What is "exopolyphosphatase"

n. (context enzyme English) Any of a class of phosphohydrolases that hydrolyse polyphosphate

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.

PPX plays an important role in the metabolism of phosphate and energy in all living organisms. It is especially important for maintenance of appropriate levels of intracellular polyphosphate, which has been implicated in a variety of cellular functions including response to stressors such as deficiencies in amino acids, orthophosphate, or nitrogen, changes in pH, nutrient downshift, and high salt, and as an inorganic molecular chaperone.
PPX is classified as a polyphosphatase, which are part of the large DHH phosphoesterase family. Both subfamilies within this super family share four N-terminus motifs but have different C-terminus moieties.

PPX activity is quantified by measuring the loss of radioactively labeled P polyphosphate. PPX is mixed with a known quantity of labeled polyphosphate, and the hydrolysis reaction is stopped with perchloric acid (HClO). The amount of remaining labeled polyphosphate is then measured by liquid scintillation counting.