Exopolyphosphatase
![Structural characterization of the Ppx protein family: crystal structure of the Aquifex aeolicus family member. Active site is indicated by the binding of the chlorides (green) and the calcium ion (purple).1T6C[7]](/uploads/202501/12/1T6C_PPX2636.png)
![Structure of an E. coli Exopolyphosphatase. 1U6Z[10]](/uploads/202501/12/1u6z2636.png)

Exopolyphosphatase (PPX) is a phosphatase enzyme which catalyzes the hydrolysis of inorganic polyphosphate, a linear molecule composed of up to 1000 or more monomers linked by phospho-anhydride bonds. PPX is a processive exophosphatase, which means that it begins at the ends of the polyphosphate chain and cleaves the phospho-anhydride bonds to release orthophosphate as it moves along the polyphosphate molecule. PPX has several characteristics which distinguish it from other known polyphosphatases, namely that it does not act on ATP, has a strong preference for long chain polyphosphate, and has a very low affinity for polyphosphate molecules with less than 15 phosphate monomers.