The effect of the replacement of E57N on MtIGPS catalysis

Mycobacterium tuberculosis is the causative agent of one of the oldest known diseases, Tuberculosis (TB), infecting nine million people and causing two million deaths worldwide each year. The efficacy of established antibiotic drugs used to treat the disease has diminished due to the increasing presence of multi-drug resistant strains of M. tuberculosis and this calls for the urgent development of a new drug. Previous literature suggests the enzyme indole-3-glycerol phosphate synthase (IGPS) in M. tuberculosis may be a potential target for new anti-TB drugs. To obtain more information on M. tuberculosis IGPS (MtIGPS), mutations in its active site were investigated. Wildtype MtIGPS and active site mutant E57N were expressed and purified, and their kinetic parameters and rate-pH profiles were analyzed. The mutant E57N revealed a K_M value of 15.7 ± 3.4 mM. This was compared to the wildtype MtIGPS K_M of 6.2 ± 2.2 mM. Reduced catalytic activity was shown in E57N compared to wildtype MtIGPS. The decreased catalytic activity of the mutant E57N indicates that E57 is catalytically important in the MtIGPS mechanism. We used this data to understand the role of the E to N mutation at the amino acid residue position 57 in MtIGPS catalysis.