The interconnected roles of residues N189, E168, and S220 in M. tuberculosis Indole-3 glycerol phosphate synthase catalysis

Tuberculosis (TB) is an infectious disease caused by the bacteria Mycobacterium tuberculosis that primarily attacks the lungs. Drug resistance for M. tuberculosis is emerging and therefore new drug targets need to be identified. One way to target TB is through the inhibition of the tryptophan biosynthetic pathway of M. tuberculosis since production of tryptophan is essential for the replication of M. tuberculosis. The enzyme Indole-3-glycerol phosphate synthase (MtIGPS) was chosen as a potential drug target due to its role in tryptophan biosynthesis. Residue N189 interacts with active site residues E168 and S220. It is inferred from previous studies done with IGPS from Sulfolobus solfataricus that E168 is important for catalysis and S220 for substrate binding. In this study, the residue N189 in MtIGPS was mutated to a variety of other amino acids. The effects were studied through steady-state kinetics and rate-pH profiles to elucidate the role of N189 in the interactions of active site residues E168 and S220. It is possible that N189 plays a role in catalysis through its proximity to E168, which is suspected to play a role in the MtIGPS mechanism. It was found that mutation of N189 to Q, D or K has a drastic impact on MtIGPS catalytic activity, indicating that N189 is catalytically important. Once the role of N189 and the other residues is better understood, then inhibitors for IGPS can be designed.