Synthesis of pyrophosphate by chromatophores of Rhodospirillum rubrum in the light and by soluble yeast inorganic pyrophosphatase in water-organic solvent mixtures

Inhibitory to non-inhibitory evolution of the ζ subunit of the F1FO-ATPase of Paracoccus denitrificans and α-proteobacteria as related to mitochondrial endosymbiosis

Introduction: The ζ subunit is a potent inhibitor of the F1FO-ATPase of Paracoccus denitrificans (PdF1FO-ATPase) and related α-proteobacteria different from the other two canonical inhibitors of bacterial (ε) and mitochondrial (IF1) F1FO-ATPases. ζ mimics mitochondrial IF1 in its inhibitory N-terminus, blocking the PdF1FO-ATPase activity as a unidirectional pawl-ratchet and allowing the PdF1FO-ATP synthase turnover. ζ is essential for the respiratory growth of P. denitrificans, as we showed by a Δζ knockout. Given the vital role of ζ in the physiology of P. denitrificans, here, we assessed the evolution of ζ across the α-proteobacteria class. Methods: Through bioinformatic, biochemical, molecular biology, functional, and structural analyses of several ζ subunits, we confirmed the conservation of the inhibitory N-terminus of ζ and its divergence toward its C-terminus. We reconstituted homologously or heterologously the recombinant ζ subunits from several α-proteobacteria into the respective F-ATPases, including free-living photosynthetic, facultative symbiont, and intracellular facultative or obligate parasitic α-proteobacteria. Results and discussion: The results show that ζ evolved, preserving its inhibitory function in free-living α-proteobacteria exposed to broad environmental changes that could compromise the cellular ATP pools. However, the ζ inhibitory function was diminished or lost in some symbiotic α-proteobacteria where ζ is non-essential given the possible exchange of nutrients and ATP from hosts. Accordingly, the ζ gene is absent in some strictly parasitic pathogenic Rickettsiales, which may obtain ATP from the parasitized hosts. We also resolved the NMR structure of the ζ subunit of Sinorhizobium meliloti (Sm-ζ) and compared it with its structure modeled in AlphaFold. We found a transition from a compact ordered non-inhibitory conformation into an extended α-helical inhibitory N-terminus conformation, thus explaining why the Sm-ζ cannot exert homologous inhibition. However, it is still able to inhibit the PdF1FO-ATPase heterologously. Together with the loss of the inhibitory function of α-proteobacterial ε, the data confirm that the primary inhibitory function of the α-proteobacterial F1FO-ATPase was transferred from ε to ζ and that ζ, ε, and IF1 evolved by convergent evolution. Some key evolutionary implications on the endosymbiotic origin of mitochondria, as most likely derived from α-proteobacteria, are also discussed.

ATP synthesis catalyzed by a V-ATPase: an alternative pathway for energy conservation operating in plant vacuoles?

The electrochemical H(+) gradient generated in tonoplast vesicles isolated from maize seeds was found to be able to drive the reversal of the catalytic cycle of both vacuolar H(+)-pumps (Façanha and de Meis, 1998). Here we describe the reversibility of the vacuolar V-type H(+)-ATPase (V-ATPase) even in the absence of the H(+) gradient in a water-Me2SO co-solvent mixture, resulting in net synthesis of [γ-(32)P]ATP from [(32)P]Pi and ADP. The water-Me2SO (5 to 20 %) media promoted inhibition of both PPi hydrolysis and synthesis reactions whereas it slightly affected the ATP hydrolysis and clearly stimulated the ATP synthesis, which was unaffected by uncoupling agents (FCCP, Triton X-100 or NH4 (+)). This effect of Me2SO on the ATP⇔(32)P exchange reaction seems to be related to a decrease of the apparent K m of the V-ATPase for Pi. The results are in accordance to the concept that the energetics of ATP synthesis catalysis depends on the solvation energies interacting in the enzyme microenvironment. A possible physiological significance of this phenomenon for the metabolism of desiccation-tolerant plant cells is discussed.

Reversibility of H+-ATPase and H+-pyrophosphatase in tonoplast vesicles from maize coleoptiles and seeds

Tonoplast-enriched vesicles isolated from maize (Zea mays L.) coleoptiles and seeds synthesize ATP from ADP and inorganic phosphate (Pi) and inorganic pyrophosphate from Pi. The synthesis is consistent with reversal of the catalytic cycle of the H+-ATPase and H+-pyrophosphatase (PPase) vacuolar membrane-bound enzymes. This was monitored by measuring the exchange reaction that leads to 32Pi incorporation into ATP or inorganic pyrophosphate. The reversal reactions of these enzymes were dependent on the proton gradient formed across the vesicle membrane and were susceptible to the uncoupler carbonyl cyanide p(trifluoromethoxy)-phenylhydrazone and the detergent Triton X-100. Comparison of the two H+ pumps showed that the H+-ATPase was more active than H+-PPase in coleoptile tonoplast vesicles, whereas in seed vesicles H+-PPase activity was clearly dominant. These findings may reflect the physiological significance of these enzymes in different tissues at different stages of development and/or differentiation.

