A structure-based analysis of the inhibition of class A beta-lactamases by sulbactam

7-alkylidenecephalosporin esters as inhibitors of human leukocyte elastase

A series of 7-alkylidenecephalosporins and 7-vinylidenecephalosporins, as their benzhydryl esters, have been tested as inhibitors of both porcine pancreatic elastase and human leukocyte elastase. Selected 7-alkylidenecephalosporin esters are found to be potent inhibitors of HLE. One category of new inhibitors is the 7-(haloalkylidene)cephalosporins. In contrast to previously reported cephalosporin-based elastase inhibitors, these haloalkylidene cephems show optimum inhibitory activity as sulfides, rather than as sulfones. They are efficient and irreversible inhibitors. A second class of active compounds is represented by the benzhydryl ester 7-(cyanomethylidene)cephalosporin sulfone. In contrast to the activity of these new inhibitors, the benzhydryl ester of the mechanism-based beta-lactamase inhibitor, 7-[(2'-pyridyl)methylidene]-cephalosporin sulfone showed little inhibitory activity as an elastase inhibitor. 7-Vinylidenecephalosporins were also relatively poor inhibitors, although the terminally unsubstituted allene sulfide showed activity as an inhibitor of PPE. A modeling analysis suggests the 7-alkylidene substituents can be readily accommodated in the S1 pocket. A potential mechanism of inhibition is proposed.

Construction and characterization of an OHIO-1 beta-lactamase bearing Met69Ile and Gly238Ser mutations

Amino acid changes that influence activity and resistance to beta-lactams and beta-lactamase inhibitors were explored by constructing the Gly238Ser and Met69Ile-Gly238Ser mutants of the OHIO-1 beta-lactamase, a class A enzyme of the SHV family. The Km values of cefotaxime and ceftazidime for OHIO-1 and Met69Ile beta-lactamases were > or = 500 microM. The Km of cefotaxime for the Gly238Ser beta-lactamase was 26 microM, and that of ceftazidime was 105 microM. The Km of cefotaxime for the Met69Ile-Gly238Ser beta-lactamase was 292 microM, and that of ceftazidime was 392 microM. For the beta-lactamase inhibitors clavulanate and sulbactam, the apparent Ki values for the Met69Ile-Gly238Ser enzyme were 0.03 and 0.15 microM, respectively. Relative Vmax values indicate that the Met69Ile-Gly238Ser mutant of the OHIO-1 beta-lactamase possesses cephalosporinase activity similar to that of the Gly238Ser mutant but diminished penicillinase activity. In an Escherichia coli DH5alpha strain that possesses a Met69Ile beta-lactamase of the OHIO-1 family, the added Gly238Ser mutation resulted in a phenotype with qualities that confer resistance to expanded-spectrum cephalosporins and, to a lesser extent, beta-lactamase inhibitors.

Variability of chromosomally encoded beta-lactamases from Klebsiella oxytoca

The beta-lactamase genes of Klebsiella oxytoca were previously divided into two main groups: bla(OXY-1) and bla(OXY-2). The two beta-lactamase groups were each represented by beta-lactamases with four different pIs. In each group, one form of beta-lactamase is more frequent than the others combined. The beta-lactamase gene of each representative beta-lactamase with a different pI that was not yet sequenced (pIs 5.7, 6.8 [OXY-2], 7.1, 8.2, and 8.8 [OXY-1]) was cloned and sequenced. The susceptibility patterns as well as relative rates and kinetic parameters for beta-lactam hydrolysis revealed that OXY-2 enzymes hydrolyzed several of the beta-lactams that were examined (carbenicillin, cephalothin, cefamandole, ceftriaxone, and aztreonam) at a greater rate than the OXY-1 enzymes did. Comparison of K. oxytoca beta-lactamases with plasmid-mediated extended-spectrum beta-lactamases MEN-1 and TOHO-1 implied that the threonine at position 168 present in OXY-2 beta-lactamase instead of the alanine in OXY-1 could be responsible for its modified substrate hydrolysis. In each group, the beta-lactamase with a variant pI differs from the main form of beta-lactamase by one to five amino acid substitutions. The substrate profile and the 50% inhibitory concentrations revealed that all substitutions differing from the main form of beta-lactamase were neutral except one difference in the OXY-1 group. This substitution of an Ala to a Gly at position 237 increases the hydrolysis of some beta-lactams, particularly aztreonam; decreases the hydrolysis of benzylpenicillin, cephaloridine, and cefamandole, and decreases the susceptibility to clavulanic acid (fivefold increase in the 50% inhibitory concentration).

Structure of an enzyme required for aminoglycoside antibiotic resistance reveals homology to eukaryotic protein kinases

Bacterial resistance to aminoglycoside antibiotics is almost exclusively accomplished through either phosphorylation, adenylylation, or acetylation of the antibacterial agent. The aminoglycoside kinase, APH(3')-IIIa, catalyzes the phosphorylation of a broad spectrum of aminoglycoside antibiotics. The crystal structure of this enzyme complexed with ADP was determined at 2.2 A. resolution. The three-dimensional fold of APH(3')-IIIa reveals a striking similarity to eukaryotic protein kinases despite a virtually complete lack of sequence homology. Nearly half of the APH(3')-IIIa sequence adopts a conformation identical to that seen in these kinases. Substantial differences are found in the location and conformation of residues presumably responsible for second-substrate specificity. These results indicate that APH(3') enzymes and eukaryotic-type protein kinases share a common ancestor.

