Membrane-bound DD-carboxypeptidases from Bacillus megaterium KM general properties, substrate specificity and sensitivity to penicillins, cephalosporins and peptide inhibitors of the activity at pH5

The interaction of nocardicin A with the penicillin-binding proteins of Bacillus megaterium KM

The inhibition of elongation of Bacillus megaterium KM growing in the presence of low concentrations of nocardicin A resulted in the production of osmotically stable, actively dividing coccal-shaped cells. Saturation of penicillin-binding proteins 3a and 3b with nocardicin A in vivo at these concentrations was correlated with the inhibition of cell elongation. Analysis of the DD-carboxypeptidase activity of isolated vegetative membranes of B. megaterium KM in vitro indicated that penicillin-binding protein 4 is not a DD-carboxypeptidase under the assay conditions used. Penicillin-binding proteins were analysed by two-dimensional gel electrophoresis and the suitability of lysozyme treatment of cells as a method of membrane preparation was investigated with regard to the detection of proteins with highly labile penicillin-binding activities in vitro.

Identification of penicillin-binding protein 5a of Bacillus megaterium KM as a DD-carboxypeptidase

Measurement of the stabilities of DD-carboxypeptidase activity and the penicillin-binding activity of proteins 5 and 5a in membranes isolated from vegetative cells and stage-V forespores suggests that the unique sporulation-specific protein 5a may be a penicillin-sensitive DD-carboxypeptidase.

Inhibition of Bacillus subtilis aminopeptidase D by beta-lactam antibiotics

Penicillins and cephalosporins inhibit the hydrolysis of D-alanyl-beta-naphthylamide by aminopeptidase D (EC 3.4.11.-) from Bacillus subtilis. The inhibition is predominantly non-competitive, although the more effective inhibitors, i.e., those with the lower Ki values, seem to exhibit a tendency toward competitive kinetics, while penicillin V, the antibiotic with the largest Ki, exhibits a strong tendency toward uncompetitive kinetics. The removal of antibiotic from the enzyme by gel filtration on Sephadex G-25 restores 100% activity to the enzyme, and suggests that the inhibition does not derive from a covalent antibiotic-enzyme complex. The antibiotics which have been studied, with their respective Ki values (mM) and inhibition type are: methicillin (1.01 +/- 0.34, mixed non-competitive-competitive); cephaloridine (2.43 +/- 0.17, non-competitive); cloxacillin (3.19 +/- 1.29, mixed non-competitive-competitive); oxacillin (6.88 +/- 0.77, non-competitive); cephalothin (8.12 +/- 0.99, non-competitive); penicillin G (20.0 +/- 3.21, non-competitive) and penicillin V (32.1 +/- 4.90, mixed non-competitive-uncompetitive). An empirical correlation exists between the Ki value and the freedom of rotation about the bond between the phenyl ring and the atom alpha to the ring in the variable side chain portion of the penicillin molecule.

Membrane-bound DD-carboxypeptidase and transpeptidase activities from Bacillus megaterium KM at pH 7. General properties, substrate specificity and inhibition by beta-lactam antibiotics

1. The membranes from Bacillus megaterium KM contained a DD-carboxypeptidase with optimum activity under the following conditions: pH 7; ionic strength, 1.3 M; temperature, 40 degrees C and below 20 degrees C. It did not require any divalent cation, but was inactivated by Cu2+ and Hg2+. It was stimulated by 2-mercaptoethanol and low concentrations of p-chloromercuribenzoate. 2. The membrane preparation also catalyzed a simple transpeptidation reaction using as carboxyl acceptors D-alanine or glycine. 3. The conditions for optimum activity, temperature-inactivation, temperature-dependence of the activity, carboxyl donor specificity, sensitivity to beta-lactam antibiotics, and insensitivity to potential peptide inhibitors of both enzyme activities, was identical. The DD-carboxypeptidase showed inhibition by D-alanine and Ac2-L-Lys-D-Ala. 4. The inhibition by beta-lactam antibiotic was reversible for both enzymic activities and the time-dependence for their recovery was identical. 5. The DD-carboxypeptidase was very sensitive to changes in the configuration and size of the side-chains of the C-terminal dipeptide of the substrate. Amino acid residues at the C-terminus that precluded the peptide from being a DD-carboxypeptidase substrate were not acceptors in the transpeptidation reaction. Dipeptides were not acceptors for the 'model transpeptidase'. 6. It is suggested that both activities are catalysed by the same enzyme molecule, whose physiological role is not the formation of peptide crosslinks during peptidoglycan biosynthesis.