Oxidative phosphorylation by membrane vesicles from Bacillus alcalophilus

A protet-based, protonic charge transfer model of energy coupling in oxidative and photosynthetic phosphorylation

Textbooks of biochemistry will explain that the otherwise endergonic reactions of ATP synthesis can be driven by the exergonic reactions of respiratory electron transport, and that these two half-reactions are catalyzed by protein complexes embedded in the same, closed membrane. These views are correct. The textbooks also state that, according to the chemiosmotic coupling hypothesis, a (or the) kinetically and thermodynamically competent intermediate linking the two half-reactions is the electrochemical difference of protons that is in equilibrium with that between the two bulk phases that the coupling membrane serves to separate. This gradient consists of a membrane potential term Δψ and a pH gradient term ΔpH, and is known colloquially as the protonmotive force or pmf. Artificial imposition of a pmf can drive phosphorylation, but only if the pmf exceeds some 150-170mV; to achieve in vivo rates the imposed pmf must reach 200mV. The key question then is 'does the pmf generated by electron transport exceed 200mV, or even 170mV?' The possibly surprising answer, from a great many kinds of experiment and sources of evidence, including direct measurements with microelectrodes, indicates it that it does not. Observable pH changes driven by electron transport are real, and they control various processes; however, compensating ion movements restrict the Δψ component to low values. A protet-based model, that I outline here, can account for all the necessary observations, including all of those inconsistent with chemiosmotic coupling, and provides for a variety of testable hypotheses by which it might be refined.

The regulation of oxidative phosphorylation pathway on Vibrio alginolyticus adhesion under adversities

Vibrio alginolyticus is one of the most important pathogens in mariculture and leading to heavy losses. After treatment with Cu²⁺, Pb²⁺, and low pH, the expression of oxidative phosphorylation pathway genes, including coxA, coxB, coxC, ccoN, ccoO, and ccoQ, was found commonly downregulated by RNA‐seq as well as quantitative real‐time PCR. RNAi significantly reduced the expression of coxA, coxB, coxC, ccoN, ccoO, and ccoQ in V. alginolyticus. Compared with the wild‐type strain, the adhesion abilities of RNAi strains of V. alginolyticus were significantly impaired, as well as their cytochrome C oxidase activity. ccoQ appeared to be more important in the regulation of bacterial adhesion in these target genes, while ccoO was relatively weak in the regulation of the adhesion. Meanwhile, the changes of temperature, salinity, pH, and starvation affected coxA, coxB, coxC, ccoN, ccoO, and ccoQ expression remarkably. These findings indicated that: the oxidative phosphorylation pathway is a critical regulator of adhesion in V. alginolyticus; coxA, coxB, coxC, ccoN, ccoO, and ccoQ regulate the bacterial adhesion in response to environmental changes such as temperature, salinity, pH, and starvation.

A low phosphorylation potential in the acetogen Acetobacterium woodii reflects its lifestyle at the thermodynamic edge of life

The anaerobic, acetogenic bacterium Acetobacterium woodii grows on hydrogen and carbon dioxide and uses the Wood-Ljungdahl pathway to fix carbon but also to synthesize ATP. The free energy change of acetogenesis from H2 + CO2 allows for synthesis of only a fraction of an ATP under environmental conditions, and A. woodii is clearly a paradigm for microbial life under extreme energy limitation. However, it was unknown how much energy is required to make ATP under these conditions. In the present study, we determined the phosphorylation potential in cells metabolizing three different acetogenic substrates. It accounts to 37.9 ± 1.3 kJ/mol ATP during acetogenesis from fructose, 32.1 ± 0.3 kJ/mol ATP during acetogenesis from H2 + CO2 and 30.2 ± 0.9 kJ/mol ATP during acetogenesis from CO, the lowest phosphorylation potential ever described. The physiological consequences in terms of energy conservation under extreme energy limitation are discussed.

