The mechanism of ATP synthase: a reassessment of the functions of the b and a subunits

F1Fo adenosine triphosphate (ATP) synthase is a potential drug target in non-communicable diseases

F₁F_o adenosine triphosphate (ATP) synthase, also known as the complex V, is the central ATP-producing unit in the cells arranged in the mitochondrial and plasma membranes. F₁F_o ATP synthase also regulates the central metabolic processes in the human body driven by proton motive force (Δp). Numerous studies have immensely contributed toward highlighting its regulation in improving energy homeostasis and maintaining mitochondrial integrity, which otherwise gets compromised in illnesses. Yet, its role in the implication of non-communicable diseases remains unknown. F₁F_o ATP synthase dysregulation at gene level leads to reduced activity and delocalization in the cristae and plasma membranes, which is directly associated with non-communicable diseases: cardiovascular diseases, diabetes, neurodegenerative disorders, cancer, and renal diseases. Individual subunits of the F₁F_o ATP synthase target ligand-based competitive or non-competitive inhibition. After performing a systematic literature review to understand its specific functions and its novel drug targets, the present article focuses on the central role of F₁F_o ATP synthase in primary non-communicable diseases. Next, it discusses its involvement through various pathways and the effects of multiple inhibitors, activators, and modulators specific to non-communicable diseases with a futuristic outlook.

A Window to the Potassium World. The Evidence of Potassium Energetics in the Mitochondria and Identity of the Mitochondrial ATP-Dependent K+ Channel

The conclusions made in the three papers published in Function by Juhaszova et al. [Function, 3, 2022, zqab065, zqac001, zqac018], can be seen as a breakthrough in bioenergetics and mitochondrial medicine. For more than half a century, it has been believed that mitochondrial energetics is solely protonic and is based on the generation of electrochemical potential of hydrogen ions across the inner mitochondrial membrane upon oxidation of respiratory substrates, resulting in the generation of ATP via reverse transport of protons through the ATP synthase complex. Juhaszova et al. demonstrated that ATP synthase transfers not only protons, but also potassium ions, with the generation of ATP. This mechanism seems logical, given the fact that in eukaryotic cells, the concentration of potassium ions is several million times higher than the concentration of protons. The transport of K+ through the ATP synthase was enhanced by the activators of mitochondrial ATP-dependent K+ channel (_mK/ATP), leading to the conclusion that ATP synthase is the material essence of _mK/ATP. Beside ATP generation, the transport of osmotically active K+ to the mitochondrial matrix is accompanied by water entry to the matrix, leading to an increase in the matrix volume and activation of mitochondrial respiration with the corresponding increase in the ATP synthesis, which suggests an advantage of such transport for energy production. The driving force for K+ transport into the mitochondria is the membrane potential; an excess of K+ is exported from the matrix by the hypothetical K+/H+ exchangers. Inhibitory factor 1 (IF1) plays an important role in the activation of _mK/ATP by increasing the chemo-mechanical efficiency of ATP synthase, which may be a positive factor in the protective anti-ischemic signaling.

ATP Synthesis by Oxidative Phosphorylation

The F1F0-ATP synthase (EC 3.6.1.34) is a remarkable enzyme that functions as a rotary motor. It is found in the inner membranes of Escherichia coli and is responsible for the synthesis of ATP in response to an electrochemical proton gradient. Under some conditions, the enzyme functions reversibly and uses the energy of ATP hydrolysis to generate the gradient. The ATP synthase is composed of eight different polypeptide subunits in a stoichiometry of α3β3γδεab2c10. Traditionally they were divided into two physically separable units: an F1 that catalyzes ATP hydrolysis (α3β3γδε) and a membrane-bound F0 sector that transports protons (ab2c10). In terms of rotary function, the subunits can be divided into rotor subunits (γεc10) and stator subunits (α3β3δab2). The stator subunits include six nucleotide binding sites, three catalytic and three noncatalytic, formed primarily by the β and α subunits, respectively. The stator also includes a peripheral stalk composed of δ and b subunits, and part of the proton channel in subunit a. Among the rotor subunits, the c subunits form a ring in the membrane, and interact with subunit a to form the proton channel. Subunits γ and ε bind to the c-ring subunits, and also communicate with the catalytic sites through interactions with α and β subunits. The eight subunits are expressed from a single operon, and posttranscriptional processing and translational regulation ensure that the polypeptides are made at the proper stoichiometry. Recent studies, including those of other species, have elucidated many structural and rotary properties of this enzyme.

