Pre-steady-state kinetics and solvent isotope effects support the "billiard-type" transport mechanism in Na+ -translocating pyrophosphatase

Rapid kinetics of H+ transport by membrane pyrophosphatase: Evidence for a "direct-coupling" mechanism

Stress resistance-conferring membrane pyrophosphatase (mPPase) found in microbes and plants couples pyrophosphate hydrolysis with H+ transport out of the cytoplasm. There are two opposing views on the energy-coupling mechanism in this transporter: the pumping is associated with either pyrophosphate binding to mPPase or the hydrolysis step. We used our recently developed stopped-flow pyranine assay to measure H+ transport into mPPase-containing inverted membrane vesicles on the timescale of a single turnover. The vesicles were prepared from Escherichia coli overproducing the H+-translocating mPPase of Desulfitobacterium hafniense. Pyrophosphate induced linear accumulation of H+ in the vesicles, without evident lag or burst. In contrast, the binding of three nonhydrolyzable pyrophosphate analogs essentially induced no H+ accumulation. These findings are inconsistent with the "pumping-before-hydrolysis" model of mPPase functioning and support the alternative model positing the hydrolysis reaction as the source of the transported H+ ions. mPPase is thus a first "directly-coupled" proton pump.

Na+ Translocation Dominates over H+-Translocation in the Membrane Pyrophosphatase with Dual Transport Specificity

Cation-pumping membrane pyrophosphatases (mPPases; EC 7.1.3.1) vary in their transport specificity from obligatory H+ transporters found in all kingdoms of life, to Na+/H+-co-transporters found in many prokaryotes. The available data suggest a unique "direct-coupling" mechanism of H+ transport, in which the transported proton is generated from nucleophilic water molecule. Na+ transport is best rationalized by assuming that the water-borne proton propels a prebound Na+ ion through the ion conductance channel ("billiard" mechanism). However, the "billiard" mechanism, in its simple form, is not applicable to the mPPases that simultaneously transport Na+ and H+ without evident competition between the cations (Na+,H+-PPases). In this study, we used a pyranine-based fluorescent assay to explore the relationship between the cation transport reactions catalyzed by recombinant Bacteroides vulgatus Na+,H+-PPase in membrane vesicles. Under appropriately chosen conditions, including the addition of an H+ ionophore to convert Na+ influx into equivalent H+ efflux, the pyranine signal measures either H+ or Na+ translocation. Using a stopped-flow version of this assay, we demonstrate that H+ and Na+ are transported by Na+,H+-PPase in a ratio of approximately 1:8, which is independent of Na+ concentration. These findings were rationalized using an "extended billiard" model, whose most likely variant predicts the kinetic limitation of Na+ delivery to the pump-loading site.

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

Membrane-bound pyrophosphatases (M-PPases) are homodimeric primary ion pumps that couple the transport of Na+- and/or H+ across membranes to the hydrolysis of pyrophosphate. Their role in the virulence of protist pathogens like Plasmodium falciparum makes them an intriguing target for structural and functional studies. Here, we show the first structure of a K+-independent M-PPase, asymmetric and time-dependent substrate binding in time-resolved structures of a K+-dependent M-PPase and demonstrate pumping-before-hydrolysis by electrometric studies. We suggest how key residues in helix 12, 13, and the exit channel loops affect ion selectivity and K+-activation due to a complex interplay of residues that are involved in subunit-subunit communication. Our findings not only explain ion selectivity in M-PPases but also why they display half-of-the-sites reactivity. Based on this, we propose, for the first time, a unified model for ion-pumping, hydrolysis, and energy coupling in all M-PPases, including those that pump both Na+ and H+.

H+-Translocating Membrane-Bound Pyrophosphatase from Rhodospirillum rubrum Fuels Escherichia coli Cells via an Alternative Pathway for Energy Generation

Inorganic pyrophosphatases (PPases) catalyze an essential reaction, namely, the hydrolysis of PP_i, which is formed in large quantities as a side product of numerous cellular reactions. In the majority of living species, PP_i hydrolysis is carried out by soluble cytoplasmic PPase (S-PPases) with the released energy dissipated in the form of heat. In Rhodospirillum rubrum, part of this energy can be conserved by proton-pumping pyrophosphatase (H⁺-PPase^Rru) in the form of a proton electrochemical gradient for further ATP synthesis. Here, the codon-harmonized gene hppa^Rru encoding H⁺-PPase^Rru was expressed in the Escherichia coli chromosome. We demonstrate, for the first time, that H⁺-PPase^Rru complements the essential native S-PPase in E. coli cells. ¹³C-MFA confirmed that replacing native PPase to H⁺-PPase^Rru leads to the re-distribution of carbon fluxes; a statistically significant 36% decrease in tricarboxylic acid (TCA) cycle fluxes was found compared with wild-type E. coli MG1655. Such a flux re-distribution can indicate the presence of an additional method for energy generation (e.g., ATP), which can be useful for the microbiological production of a number of compounds, the biosynthesis of which requires the consumption of ATP.

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.