A barrel in the stalk

Intrinsic uncoupling in the ATP synthase of Escherichia coli

The ATP hydrolysis activity and proton pumping of the ATP synthase of Escherichia coli in isolated native membranes have been measured and compared as a function of ADP and Pi concentration. The ATP hydrolysis activity was inhibited by Pi with an half-maximal effect at 140 microM, which increased progressively up in the millimolar range when the ADP concentration was progressively decreased by increasing amounts of an ADP trap. In addition, the relative extent of this inhibition decreased with decreasing ADP. The half-maximal inhibition by ADP was found in the submicromolar range, and the extent of inhibition was enhanced by the presence of Pi. The parallel measurement of ATP hydrolysis activity and proton pumping indicated that, while the rate of ATP hydrolysis was decreased as a function of either ligand, the rate of proton pumping increased. The latter showed a biphasic response to the concentration of Pi, in which an inhibition followed the initial stimulation. Similarly as previously found for the ATP synthase from Rhodobacter caspulatus [P. Turina, D. Giovannini, F. Gubellini, B.A. Melandri, Physiological ligands ADP and Pi modulate the degree of intrinsic coupling in the ATP synthase of the photosynthetic bacterium Rhodobacter capsulatus, Biochemistry 43 (2004) 11126-11134], these data indicate that the E. coli ATP synthase can operate at different degrees of energetic coupling between hydrolysis and proton transport, which are modulated by ADP and Pi.

Bacterial Na+- or H+-coupled ATP Synthases Operating at Low Electrochemical Potential

In certain strictly anaerobic bacteria, the energy for growth is derived entirely from a decarboxylation reaction. A prominent example is Propionigenium modestum, which converts the free energy of the decarboxylation of (S)-methylmalonyl-CoA to propionyl-CoA (DeltaG degrees =-20.6 kJ/mol) into an electrochemical Na(+) ion gradient across the membrane. This energy source is used as a driving force for ATP synthesis by a Na(+)-translocating F(1)F(0) ATP synthase. According to bioenergetic considerations, approximately four decarboxylation events are necessary to support the synthesis of one ATP. This unique feature of using Na(+) instead of H(+) as the coupling ion has made this ATP synthase the paradigm to study the ion pathway across the membrane and its relationship to rotational catalysis. The membrane potential (Deltapsi) is the key driving force to convert ion translocation through the F(0) motor components into torque. The resulting rotation elicits conformational changes at the catalytic sites of the peripheral F(1) domain which are instrumental for ATP synthesis. Alkaliphilic bacteria also face the challenge of synthesizing ATP at a low electrochemical potential, but for entirely different reasons. Here, the low potential is not the result of insufficient energy input from substrate degradation, but of an inverse pH gradient. This is a consequence of the high environmental pH where these bacteria grow and the necessity to keep the intracellular pH in the neutral range. In spite of this unfavorable bioenergetic condition, ATP synthesis in alkaliphilic bacteria is coupled to the proton motive force (DeltamuH(+)) and not to the much higher sodium motive force (DeltamuNa(+)). A peculiar feature of the ATP synthases of alkaliphiles is the specific inhibition of their ATP hydrolysis activity. This inhibition appears to be an essential strategy for survival at high external pH: if the enzyme were to operate as an ATPase, protons would be pumped outwards to counteract the low DeltamuH(+), thus wasting valuable ATP and compromising acidification of the cytoplasm at alkaline pH.

Effect of the epsilon-subunit on nucleotide binding to Escherichia coli F1-ATPase catalytic sites

The influence of the epsilon-subunit on the nucleotide binding affinities of the three catalytic sites of Escherichia coli F1-ATPase was investigated, using a genetically engineered Trp probe in the adenine-binding subdomain (beta-Trp-331). The interaction between epsilon and F1 was not affected by the mutation. Kd for binding of epsilon to betaY331W mutant F1 was approximately 1 nM, and epsilon inhibited ATPase activity by 90%. The only nucleotide binding affinities that showed significant differences in the epsilon-depleted and epsilon-replete forms of the enzyme were those for MgATP and MgADP at the high-affinity catalytic site 1. Kd1(MgATP) and Kd1(MgADP) were an order of magnitude higher in the absence of epsilon than in its presence. In contrast, the binding affinities for MgATP and MgADP at sites 2 and 3 were similar in the epsilon-depleted and epsilon-replete enzymes, as were the affinities at all three sites for free ATP and ADP. Comparison of MgATP binding and hydrolysis parameters showed that in the presence as well as the absence of epsilon, Km equals Kd3. Thus, in both cases, all three catalytic binding sites have to be occupied to obtain rapid (Vmax) MgATP hydrolysis rates.