A tridecameric c ring of the adenosine triphosphate (ATP) synthase from the thermoalkaliphilic Bacillus sp. strain TA2.A1 facilitates ATP synthesis at low electrochemical proton potential

Despite the thermodynamic problem imposed on alkaliphilic bacteria of synthesizing adenosine triphosphate (ATP) against a large inverted pH gradient and consequently a low electrochemical proton potential, these bacteria still utilize a proton-coupled F(1)F(o)-ATP synthase to synthesize ATP. One potential solution to this apparent thermodynamic problem would be the operation of a larger oligomeric c ring, which would raise the ion to ATP ratio, thus facilitating the conversion of a low electrochemical potential into a significant phosphorylation potential. To address this hypothesis, we have purified the oligomeric c ring from the thermoalkaliphilic bacterium Bacillus sp. strain TA2.A1 and determined the number of c-subunits using a novel mass spectrometry method, termed 'laser-induced liquid bead ion desorption' (LILBID). This technique allows the mass determination of non-covalently assembled, detergent-solubilized membrane protein complexes, and hence enables an accurate determination of c ring stoichiometries. We show that the Bacillus sp. strain TA2.A1 ATP synthase harbours a tridecameric c ring. The operation of a c ring with 13 subunits renders the thermodynamic problem of ATP synthesis at alkaline pH less severe and may represent a strategy for ATP synthesis at low electrochemical potential.

Bacterial energetics at high pH: what happens to the H+ cycle when the extracellular H+ concentration decreases?

A decrease in the extracellular H+ concentration creates difficulties for membrane-linked energetics in bacteria employing H+ as the coupling ion. At high extracellular pH (pHo), H+ ions pumped from the cell by, say, the respiratory chain, are immediately neutralized by the alkaline extracellular medium. Under such conditions, the only driving force that might compel outer H+ ions to return to cytosol and perform their function is the electric potential difference across the cytoplasmic membrane (delta psi). However, when delta pH in the opposite direction is equal to, e.g., 2 pH units (intracellular pH = 7.5 at pHo = 9.5), delta psi would be so high that the risk of membrane electric breakdown would increase. This is why some bacteria deal with high pH by, for example, replacing H+ by Na+ as the coupling ion rather than by increasing delta psi. It has been shown in several species of bacteria that the alkalinization of the growth medium induces primary Na+ pumps (e.g. Na(+)-motive respiratory chain enzymes and Na+ ATPase). Electrogenic Na+ efflux via these pumps produces an electrochemical Na+ potential difference (delta mu Na+) composed of delta psi and delta pNa+. delta mu Na+ can be used to perform various types of membrane-linked work. The delta psi constituent of delta mu Na+ may maintain electrophoretic influx of H+ such that the alkalinization of cytoplasm is prevented. The latter function may be supported by a mechanism based on the uphill influx of Cl- instead of Na+. This seems to be the case for alkaliphilic and halophilic Natronobacter pharaonis. There is an indication that not only Na+ but also Ca2+ may substitute for H+ in Gleobacter violaceus growing at high pH.

Instability of the protoplast membrane of facultative alkaliphilic Bacillus sp. C-125 at alkaline pH values below the pH optimum for growth

Cell walls of facultative alkaliphilic Bacillus sp. C-125 consist of three polymers (peptidoglycan, teichuronopeptide and teichuronic acid). Protoplasts prepared from the strain with egg-white lysozyme regenerated cell walls at neutral pH, but not at pH above 8.5. The protoplasts were susceptible to lysis at alkaline pH. The protoplasts exposed to alkaline pH rapidly burst and lost ability to regenerate their cell walls. The alkali-instability was similar to that of protoplasts from neutrophilic Bacillus subtilis 168. The membrane vesicles were also labile at alkaline pH. The acidic wall components of strain C-125 may contribute to stabilization of the cytoplasmic membrane of cells growing at alkaline pH, probably by shielding the membrane from direct exposure to an alkaline environment.

