Proton-coupled bioenergetic processes in extremely alkaliphilic bacteria

Oxidative Phosphorylation Does Not Violate the Second Law of Thermodynamics

In a recent series of papers, James W. Lee reported that mitochondrial oxidative phosphorylation violates the second law of thermodynamics and that it is allowed to do so because it is a "Type-B" process that features lateral and longitudinal membrane asymmetry. We show here that these contentions are based on problematic interpretations of the literature. More reliable values of ΔGredox and ΔGATP synthesis show that the second law is not violated. More recent reports on the structures of the redox-driven proton pumps (Complexes I, III, and IV) suggest that longitudinal membrane asymmetry does not exist. Finally, Lee's predictions for the concentration of protons localized at the P-side surface of the bioenergetic membrane are likely to be much too high due to several errors; thus, his predicted high values of ΔpHsurface that violate the second law are likely to be wrong. There is currently no strong experimental or theoretical evidence to support the contention that oxidative phosphorylation violates the second law of thermodynamics.

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.

H(+) V-ATPase-energized transporters in brush border membrane vesicles from whole larvae of Aedes aegypti

Brush border membrane vesicles (BBMVs) from Whole larvae of Aedes aegypti (AeBBMVWs) contain an H(+) V-ATPase (V), a Na(+)/H(+) antiporter, NHA1 (A) and a Na(+)-coupled, nutrient amino acid transporter, NAT8 (N), VAN for short. All V-ATPase subunits are present in the Ae. aegypti genome and in the vesicles. AgNAT8 was cloned from Anopheles gambiae, localized in BBMs and characterized in Xenopus laevis oocytes. AgNHA1 was cloned and localized in BBMs but characterization in oocytes was compromised by an endogenous cation conductance. AeBBMVWs complement Xenopus oocytes for characterizing membrane proteins, can be energized by voltage from the V-ATPase and are in their natural lipid environment. BBMVs from caterpillars were used in radio-labeled solute uptake experiments but approximately 10,000 mosquito larvae are needed to equal 10 caterpillars. By contrast, functional AeBBMVWs can be prepared from 10,000 whole larvae in 4h. Na(+)-coupled (3H)phenylalanine uptake mediated by AeNAT8 in AeBBMVs can be compared to the Phe-induced inward Na(+) currents mediated by AgNAT8 in oocytes. Western blots and light micrographs of samples taken during AeBBMVW isolation are labeled with antibodies against all of the VAN components. The use of AeBBMVWs to study coupling between electrogenic V-ATPases and the electrophoretic transporters is discussed.

Response to alkaline stress by root canal bacteria in biofilms

AIM

To determine whether bacteria isolated from infected root canals survive alkaline shifts better in biofilms than in planktonic cultures.

METHODOLOGY

Clinical isolates of Enterococcus faecalis, Lactobacillus paracasei, Olsenella uli, Streptococcus anginosus, S. gordonii, S. oralis and Fusobacterium nucleatum in biofilm and planktonic cultures were stressed at pH 10.5 for 4 h, and cell viability determined using the fluorescent staining LIVE/DEAD BacLight bacterial viability kit. In addition, proteins released into extracellular culture fluids were identified by Western blotting.

RESULTS

Enterococcus faecalis, L. paracasei, O. uli and S. gordonii survived in high numbers in both planktonic cultures and in biofilms after alkaline challenge. S. anginosus, S. oralis and F. nucleatum showed increased viability in biofilms compared with planktonic cultures. Alkaline exposure caused all planktonic cultures to aggregate into clusters and resulted in a greater extrusion of cellular proteins compared with cells in biofilms. Increased levels of DnaK, HPr and fructose-1,6-bisphosphate aldolase were observed in culture fluids, especially amongst streptococci.

CONCLUSIONS

In general, bacteria isolated from infected roots canals resisted alkaline stress better in biofilms than in planktonic cultures, however, planktonic cells appeared to use aggregation and the extracellular transport of specific proteins as survival mechanisms.

