Biochemical, structural and physiological characteristics of vacuolar h+-pyrophosphatase

Abstract

Proton-translocating inorganic pyrophosphatase (H+-PPase) actively translocates protons across membranes coupled with the hydrolysis of inorganic pyrophosphate (PPi). H+-PPase, which is composed of a single protein and uses a simple compound as a substrate, has been recognized as a new type of ion pump in addition to the P-, F- A nd V-type ion-translocating ATPases. H+- A nd Na+-PPases are distributed in various organisms including plants, parasitic protozoa, Archaebacteria and bacteria, but are not present in animals or yeast. Vacuolar H+-PPase has dual functions in plant cells: Hydrolysis of cytosolic PPi to maintain the levels of PPi, and translocation of protons into vacuoles to maintain the acidity of the vacuolar lumen. Acidification performed with the vacuolar-type H+-ATPase and H+-PPase is essential to maintain acidic conditions, which are necessary for vacuolar hydrolytic enzymes and for supplying energy to secondary active transporters. Recent studies using loss-offunction mutant lines of H+-PPase and complementation lines with soluble PPases have emphasized the physiological importance of the scavenging role of PPi. An overview of the main features of H+-PPases present in the vacuolar membrane is provided in terms of tissue distribution in plants, intracellular localization, structure-function relationship, biochemical potential as a proton pump and functional stability.