Sodium-coupled ATP synthesis in the bacterium Vitreoscilla

The bacterium Vitreoscilla generates an electrical potential gradient due to sodium ion (delta psi Na+) across its membrane via respiratory-driven primary Na+ pump(s). The role of the delta psi Na+ as a driving force for ATP synthesis was, therefore, investigated. In respiring starved cells pulsed with 100 mM external Na+ [( Na+]o) there was a 167% net increase in cellular ATP concentration over basal levels compared with 0, 56, 78, and 78% for no addition, choline, Li+, and K+ controls, respectively. Doubling the [Na+]o to 200 mM boosted the net increase to 244% but a similar doubling of the choline caused only an increase to 78%. When the initial condition was intracellular Na+ ([Na+]i) = [Na+]o = 100 mM, there was a 94% net increase in cellular ATP compared with only 18 and 11% for Li+ and K+ controls, respectively, indicating that Nai+ may be the only cation tested that the cells extruded to generate the electrochemical gradient required to drive ATP synthesis. The Na(+)-dependent ATP synthesis was inhibited completely by monensin (12 microM), but only transiently by the protonophore 3,5-di-tert-butyl-4-hydroxybenzaldehyde (100 microM), further evidence that the Na+ gradient and not a H+ gradient was driving the ATP synthesis. ATP synthesis in response to an artificially imposed H+ gradient (delta pH approximately 3) in the absence of an added cation, or in the presence of Li+, K+, or choline, yielded similar delta ATP/delta pH ratios of 0.98-1.22. In the presence of Na+, however, this ratio dropped to 0.23, indicating that Na+ inhibited H(+)-coupling to ATP synthesis and possibly that H+ and Na+ coupling to ATP synthesis share a common catalyst. The above evidence adds to previous findings that under normal growth conditions Na+ is probably the main coupling cation for ATP synthesis in Vitreoscilla.

The sodium cycle: a novel type of bacterial energetics

The progress of bioenergetic studies on the role of Na+ in bacteria is reviewed. Experiments performed over the past decade on several bacterial species of quite different taxonomic positions show that Na+ can, under certain conditions, substitute for H+ as the coupling ion. Various primary Na+ pumps (delta mu Na+ generators) are described, i.e., Na+ -motive decarboxylases, NADH-quinone reductase, terminal oxidase, and ATPase. The delta mu Na+ formed is shown to be consumed by Na+ driven ATP-synthase, Na+ flagellar motor, numerous Na+, solute symporters, and the methanogenesis-linked reverse electron transfer system. In Vibrio alginolyticus, it was found that delta mu Na+, generated by NADH-quinone reductase, can be utilized to support all three types of membrane-linked work, i.e., chemical (ATP synthesis), osmotic (Na+, solute symports), and mechanical (rotation of the flagellum). In Propionigenum modestum, circulation of Na+ proved to be the only mechanism of energy coupling. In other species studied, the Na+ cycle seems to coexist with the H+ cycle. For instance, in V. alginolyticus the initial and terminal steps of the respiratory chain are Na+ - and H+ -motive, respectively, whereas ATP hydrolysis is competent in the uphill transfer of Na+ as well as of H+. In the alkalo- and halotolerant Bacillus FTU, there are H+ - and Na+ -motive terminal oxidases. Sometimes, the Na+ -translocating enzyme strongly differs from its H+ -translocating homolog. So, the Na+ -motive and H+ -motive NADH-quinone reductases are composed of different subunits and prosthetic groups. The H+ -motive and Na+ -motive terminal oxidases differ in that the former is of aa3-type and sensitive to micromolar cyanide whereas the latter is of another type and sensitive to millimolar cyanide. At the same time, both Na+ and H+ can be translocated by one and the same P. modestum ATPase which is of the F0F1-type and sensitive to DCCD. The sodium cycle, i.e., a system composed of primary delta mu Na+ generator(s) and delta mu Na+ consumer(s), is already described in many species of marine aerobic and anaerobic eubacteria and archaebacteria belonging to the following genera: Vibrio, Bacillus, Alcaligenes, Alteromonas, Salmonella, Klebsiella, Propionigenum, Clostridium, Veilonella, Acidaminococcus, Streptococcus, Peptococcus, Exiguobacterium, Fusobacterium, Methanobacterium, Methanococcus, Methanosarcina, etc. Thus, the "sodium world" seems to occupy a rather extensive area in the biosphere.

