An assay for exogenous sources of purified MurG, enabled by the complementation of Escherichia coli murG(Ts) by the Mycobacterium tuberculosis homologue.

The Mycobacterium tuberculosis murG gene, Rv2153, was expressed in Escherichia coli murG(Ts) strain OV58 on a plasmid under the control of the arabinose-inducible araBAD promoter. Mycobacterium tuberculosis murG rescued the growth of E. coli murG(Ts) at the nonpermissive temperature: transformants were only obtained in the presence of 0.2% arabinose at 42 °C, and their growth rate was dependent on arabinose concentrations. However, no MurG activity was detected in membranes from the transformant grown in arabinose at 42 °C, while MraY activity was normal. This observation led to the development of a membrane-based scintillation proximity assay for exogenous sources of MurG. Addition of purified E. coli MurG resulted in the reconstitution of MurG and peptidoglycan synthesis in these membranes. MurG is an attractive target for drug discovery, but assays to measure the activity of purified MurG are challenging. This presents an easy method to measure the activity of exogenous sources of MurG for structure-activity studies of mutant MurG proteins. It can also be used to compare the activity of, or effect of inhibitors on, MurG from other bacterial species.

Screening, identification, and characterization of mechanistically diverse inhibitors of the Mycobacterium tuberculosis enzyme, pantothenate kinase (CoaA).

The authors describe the discovery of anti-mycobacterial compounds through identifying mechanistically diverse inhibitors of the essential Mycobacterium tuberculosis (Mtb) enzyme, pantothenate kinase (CoaA). Target-driven drug discovery technologies often work with purified enzymes, and inhibitors thus discovered may not optimally inhibit the form of the target enzyme predominant in the bacterial cell or may not be available at the desired concentration. Therefore, in addition to addressing entry or efflux issues, inhibitors with diverse mechanisms of inhibition (MoI) could be prioritized before hit-to-lead optimization. The authors describe a high-throughput assay based on protein thermal melting to screen large numbers of compounds for hits with diverse MoI. Following high-throughput screening for Mtb CoaA enzyme inhibitors, a concentration-dependent increase in protein thermal stability was used to identify true binders, and the degree of enhancement or reduction in thermal stability in the presence of substrate was used to classify inhibitors as competitive or non/uncompetitive. The thermal shift-based MoI assay could be adapted to screen hundreds of compounds in a single experiment as compared to traditional biochemical approaches for MoI determination. This MoI was confirmed through mechanistic studies that estimated K(ie) and K(ies) for representative compounds and through nuclear magnetic resonance-based ligand displacement assays.

Glycine and alanine dehydrogenase activities are catalyzed by the same protein in Mycobacterium smegmatis: upregulation of both activities under microaerophilic adaptation.

Microaerophilic adaptation has been described as one of the in vitro dormancy models for tuberculosis. Studies on Mycobacterium tuberculosis adapted to low oxygen levels showed an enhancement of glycine dehydrogenase (deaminating) activity. We studied the physiology of the fast-growing, nonpathogenic strain of Mycobacterium smegmatis ATCC 607 under low oxygen by shifting the actively growing M. smegmatis cells to static microaerophilic growth conditions. This shifting of M. smegmatis culture resulted in a similar phenomenon as seen with M. tuberculosis, i.e., elevated glycine dehydrogenase activity. Further purification of glycine dehydrogenase from M. smegmatis demonstrated glyoxylate amination, but failed to demonstrate glycine deamination, even in the purified fraction. Moreover, the purified protein showed pyruvate amination as well as L-alanine deamination activities. By activity staining, the protein band positive for glyoxylate amination demonstrated only pyruvate amination in the presence of NAD. Absence of glycine deamination activity strongly suggested that alanine dehydrogenase of M. smegmatis was responsible for glyoxylate amination in the cell lysate. This was further confirmed by demonstrating the similar level of upregulation of both glyoxylate and pyruvate amination activities in the cell lysate of the adapted culture.