Bioenergetic properties of the thermoalkaliphilic Bacillus sp. strain TA2.A1

The thermoalkaliphilic Bacillus sp. strain TA2.A1 was able to grow in pH-controlled batch culture containing a nonfermentable growth substrate from pH 7.5 to 10.0 with no significant change in its specific growth rate, demonstrating that this bacterium is a facultative alkaliphile. Growth at pH 10.0 was sensitive to the protonophore carbonyl cyanide m-chlorophenylhydrazone, suggesting that a proton motive force (Deltap) generated via aerobic respiration was an obligate requirement for growth of strain TA2.A1. Strain TA2.A1 exhibited intracellular pH homeostasis as the external pH increased from 7.5 to 10.0; however, the maximum DeltapH generated over this pH range was only 1.1 units at an external pH of 9.5. The membrane potential (Deltapsi) was maintained between -114 mV and -150 mV, and little significant change was observed over the pH range for growth. In contrast, the Deltap declined from -164 mV at pH 7.5 to approximately -78 mV at pH 10.0. An inwardly directed sodium motive force (DeltapNa(+)) of -100 mV at pH 10.0 indicated that cellular processes (i.e., solute transport) dependent on a sodium gradient would not be affected by the adverse Deltap. The phosphorylation potential of strain TA2.A1 was maintained between -300 mV and -418 mV, and the calculated H(+)/ATP stoichiometry of the ATP synthase increased from 2.0 at pH 7.5 to 5.7 at pH 10.0. Based on these data, vigorous growth of strain TA2.A1 correlated well with the DeltapNa(+), phosphorylation potential, and the ATP/ADP ratio, but not with Deltap. This communication represents the first report on the bioenergetics of an extremely thermoalkaliphilic aerobic bacterium.

Sodium-dependent glutamate uptake by an alkaliphilic, thermophilic Bacillus strain, TA2.A1

A strain of Bacillus designated TA2.A1, isolated from a thermal spring in Te Aroha, New Zealand, grew optimally at pH 9.2 and 70 degrees C. Bacillus strain TA2.A1 utilized glutamate as a sole carbon and energy source for growth, and sodium chloride (>5 mM) was an obligate requirement for growth. Growth on glutamate was inhibited by monensin and amiloride, both inhibitors that collapse the sodium gradient (DeltapNa) across the cell membrane. N, N-Dicyclohexylcarbodiimide inhibited the growth of Bacillus strain TA2.A1, suggesting that an F1F0-ATPase (H type) was being used to generate cellular ATP needed for anabolic reactions. Vanadate, an inhibitor of V-type ATPases, did not affect the growth of Bacillus strain TA2.A1. Glutamate transport by Bacillus strain TA2.A1 could be driven by an artificial membrane potential (DeltaPsi), but only when sodium was present. In the absence of sodium, the rate of DeltaPsi-driven glutamate uptake was fourfold lower. No glutamate transport was observed in the presence of DeltapNa alone (i.e., no DeltaPsi). Glutamate uptake was specifically inhibited by monensin, and the Km for sodium was 5.6 mM. The Hill plot had a slope of approximately 1, suggesting that sodium binding was noncooperative and that the glutamate transporter had a single binding site for sodium. Glutamate transport was not affected by the protonophore carbonyl cyanide m-chlorophenylhydrazone, suggesting that the transmembrane pH gradient was not required for glutamate transport. The rate of glutamate transport increased with increasing glutamate concentration; the Km for glutamate was 2.90 microM, and the Vmax was 0.7 nmol. min-1 mg of protein. Glutamate transport was specifically inhibited by glutamate analogues.

The Intracellular pH of Clostridium paradoxum, an Anaerobic, Alkaliphilic, and Thermophilic Bacterium

When the extracellular pH was increased from 7.6 to 9.8, Clostridium paradoxum, a novel alkalithermophile, increased its pH gradient across the cell membrane ((Delta)pH, pH(infin) - pH(infout)) by as much as 1.3 U. At higher pH values (>10.0), the (Delta)pH and membrane potential ((Delta)(psi)) eventually declined, and the intracellular pH increased significantly. Growth ceased when the extracellular pH was greater than 10.2 and the intracellular pH increased to above 9.8. The membrane potential increased to 110 (plusmn) 8.6 mV at pH 9.1, but the total proton motive force ((Delta)p) declined from about 65 mV at pH 7.6 to 25 mV at pH 9.8. Between the extracellular pH of 8.0 and 10.3, the intracellular ATP concentration was around 1 mM and decreased at lower and higher pH values concomitantly with a decrease in growth rate.