Fo-driven Rotation in the ATP Synthase Direction against the Force of F1 ATPase in the FoF1 ATP Synthase

Living organisms rely on the FoF1 ATP synthase to maintain the non-equilibrium chemical gradient of ATP to ADP and phosphate that provides the primary energy source for cellular processes. How the Fo motor uses a transmembrane electrochemical ion gradient to create clockwise torque that overcomes F1 ATPase-driven counterclockwise torque at high ATP is a major unresolved question. Using single FoF1 molecules embedded in lipid bilayer nanodiscs, we now report the observation of Fo-dependent rotation of the c10 ring in the ATP synthase (clockwise) direction against the counterclockwise force of ATPase-driven rotation that occurs upon formation of a leash with Fo stator subunit a. Mutational studies indicate that the leash is important for ATP synthase activity and support a mechanism in which residues aGlu-196 and cArg-50 participate in the cytoplasmic proton half-channel to promote leash formation.

Half a century of molecular bioenergetics

Molecular bioenergetics deals with the construction, function and regulation of the powerhouses of life. The present overview sketches scenes and actors, farsighted goals and daring hypotheses, meticulous tool-making, painstaking benchwork, lucky discovery, serious scepticism, emphatic believing and strong characters with weak and others with hard arguments, told from a personal, admittedly limited, perspective. Bioenergetics will blossom further with the search focused on both where there is bright light for ever-finer detail and the obvious dark spots for surprise and discovery.

F1FO ATPase vesicle preparation and technique for performing patch clamp recordings of submitochondrial vesicle membranes

Mitochondria are involved in many important cellular functions including metabolism, survival(1), development and, calcium signaling(2). Two of the most important mitochondrial functions are related to the efficient production of ATP, the energy currency of the cell, by oxidative phosphorylation, and the mediation of signals for programmed cell death(3). The enzyme primarily responsible for the production of ATP is the F1FO-ATP synthase, also called ATP synthase(4-5). In recent years, the role of mitochondria in apoptotic and necrotic cell death has received considerable attention. In apoptotic cell death, BCL-2 family proteins such as Bax enter the mitochondrial outer membrane, oligomerize and permeabilize the outer membrane, releasing pro-apoptotic factors into the cytosol(6). In classic necrotic cell death, such as that produced by ischemia or excitotoxicity in neurons, a large, poorly regulated increase in matrix calcium contributes to the opening of an inner membrane pore, the mitochondrial permeability transition pore or mPTP. This depolarizes the inner membrane and causes osmotic shifts, contributing to outer membrane rupture, release of pro-apoptotic factors, and metabolic dysfunction. Many proteins including Bcl-xL(7) interact with F1FO ATP synthase, modulating its function. Bcl-xL interacts directly with the beta subunit of F1FO ATP synthase, and this interaction decreases a leak conductance within the F1FOATPasecomplex, increasing the net transport of H+ by F1FO during F1FO ATPase activity(8) and thereby increasing mitochondrial efficiency. To study the activity and modulation of the ATP synthase, we isolated from rodent brain submitochondrial vesicles (SMVs) containing F1FO ATPase. The SMVs retain the structural and functional integrity of the F1FO ATPase as shown in Alavian et al. Here, we describe a method that we have used successfully for the isolation of SMVs from rat brain and we delineate the patch clamp technique to analyze channel activity (ion leak conductance) of the SMVs.

