenesis reveals the substrate determinants of guanine deaminase

Guanine deaminases (GDs) are essential enzymes that regulate the overall nucleobase pool. Since the deamination of guanine to xanthine results in the production of a mutagenic base, these enzymes have evolved to be very specific in nature. Surprisingly, they accept structurally distinct triazine ammeline, an intermediate in the melamine pathway, as one of the moonlighting substrates. Here, by employing NE0047 (a GD from Nitrosomonas europaea), we delineate the nuance in the catalytic mechanism that allows these two distinct substrates to be catalyzed. A combination of enzyme kinetics, X-ray crystallographic, and calorimetric studies reveal that GDs operate via a dual proton shuttle mechanism with two glutamates, E79 and E143, crucial for deamination. Additionally, N66 appears to be central for substrate anchoring and participates in catalysis. The study highlights the importance of closure of the catalytic loop and of maintenance of the hydrophobic core by capping residues like F141 and F48 for the creation of an apt environment for activation of the zinc-assisted catalysis. This study also analyzes evolutionarily distinct GDs and asserts that GDs incorporate subtle variations in the active site architectures while keeping the most critical active site determinants conserved.     

Structural insight into the kinetics and DeltaCp of interactions between TEM-1 beta-lactamase and beta-lactamase inhibitory protein (BLIP)

In a previous study, we examined thermodynamic parameters for 20 alanine mutants in beta-lactamase inhibitory protein (BLIP) for binding to TEM-1 beta-lactamase. Here we have determined the structures of two thermodynamically distinctive complexes of BLIP mutants with TEM-1 beta-lactamase. The complex BLIP Y51A-TEM-1 is a tight binding complex with the most negative binding heat capacity change (DeltaG = approximately -13 kcal mol(-1) and DeltaCp = approximately -0.8 kcal mol(-1) K(-1)) among all of the mutants, whereas BLIP W150A-TEM-1 is a weak complex with one of the least negative binding heat capacity changes (DeltaG = approximately -8.5 kcal mol(-1) and DeltaCp = approximately -0.27 kcal mol(-1) K(-1)). We previously determined that BLIP Tyr51 is a canonical and Trp150 an anti-canonical TEM-1-contact residue, where canonical refers to the alanine substitution resulting in a matched change in the hydrophobicity of binding free energy. Structure determination indicates a rearrangement of the interactions between Asp49 of the W150A BLIP mutant and the catalytic pocket of TEM-1. The Asp49 of W150A moves more than 4 angstroms to form two new hydrogen bonds while losing four original hydrogen bonds. This explains the anti-canonical nature of the Trp150 to alanine substitution, and also reveals a strong long distance coupling between Trp150 and Asp49 of BLIP, because these two residues are more than 25 angstroms apart. Kinetic measurements indicate that the mutations influence the dissociation rate but not the association rate. Further analysis of the structures indicates that an increased number of interface-trapped water molecules correlate with poor interface packing in a mutant. It appears that the increase of interface-trapped water molecules is inversely correlated with negative binding heat capacity changes.     

Active TEM-1 beta-lactamase mutants with random peptides inserted in three contiguous surface loops

Engineering of alternative binding sites on the surface of an enzyme while preserving the enzymatic activity would offer new opportunities for controlling the activity by binding of non-natural ligands. Loops and turns are the natural substructures in which binding sites might be engineered with this purpose. We have genetically inserted random peptide sequences into three relatively rigid and contiguous loops of the TEM-1 beta-lactamase and assessed the tolerance to insertion by the percentage of active mutants. Our results indicate that tolerance to insertion could not be correlated to tolerance to mutagenesis. A turn between two beta-strands bordering the active site was observed to be tolerant to random mutagenesis but not to insertions. Two rigid loops comprising rather well-conserved amino acid residues tolerated insertions, although with some constraints. Insertions between the N-terminal helix and the first beta-strand generated active libraries if cysteine residues were included at both ends of the insert, suggesting the requirement for a stabilizing disulfide bridge. Random sequences were relatively well accommodated within the loop connecting the final beta-strand to the C-terminal helix, particularly if the wild-type residue was retained at one of the loops' end. This suggests two strategies for increasing the percentage of active mutants in insertion libraries. The amino acid distribution in the engineered loops was analyzed and found to be less biased against hydrophobic residues than in natural medium-sized loops. The combination of these activity-selected libraries generated a huge library containing active hybrid enzymes with all three loops modified.     

Site-saturation mutagenesis for β-glucosidase 1 from Aspergillus aculeatus to accelerate the saccharification of alkaline-pretreated bagasse

Applied Microbiology and Biotechnology - Aspergillus aculeatus β-glucosidase 1 (AaBGL1) is one of the best cellobiose hydrolytic enzymes without transglycosylation products, among...     