Purification of three catalase isozymes from facultatively alkaliphilic Bacillus firmus OF4

Cell extracts of facultatively alkaliphilic B. firmus OF4 were assayed for catalase activity and their catalase isozyme content was analyzed on native polyacrylamide gels stained for catalase activity. pH-10.5-grown cells had about twice the specific catalase activity of pH-7.5-grown cells. The higher activity, however, did not confer resistance to exogenous hydrogen peroxide challenge relative to pH-7.5-grown cells, and in fact, the pH-10.5-grown cells were much more sensitive to the challenge. Electrophoresis resolved three catalase isozymes in cell extracts. The isozymes, labeled I-III in order of decreasing electrophoretic mobility, were purified and their Nterminal amino acid sequences were obtained. Isozyme III corresponded to the product of a cloned gene fragment that had been shown to possess substantial sequence similarity to the KatE (HP-II) catalase of E. coli (Quirk, P.G., Krulwich, T.A. and Hicks, D.B. (1993) Biophys J. 64, 164A) and which had similar biochemical properties to HP-II, i.e., it was a chlorin-containing enzyme expressed only in stationary phase. Isozyme II, a protoheme enzyme, was responsible for the higher activity of alkaline-grown cells and was induced in cells treated with hydrogen peroxide or ascorbate. It showed sequence similarity to katA of Bacillus subtilis (Bol, D. and Yasbin, R. (1991) Gene 109, 31-37). Isozyme I was the only isozyme that exhibited detectable levels of peroxidase activity in addition to catalase activity, resembling a catalase enzyme purified from a different alkaliphile, Bacillus YN-2000 (Yumoto, I., Fukumori, Y. and Yamanaka, T. (1990) J. Biochem. 108, 583-587), to which it showed some sequence similarity.

Alkaliphiles: 'basic' molecular problems of pH tolerance and bioenergetics

Alkaliphilic Bacillus species provide experimental opportunities for examination of physiological processes under conditions in which the stress of the extreme environment brings issues of general biological importance into special focus. The alkaliphile, like many other cells, uses Na+/H+ antiporters in pH regulation, but its array of these porters, and other ion-flux pathways that energize and support their activity, result in an extraordinary capacity for pH homeostasis; this process nonetheless becomes the factor that limits growth at the upper edge of the pH range. Above pH 9.5, aerobic alkaliphiles maintain a cytoplasmic pH that is two or more units below the external pH. This chemiosmotically adverse delta pH is bypassed by use of an electrochemical gradient of Na+ rather than of protons to energize solute uptake and motility. By contrast, ATP synthesis occurs via completely proton-coupled oxidative phosphorylation that proceeds just as well, or better, at pH 10 and above as it does in the same bacteria growing at lower pH, without the adverse pH gradient. Various mechanisms that might explain this conundrum are described, and the current state of the evidence supporting them is summarized.

The abundance of atp gene transcript and of the membrane F1F0-ATPase as a function of the growth pH of alkaliphilic Bacillus firmus OF4

Molecular biological and biochemical studies of the F(1)F(0)-ATP synthase of alkaliphilic Bacillus firmus OF4 show that the enzyme used at pH 7.5 and pH 10.5 is a unique product of the atp operon, expressed at the same levels and yielding an enzyme with the same subunit properties and c-subunit/holoenzyme stoichiometry.

