Properties and function of clostridial membrane ATPase

Genome-scale model for Clostridium acetobutylicum: Part II. Development of specific proton flux states and numerically determined sub-systems

A regulated genome-scale model for Clostridium acetobutylicum ATCC 824 was developed based on its metabolic network reconstruction. To aid model convergence and limit the number of flux-vector possible solutions (the size of the phenotypic solution space), modeling strategies were developed to impose a new type of constraint at the endo-exo-metabolome interface. This constraint is termed the specific proton flux state, and its use enabled accurate prediction of the extracellular medium pH during vegetative growth of batch cultures. The specific proton flux refers to the influx or efflux of free protons (per unit biomass) across the cell membrane. A specific proton flux state encompasses a defined range of specific proton fluxes and includes all metabolic flux distributions resulting in a specific proton flux within this range. Effective simulation of time-course batch fermentation required the use of independent flux balance solutions from an optimum set of specific proton flux states. Using a real-coded genetic algorithm to optimize temporal bounds of specific proton flux states, we show that six separate specific proton flux states are required to model vegetative-growth metabolism and accurately predict the extracellular medium pH. Further, we define the apparent proton flux stoichiometry per weak acids efflux and show that this value decreases from approximately 3.5 mol of protons secreted per mole of weak acids at the start of the culture to approximately 0 at the end of vegetative growth. Calculations revealed that when specific weak acids production is maximized in vegetative growth, the net proton exchange between the cell and environment occurs primarily through weak acids efflux (apparent proton flux stoichiometry is 1). However, proton efflux through cation channels during the early stages of acidogenesis was found to be significant. We have also developed the concept of numerically determined sub-systems of genome-scale metabolic networks here as a sub-network with a one-dimensional null space basis set. A numerically determined sub-system was constructed in the genome-scale metabolic network to study the flux magnitudes and directions of acetylornithine transaminase, alanine racemase, and D-alanine transaminase. These results were then used to establish additional constraints for the genome-scale model.

Characterization and purification of the membrane-bound ATPase of the archaebacterium Methanosarcina barkeri

Membrane-bound ATPase was found in membranes of the archaebacterium Methanosarcina barkeri. The ATPase activity required divalent cations, Mg2+ or Mn2+, and maximum activity was obtained at pH 5.2. The activity was specifically stimulated by HSO3- with a shift of optimal pH to 5.8, and N,N'-dicyclohexylcarbodiimide inhibited ATP hydrolysis. The enzyme could be solubilized from membranes by incubation in 1 mM Tris-maleate buffer (pH 6.9) containing 0.5 mM EDTA. The solubilized ATPase was purified by DEAE-Sepharose and Sephacryl S-300 chromatography. The molecular weight of the purified enzyme was estimated to be 420,000 by gel filtration through Sephacryl S-300. Polyacrylamide gel electrophoresis in sodium dodecyl sulfate revealed two classes of subunit, Mr 62,000 (alpha) and 49,000 (beta) associated in the molar ratio 1:1. These results suggest that the ATPase of M. barkeri is similar to the F0F1 type ATPase found in many eubacteria.

Effects of butanol on Clostridium acetobutylicum

The internal pH of Clostridium acetobutylicum was determined at various stages during the growth of the organism. Even in the presence of significant quantities of acetic, butyric, and lactic acids, an internal pH of 6.2 was maintained. Experiments using N,N'-dicyclohexylcarbodiimide indicated that a functioning H+-ATPase is necessary for internal pH control. Butanol, one of the end products of the fermentation, had numerous harmful effects on C. acetobutylicum. At a concentration high enough to inhibit growth, butanol destroyed the ability of the cell to maintain internal pH, lowered the intracellular level of ATP, and inhibited glucose uptake. Experiments done at two different external pH values suggested that the butanol-mediated decrease in ATP concentration was independent of the drop in internal pH. Glucose uptake was not affected by arsenate, suggesting that uptake was not ATP dependent. The effects of butanol on C. acetobutylicum are complex, inhibiting several interrelated membrane processes.

Properties and function of the proton-translocating adenosine triphosphatase of Clostridium perfringens

Growth of Clostridium perfringens was inhibited by compounds which dissipate or prevent the formation of electrochemical proton gradients. Membrane vesicles prepared from this organism exhibited Mg2+-dependent adenosine triphosphatase (ATPase) activity sensitive to N,N'-dicyclohexylcarbodiimide. Mg2+-ATPase activity was optimal of 50 degrees C, but no discrete pH optimum was observed. Adenosine triphosphate-dependent quenching of the fluorescence of the weak base quinacrine by everted membrane vesicles suggested that the Mg2+-ATPase is a proton pump capable of generating an electrochemical proton gradient. Adenosine triphosphate-dependent transport of Ca2+ by everted vesicles was sensitive to uncouplers and inhibitors of the Mg2+-ATPase.

The proton-translocating adenosine triphosphatase of the obligately anaerobic bacterium Clostridium pasteurianum. 1. ATP phosphohydrolase activity