Mitochondrial genome sequences of Artemia tibetiana and Artemia urmiana: assessing molecular changes for high plateau adaptation

Brine shrimps, Artemia (Crustacea, Anostraca), inhabit hypersaline environments and have a broad geographical distribution from sea level to high plateaus. Artemia therefore possess significant genetic diversity, which gives them their outstanding adaptability. To understand this remarkable plasticity, we sequenced the mitochondrial genomes of two Artemia tibetiana isolates from the Tibetan Plateau in China and one Artemia urmiana isolate from Lake Urmia in Iran and compared them with the genome of a low-altitude Artemia, A. franciscana. We compared the ratio of the rate of nonsynonymous (Ka) and synonymous (Ks) substitutions (Ka/Ks ratio) in the mitochondrial protein-coding gene sequences and found that atp8 had the highest Ka/Ks ratios in comparisons of A. franciscana with either A. tibetiana or A. urmiana and that atp6 had the highest Ka/Ks ratio between A. tibetiana and A. urmiana. Atp6 may have experienced strong selective pressure for high-altitude adaptation because although A. tibetiana and A. urmiana are closely related they live at different altitudes. We identified two extended termination-associated sequences and three conserved sequence blocks in the D-loop region of the mitochondrial genomes. We propose that sequence variations in the D-loop region and in the subunits of the respiratory chain complexes independently or collectively contribute to the adaptation of Artemia to different altitudes.

Structure of the yeast F1Fo-ATP synthase dimer and its role in shaping the mitochondrial cristae

We used electron cryotomography of mitochondrial membranes from wild-type and mutant Saccharomyces cerevisiae to investigate the structure and organization of ATP synthase dimers in situ. Subtomogram averaging of the dimers to 3.7 nm resolution revealed a V-shaped structure of twofold symmetry, with an angle of 86° between monomers. The central and peripheral stalks are well resolved. The monomers interact within the membrane at the base of the peripheral stalks. In wild-type mitochondria ATP synthase dimers are found in rows along the highly curved cristae ridges, and appear to be crucial for membrane morphology. Strains deficient in the dimer-specific subunits e and g or the first transmembrane helix of subunit 4 lack both dimers and lamellar cristae. Instead, cristae are either absent or balloon-shaped, with ATP synthase monomers distributed randomly in the membrane. Computer simulations indicate that isolated dimers induce a plastic deformation in the lipid bilayer, which is partially relieved by their side-by-side association. We propose that the assembly of ATP synthase dimer rows is driven by the reduction in the membrane elastic energy, rather than by direct protein contacts, and that the dimer rows enable the formation of highly curved ridges in mitochondrial cristae.

Evidence of the proximity of ATP synthase subunits 6 (a) in the inner mitochondrial membrane and in the supramolecular forms of Saccharomyces cerevisiae ATP synthase

The involvement of subunit 6 (a) in the interface between yeast ATP synthase monomers has been highlighted. Based on the formation of a disulfide bond and using the unique cysteine 23 as target, we show that two subunits 6 are close in the inner mitochondrial membrane and in the solubilized supramolecular forms of the yeast ATP synthase. In a null mutant devoid of supernumerary subunits e and g that are involved in the stabilization of ATP synthase dimers, ATP synthase monomers are close enough in the inner mitochondrial membrane to make a disulfide bridge between their subunits 6, and this proximity is maintained in detergent extract containing this enzyme. The cross-linking of cysteine 23 located in the N-terminal part of the first transmembrane helix of subunit 6 suggests that this membrane-spanning segment is in contact with its counterpart belonging to the ATP synthase monomer that faces it and participates in the monomer-monomer interface.