The Na+ cycle of extreme alkalophiles: a secondary Na+/H+ antiporter and Na+/solute symporters

Extremely alkalophilic bacteria that grow optimally at pH 10.5 and above are generally aerobic bacilli that grow at mesophilic temperatures and moderate salt levels. The adaptations to alkalophily in these organisms may be distinguished from responses to combined challenges of high pH together with other stresses such as salinity or anaerobiosis. These alkalophiles all possess a simple and physiologically crucial Na+ cycle that accomplishes the key task of pH homeostasis. An electrogenic, secondary Na+/H+ antiporter is energized by the electrochemical proton gradient formed by the proton-pumping respiratory chain. The antiporter facilitates maintenance of a pHin that is two or more pH units lower than pHout at optimal pH values for growth. It also largely converts the initial electrochemical proton gradient formed by respiration into an electrochemical sodium gradient that energizes motility as well as a plethora of Na+ solute symporters. These symporters catalyze solute accumulation and, importantly, reentry of Na+. The extreme nonmarine alkalophiles exhibit no primary sodium pumping dependent upon either respiration or ATP. ATP synthesis is not part of their Na+ cycle. Rather, the specific details of oxidative phosphorylation in these organisms are an interesting analogue of the same process in mitochondria, and may utilize some common features to optimize energy transduction.

The Na+-motive terminal oxidase activity in an alkalo- and halo-tolerant Bacillus

An alkalo- and halo-tolerant aerobic microorganism has been isolated which, according to microbiological analysis data and the ribosomal 5S RNA sequence, is a Bacillus similar, but not identical, to B. licheniformis and B. subtilis. The microorganism, called Bacillus FTU, proved to be resistant to the protonophorous uncoupler carbonylcyanide m-chlorophenylhydrazone (CCCP). The fast growth of Bacillus FTU in the presence of CCCP was shown to require a high Na+ concentration in the medium. A procedure was developed to exhaust endogenous respiratory substrates in Bacillus FTU cells so that fast oxygen consumption by the cells was observed only when an exogenous respiratory substrate was added. The exhausted cells were found to oxidize ascorbate in the presence of N,N,N',N'-tetramethyl-p-phenylenediamine (TMPD) in a cyanide-sensitive fashion. The ascorbate oxidation was coupled to the uphill Na+ extrusion which was stimulated by CCCP and a penetrating weak base, diethylamine, as well as by valinomycin with or without diethylamine. Operation of the Bacillus FTU terminal oxidase resulted in the generation of a delta psi which, in the Na+ medium, was slightly decreased by CCCP and strongly decreased by CCCP + diethylamine. In the K+ medium, CCCP discharged delta psi even without diethylamine. Ascorbate oxidation was competent in ATP synthesis which was resistant to CCCP in the Na+ medium and sensitive to CCCP in the K+ medium as if Na+- and H+-coupled oxidative phosphorylations were operative in the Na+ and K+ media, respectively. Inside-out subcellular vesicles of Bacillus FTU were found to be competent in the Na+ uptake supported by oxidation of ascorbate + TMPD or diaminodurene. CCCP or valinomycin + K+ increased the Na+ uptake very strongly. The process was completely inhibited by cyanide or monensin, the former, but not the latter, being inhibitory for respiration. The data obtained indicate that in Bacillus FTU there is not only H+-motive but also Na+-motive terminal oxidase activity.

The bioenergetics of alkalophilic bacilli

A summary, cum speculation, of the major bioenergetic characteristics of alkalophilic bacilli is presented in Figure 5. Further progress will depend heavily on the purification and characterization of the relevant proteins that catalyze the ion fluxes and on the development of much more potent genetic approaches to the outstanding issues of this interesting group of bacteria.

