Hpyrophosphatases

H+-pyrophosphatases, often abbreviated H+-PPases, are enzymes that hydrolyze inorganic pyrophosphate (PPi) to two phosphate ions (Pi). In their membrane-bound forms, the energy released by PPi hydrolysis is coupled to the translocation of protons across a membrane, creating a proton motive force used to drive secondary transport and contribute to organellar or cellular acidification.

Two main classes are recognized. H+-translocating pyrophosphatases are integral membrane proteins that actively pump H+ across membranes using the energy from PPi hydrolysis. Soluble inorganic pyrophosphatases (sPPases) reside in the cytosol and hydrolyze PPi to prevent its accumulation, thereby supporting biosynthetic reactions and PPi-dependent metabolic processes.

Distribution and function vary across life. H+-PPases are found in many bacteria, archaea, plants and some fungi, typically localized to vacuolar or acidocalcisome membranes. Vertebrates generally lack H+-PPases and rely on soluble PPases for PPi metabolism. In plants and fungi, vacuolar H+-PPases help acidify compartments and energize transport processes essential for growth and stress responses.

Structure and mechanism differ between the two classes. H+-PPases are multi-pass membrane proteins with conserved catalytic sites that couple PPi hydrolysis to proton translocation. Soluble PPases are metal-dependent enzymes, usually requiring Mg2+ or Mn2+, and function to hydrolyze PPi in the cytosol.

Relevance and applications. Understanding H+-PPases informs plant physiology, nutrient transport and cellular energy management. Their activity is of interest in fields such as agriculture and microbial bioenergetics, and they have been considered in discussions of herbicide targets and metabolic engineering. Note that the term Hpyrophosphatases may refer to H+-PPases, but spelling variations can occur in the literature.