The cytoplasmic loops of subunit a of Escherichia coli ATP synthase may participate in the proton translocating mechanism

Subunit a plays a key role in promoting H(+) transport and the coupled rotary motion of the subunit c ring in F(1)F(0)-ATP synthase. H(+) binding and release occur at Asp-61 in the middle of the second transmembrane helix (TMH) of F(0) subunit c. H(+) are thought to reach Asp-61 via aqueous pathways mapping to the surfaces of TMHs 2-5 of subunit a based upon the chemical reactivity of Cys substituted into these helices. Here we substituted Cys into loops connecting TMHs 1 and 2 (loop 1-2) and TMHs 3 and 4 (loop 3-4). A large segment of loop 3-4 extending from loop residue 192 loop to residue 203 in TMH4 at the lipid bilayer surface proved to be very sensitive to inhibition by Ag(+). Cys-161 and -165 at the other end of the loop bordering TMH3 were also sensitive to inhibition by Ag(+). Further Cys substitutions in residues 86 and 93 in the middle of the 1-2 loop proved to be Ag(+)-sensitive. We next asked whether the regions of Ag(+)-sensitive residues clustered together near the surface of the membrane by combining Cys substitutions from two domains and testing for cross-linking. Cys-161 and -165 in loop 3-4 were found to cross-link with Cys-202, -203, or -205, which extend into TMH4 from the cytoplasm. Further Cys at residues 86 and 93 in loop 1-2 were found to cross-link with Cys-195 in loop 3-4. We conclude that the Ag(+)-sensitive regions of loops 1-2 and 3-4 may pack in a single domain that packs at the ends of TMHs 3 and 4. We suggest that the Ag(+)-sensitive domain may be involved in gating H(+) release at the cytoplasmic side of the aqueous access channel extending through F(0).

The structure and function of mitochondrial F1F0-ATP synthases

We review recent advances in understanding of the structure of the F(1)F(0)-ATP synthase of the mitochondrial inner membrane (mtATPase). A significant achievement has been the determination of the structure of the principal peripheral or stator stalk components bringing us closer to achieving the Holy Grail of a complete 3D structure for the complex. A major focus of the field in recent years has been to understand the physiological significance of dimers or other oligomer forms of mtATPase recoverable from membranes and their relationship to the structure of the cristae of the inner mitochondrial membrane. In addition, the association of mtATPase with other membrane proteins has been described and suggests that further levels of functional organization need to be considered. Many reports in recent years have concerned the location and function of ATP synthase complexes or its component subunits on the external surface of the plasma membrane. We consider whether the evidence supports complete complexes being located on the cell surface, the biogenesis of such complexes, and aspects of function especially related to the structure of mtATPase.

Biological application of multi-component nanowires in hybrid devices powered by F1-ATPase motors

In this paper, construction of hybrid device by integrating nanowires with F(1)-ATPase motors is described. The nickel nanowires and multi-segment nanowires, including gold and nickel, were fabricated by electrochemical deposition in nanoporous templates. The nickel nanowires functionalized by biotinylated peptide can be assembled directly onto F(1)-ATPase motors to act as the propellers. If the multi-component nanowires, including gold and nickel, were selectively functionalized by the thiol group modified ssDNA and the synthetic peptide, respectively, the biotinylated F(1)-ATPase motors can be attached to the biotinylated peptide on nickel segment of the nanowires. Then, the multi-component nanowires can also be used as the propellers, and one may observe the rotations of the multi-component nanowires driven by F(1)-ATPase motors. Therefore, introduction of multiple segments along the length of a nanowire can lead to a variety of multiple chemical functionalities, which can be selectively bound to cells and special biomolecules. This method provides an insight for the construction of other hybrid devices with its controlling arrangement of different biomolecule on designed nanometer scale structures.

Molecular mechanisms of energy transduction in cells: engineering applications and biological implications

The synthesis of ATP from ADP and inorganic phosphate by F1F0-ATP synthase, the universal enzyme in biological energy conversion, using the energy of a transmembrane gradient of ions, and the use of ATP by the myosin-actin system to cause muscular contraction are among the most fundamental processes in biology. Both the ATP synthase and the myosin-actin may be looked upon as molecular machines. A detailed analysis of the molecular mechanisms of energy transduction by these molecular machines has been carried out in order to understand the means by which living cells produce and consume energy. These mechanisms have been compared with each other and their biological implications have been discussed. The thermodynamics of energy coupling in the oxidative phosphorylation process has been developed and the consistency of the mechanisms with the thermodynamics has been explored. Novel engineering applications that can result have been discussed in detail and several directions for future work have been pointed out.