Bioenergetic properties of alkalophilic Bacillus sp. strain C-59 on an alkaline medium containing K2CO3

Alkalophilic Bacillus sp. strain C-59 could grow well on an alkaline medium containing K2CO3, as well as Na2CO3, but did not grow on K+-depleted medium. Right-side-out membrane vesicles, energized in the absence of Na+, however, could not take up [14C]methylamine actively, while vesicles equilibrated with 10 mM NaCl actively took up [14C]methylamine. The uptake of [14C]serine was also stimulated by the addition of Na+, and the imposition of a sodium gradient caused transient uptake. These results indicated that an Na+/H+ antiporter was involved in pH homeostasis and generation of an electrochemical sodium gradient in strain C-59 even though a growth requirement for Na+ was not evident. The efflux of 22Na+ from 22Na+-loaded vesicles was more rapid at pH 9.5 than at pH 7 in the presence of an electron donor. On the other hand, vesicles at pH 7 showed more rapid efflux than at pH 9.5 when the antiporter was energized by a valinomycin-mediated K+ diffusion potential (inside negative).

Can free energy be transduced from electric noise?

Recently, it was shown that free energy can be transduced from a regularly oscillating electric field to do chemical or transport work when coupled through an enzyme with appropriate electrical characteristics. Here we report that randomly pulsed electric fields can also lead to work being done, giving rise to speculation as to whether appropriately designed enzymes can extract and convert free energy from the inherent fluctuations in their environment. The paradox is resolved by showing that equilibrium electrical noise resulting from the environment around an enzyme cannot be completely random but is correlated to the state that the enzyme is in. If the noise has the appropriate reciprocal interaction with the enzyme, its potential to serve as a free-energy source disappears. This is shown by Monte Carlo and other numerical calculations and is proven analytically by use of the diagram method. This method also is used to provide an explicit equation showing that, under a range of conditions, our model enzyme will be induced by uncorrelated ("autonomous") noise to undergo net cyclic flux. That work can be transduced from the "random" noise is demonstrated by using numerical methods.

Oxidative phosphorylation and energy buffering in cyanobacteria

The onset of respiration in the cyanobacteria Anacystis nidulans and Nostoc sp. strain Mac upon a shift from dark anaerobic to aerobic conditions was accompanied by rapid energization of the adenylate pool (owing to the combined action of ATP synthase and adenylate kinase) and also the guanylate, uridylate, and cytidylate pools (owing to nucleoside diphosphate and nucleoside monophosphate kinases). Rates of the various transphosphorylation reactions were comparable to the rate of oxidative phosphorylation, thus explaining, in part, low approximately P/O ratios which incorporate adenylates only. The increase of ATP, GTP, UTP, and CTP levels (nanomoles per minute per milligram [dry weight]) in oxygen-pulsed cells of A. nidulans and Nostoc species was calculated to be, on average, 2.3, 1.05, 0.8, and 0.57, respectively. Together with aerobic steady-state pool sizes of 1.35, 0.57, 0.5, and 0.4 nmol/mg (dry weight) for these nucleotides, a fairly uniform turnover of 1.3 to 1.5 min-1 was derived. All types of nucleotides, therefore, may be conceived of as being in equilibrium with each other, reflecting the energetic homeostasis or energy buffering of the (respiring) cyanobacterial cell. For the calculation of net efficiencies of oxidative phosphorylation in terms of approximately P/O ratios, this energy buffering was taken into account. Moreover, in A. nidulans an additional 30% of the energy initially conserved in ATP by oxidative phosphorylation was immediately used up by a plasma membrane-bound reversible H+-ATPase for H+ extrusion. Consequently, by allowing for energy buffering and ATPase-linked H+ extrusion, maximum P/O ratios of 2.6 to 3.3 were calculated. By contrast, in Nostoc sp. all the H+ extrusion, appeared to be linked to a plasma membrane-bound respiratory chain, thus bypassing any ATP formation and leading to P/O ratios of only 1.3 to 1.5 despite the correction for energy buffering.

