Allosteric cooperation in ß-lactam binding to a non-classical transpeptidase

Uptake of Biomimetic Nanovesicles by Granuloma for Photodynamic Therapy of Tuberculosis

The antimicrobial resistance of Mycobacterium tuberculosis (M. tuberculosis) is a challenge in the antibiotic treatment of tuberculosis (TB). Herein, we aimed to examine a photodynamic therapy for TB that has a low risk of drug resistance and involves biomimetic macrophage membranes combined with a photosensitizer, chlorin e6 (Ce6; hereinafter, C-MV). We used Mycobacterium marinum (M. marinum), a waterborne pathogen closely related to M. tuberculosis, which causes TB-like infections in ectotherms but not in humans. The mouse tail granuloma model induced by M. marinum is a relatively mature TB model developed by our team. C-MV nanoparticles were prepared and injected intravenously, showing longevity in circulation due to the properties of the macrophage membrane, which protects them from being eliminated from the blood. They were then guided to tuberculous granulomas, helping deliver precise photodynamic therapy. Ce6 is a classical photosensitizer that triggers the production of reactive oxygen species under laser irradiation, causing M. marinum death. The C-MV nanoparticles showed good compatibility and a long circulation time, effectively inhibiting the proliferation and infiltration of M. marinum, providing a new paradigm for TB treatment.

Biochemical and crystallographic studies of L,D-transpeptidase 2 from Mycobacterium tuberculosis with its natural monomer substrate

The essential L,D-transpeptidase of Mycobacterium tuberculosis (LdtMt2) catalyses the formation of 3 → 3 cross-links in cell wall peptidoglycan and is a target for development of antituberculosis therapeutics. Efforts to inhibit LdtMt2 have been hampered by lack of knowledge of how it binds its substrate. To address this gap, we optimised the isolation of natural disaccharide tetrapeptide monomers from the Corynebacterium jeikeium bacterial cell wall through overproduction of the peptidoglycan sacculus. The tetrapeptides were used in binding / turnover assays and biophysical studies on LdtMt2. We determined a crystal structure of wild-type LdtMt2 reacted with its natural substrate, the tetrapeptide monomer of the peptidoglycan layer. This structure shows formation of a thioester linking the catalytic cysteine and the donor substrate, reflecting an intermediate in the transpeptidase reaction; it informs on the mode of entrance of the donor substrate into the LdtMt2 active site. The results will be useful in design of LdtMt2 inhibitors, including those based on substrate binding interactions, a strategy successfully employed for other nucleophilic cysteine enzymes.

The l,d-Transpeptidase LdtAb from Acinetobacter baumannii Is Poorly Inhibited by Carbapenems and Has a Unique Structural Architecture

l,d-Transpeptidases (LDTs) are enzymes that catalyze reactions essential for biogenesis of the bacterial cell wall, including formation of 3-3 cross-linked peptidoglycan. Unlike the historically well-known bacterial transpeptidases, the penicillin-binding proteins (PBPs), LDTs are resistant to inhibition by the majority of β-lactam antibiotics, with the exception of carbapenems and penems, allowing bacteria to survive in the presence of these drugs. Here we report characterization of LdtAb from the clinically important pathogen, Acinetobacter baumannii. We show that A. baumannii survives inactivation of LdtAb alone or in combination with PBP1b or PBP2, while simultaneous inactivation of LdtAb and PBP1a is lethal. Minimal inhibitory concentrations (MICs) of all 13 β-lactam antibiotics tested decreased 2- to 8-fold for the LdtAb deletion mutant, while further decreases were seen for both double mutants, with the largest, synergistic effect observed for the LdtAb + PBP2 deletion mutant. Mass spectrometry experiments showed that LdtAb forms complexes in vitro only with carbapenems. However, the acylation rate of these antibiotics is very slow, with the reaction taking longer than four hours to complete. Our X-ray crystallographic studies revealed that LdtAb has a unique structural architecture and is the only known LDT to have two different peptidoglycan-binding domains.