The ATP-waiting conformation of rotating F1-ATPase revealed by single-pair fluorescence resonance energy transfer

F1-ATPase is an ATP-driven rotary motor in which a rod-shaped gamma subunit rotates inside a cylinder made of alpha3beta3 subunits. To elucidate the conformations of rotating F1, we measured fluorescence resonance energy transfer (FRET) between a donor on one of the three betas and an acceptor on gamma in single F1 molecules. The yield of FRET changed stepwise at low ATP concentrations, reflecting the stepwise rotation of gamma. In the ATP-waiting state, the FRET yields indicated a gamma position approximately 40 degrees counterclockwise (= direction of rotation) from that in the crystal structures of mitochondrial F1, suggesting that the crystal structures mimic a metastable state before product release.

ATP synthesis driven by proton transport in F1F0-ATP synthase

Topical questions in ATP synthase research are: (1) how do protons cause subunit rotation and how does rotation generate ATP synthesis from ADP+Pi? (2) How does hydrolysis of ATP generate subunit rotation and how does rotation bring about uphill transport of protons? The finding that ATP synthase is not just an enzyme but rather a unique nanomotor is attracting a diverse group of researchers keen to find answers. Here we review the most recent work on rapidly developing areas within the field and present proposals for enzymatic and mechanoenzymatic mechanisms.

Voltage-gated proton channels and other proton transfer pathways

Proton channels exist in a wide variety of membrane proteins where they transport protons rapidly and efficiently. Usually the proton pathway is formed mainly by water molecules present in the protein, but its function is regulated by titratable groups on critical amino acid residues in the pathway. All proton channels conduct protons by a hydrogen-bonded chain mechanism in which the proton hops from one water or titratable group to the next. Voltage-gated proton channels represent a specific subset of proton channels that have voltage- and time-dependent gating like other ion channels. However, they differ from most ion channels in their extraordinarily high selectivity, tiny conductance, strong temperature and deuterium isotope effects on conductance and gating kinetics, and insensitivity to block by steric occlusion. Gating of H(+) channels is regulated tightly by pH and voltage, ensuring that they open only when the electrochemical gradient is outward. Thus they function to extrude acid from cells. H(+) channels are expressed in many cells. During the respiratory burst in phagocytes, H(+) current compensates for electron extrusion by NADPH oxidase. Most evidence indicates that the H(+) channel is not part of the NADPH oxidase complex, but rather is a distinct and as yet unidentified molecule.

The Saccharomyces cerevisiae ATP synthase

The ATP synthase of the yeast Saccharomyces cerevisiae is composed of 20 different subunits whose primary structure is known. The organization of proteins that constitute the membranous domain is now under investigation. Cysteine insertions combined with the use of nonpermeant maleimide reagents and cross-linking reagents showing different lengths and specificity contribute to the knowledge of the location of the N- and C-termini of the subunits involved in the stator of the enzyme and their organization. This review summarizes data on yeast ATP synthase obtained in our laboratory since 1980.

Organisation of the yeast ATP synthase F(0):a study based on cysteine mutants, thiol modification and cross-linking reagents

A topological study of the yeast ATP synthase membranous domain was undertaken by means of chemical modifications and cross-linking experiments on the wild-type complex and on mutated enzymes obtained by site-directed mutagenesis of genes encoding ATP synthase subunits. The modification by non-permeant maleimide reagents of the Cys-54 of mutated subunit 4 (subunit b), of the Cys-23 in the N-terminus of subunit 6 (subunit a) and of the Cys-91 in the C-terminus of mutated subunit f demonstrated their location in the mitochondrial intermembrane space. Near-neighbour relationships between subunits of the complex were demonstrated by means of homobifunctional and heterobifunctional reagents. Our data suggest interactions between the first transmembranous alpha-helix of subunit 6, the two hydrophobic segments of subunit 4 and the unique membrane-spanning segments of subunits i and f. The amino acid residue 174 of subunit 4 is close to both oscp and the beta-subunit, and the residue 209 is close to oscp. The dimerisation of subunit 4 in the membrane revealed that this component is located in the periphery of the enzyme and interacts with other ATP synthase complexes.

