The Mechanism of Energy Coupling in H+/Na+-Pumping Membrane Pyrophosphatase-Possibilities and Probabilities

Membrane pyrophosphatases (mPPases) found in plant vacuoles and some prokaryotes and protists are ancient cation pumps that couple pyrophosphate hydrolysis with the H+ and/or Na+ transport out of the cytoplasm. Because this function is reversible, mPPases play a role in maintaining the level of cytoplasmic pyrophosphate, a known regulator of numerous metabolic reactions. mPPases arouse interest because they are among the simplest membrane transporters and have no homologs among known ion pumps. Detailed phylogenetic studies have revealed various subtypes of mPPases and suggested their roles in the evolution of the sodium and proton bioenergetics. This treatise focuses on the mechanistic aspects of the transport reaction, namely, the coupling step, the role of the chemically produced proton, subunit cooperation, and the relationship between the proton and sodium ion transport. The available data identify H+-PPases as the first non-oxidoreductase pump with a direct-coupling mechanism, i.e., the transported proton is produced in the coupled chemical reaction. They also support a billiard hypothesis, which unifies the H+ and Na+ transport mechanisms in mPPase and, probably, other transporters.

Automated summary

Membrane pyrophosphatases (mPPases) are ancient membrane transporters found in plant vacuoles and some prokaryotes and protists. They couple pyrophosphate hydrolysis with the transport of H+ and/or Na+ out of the cytoplasm, and play a significant role in regulating the level of cytoplasmic pyrophosphate.