The sodium cycle. II. Na+-coupled oxidative phosphorylation in Vibrio alginolyticus cells

The role of Na+ in Vibrio alginolyticus oxidative phosphorylation has been studied. It has been found that the addition of a respiratory substrate, lactate, to bacterial cells exhausted in endogenous pools of substrates and ATP has a strong stimulating effect on oxygen consumption and ATP synthesis. Phosphorylation is found to be sensitive to anaerobiosis as well as to HQNO, an agent inhibiting the Na+-motive respiratory chain of V. alginolyticus. Na+ loaded cells incubated in a K+ or Li+ medium fail to synthesize ATP in response to lactate addition. The addition of Na+ at a concentration comparable to that inside the cell is shown to abolish the inhibiting effect of the high intracellular Na+ level. Neither lactate oxidation nor delta psi generation coupled with this oxidation is increased by external Na+ in the Na+-loaded cells. It is concluded that oxidative ATP synthesis in V. alginolyticus cells is inhibited by the artificially imposed reverse delta pNa, i.e., [Na+]in greater than [Na+]out. Oxidative phosphorylation is resistant to a protonophorous uncoupler (0.1 mM CCCP) in the K+-loaded cells incubated in a high Na+ medium, i.e., when delta pNa of the proper direction [( Na+]in less than [Na+]out) is present. The addition of monensin in the presence of CCCP completely arrests the ATP synthesis. Monensin without CCCP is ineffective. Oxidative phosphorylation in the same cells incubated in a high K+ medium (delta pNa is low) is decreased by CCCP even without monensin. Artificial formation of delta pNa by adding 0.25 M NaCl to the K+-loaded cells (Na+ pulse) results in a temporary increase in the ATP level which spontaneously decreases again within a few minutes. Na+ pulse-induced ATP synthesis is completely abolished by monensin and is resistant to CCCP, valinomycin and HQNO. 0.05 M NaCl increases the ATP level only slightly. Thus, V. alginolyticus cells at alkaline pH represent the first example of an oxidative phosphorylation system which uses Na+ instead of H+ as the coupling ion.

Bioenergetics of alkalophilic bacteria

The central problem for organisms which grow optimally, and in some cases obligately, at pH values of 10 to 11, is the maintenance of a relatively acidified cytoplasm. A key component of the pH homeostatic mechanism is an electrogenic Na+/H+ antiporter which--by virtue of kinetic properties and/or its concentration in the membrane--catalyzes net proton uptake while the organisms extrude protons during respiration. The antiporter is also capable of maintaining a constant pHin during profound elevations in pHout as long as Na+ entry is facilitated by the presence of solutes which are taken up with Na+. Secondary to the problem of acidifying the interior is the adverse effect of the large pH gradient, acid in, on the total pmf of alkalophile cells. For the purposes of solute uptake and motility, the organisms appear to largely bypass the problem of a low pmf by utilizing a sodium motive force for energization. However, ATP synthesis appears not to resolve the energetics problem by using Na+ or by incorporating the proton-translocating ATPase into intracellular organelles. The current data suggest that effective proton pumping carried out by the alkalophile respiratory chain at high pH may deliver at least some portion of the protons to the proton-utilizing catalysts, i.e., the F1F0-ATPase and the Na+/H+ antiporter, by some localized pathway.

Free energy coupling between H+-generating and H+-consuming pumps. Ratio between output and input forces

The delta Gp/delta mu H ratio has been measured in mitochondria close to state 4 in the presence of various uncoupler or K+/valinomycin concentrations in media containing either 1 mM or 50 mM Pi. Care has been taken to control the factors affecting delta Gp and delta mu H which could lead to an artefactual increase of the delta Gp/delta mu H ratio above the highest accepted value for the H+/ATP stoichiometry (n = 4, synthesis + transport). In particular, to avoid overestimation of delta Gp due to inactivation of the ATPases at low delta mu H or to the presence of adenylate kinase, the static head state was approached from the side of net ATP synthesis and delta Gp was measured in a state close to static head but still maintaining a residual rate of aerobic phosphorylation. For each concentration of uncoupler or K+, the Pi concentration and/or the adenylate energy charge (EC) as a function of time have been measured as indicators of net ATP synthesis. Only the values of delta Gp measured during a decrease in Pi concentration and/or an increase in EC have been considered to be meaningful for calculations of delta Gp/delta mu H ratios. Both uncouplers and K+ transport cause a marked depression of delta mu H and a parallel depression of the rate of ATP synthesis. However the low rate of ATP synthesis taking place under conditions of low delta mu H eventually results, especially at high Pi concentrations, in a relatively large delta Gp. The delta Gp/delta mu H ratios obtained at the lower delta mu H values exceed 4 and approach 6. Although slightly higher delta Gp/delta mu H ratios are obtained with valinomycin-treated than with uncoupler-treated mitochondria, the pattern of the rise of the force ratio as delta mu H decreases is similar in both cases. An increase of the delta Gp/delta mu H ratio above 4, the maximal accepted H+/ATP stoichiometry is thermodynamically incompatible with the delocalized protonic coupling model.