Molecular models of the structural arrangement of subunits and the mechanism of proton translocation in the membrane domain of F(1)F(0) ATP synthase

Subunit c of the proton-transporting ATP synthase of Escherichia coli forms an oligomeric complex in the membrane domain that functions in transmembrane proton conduction. The arrangement of subunit c monomers in this oligomeric complex was studied by scanning mutagenesis. On the basis of these studies and structural information on subunit c, different molecular models for the potential arrangement of monomers in the c-oligomer are discussed. Intersubunit contacts in the F(0) domain that have been analysed in the past by chemical modification and mutagenesis studies are summarised. Transient contacts of the c-oligomer with subunit a might play a crucial role in the mechanism of proton translocation. Schematic models presented by several authors that interpret proton transport in the F(0) domain by a relative rotation of the c-subunit oligomer against subunit a are reviewed against the background of the molecular models of the oligomer.

A model for the structure of subunit a of the Escherichia coli ATP synthase and its role in proton translocation

Most of what is known about the structure and function of subunit a, of the ATP synthase, has come from the construction and isolation of mutations, and their analysis in the context of the ATP synthase complex. Three classes of mutants will be considered in this review. (1) Cys substitutions have been used for structural analysis of subunit a, and its interactions with subunit c. (2) Functional residues have been identified by extensive mutagenesis. These studies have included the identification of second-site suppressors within subunit a. (3) Disruptive mutations include deletions at both termini, internal deletions, and single amino acid insertions. The results of these studies, in conjunction with information about subunits b and c, can be incorporated into a model for the mechanism of proton translocation in the Escherichia coli ATP synthase.

The ATP synthase of Escherichia coli: structure and function of F(0) subunits

In this review we discuss recent work from our laboratory concerning the structure and/or function of the F(0) subunits of the proton-translocating ATP synthase of Escherichia coli. For the topology of subunit a a brief discussion gives (i) a detailed picture of the C-terminal two-thirds of the protein with four transmembrane helices and the C terminus exposed to the cytoplasm and (ii) an evaluation of the controversial results obtained for the localization of the N-terminal region of subunit a including its consequences on the number of transmembrane helices. The structure of membrane-bound subunit b has been determined by circular dichroism spectroscopy to be at least 75% alpha-helical. For this purpose a method was developed, which allows the determination of the structure composition of membrane proteins in proteoliposomes. Subunit b was purified to homogeneity by preparative SDS gel electrophoresis, precipitated with acetone, and redissolved in cholate-containing buffer, thereby retaining its native conformation as shown by functional coreconstitution with an ac subcomplex. Monoclonal antibodies, which have their epitopes located within the hydrophilic loop region of subunit c, and the F(1) part are bound simultaneously to the F(0) complex without an effect on the function of F(0), indicating that not all c subunits are involved in F(1) interaction. Consequences on the coupling mechanism between ATP synthesis/hydrolysis and proton translocation are discussed.

Insights into ATP synthase assembly and function through the molecular genetic manipulation of subunits of the yeast mitochondrial enzyme complex

Development of an increasingly detailed understanding of the eucaryotic mitochondrial ATP synthase requires a detailed knowledge of the stoichiometry, structure and function of F(0) sector subunits in the contexts of the proton channel and the stator stalk. Still to be resolved are the precise locations and roles of other supernumerary subunits present in mitochondrial ATP synthase complexes, but not found in the bacterial or chloroplast enzymes. The highly developed system of molecular genetic manipulation available in the yeast Saccharomyces cerevisiae, a unicellular eucaryote, permits testing for gene function based on the effects of gene disruption or deletion. In addition, the genes encoding ATP synthase subunits can be manipulated to introduce specific amino acids at desired positions within a subunit, or to add epitope or affinity tags at the C-terminus, enabling questions of stoichiometry, structure and function to be addressed. Newly emerging technologies, such as fusions of subunits with GFP are being applied to probe the dynamic interactions within mitochondrial ATP synthase, between ATP synthase complexes, and between ATP synthase and other mitochondrial enzyme complexes.

