Differential flap dynamics in L,D-transpeptidase2 from mycobacterium tuberculosis revealed by molecular dynamics

Despite the advances in tuberculosis treatment, TB is still one the most deadly infectious diseases and remains a major global health quandary. Mycobacterium tuberculosis (Mtb) is the only known mycobacterium with a high content of 3 -> 3 crosslinks in the biosynthesis of peptidoglycan, which is negligible in most bacterial species. An Mtb lacking Ldt(Mt2) leads to alteration of the colony morphology and loss of virulence which makes this enzyme an attractive target. Regardless of the vital role of Ldt(Mt2) for cell wall survival, the impact of ligand binding on the dynamics of the beta-hairpin flap is still unknown. Understanding the structural and dynamical behaviour of the flap regions provides clear insight into the design of the effective inhibitors against Ldt(Mt2). Carbapenems, an specific class of beta-lactam family, have been shown to inactivate this enzyme. Herein a comprehensive investigation of the flap dynamics of Ldt(Mt2) complex with substrate and three carbapenems namely, ertapenem, imipenem and meropenem is discussed and analyzed for the first account using 140 ns molecular dynamics simulations. The structural features (RMSD, RMSF and R-g) derived by MD trajectories were analyzed. Distance analysis, particularly tip-tip SER135-ASN167 index, identified conformational changes in terms of flap opening and closure within binding process. Principal component analysis (PCA) was employed to qualitatively understand the divergent effects of different inhibitors on the dominant motion of each residue. To probe different internal dynamics induced by ligand binding, dynamic cross-correlation marix (DCCM) analysis was used. The binding free energies of the selected complexes were assessed using MM-GBSA method and per residue free energy decomposition analysis were performed to characterize the contribution of the key residues to the total binding free energies.