Site-saturation mutagenesis library construction and screening for specific broad-spectrum single-domain antibodies against multiple Cry1 toxins

Potential ecological environmental and food safety risks of various Cry toxins of Bacillus thuringiensis (Bt) in transgenic food have received gradually increasing attention, which urged to establish an efficient and broad-spectrum detection technology for Cry toxins. Based on the single-domain antibody (sdAb) A8 against Bt Cry1Ab toxin screened from the humanized domain antibody library, the key amino acids of sdAb (A8) binding five kinds of Cry1 toxins were predicted using homology modeling and molecular docking technology, and the results showed that 105th asparagine, 106th arginine, 107th valine, and 114th arginine, respectively, located in heavy-chain complementarity-determining region 3 were common key amino acid sites. Subsequently, site-saturation cooperative mutagenesis of the four key sites was performed using overlap extension PCR, and multiple site-saturation mutagenesis sdAb library with the capacity of 1.2 × 10⁵ colony-forming units (CFU) was successfully constructed. With alternating five Cry1 toxins as coating antigen, two generic sdAbs (2-C1, 2-C9) were screened out from the mutagenesis library, which could detect six kinds of Cry1 toxins at least. Through ELISA analysis, the binding activity of 2-C9 was significantly enhanced, and its OD values versus Cry1Aa, Cry1Ab, Cry1B, Cry1C, and Cry1E increased to 1.34, 1.53, 1.82, 2.39, and 2.7 times, respectively, compared with maternal antibody A8. The IC50 values of 2-C9 against Cry1Aa, Cry1Ab, Cry1B, and Cry1C were lower than that of A8, which showed that the affinity of 2-C9 against Cry1 toxins was enhanced. The results were beneficial to developing high-throughput and high-sensitive immune-detecting technology for Cry toxins.     

The role of OXA-1 beta-lactamase Asp(66) in the stabilization of the active-site carbamate group and in substrate turnover

The OXA-1 beta-lactamase is one of the few class D enzymes that has an aspartate residue at position 66, a position that is proximal to the active-site residue Ser(67). In class A beta-lactamases, such as TEM-1 and SHV-1, residues adjacent to the active-site serine residue play a crucial role in inhibitor resistance and substrate selectivity. To probe the role of Asp(66) in substrate affinity and catalysis, we performed site-saturation mutagenesis at this position. Ampicillin MIC (minimum inhibitory concentration) values for the full set of Asp(66) mutants expressed in Escherichia coli DH10B ranged from < or =8 microg/ml for cysteine, proline and the basic amino acids to > or =256 microg/ml for asparagine, leucine and the wild-type aspartate. Replacement of aspartic acid by asparagine at position 66 also led to a moderate enhancement of extended-spectrum cephalosporin resistance. OXA-1 shares with other class D enzymes a carboxylated residue, Lys(70), that acts as a general base in the catalytic mechanism. The addition of 25 mM bicarbonate to Luria-Bertani-broth agar resulted in a > or =16-fold increase in MICs for most OXA-1 variants with amino acid replacements at position 66 when expressed in E. coli. Because Asp(66) forms hydrogen bonds with several other residues in the OXA-1 active site, we propose that this residue plays a role in stabilizing the CO2 bound to Lys(70) and thereby profoundly affects substrate turnover.     

Site-directed mutagenesis at the active site of Escherichia coli TEM-1 beta-lactamase. Suicide inhibitor-resistant mutants reveal the role of arginine 244 and methionine 69 in catalysis.

Arginine 244 is a highly conserved residue in Class A beta-lactamases, while methionine 69 is not. Informational suppression experiments show that replacement of M69 by a leucine, or that of R244 by most other amino acids lead to clavulanic acid-resistant phenotypes. The arginyl 244 side chain is tightly held in a network of interactions within the active site. Its replacement by a glutamine or a threonine perturbs the enzyme kinetics but to a smaller extent than would have been predicted if it were directly involved in substrate binding. Clavulanic acid and sulbactam still interact specifically with the mutant enzymes but are much less efficiently metabolized. Substitutions at position 244 also unveil interactions between the C6 substituent of substrates and the Asn132/Glu104 region of the active site. Methionine 69 is located in a region of strong structural constraints and presents an unusual conformation. Molecular dynamics simulation showed that its replacement by a leucine does not release the strain in this area and induces only minor structural changes. Accordingly, the kinetic behavior of the mutant is only marginally perturbed, except for suicide inhibitors. Both clavulanic acid and sulbactam are well degraded by the mutant enzyme, while irreversible inactivation is dramatically decreased. The contribution of both residues to catalysis is discussed in the light of the kinetic and structural data.     

Overexpression and biosynthetic deuterium enrichment of TEM-1 beta-lactamase for structural characterization by magnetic resonance methods

An expression system has been developed that allows high levels of production of TEM-1 beta-lactamase with ease of biosynthetic incorporation of nuclear isotopes. The gene for mature TEM-1 beta-lactamase fused to the leader sequence of the ompA protein was subcloned into the pET-24a(+) vector by introduction of an NdeI restriction site at the first codon of the fused genes and transformed into Escherichia coli BL21 (DE3) cells. With protein induction at 25 degrees C supported by LB medium supplemented with osmolytes (300 mM sucrose and 2.5 mM betaine), the extracellular, mature form of wild-type TEM-1 beta-lactamase was recovered at a level of 140 mg/L. The production level of E166N, E240C, E104C, and M272C mutants depended on the mutation but was invariably higher than reported by others for expression systems of the wild-type enzyme. Comparison of different carbon sources on the efficiency of biosynthetic incorporation of covalent deuterium showed maximal (90%) incorporation with minimal medium containing 99% (2)H(2)O and sodium d(3)-acetate (99 atom% (2)H). The yield of deuterium-enriched wild-type enzyme was 80 mg/L with yields for mutants proportionally reduced. The high level of protein deuteration achieved with this system allowed detection of the hyperfine coupling between the paramagnetic nitroxyl group of a spin-labeled penicillin substrate and hydrogens on the penicillin moiety in a cryokinetically isolated acylenzyme reaction intermediate because of the decrease in overlapping resonances of active site residues. The overexpression system is readily adaptable for other target proteins and facilitates studies requiring large quantities of protein in isotopically enriched forms.