An Unusual, His-dependent Family I Pyrophosphatase from Mycobacterium tuberculosis
Soluble inorganic pyrophosphatases (PPases) comprise two evolutionarily unrelated families (I and II). These two families have different specificities for metal cofactors, which is thought to be because of the fact that family II PPases have three active site histidines, whereas family I PPases have none. Here, we report the structural and functional characterization of a unique family I PPase from Mycobacterium tuberculosis (mtPPase) that has two His residues (His21 and His86) in the active site. The 1.3-A three-dimensional structure of mtPPase shows that His86 directly interacts with bound sulfate, which mimics the product phosphate. Otherwise, mtPPase is structurally very similar to the well studied family I hexameric PPase from Escherichia coli, although mtPPase lacks the intersubunit metal binding site found in E. coli PPase. The cofactor specificity of mtPPase resembles that of E. coli PPase in that it has high activity in the presence of Mg2+, but it differs from the E. coli enzyme and family II PPases because it has much lower activity in the presence of Mn2+ or Zn2+. Replacements of His21 and His86 in mtPPase with the residues found in the corresponding positions of E. coli PPase had either no effect on the Mg2+- and Mn2+-supported reactions (H86K) or reduced Mg2+-supported activity (H21K). However, both replacements markedly increased the Zn2+-supported activity of mtPPase (up to 11-fold). In the double mutant, Zn2+ was a 2.5-fold better cofactor than Mg2+. These results show that the His residues in mtPPase are not essential for catalysis, although they determine cofactor specificity.
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