page_title: Exopolyphosphatase
section_title: Structure
hierarchical_section_title: Exopolyphosphatase / Structure
caption_attribution_description: English: Structural characterization of the Ppx protein family: crystal structure of the Aquifex aeolicus family member
context_page_description: Exopolyphosphatase 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.
context_section_description: The structure of PPX is characterized by the actin-like ATPase domain that is a part of this superfamily.  In Aquifex aeolicus it contains a ribonuclease H-like motif that is made up of a five-stranded ß-sheet with the second strand antiparallel to the rest.  A few of the strands are connected by helical segments that are longer in the C-terminal domain than in the N-terminal domain.  Five alpha-helices are located in the C-terminal domain and only two are located in the N-terminal domain.  The closed configuration of the enzyme is referred to as the type I structure.  This configuration shares similar features to other members of this superfamily, including the N-terminal and C-terminal domains being separated by two alpha-helices centered on the structure.  The more open arrangement of the domains displays rotational movement of the two domains around a single hinge region.  The structural flexibility has been described as a "butterfly like" cleft opening around the active site.
  In E. coli, exopolyphosphatase exists as a dimer, with each monomer consisting of four domains.  The first two domains consist of three beta-sheets followed by an alpha-beta-alpha-beta-alpha fold.  This is different from the previously described Aquifex aeolicus homolog which lacks the third and fourth domains.  To date, 4 structures have been solved for this class of enzymes, with Protein Data Bank accession codes 1T6C, 1T6D, 1U6Z, and 2FLO.
caption: 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]
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