The roles of N189 and E168 in M. tuberculosis IGP Synthase Catalysis

Tuberculosis is one of the world’s leading infectious diseases. New strains of drug resistant tuberculosis have emerged, making current treatments virtually ineffective. The bacterium that causes tuberculosis is M. tuberculosis. One enzyme inside this bacteria is IGP Synthase (MtIGPS). When MtIGPS interacts with its substrate (CdRP), it produces IGP, which is an intermediate in the production of tryptophan, an essential amino acid for tuberculosis. We study MtIGPS to learn and understand how it operates and to potentially predict possible inhibitors that would interfere with the interaction between MtIGPS and its substrate. I studied the N189D IGPS, in which the 189th amino acid was mutated from asparagine (N) to aspartic acid (D) changing the neutral residue to be negatively charged. This resulted in N189D IGPS having 3,300-fold lower catalytic activity than the natural enzyme. N189 is also located next to glutamic acid 168 (E168), which bears a negative charge. It is possible that the new negative charge on N189D repels E168 and that this repulsion is related to the large decrease in catalytic activity. Now, I am studying the double mutant E168D/E219Q, in which residue 168 keeps its negative charge but is shorter and residue 219 loses its negative charge. It is possible that E168D/E219Q IGPS will not have a repulsion between residues 168 and 219 and this could stabilize the structure, either increasing or decreasing activity. Understanding the roles of electrostatic interactions in the active site will provide a deeper understanding of MtIGPS catalysis and substrate binding.