Fragment screening against an essential metabolic enzyme in Mycobacterium tuberculosis, DAH7PS

Abstract

Tuberculosis (TB) claims one of the highest annual mortality rates among all infectious diseases. The rapidly increasing prevalence of drug resistant TB and limited treatment options available necessitate urgent identification of new potent drug targets in Mycobacterium tuberculosis and development of new, more potent antimycobacterial agents. DAH7PS is an essential enzyme in M. tuberculosis, involved in a metabolic pathway responsible for the biosynthesis of the precursor for a wide variety of essential aromatic compounds, including the aromatic amino acids, aromatic cofactors and an essential siderophore. One of the emerging approaches towards new drug development is called fragment screening. This method involves structure determination of the potential drug target, the screening of small chemical fragments against it to identify the binding sites of those with binding affinity and subsequent inhibitor design based on the fragments identified. This research used fragment screening as an efficient approach to identify and characterise small ligands that bind to DAH7PS, in order to aid in the collaborative development of potent inhibitors against it. Differential scanning fluorimetry (DSF) was used for the initial screening of fragments for binding. This was followed by isothermal titration calorimetry (ITC) to measure the binding affinities and rank the fragments identified, and X-ray crystallography to structurally determine the binding interactions. This fragmentbased drug development exploits the value of protein structural information in facilitating the development of new antimycobacterial agents. The initial screening against 352 fragments using DSF obtained sharply contrasting responses of DAH7PS to the aromatic fragments, compared with the aromatic amino acids, which are the native ligands of the enzyme. All fragments gave weak negative thermal shifts, whereas the aromatic amino acids gave strong positive thermal shifts. These strongly contrasting results led to propose that the allosteric regulatory binding sites of DAH7PS, where the aromatic amino acids bind, are of exceptional binding specificity, with an assumption that the aromatic fragments should have given some hits of similar thermal stability responses with the aromatic amino acids, given their close similarity. The limited diversity of the Zenobia fragment library proved to be unsuitable for fragment binding to DAH7PS.