Exopolyphosphatasen

Exopolyphosphatasen, or exopolyphosphatases, are enzymes that catalyze the hydrolysis of terminal phosphate groups from polyphosphates (polyP), releasing inorganic phosphate and shortening the polymer chain. They are exo-acting phosphatases that remove phosphate from the end of a polyP chain rather than from internal bonds.

Distribution and genetics: Exopolyphosphatasen genes are found in bacteria, archaea, and eukaryotes, and in bacteria they are commonly referred to by ppx or related gene names. In bacteria, these enzymes form part of the polyphosphate metabolism network, balancing synthesis by polyphosphate kinases (PPK) and degradation to regulate phosphate reserves and energy-related processes. In eukaryotes, polyphosphate metabolism involves various PPX-like enzymes and organellar forms that participate in phosphate homeostasis and stress responses.

Enzymatic properties and mechanism: Exopolyphosphatasen activity typically requires divalent metal ions such as Mg2+ or Mn2+. The catalytic reaction converts (polyP)n + H2O into (polyP)n-1 and Pi. Some exopolyphosphatases show processive action, producing Pi or short polyP fragments as products. Structural features vary among families, but many enzymes contain motifs characteristic of metal-dependent phosphohydrolases, and certain subfamilies are associated with DHH/DHHA1-like domains.

Biological roles: The enzymes help regulate intracellular polyP pools, contributing to phosphate homeostasis, energy storage, and stress tolerance. In pathogenic bacteria, exopolyphosphatases can influence virulence and persistence by modulating polyP-dependent stress responses and regulatory networks. Expression and activity are often responsive to phosphate availability, nutritional status, and environmental conditions.

Regulation and localization: In bacteria, exopolyphosphatasen activity integrates with the broader polyP metabolism, including synthesis by PPK and turnover by other phosphatases. Localization is generally cytosolic, though some organisms possess organellar or periplasmic forms that broaden functional roles.