Membrane-linked energy transductions. Bioenergetic functions of sodium: H+ is not unique as a coupling ion

The concept is developed according to which Na+, like H+, can play the role of a coupling ion in energy-transducing biomembranes. This idea is based on observations that (i) Na+ can be extruded from the cell by primary pumps (Na-motive NADH-quinone reductase, decarboxylase or ATPase), and (ii) the downhill Na+ flux into the cell can be coupled with the performance of all the three types of membrane-linked work i.e. chemical (ATP synthesis), osmotic (accumulation of solutes) and mechanical (motility). Marine alkalotolerant Vibrio alginolyticus represents the first example of such a complete sodium cycle pattern. Simplified versions of the sodium cycle or some of its constituents are found in the cytoplasmic membrane of a great variety of taxa including anaerobic, aerobic and photosynthetic bacteria, cyanobacteria and animals; this fact indicates that Na+ energetics should be regarded as a common case, rather than a rare exception applied to some natural niches only.

Failure of an alkalophilic bacterium to synthesize ATP in response to a valinomycin-induced potassium diffusion potential at high pH

Starved whole cells of the obligately alkalophilic Bacillus firmus RAB synthesize ATP upon addition of L-malate at pH 9.0 as expected of an aerobic organism that grows oxidatively on nonfermentable carbon sources at pH values as high as 11.0. The current study was a detailed examination of the perplexing inability of such cells to exhibit ATP synthesis in response to a valinomycin-mediated potassium diffusion potential at pH 9.0. While there were minor differences in the patterns of generation of the potential and the proton influx that accompanies its generation in the three different buffering systems employed, the magnitude of the transmembrane electro-chemical potential of protons was at least as high as pH 9.0 as at pH 7.0. Nevertheless, a diffusion potential consistently energized ATP synthesis at pH 7.0 but not at 9.0; these findings were independent of the presence or absence of Tris or of Na+. By contrast, the artificial electron donor ascorbate, in the presence of phenazine methosulfate, energized ATP synthesis by the starved whole cells at both pH values. The same phenomenon, i.e., efficacy of a respiration-derived potential but not of a diffusion potential at pH 9.0, was demonstrated in ADP + Pi-loaded membrane vesicles. On the other hand, electrogenic Na+-coupled solute transport could be energized by both ascorbate/phenazine and methosulfate and a diffusion potential in the vesicles at pH 9.0. The results are discussed in connection with models of a localized path of proton flow between proton pumps and the ATP synthase.

A minimal hypothesis for membrane-linked free-energy transduction. The role of independent, small coupling units

Experimental data are reviewed that are not in keeping with the scheme of 'delocalized' protonic coupling in membrane-linked free-energy transduction. It turns out that there are three main types of anomalies: (i) rates of electron transfer and of ATP synthesis do not solely depend on their own driving force and on delta mu H, (ii) the ('static head') ratio of delta Gp to delta mu H varies with delta mu H and (iii) inhibition of either some of the electron-transfer chains or some of the H+-ATPases, does not cause an overcapacity in the other, non-inhibited proton pumps. None of the earlier free-energy coupling schemes, alternative to delocalized protonic coupling, can account for these three anomalies. We propose to add a fifth postulate, namely that of the coupling unit, to the four existing postulates of 'delocalized protonic coupling' and show that, with this postulate, protonic coupling can again account for most experimental observations. We also discuss: (i) how experimental data that might seem to be at odds with the 'coupling unit' hypothesis can be accounted for and (ii) the problem of the spatial arrangement of the electrical field in the different free-energy coupling schemes.

