Synthesis, purification and kinetic properties of fluorescein-labelled penicillins

The synthesis and properties of six fluorescein-labelled penicillins are reported. The two isomers of fluoresceyl-glycyl-6-amino-penicillanic acid are probably the best compounds to use for detection of all the penicillin-binding proteins (PBPs) present in a bacterial membrane preparation. However, the derivatives of ampicillin were much more efficient against Enterobacter aerogenes PBP3. The two isomers obtained when a commercial mixture of the two isomers of carboxyfluorescein was used most often exhibited similar properties, but the Streptomyces R61 extracellular DD-peptidase was only efficiently acylated by the 5'-carboxyfluorescein derivative of glycyl-6-aminopenicillanic acid.

A class-A beta-lactamase from Pseudomonas stutzeri that is highly active against monobactams and cefotaxime

A beta-lactamase produced by Pseudomonas stutzeri was purified to protein homogeneity, and its physicochemical and catalytic properties were determined. Its profile was unusual since, in addition to penicillins, the enzyme hydrolysed second- and third-generation 'beta-lactamase-stable' cephalosporins and monobactams with similar efficiencies. On the basis of the characteristics of the interaction with beta-iodopenicillanic acid, the enzyme could be classified as a class-A beta-lactamase. However, when compared with most class-A beta-lactamases, it exhibited significantly lower kcat./Km values for the compounds usually considered to be the best substrates of these enzymes.

A new, highly sensitive method for the detection and quantification of penicillin-binding proteins

A new method for the identification and quantification of penicillin-binding proteins is described which uses fluorescein-coupled penicillins. It allows the rapid detection of 0.2 pmol with the naked eye and 2 fmol with the help of an A.L.F. automatic DNA sequencer. Direct labelling can also be performed on whole bacterial cells.

Acyltransferase activities of the high-molecular-mass essential penicillin-binding proteins

The high-molecular-mass penicillin-binding proteins (HMM-PBPs), present in the cytoplasmic membranes of all eubacteria, are involved in important physiological events such as cell elongation, septation or shape determination. Up to now it has, however, been very difficult or impossible to study the catalytic properties of the HMM-PBPs in vitro. With simple substrates, we could demonstrate that several of these proteins could catalyse the hydrolysis of some thioesters or the transfer of their acyl moiety on the amino group of a suitable acceptor nucleophile. Many of the acyl-donor substrates were hippuric acid or benzoyl-D-alanine derivatives, and their spectroscopic properties enabled a direct monitoring of the enzymic reaction. In their presence, the binding of radioactive penicillin to the PBPs was also inhibited.

Coordinate regulation of beta-lactamase induction and peptidoglycan composition by the amp operon

The amp operon, which is located on the Escherichia coli chromosome, modulates the induction of plasmid-borne beta-lactamase genes by extracellular beta-lactam antibiotics. This suggests that the gene products AmpD and AmpE may function in the transduction of external signals. beta-Lactam antibiotics are analogs of cell wall components that can be released during cell wall morphogenesis of enterobacteria. The amp operon was studied to determine its importance in signal transduction during cell wall morphogenesis. The peptidoglycan compositions of amp mutants were determined by high-performance liquid chromatography and fast atom bombardment mass spectrometry. When a chromosomal or plasmid-borne copy of ampD was present, the amount of pentapeptide-containing muropeptides in the cell wall increased upon addition of the cell wall constituent diaminopimelic acid to the growth medium. These results suggest that beta-lactamase induction and modulation of the composition of the cell wall share elements of a regulatory circuit that involves AmpD. Escherichia coli requires AmpD to respond to extracellular signaling amino acids, such as diaminopimelic acid, and this signal transduction system may regulate peptidoglycan composition in response to cell wall turnover products.

Signalling proteins in enterobacterial AmpC beta-lactamase regulation

The cloned Citrobacter freundii ampC beta-lactamase is inducible in the presence of its regulatory gene ampR in Escherichia coli (Lindberg et al., 1985). The basal level of expression and inducibility are affected by two E. coli proteins encoded by the closely linked ampD and ampE genes. Deletion of both genes led to constitutive ampR-dependent overproduction of beta-lactamase, whereas an out-of-frame deletion in AmpD caused the basal expression to increase two-fold. This ampD1 mutant was inducible at lower beta-lactam concentrations than the wild type. An IS1 insertion in ampD was polar on ampE expression and increased basal beta-lactamase expression 30-fold while mediating a semi-constitutive phenotype. AmpE expressed from a recombinant plasmid in an ampD-ampE deletion mutant reduced basal beta-lactamase expression to wild-type levels but did not markedly reduce beta-lactam resistance since the cells became hyperinducible. In the absence of AmpD, increasing levels of AmpE therefore decrease the basal expression of AmpC beta-lactamase in an AmpR-dependent manner. AmpD modulated the response exerted on beta-lactamase expression by AmpE. The ampD gene encodes a 20.5kD cytoplasmic protein while the 32.1kD ampE gene product is an integral membrane protein with a likely ATP-binding site between the second and third putative transmembrane region. Since neither AmpD nor AmpE are needed for beta-lactam induction and since these proteins could not be covalently labelled by benzylpenicillin, they are not thought to act as beta-lactam-binding sensory transducers. Instead it is suggested that AmpD and AmpE sense the effect of beta-lactam action on peptidoglycan biosynthesis and relay this signal to AmpR.