Emergence of an inhibitor-resistant beta-lactamase (SHV-10) derived from an SHV-5 variant

An inhibitor-resistant beta-lactamase (SHV-10), derived from an SHV-5 variant (SHV-9), was found in an Escherichia coli clinical isolate. In SHV-10, Ser-130 was replaced by Gly. The enzyme partially retained its ability to hydrolyze penicillins, but its activity against cephalosporins was drastically reduced. A Ser-130-->Gly mutant of the prototype SHV-5, obtained by site-directed mutagenesis, had properties similar to those of SHV-10.

Design, synthesis, and evaluation of 2 beta-alkenyl penam sulfone acids as inhibitors of beta-lactamases

A general method for synthesis of 2 beta-alkenyl penam sulfones has been developed. The new compounds inhibited most of the common types of beta-lactamase. The level of activity depended very strongly on the nature of the substituent in the 2 beta-alkenyl group. The inhibited species formed with the beta-lactamase from Citrobacter freundii 1205 was sufficiently stable for X-ray crystallographic studies. These, together with UV absorption spectroscopy and studies of chemical degradation, suggested a novel reaction mechanism for the new inhibitors that might account for their broad spectrum of action. The (Z)-2 beta-acrylonitrile penam sulfone Ro 48-1220 was the most active inhibitor from this class of compound. The inhibitor enhanced the action of, for example, ceftriaxone against a broad selection of organisms producing beta-lactamases. The organisms included strains of Enterobacteriaceae that produce cephalosporinases, which is an exceptional activity for penam sulfones.

Cloning and sequence of the gene encoding a cefotaxime-hydrolyzing class A beta-lactamase isolated from Escherichia coli

Escherichia coli TUH12191, which is resistant to piperacillin, cefazolin, cefotiam, ceftizoxime, cefuzonam, and aztreonam but is susceptible to cefoxitin, latamoxef, flomoxef, and imipenem, was isolated from the urine of a patient treated with beta-lactam antibiotics. The beta-lactamase (Toho-1) purified from the bacteria had a pI of 7.8, had a molecular weight of about 29,000, and hydrolyzed beta-lactam antibiotics such as penicillin G, ampicillin, oxacillin, carbenicillin, piperacillin, cephalothin, cefoxitin, cefotaxime, ceftazidime, and aztreonam. Toho-1 was markedly inhibited by beta-lactamase inhibitors such as clavulanic acid and tazobactam. Resistance to beta-lactams, streptomycin, spectinomycin, sulfamethoxazole, and trimethoprim was transferred by conjugational transfer from E. coli TUH12191 to E. coli ML4903, and the transferred plasmid was about 58 kbp, belonging to incompatibility group M. The cefotaxime resistance gene for Toho-1 was subcloned from the 58-kbp plasmid by transformation of E. coli MV1184. The sequence of the gene for Toho-1 was determined, and the open reading frame of the gene consisted of 873 or 876 bases (initial sequence, ATGATG). The nucleotide sequence of the gene (DDBJ accession number D37830) was found to be about 73% homologous to the sequence of the gene encoding a class A beta-lactamase produced by Klebsiella oxytoca E23004. According to the amino acid sequence deduced from the DNA sequence, the precursor consisted of 290 or 291 amino acid residues, which contained amino acid motifs common to class A beta-lactamases (70SXXK, 130SDN, and 234KTG). Toho-1 was about 83% homologous to the beta-lactamase mediated by the chromosome of K. oxytoca D488 and the beta-lactamase mediated by the plasmid of E. coli MEN-1. Therefore, the newly isolated beta-lactamase Toho-1 produced by E. coli TUH12191 is similar to beta-lactamases produced by K. oxytoca D488, K. oxytoca E23004, and E. coli MEN-1 rather than to mutants of TEM or SHV enzymes. Toho-1 has shown the highest degree of similarity to K. oxytoca class A beta-lactamase. Detailed comparison of Toho-1 with other beta-lactamases implied that replacement of Asn-276 by Arg with the concomitant substitution of Thr for Arg-244 is an important mutation in the extension of the substrate specificity.

beta-Lactamase mutations far from the active site influence inhibitor binding

Analysis of the three dimensional structure of the class A beta-lactamases shows that Arg-244, a spatially conserved residue important for inactivation by clavulanic acid, is held in place by a hydrogen (H) bond from the residue at 276. An Asn276-Gly mutant of OHIO-1, an SHV family class A enzyme, was constructed to investigate the importance of that interaction. Compared to a strain expressing the wild type enzyme, OHIO-1, the MIC of the Asn276-Gly mutant strain was more resistant to clavulanate (0.25 vs. 2.0 micrograms/ml) in the presence of ampicillin (16 micrograms/ml) but was as susceptible to sulbactam or tazobactam plus ampicillin as the OHIO-1 bearing strain. No difference in MICs was observed when other beta-lactams were tested. Consistent with the susceptibility test results, the apparent Ki of clavulanate for the Asn276-Gly enzyme (4.5 microM) was 10-fold greater than OHIO-1 (0.4 microM). For sulbactam and tazobactam the apparent Ki decreased for Asn276-Gly enzyme (1.0 and 0.1 micrograms/ml, respectively) compared to the wild-type parent (17 and 0.7 micrograms/ml, respectively). Comparing the Asn276-Gly heta-lactamase with OHIO-1, the Vmax for most substrates except cephaloridine did not change substantially. There was a 2-15 fold decreased affinity (Km) and catalytic efficiency (Vmax/Km) for beta-lactam substrates. These data support the observation and emphasize the role for this H bonding residue in orienting Arg-244 towards the active site.