Abstract
The 1.5 Å resolution crystal structure of the Mycobacterium tuberculosis soluble inorganic pyrophosphatase Rv3628 at pH 7.0 is reported. The M. tuberculosis and M. leprae genomes include genes for the only two family I inorganic pyrophosphatases known to contain two histidines in the active site. The role of these two residues in catalysis is not fully understood. Mutational and functional studies of the M. tuberculosis enzyme showed that His21 and His86 are not essential for pyrophosphate hydrolysis, but are responsible for a shift in the optimal pH for the reaction compared with the Escherichia coli enzyme. Comparison with the structure previously reported at pH 5.0 provides further insight into the role of the two histidines. Two potassium-binding sites are found as a result of the high potassium concentration in the mother liquor.