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D-alanyl-D-alanine carboxypeptidase in the bacterial form and L-form of Proteus mirabilis
Abstract:
Membranes of the bacterial form and the stable and unstable L-forms of Proteus mirabilis contain LD and DD-carboxypeptidase. The DD-carboxypeptidase is inhibited non-competitively by penicillin G. The enzyme of the bacterial form is highly penicillin-sensitive (Ki - 4 X 10(-9) M penicillin G). Inhibition is only partly reversible by treatment with penicillinase or by dialysis against buffer. In contrast, the DD-carboxypeptidase of the unstable L-form, grown in the presence of penicillin, is 175-fold less penicillin-sensitive (Ki = 7 X 10(7) M penicillin G). Inhibition is completely reversed by penicillinase or dialysis. After inhibition by penicillin and subsequent reactivation the penicillin sensitivity of the bacterial DD-carboxtpeptidase is similar to the sensitivity of the enzyme of the unstable L-form. The hypothesis is proposed that P. mirabilis contains two DD-carboxypeptidases of different penicillin sensitivity and with different mechanisms of penicillin binding. Peptidoglycan synthesis in the cell walls of the unstable L-form is probably carried out with the help of only one DD-carboxypeptidase, viz. the completely reactivatable enzyme with the lower penicillin sensitivity.
Insights
Proteus mirabilis bacteria possess two DD-carboxypeptidases with varying penicillin sensitivity. The unstable L-form utilizes a less sensitive enzyme for peptidoglycan synthesis, crucial for bacterial cell wall formation.
Area of Science:
- Microbiology
- Biochemistry
- Molecular Biology
Background:
- Proteus mirabilis exists in bacterial and L-form states, including stable and unstable variants.
- Cell wall synthesis, particularly peptidoglycan, is essential for bacterial viability and is a target for antibiotics like penicillin.
Purpose of the Study:
- To investigate the characteristics of DD-carboxypeptidases in bacterial and L-form Proteus mirabilis.
- To determine the differential sensitivity and penicillin-binding mechanisms of these enzymes.
Main Methods:
- Enzyme assays were performed on membranes from bacterial and L-form Proteus mirabilis.
- Penicillin G inhibition kinetics were analyzed, including non-competitive inhibition.
- Reversibility of inhibition was tested using penicillinase and dialysis.
Main Results:
- Both bacterial and L-form Proteus mirabilis membranes contain LD and DD-carboxypeptidases.
- Bacterial DD-carboxypeptidase shows high penicillin sensitivity (Ki = 4 x 10^-9 M) with partially reversible inhibition.
- Unstable L-form DD-carboxypeptidase exhibits significantly lower sensitivity (Ki = 7 x 10^-7 M) with fully reversible inhibition.
Conclusions:
- Proteus mirabilis likely possesses two distinct DD-carboxypeptidases with differing penicillin sensitivities and binding mechanisms.
- The unstable L-form appears to rely on the less penicillin-sensitive DD-carboxypeptidase for peptidoglycan synthesis.