A detailed understanding of the catalytic mechanism of enzyme drug targets is required to guide structure-based drug discovery. In my thesis research I have primarily focused on the catalytic mechanism and inhibition of MenE the OSB-CoA synthetase from three different organisms: M. tuberculosis, S. aureus and E. coli. MenE catalyzes the formation of OSB-CoA through a two step reaction that proceeds via acyl adenylate intermediate and utilizes ATP and CoA as cofactors. In chapter 2, I discuss the catalytic mechanism of MenE from S. aureus (saMenE) where steady state kinetics coupled with site-directed mutagenesis has been used to identify key catalytic residues in the active site of MenE and to elucidate the molecular details of each half reaction. In chapter 3, I present data on the inhibition of saMenE as well as the enzymes from M. tuberculosis (tbMenE) and E. coli (ecMenE). These studies have focused on a series of OSB-AMP intermediate analogs synthesized by our collaborator Dr. Tan at Memorial Sloan-Kettering Cancer Center. The most potent compound inhibits MenE with a Kid value of 11.2ñ0.9 nM. While the majority of my work has focused on MenE, I have also undertaken kinetic studies of the charismata-dependent enzymes Men from E. coli and MbtI from M. tuberculosis (chapter 4). These enzymes catalyze the initial reaction in menaquinone biosynthetic pathway and mycobactin biosynthetic pathway, respectively. Interestingly, in the absence of Mg2+, MenF and MbtI show chorismate mutase activity, although these enzymes have no similarity to the AroQ/H chorismate mutase. Because tbEntC, which is annotated as isochorismate synthase in M. tuberculosis menaquinone pathway, cannot yet be well expressed by using a heterologous E. coli expression system, I propose that MbtI might have a dual function and produce isochorismate for menaquinone in vitro.