Growth and bioenergetics of alkaliphilic Bacillus firmus OF4 in continuous culture at high pH

The effect of external pH on growth of alkaliphilic Bacillus firmus OF4 was studied in steady-state, pH-controlled cultures at various pH values. Generation times of 54 and 38 min were observed at external pH values of 7.5 and 10.6, respectively. At more alkaline pH values, generation times increased, reaching 690 min at pH 11.4; this was approximately the upper limit of pH for growth with doubling times below 12 h. Decreasing growth rates above pH 11 correlated with an apparent decrease in the ability to tightly regulate cytoplasmic pH and with the appearance of chains of cells. Whereas the cytoplasmic pH was maintained at pH 8.3 or below up to external pH values of 10.8, there was an increase up to pH 8.9 and 9.6 as the growth pH was increased to 11.2 and 11.4, respectively. Both the transmembrane electrical potential and the phosphorylation potential (delta Gp) generally increased over the total pH range, except for a modest fall-off in the delta Gp at pH 11.4. The capacity for pH homeostasis rather than that for oxidative phosphorylation first appeared to become limiting for growth at the high edge of the pH range. No cytoplasmic or membrane-associated organelles were observed at any growth pH, confirming earlier conclusions that structural sequestration of oxidative phosphorylation was not used to resolve the discordance between the total electrochemical proton gradient (delta p) and the delta Gp as the external pH is raised. Were a strictly bulk chemiosmotic coupling mechanism to account for oxidative phosphorylation over the entire range, the deltaGp/deltap ration (which would equal the H+/ATP ratio) would rise from about 3 at pH 7.5 to 13 at pH 11.2, dropping to 7 at pH 11.4 only because of the rise in cytoplasmic pH relative to other parameters. Moreover, the molar growth yields on malate were higher at pH 10.5 than at pH 7.5, indicating greater rather than lesser efficiency in the use of substrate at the more alkaline pH.

Na+/H+ antiporters, molecular devices that couple the Na+ and H+ circulation in cells

Na+/H+ antiporters are universal devices involved in the Na+ and H+ circulation of both eukaryotes and prokaryotes, thus playing an essential role in the pH and Na+ homeostasis of cells. This review focuses on the major impact of the application of molecular biology tools in the study of the antiporters. These tools permit the verification of the role of the antiporters and provide insights into their unique biology. A novel signal transduction to Na+ involving nhaR, a positive regulator, controls the expression of nhaA in E. coli. A "pH sensor" regulates the activity of Na+/H+ antiporters, both in eukaryotes and prokaryotes. A most intricate signal transduction to pH involving phosphorylation steps controls the activity of nhel in higher mammals. The identification of Histidine 226 in the "pH sensor" of NhaA is a step forward towards the understanding of the pH regulation of these proteins.

On the activation mechanism of the H(+)-ATP synthase and unusual thermodynamic properties in the alkalophilic cyanobacterium Spirulina platensis

The activation requirements and thermodynamic characteristics of ATP synthase from the alkalophilic cyanobacterium Spirulina platensis were studied in coupled membrane vesicles. Activation by methanol increased the Vmax, while the Km for MgATP was unaffected (0.7 mM). We propose that in Sp. platensis, as in chloroplasts, the activating effect of methanol is based on perturbation of the gamma-epsilon subunit interaction. Light-driven ATP synthesis by membrane vesicles of Sp. platensis was stimulated by dithiothreitol. The characteristics of the activation of the ATP synthase by the proton electrochemical potential difference (delta mu H+) were analyzed on the basis of the uncoupled rates of ATP hydrolysis as a function of a previously applied proton gradient. Two values of delta mu H+, at which 50% of the enzyme is active, were found; 13-14 kJ.mol-1 for untreated membrane vesicles, and 4-8 kJ.mol-1 for light-treated and dithiothreitol-treated membrane vesicles. These values are lower than the corresponding values for the oxidized and reduced forms, respectively, of the chloroplast enzyme. Although no bulk proton gradient could be observed, membrane vesicles of Sp. platensis were able to maintain an equilibrium phosphate potential (delta Gp) of 40-43.5 kJ.mol-1, comparable to values found for Synechococcus 6716 and Anabaena 7120 membrane vesicles. Acid/base-transition experiments showed that the thermodynamic threshold, delta mu H+, for ATP synthesis, catalyzed by light-treated and dithiothreitol-treated Spirulina membrane vesicles, was less than 5 kJ.mol-1. The activation characteristics and the low thermodynamic threshold allow ATP synthesis to occur at low delta mu H+ values. The findings are discussed, both with respect to differences and similarities with the enzymes from chloroplasts and other cyanobacteria, and with respect to the alkalophilic properties of Sp. platensis.