Kinetic characterization of the hydrolytic activity of the H+-pyrophosphatase of Rhodospirillum rubrum in membrane-bound and isolated states

Substrate hydrolysis by the H+-pyrophosphatase (pyrophosphate phosphohydrolase, H+-PPase) of the photosynthetic bacterium Rhodospirillum rubrum follows a two-pathway reaction scheme in which preformed 1:1 and 1:2 . enzyme . Mg2+ complexes (EMg and EMg2) convert dimagnesium pyrophosphate (the substrate). This scheme is applicable to isolated enzyme, uncoupled chromatophores and chromatophores energized by a K+/valinomycin diffusion potential. Tris and other amine buffers exert a specific effect on the bacterial H+-PPase by increasing the Michaelis constant for substrate binding to EMg by a factor of 26-32, while having only small effect on substrate binding to EMg2. Formation of EMg requires a basic group with pKa of 7.2-7.7 and confers resistance against inactivation by mersalyl and N-ethylmaleimide to H+-PPase. The dissociation constants governing EMg and EMg2 formation, as estimated from the mersalyl-protection assays and steady-state kinetics of PPi hydrolysis, respectively, differ by an order of magnitude. Comparison with the data on soluble PPases suggests that, in spite of gross structural differences between H+-PPase and soluble PPases and the added ability of H+-PPase to act as a proton pump, the two classes of enzyme utilize the same reaction mechanism in PPi hydrolysis.

Isolation of the catalytic subunit of a membrane-bound H(+)-pyrophosphatase from pea stem mitochondria

The catalytic subunit of a membrane-bound pyrophosphatase was purified by electroendosmotic preparative electrophoresis from etiolated pea stem mitochondria. The enzyme was identified as a single peak relatively pure, because only a very limited number of polypeptides were detectable by SDS/PAGE of the active fractions. The pyrophosphatase was associated to a band with a molecular mass of 35 kDa, showing a specific activity of 0.7 mumol Pi . mg-1 protein . min-1 (37 degrees C, pH 8.0) and an apparent Km value of 200 microM. The hydrolytic activity required Mg2+, was inhibited by imidodiphosphate (HNO6P2Na4), Ca2+, F- and was stimulated by phospholipids. Cardiolipin, phophatidylcholine and phosphatidylethanolamine had the maximal activating effect. The isolated protein is very similar to the catalytic subunit of pyrophosphatases isolated from rat liver (beta-subunit) and Saccharomyces cerevisiae mitochondria.

Inhibition of mitochondrial F1 ATPase and sarcoplasmic reticulum ATPase by hydrophobic molecules

The hydrophobic nature of the active site of two energy-transducing ATPases was explored by comparing interactions between Pi and each of three hydrophobic drugs in the absence and presence of organic solvents. The drugs tested were the Fe . bathophenanthroline complex and the anticalmodulin drugs, calmidazolium and trifluoperazine. All inhibit the Pi in equilibrium with ATP exchange reaction catalyzed by submitochondrial particles and the ATPase activity of both submitochondrial particles and soluble F1 ATPase. The inhibition by the three drugs is reversed by either raising the Pi concentration or by adding organic solvent (dimethylsulfoxide, ethyleneglycol or methanol) to the medium. The inhibition of the Pi in equilibrium with ATP exchange by trifluoperazine becomes more pronounced when the electrochemical proton gradient formed across the membrane of the submitochondrial particles is decreased by the addition to the medium of the proton ionophore carbonylcyanide p-trifluoromethoxyphenylhydrazone. The ATPase activity and the Ca2+ uptake by sarcoplasmic reticulum vesicles are inhibited by the Fe . bathophenanthroline complex, calmidazolium and trifluoperazine. Phosphorylation of the ATPases by Pi, synthesis of ATP from ADP and Pi and the fast efflux of Ca2+ observed during reversal of the Ca2+ pump are inhibited by the three drugs. The inhibition is reversed by raising the concentration of Pi or dimethylsulfoxide. The three drugs tested appear to compete with Pi for a common binding site on the Ca2+-ATPase. The data presented are interpreted according to the proposal that the catalytic site of an enzyme involved in energy transduction undergoes a hydrophobic-hydrophilic transition during the catalytic cycle.

