Positional isotope exchange analysis of the Mycobacterium smegmatis cysteine ligase (MshC)

MshC catalyzes the ATP-dependent condensation of GlcN-Ins and cysteine to form Cys-GlcN-Ins, which is an intermediate in the biosynthetic pathway of mycothiol, i.e., 1-D-myo-inosityl-2-(N-acetyl-L-cysteinyl)amido-2-deoxy-alpha-D-glucopyranoside (MSH or AcCys-GlcN-Ins). MSH is produced by Mycobacterium tuberculosis, members of the Actinomycetes family, to maintain an intracellular reducing environment and protect against oxidative and antibiotic induced stress. The biosynthesis of MSH is essential for cell growth, and therefore, the MSH biosynthetic enzymes present potential targets for inhibitor design. The formation of kinetically competent adenylated intermediates was suggested by the observation of positional isotope exchange (PIX) reaction using [betagamma-(18)O6]-ATP in the presence of cysteine. The PIX rate depends on the presence of cysteine and increases with concentrations of cysteine. The loss of PIX activity upon the addition of small concentrations of pyrophosphatase suggests that the PP(i) is free to dissociate from the active site of cysteine ligase into the bulk solution. The PIX activity is also eliminated at high concentrations of GlcN-Ins, consistent with the mechanism in which GlcN-Ins binds after cysteine-adenylate formation. This PIX analysis confirms that MshC catalyzes the formation of a kinetically competent cysteinyl-adenylate intermediate after the addition of ATP and cysteine.