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

Abstract

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.

Functional and structural asymmetry suggest a unifying principle for catalysis in membrane-bound pyrophosphatases

“…Conversely, two recently published papers from Baykov and co-workers (Baykov et al, 2022 ; Malinen et al, 2022 ) continue to posit their billiard-type mechanism, where hydrolysis precedes pumping. By using quenched-flow measurements on Tm PPase at 40 °C, they demonstrate that hydrolysis is the most likely rate-determining step, which is consistent with our electrometric measurements on Tm PPase at 20 °C.…”

Functional and structural asymmetry suggest a unifying principle for catalysis in integral membrane-bound pyrophosphatases

“…Conversely, two recently-published papers from Baykov and co-workers (Baykov et al, 2022; Malinen et al, 2022) continue to posit their “billiard-type” mechanism, where hydrolysis precedes pumping. By using stopped-flow measurements on Tm PPase at 40 °C, they demonstrate that hydrolysis is the most likely rate-determining step, which is consistent with our SURFE 2 R N1 data on Tm PPase at 20 °C.…”

The inorganic pyrophosphatases of microorganisms: a structural and functional review

“…However, m-PPases are part of the PPi degrading machinery and are capable of pumping either H + or Na + ions. m-PPases, simple energy-supplying enzymes, have no known homologs to ion pumps, suggesting a unique ion transport coupling mechanism ( Baykov et al, 2022 ). The origin of H + PPases has been analyzed and they are suggested to have been derived from Na + /H + cotransporter m-PPases, the most ancestral form of the enzyme ( Luoto et al, 2011 ).…”