Pi in equilibrium ATP exchange in the absence of proton gradient by the H+-ATPase from yeast plasma membranes

Purified soluble H+-ATPase from Schizosaccharomyces pombe catalyzes a Pi in equilibrium ATP exchange in the absence of a H+ gradient. When the pH of the assay medium is raised from 5.5 to 8.0 there is a decrease of the ATPase activity and an increase of the rate of Pi in equilibrium ATP exchange. At pH 7.0 the addition of the organic solvent dimethyl sulfoxide (20%, v/v) promotes a decrease of ATPase activity and an increase of the Pi in equilibrium ATP exchange reaction. The effect of the organic solvent on the Pi in equilibrium ATP exchange is related to a decrease of the apparent Km for Pi.

Synthesis of ATP by soluble mitochondrial F1 ATPase and F1-inhibitor-protein complex in the presence of organic solvents

The F1 and F1-inhibitor-protein complex synthesized tightly bound ATP from ADP and Pi when the organic solvents dimethylsulfoxide (20-50% v/v), ethylene glycol (20-60% v/v) or poly(ethylene glycol) 4000 and 8000 (30-50% w/v) were included in the assay media. There was no synthesis of tightly bound ATP in the absence of organic solvents. In the presence of 50% dimethylsulfoxide, maximal synthesis of ATP was obtained at pH values between 6.5 and 7.7. In both F1 and F1-inhibitor-protein there was no synthesis of ATP in the absence of MgCl2. The rate of ATP synthesis became faster as the MgCl2 concentration in the medium was raised from 0.1-10 mM. The Km for Pi of F1 was in the range of 0.8-1.5 mM. The Km for Pi of the F1-inhibitor-protein was much higher than that of F1 and could not be measured. In the presence of 10 mM MgCl2 and 2 mM Pi, the rate constants of ATP synthesis by F1 and F1-inhibitor-protein were 5.2-10.4 h-1 and 3.5-5.9 h-1 respectively. For both enzymes the rate constant of ATP hydrolysis was 0.69 h-1. The tightly bound ATP of F1 and F1-inhibitor-protein were hydrolyzed at a much slower rate when either the Pi concentration or the MgCl2 concentration was suddenly decreased. Both in presence and absence of Mg2+, 40-60% of the radioactive tightly bound ATP synthesized by F1 was hydrolyzed when non-radioactive ATP was added to the assay medium. This was not observed when F1-inhibitor-protein was used.

Orthophosphate-pyrophosphate exchange catalyzed by soluble and membrane-bound inorganic pyrophosphatases. Role of H+ gradient

A comparative study of the orthophosphate-pyrophosphate exchange reaction catalyzed by the soluble pyrophosphatase from baker's yeast and by the membrane-bound pyrophosphatase of Rhodospirillum rubrum chromatophores was performed. In both systems the rate of exchange increased when the pH of the medium was raised from 6.0 to 7.8 and when the MgCl2 concentration was raised from 0.1 mM to 20 mM. For the yeast pyrophosphatase the exchange rates measured at different pH values and in the presence of 6.7 to 8.8 mM free Mg2+ superimposed as a single curve when plotted as a function of the concentrations of either HPO4(2-) or MgHPO4. This was not observed with the use of R. rubrum chromatophores. With yeast pyrophosphatase, the Km for Pi was higher than 10 mM and could not be measured when the free Mg2+ concentration in the medium was lower than 0.5 mM. There was a decrease in the Km for Pi when the free Mg2+ concentration was raised to 6.7-8.8 mM or when, in the presence of low free Mg2+, the organic solvents dimethylsulfoxide (20% v/v) or ethyleneglycol (40% v/v) were included in the assay medium. In the presence of 6.7-8.8 mM free Mg2+ the Km for total Pi was 7 mM at pH 7.0 and 12 mM at pH 7.8. For the ionic species HPO4(2-) and MgHPO4, the Km values were 5.8 mM and 4.2 mM respectively. In the presence of 0.24-0.42 mM free Mg2+ and either 20% (v/v) dimethylsulfoxide or 40% (v/v) ethyleneglycol the Km values for total Pi, HPO4(2-) and MgHPO4 were 7.6, 3.5 and 0.5 mM respectively. With R. rubrum chromatophores, the Km for Pi in the presence of 5.5-7.5 mM free Mg2+ was very high and could not be measured. In the presence of 0.24-0.45 mM free Mg2+ the ratio between the velocities of hydrolysis and synthesis of pyrophosphate measured at pH 7.8 with yeast pyrophosphatase and chromatophores of R. rubrum were practically the same. When the free Mg2+ concentration was raised to 5.5-8.8 mM this ratio decreased from 1028 to 540 when the yeast pyrophosphatase was used and from 754 to 46 when chromatophores were used.