A survey of the kinetic parameters of class C beta-lactamases. Cephalosporins and other beta-lactam compounds

Various cephalosporins, cefoxitin, moxalactam, imipenem and aztreonam were studied as substrates of six class C beta-lactamases. Nitrocefin, cephaloridine, cefazolin, cephalothin and cephalexin were good substrates, with kcat. values ranging from 27 to 5000 s-1. Cefuroxime, cefotaxime and cefoxitin exhibited low kcat. values (0.010-1.7 s-1) and low Km values, which suggested a rate-limiting deacylation. Imipenem and aztreonam were even poorer substrates (kcat. 2 x 10(-4)-3 x 10(-2) s-1) and, in the presence of a reporter substrate, behaved as transient inactivators. With moxalactam, biphasic kinetics were observed, indicating a possible rearrangement of the acyl-enzyme.

A survey of the kinetic parameters of class C beta-lactamases. Penicillins

The interaction between six class C beta-lactamases and various penicillins has been studied. All the enzymes behaved in a very uniform manner. Benzylpenicillin exhibited relatively low kcat. values (14-75 s-1) but low values of Km resulted in high catalytic efficiencies [kcat./Km = 10 X 10(6)-75 X 10(6) M-1.s-1]. The kcat. values for ampicillin were 10-100-fold lower. Carbenicillin, oxacillin cloxacillin and methicillin were very poor substrates, exhibiting kcat. values between 1 x 10(-3) and 0.1 s-1. The Km values were correspondingly small. It could safely be hypothesized that, with all the tested substrates, deacylation was rate-limiting, resulting in acyl-enzyme accumulation.

Chromosome-encoded beta-lactamases of Citrobacter diversus. Interaction with beta-iodopenicillanate and labelling of the active site

Both forms of the chromosome-encoded beta-lactamase of Citrobacter diversus react with beta-iodopenicillanate at a rate characteristic of class A beta-lactamases. The active site of form I was labelled with the same reagent. The sequence of the peptide obtained after trypsin hydrolysis is identical with that of a peptide obtained in a similar manner from the chromosome-encoded beta-lactamase of Klebsiella pneumoniae.

Citrobacter diversus ULA-27 beta-lactamases. Improved purification and general properties

Two chromosome-encoded beta-lactamases have been purified from Citrobacter diversus ULA-27. They exhibited slightly different isoelectric points (6.8 and 6.2) and very similar Mr values (congruent to 29,000). Their specificity spectrum was rather wide, since they hydrolysed some cephalosporins with kcat: values similar to those observed with the best penicillin substrates. Cloxacillin, methicillin and imipenem were hydrolysed very slowly. Hydrolysis of azthreonam could not be detected.

Sequence and comparative analysis of three Enterobacter cloacae ampC beta-lactamase genes and their products

The sequences of three Enterobacter cloacae ampC beta-lactamase genes have been determined. The deduced amino acid sequences are very similar: out of a total of 361 residues, only eight positions were found to be variable, and several mutations yielded residues with very similar properties. The kinetic properties of two of the enzymes were not significantly different. The three enzymes also exhibited a high degree of homology (greater than 70%) with the ampC beta-lactamases of Escherichia coli K12 and Citrobacter freundii, confirming the homogeneity of class-C beta-lactamases.

The K1 beta-lactamase of Klebsiella pneumoniae

beta-Lactamase K1 was purified from Klebsiella pneumoniae SC10436. It is very similar to the enzyme produced by Klebsiella aerogenes 1082E and described by Emanuel, Gagnon & Waley [Biochem. J. (1986) 234, 343-347]. An active-site peptide was isolated after labelling of the enzyme with tritiated beta-iodopenicillanate. A cysteine residue was found just before the active-site serine residue. This result could explain the properties of the enzyme after modification by thiol-blocking reagents. The sequence of the active-site peptide clearly established the enzyme as a class A beta-lactamase.

Properties of a class C beta-lactamase from Serratia marcescens

A beta-lactamase produced by a penicillin-resistant strain of Serratia marcescens was isolated and purified. The kcat. value for benzylpenicillin was about 5% of that observed for the best cephalosporin substrates. However, the low Km of the penam resulted in a high catalytic efficiency (kcat./Km) and the classification of the enzyme as a cephalosporinase might not be completely justified. It also exhibited a low but measurable activity against cefotaxime, cefuroxime, cefoxitin and moxalactam. Substrate-induced inactivation was observed both with a very good (cephalothin) or a very bad (moxalactam) substrate. The active site was labelled by beta-iodopenicillanate. Trypsin digestion produced a 19-residue active-site peptide whose sequence clearly allowed the classification of the enzyme as a class C beta-lactamase.