Ebselen: a covalent inhibitor of the Antigen 85 complex from Mycobacterium tuberculosis

Abstract

The World Health Organization (WHO) reported 8.7 million new cases of tuberculosis (TB) and 1.4 million deaths in 2011. Co‐infection with Human Immunodeficiency Virus (HIV), as well as the appearance of multidrug and extensively drug‐resistance tuberculosis (MDR‐TB or XDR‐TB), stress the need of discovering new drugs to treat TB. The antigen 85 complex (Ag85) of Mycobacterium tuberculosis (M. tb) includes three mycolyltransferases: Ag85A, Ag85B and Ag85C, involved in the cell wall biosynthesis. The enzymes catalyze the transfer of mycolic acids to trehalose monomycolate (TMM) producing the cord factor trehalose dimycolate (TDM) through a transesterification reaction. The Ag85 complex appears as a valuable drug target to treat TB as inactivation of one of the enzymes affects mycobacterial cell growth in the host; cell wall organization is disrupted and an increase in sensitivity to drugs targeting the cell wall biosynthesis is observed. This study aims to improve methods for inhibitor screening using Ag85C. We report the development of a new high‐throughput assay using resorufin butyrate as an acyl donor. This fluorescence‐based assay provided a Z′ value of 0.82, validating the assay for inhibitor screening. The screening of the NIH clinical collection identified ebselen as a potent inhibitor of M. tb Ag85C. Initial dose‐dependence studies suggested an inhibitory activity in the nanomolar range. However, characterization using mass spectrometry and X‐ray crystallography, gave further insight into the mechanism of action; ebselen binds covalently to the lone cysteine located near the active site of Ag85C, locking the enzyme in an inactive conformation.Funding from the National Institutes of Health (R15AI089653–01) supported this work.