1. The cell-membrane ATP phosphohydrolase of vegetatively grown Clostridium pasteurianum was specifically Mg2+-dependent, but demonstrated significant activity with GTP, CTP and UTP. It displayed approximate Michaelis-Menten kinetics only in the presence of certain effectors (e.g. phosphoenolpyruvate, fructose 1,6-bis-phosphate) which decreased the Km for ATP (to below 2 mM) but also V, whilst extending to pH 5.8 the effective pH range of activity of the enzyme. 2. ATP phosphohydrolase activity of the membrane ATPase (BF0F1) was inhibited by N,N'-dicyclohexylcarbodiimide, butyricin 7423, Dio-9, 4-chloro-7-nitrobenzofurazan, efrapeptin, leucinostatin and quercetin, and to a lesser degree by aurovertin and citreoviridin. The enzyme was not inhibited by oligomycin, spegazzinine, tributyl tin, triethyl tin or venturicidin. The soluble ATPase (BF1) component differed in not being inhibited by N,N'-dicyclohexylcarbodiimide, butyricin 7423 or leucinostatin. 3. The ATPase (BF0F1) complex and its soluble (BF1) component were separately purified. 4. Dodecylsulphate/polyacrylamide gel electrophoresis separated only four polypeptide components in the purified ATPase (BF0F1), with approximate molecular weights (+/- 10%) as follows: subunit a, 65 500; subunit c, 57 500; subunit da, 43 000; subunit fa, 15 000. The soluble (BF1 component contained only the three polypeptide subunits a, c and da. These were present in the BF0F1 preparation in the ratio 2 : 1 : 2; the contribution of subunit fa could not satisfactorily be quantified. 5. Subunit a was identified as the component binding 4-chloro-7-nitrobenzofurazan and subunit fa as the component binding N,N'-dicyclohexylcarbodiimide. The ATP phosphohydrolase activity of the membrane ATPase was not activated by trypsin treatment and the ATPase (BF0F1) contained no trypsin-sensitive inhibitor protein subunit. 6. Purified ATPase (BF0F1) was incorporated into artificial proteoliposomes which demonstrated ATP-dependent enhancement of 8-anilinonaphthalene-1-sulphonate fluorescence and ATP-dependent proton influx. These reactions were abolished by proton conductors (e.g. carbonylcyanide m-chlorophenylhydrazone) by valinomycin in the presence of a high external concentration of K+, or by N,N'-dicyclohexylcarbodiimide, butyricin 7423, Dio-9, 4-chloro-7-nitrobenzofurazan or leucinostatin. Oligomycin, tributyl tin, triethyl tin and venturicidin were not inhibitory. 7. When stripped of the soluble BF1 component, such ATPase-proteoliposomes demonstrated nil ATP phosphohydrolase activity and did not display ATP-dependent enhancement of 8-anilino-naphthalene-1-sulphonate fluorescence or ATP-dependent protein influx. All of these activities were restored by incubation of the BF1-depleted proteoliposomes with a purified preparation of the soluble BF1 component.

Structure and function of H+-ATPase

(1) Extensive studies on proton-translocating ATPase (H+-ATPase) revealed that H+-ATPase is an energy transforming device universally distributed in membranes of almost all kinds of cells. (2) Crystallization of the catalytic portion (F1) of H+-ATPase showed that F1 is a hexagonal molecule with a central hole. The diameter of F1 is about 90 A and its molecular weight is about 380,000. (3) Use of thermophilic F1 permits the complete reconstitution of F1 from its five subunits (alpha, beta, gamma, delta, epsilon) and demonstration of the gate function of the gamma delta epsilon-complex, the catalytic function of beta (supported by alpha and gamma), and the H+-translocating functions of all five subunits. (4) Studies using purified thermostable F0 showed that F0 is an H+-channel portion of H+-ATPase. The direct measurement of H+-flux through F0, sequencing of DCCD-binding protein, and isolation of F1-binding protein are described. (5) The subunit stoichiometry of F1 may be alpha 3 beta 3 gamma delta epsilon. (6) Reconstitution of stable H+-ATPase-liposomes revealed that ATP is directly synthesized by the flow of H+ driven by an electrochemical potential gradient and that H+ is translocated by ATP hydrolysis. This rules out functions for all the hypothetical components that do not belong to H+-ATPase in H+-driven ATP synthesis. The roles of conformation change and other phenomena in ATP synthesis are also discussed.

Hydrolysis and synthesis of ATP by membrane-bound ATPase from a motile Streptococcus

ATPase was detected in the membranes of a motile Streptococcus. Maximal enzymic activity was observed at pH 8 and ATP/Mg2+ ratio of 2. Mn2+ and Ca2+ could replace Mg2+ to some extent. Besides ATP, GTP and ITP were substrates. The enzyme was inhibited by N,N'-dicyclohexylcarbodiimide but not by sodium azide, uncouplers or bathophenanthroline. An electrochemical gradient of protons, which was artificially imposed across the membranes of Streptococcus cells by manipulation of either the K+ diffusion potential or the transmembrane pH gradient, led to ATP synthesis. ATP synthesis was abolished by proton conductors, an inhibitor of the ATPase or an increase in the extracellular K+ concentration. A comparison between the phosphate potential and the electrochemical proton gradient showed that the data found are in agreement with a stoichiometry of 2 protons translocated per molecule ATP synthesized.

ATP hydrolysis and synthesis by the membrane-bound ATP synthetase complex of Methanobacterium thermoautotrophicum

The membrane-bound ATP synthetase complex of Methanobacterium thermoautotrophicum showed maximum activity for ATP hydrolysis at pH 8, at temperatures between 65 and 70 degrees C, and at an ATP-Mg2+ ratio of 0.5. Anaerobic conditions were not prerequisite for enzyme activity. The enzyme showed a Km value for ATP of 2 mM, and activity was Mg2+ dependent; Mn2+, Co2+, Ca2+, and Zn2+ could replace Mg2+ to some extent. Other nucleoside triphosphates could be hydrolyzed. N,N'-dicyclohexylcarbodiimide inhibited ATP hydrolysis. A proton-motive force, artificially imposed by a pH shift or valinomycin, resulted in ATP synthesis in whole cells. The ATP synthetase complex of the thermophilic methanogenic bacterium is similar to those described in aerobic and anaerobic microorganisms.

Energy conservation in chemotrophic anaerobic bacteria

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