The Mycobacterium tuberculosis protein tyrosine phosphatase MptpA features a pH dependent activity overlapping the bacterium sensitivity to acidic conditions.

The Mycobacterium tuberculosis low-molecular weight protein tyrosine phosphatase (MptpA) is responsible for the inhibition of phagosome-lysosome fusion and is essential for the bacterium pathogenicity. This inhibition implies that M. tuberculosis is not exposed to a strongly acidic environment in vivo, enabling successful propagation in host cells. Remarkably, MptpA has been previously structurally and functionally investigated, with special emphasis devoted to the enzyme properties at pH 8.0. Considering that the virulence of M. tuberculosis is strictly dependent on the avoidance of acidic conditions in vivo, we analysed the pH-dependence of the structural and catalytic properties of MptpA. Here we show that this enzyme undergoes pronounced conformational rearrangements when exposed to acidic pH conditions, inducing a severe decrease of the enzymatic catalytic efficiency at the expense of phosphotyrosine (pTyr). In particular, a mild decrease of pH from 6.5 to 6.0 triggers a significant increase of K of MptpA for phosphotyrosine, the phosphate group of which we determined to feature a pKa equal to 5.7. Surface plasmon resonance experiments confirmed that MptpA binds poorly to pTyr at pH values < 6.5. Notably, the effectiveness of the MptpA competitive inhibitor L335-M34 at pH 6 does largely outperform the inhibition exerted at neutral or alkaline pH values. Overall, our observations indicate a pronounced sensitivity of MptpA to acidic pH conditions, and suggest the search for competitive inhibitors bearing a negatively charged group featuring pKa values lower than that of the substrate phosphate group.