Relationship between phosphorylation potential and electrochemical H+ gradient during glycolysis in Streptococcus lactis

Assays of intracellular ATP, ADP, and inorganic phosphate allowed calculation of the phosphorylation potential (delta G'ATP/F) maintained during glycolysis by Streptococcus lactis. At the same time, the electrochemical H+ gradient (delta mu-H+/F) was evaluated by distribution methods, using radioactive tetraphenylphosphonium bromide as a probe for the membrane potential and salicylic acid as a probe for the pH gradient. Detailed comparisons were made at pH 5, when the reaction mediated by the proton-translocating ATPase (BF0F1) was likely to have been poised near equilibrium; for those conditions, the ratio delta G'ATP/delta mu-H+ was used to estimate stoichiometry for BF0F1 during ATP hydrolysis. At an external pH of 5, in the presence or absence of valinomycin, this ratio was close to 3, over a range of 370 to 510 mV (8.5 to 11.7 kcal/mol) for delta G'ATP/F and a range of 128 to 167 mV for delta mu-H+/F. Other work suggested that delta G'ATP/delta mu-H+ increased from its minimum value of 3 to 4.3 as the external pH changed from pH 5 to 7.

Bioenergetic properties and viability of alkalophilic Bacillus firmus RAB as a function of pH and Na+ contents of the incubation medium

The bioenergetic properties and viability of obligately alkalophilic Bacillus firmus RAB have been examined upon incubation in alkaline and neutral buffers in the presence or absence of added Na+. At pH 10.5, cells incubated in the absence of Na+ exhibited an immediate rise in cytoplasmic pH from less than 9.5 to 10.5, and they lost viability very rapidly. Viability experiments in the presence or absence of an energy source further suggested that the Na+-dependent mechanism for pH homeostasis is an energy-requiring function. The Na+/H+ antiporter, which catalyzes the vital proton accumulation at alkaline pH, was only slightly operational at pH 7.0; both whole cells and vesicles exhibited net proton extrusion even in the presence of Na+. Moreover, cells incubated in buffer at pH 7.0 were actually more viable in the presence of Na+ than in its absence. Thus, the inability of B. firmus RAB to grow at neutral pH is not due to excessive acidification of the cytoplasm. Rather, the transmembrane electrical potential, delta psi, generated at pH 7.0 was found to be much lower than at alkaline pH. The very low delta psi compromised several cell functions, e.g., Na+/solute symport and motility, which in this and other alkalophiles specifically depend upon delta psi and Na+.

On the relationship between rate of ATP synthesis and H+ electrochemical gradient in rat-liver mitochondria

The relationship between rate of ATP synthesis, JATP, and value of the proton electrochemical gradient, delta mu H, has been analyzed in intact mitochondria. Onset of phosphorylation causes a depression of delta mu H of 1.5 kJ/mol. There is a close parallelism between inhibition of JATP and restoration of delta mu H to its state-4 value during titrations with oligomycin or atractyloside. Titrations with ionophores display the following features: (a) delta mu H can be depressed by 3-4 kJ/mol by valinomycin + K+ without affecting the rate of ATP synthesis; (b) uncouplers abolish JATP completely while depressing delta mu H by 3 kJ/mol; (c) complete abolition of ATP synthesis by inhibitors of electron transport is accompanied by a depression of delta mu H of only 1 kJ/mol. The results indicate that: (a) there is a close functional relationship between redox and ATPase H+ pumps, whereby inhibition of electron transfer is accompanied by simultaneous inhibition of the ATPase H+ pumps; and (b) uncoupling of oxidative phosphorylation is not due to depression of delta mu H per se. The consistence of the present data with either a chemiosmotic model where delta mu H is the sole and obligatory intermediate for energy coupling, or models where there is a direct transfer of energy between the two pumps is discussed.