Intergenic suppression of the gammaM23K uncoupling mutation in F0F1 ATP synthase by betaGlu-381 substitutions: the role of the beta380DELSEED386 segment in energy coupling

We previously demonstrated that the Escherichia coli F0F1-ATP synthase mutation, gammaM23K, caused increased energy of interaction between gamma- and beta-subunits which was correlated to inefficient coupling between catalysis and transport [Al-Shawi, Ketchum and Nakamoto (1997) J. Biol. Chem. 272, 2300-2306]. Based on these results and the X-ray crystallographic structure of bovine F1-ATPase [Abrahams, Leslie, Lutter and Walker (1994) Nature (London) 370, 621-628] gammaM23K is believed to form an ionized hydrogen bond with betaGlu-381 in the conserved beta380DELSEED386 segment. In this report, we further test the role of gamma-beta-subunit interactions by introducing a series of substitutions for betaGlu-381 and gammaArg-242, the residue which forms a hydrogen bond with betaGlu-381 in the wild-type enzyme. betaE381A, D, and Q were able to restore efficient coupling when co-expressed with gammaM23K. All three mutations reversed the increased transition state thermodynamic parameters for steady state ATP hydrolysis caused by gammaM23K. betaE381K by itself caused inefficient coupling, but opposite from the effect of gammaM23K, the transition state thermodynamic parameters were lower than wild-type. These results suggest that the betaE381K mutation perturbs the gamma-beta-subunit interaction and the local conformation of the beta380DELSEED386 segment in a specific way that disrupts the communication of coupling information between transport and catalysis. betaE381A, L, K, and R, and gammaR242L and E mutations perturbed enzyme assembly and stability to varying degrees. These results provide functional evidence that the beta380DELSEED386 segment and its interactions with the gamma-subunit are involved in the mechanism of coupling.

Cross-linking of chloroplast F0F1-ATPase subunit epsilon to gamma without effect on activity. Epsilon and gamma are parts of the rotor

Cys residues were directed into positions 17, 28, 41 and 85 of a Cys6-->Ser mutant of subunit epsilon of spinach chloroplast F0F1 ATP synthase. Wild-type and engineered epsilon were expressed in Escherichia coli, purified in the presence of urea, refolded and reassembled with spinach chloroplast F1 lacking the epsilon subunit [F1(-epsilon)]. Cys-containing epsilon variants were modified with a sulfhydryl-reactive photolabile cross-linker. Photocross-linking of epsilon to F1(-epsilon) yielded the same SDS gel pattern of cross-link products independent of the presence or absence of Mg2+ x ADP, phosphate and Mg2+ x ATP. Epsilon (wild type) [Ser6,Cys28]epsilon and [Ser6,Cys41]epsilon were cross-linked with subunit gamma. With chloroplast F0F1 the same cross-link pattern was obtained, except for one extra cross-link, probably between [Ser6,Cys28]epsilon and F0 subunit III. [Ser6,Cys17]epsilon and [Ser6,Cys85]epsilon did not produce cross-links. Cross-linking of epsilon, [Ser6,Cys28]epsilon, [Ser6,Cys41]epsilon to gamma in soluble chloroplast F1 impaired the ability of epsilon to inhibit Ca2+-ATPase activity. The Mg2+-ATPase activity of soluble F1 (measured in the presence of 30% MeOH) was not affected by cross-linking epsilon with gamma. Functional reconstitution of photophosphorylation in F1-depleted thylakoids was observed with F1 in which gamma was cross-linked to [Ser6,Cys28]epsilon or [Ser6,Cys41]epsilon but not with wild-type epsilon. In view of the intersubunit rotation of gamma relative to (alphabeta)3, which is driven by ATP hydrolysis, gamma and epsilon would seem to act concertedly as parts of the 'rotor' relative to the 'stator' (alphabeta)3.

The ATP synthase--a splendid molecular machine

An X-ray structure of the F1 portion of the mitochondrial ATP synthase shows asymmetry and differences in nucleotide binding of the catalytic beta subunits that support the binding change mechanism with an internal rotation of the gamma subunit. Other structural and mutational probes of the F1 and F0 portions of the ATP synthase are reviewed, together with kinetic and other evaluations of catalytic site occupancy and behavior during hydrolysis or synthesis of ATP. Subunit function as related to proton translocation and rotational catalysis is considered. Physical demonstrations of the gamma subunit rotation have been achieved. The findings have implications for other enzymatic catalyses.

The F0F1-type ATP synthases of bacteria: structure and function of the F0 complex

Membrane-bound ATP synthases (F0F1-ATPases) of bacteria serve two important physiological functions. The enzyme catalyzes the synthesis of ATP from ADP and inorganic phosphate utilizing the energy of an electrochemical ion gradient. On the other hand, under conditions of low driving force, ATP synthases function as ATPases, thereby generating a transmembrane ion gradient at the expense of ATP hydrolysis. The enzyme complex consists of two structurally and functionally distinct parts: the membrane-integrated ion-translocating F0 complex and the peripheral F1 complex, which carries the catalytic sites for ATP synthesis and hydrolysis. The ATP synthase of Escherichia coli, which has been the most intensively studied one, is composed of eight different subunits, five of which belong to F1, subunits alpha, beta, gamma, delta, and epsilon (3:3:1:1:1), and three to F0, subunits a, b, and c (1:2:10 +/- 1). The similar overall structure and the high amino acid sequence homology indicate that the mechanism of ion translocation and catalysis and their mode of coupling is the same in all organisms.