Structural interpretations of F(0) rotary function in the Escherichia coli F(1)F(0) ATP synthase

F(1)F(0) ATP synthases are known to synthesize ATP by rotary catalysis in the F(1) sector of the enzyme. Proton translocation through the F(0) membrane sector is now proposed to drive rotation of an oligomer of c subunits, which in turn drives rotation of subunit gamma in F(1). The primary emphasis of this review will be on recent work from our laboratory on the structural organization of F(0), which proves to be consistent with the concept of a c(12) oligomeric rotor. From the NMR structure of subunit c and cross-linking studies, we can now suggest a detailed model for the organization of the c(12) oligomer in F(0) and some of the transmembrane interactions with subunits a and b. The structural model indicates that the H(+)-carrying carboxyl of subunit c is located between subunits of the c(12) oligomer and that two c subunits pack in a front-to-back manner to form the proton (cation) binding site. The proton carrying Asp61 side chain is occluded between subunits and access to it, for protonation and deprotonation via alternate entrance and exit half-channels, requires a swiveled opening of the packed c subunits and stepwise association with different transmembrane helices of subunit a. We suggest how some of the structural information can be incorporated into models of rotary movement of the c(12) oligomer during coupled synthesis of ATP in the F(1) portion of the molecule.

Catalytic activities of mitochondrial ATP synthase in patients with mitochondrial DNA T8993G mutation in the ATPase 6 gene encoding subunit a

We investigated the biochemical phenotype of the mtDNA T8993G point mutation in the ATPase 6 gene, associated with neurogenic muscle weakness, ataxia, and retinitis pigmentosa (NARP), in three patients from two unrelated families. All three carried >80% mutant genome in platelets and were manifesting clinically various degrees of the NARP phenotype. Coupled submitochondrial particles prepared from platelets capable of succinate-sustained ATP synthesis were studied using very sensitive and rapid luminometric and fluorescence methods. A sharp decrease (>95%) in the succinate-sustained ATP synthesis rate of the particles was found, but both the ATP hydrolysis rate and ATP-driven proton translocation (when the protons flow from the matrix to the cytosol) were minimally affected. The T8993G mutation changes the highly conserved residue Leu(156) to Arg in the ATPase 6 subunit (subunit a). This subunit, together with subunit c, is thought to cooperatively catalyze proton translocation and rotate, one with respect to the other, during the catalytic cycle of the F(1)F(0) complex. Our results suggest that the T8993G mutation induces a structural defect in human F(1)F(0)-ATPase that causes a severe impairment of ATP synthesis. This is possibly due to a defect in either the vectorial proton transport from the cytosol to the mitochondrial matrix or the coupling of proton flow through F(0) to ATP synthesis in F(1). Whatever mechanism is involved, this leads to impaired ATP synthesis. On the other hand, ATP hydrolysis that involves proton flow from the matrix to the cytosol is essentially unaffected.

Association of a novel human mtDNA ATPase6 mutation with immature sperm cells

This study reports the first clearly defined heteroplasmic mutation in immature human sperm cells. The human sperm mitochondrial genome from residue 8186-9341 was analysed with the aim of identifying point mutations which may be associated with human male infertility. The semen samples analysed were obtained from 88 fertile men, 19 with oligozoospermia, and 12 with severe oligozoospermia. Using single strand conformation polymorphism analysis a heteroplasmic T to C transition was detected in the ATPase6 gene, at nucleotide position 8821, in semen samples from one out of 12 (8%) severely oligozoospermic men, but not in oligozoospermic men or normospermic men. This mutation changed the amino acid serine to proline at residue 99 of the mitochondrial ATPase6 in a region which is highly conserved in other vertebrates including rat, bovine, chicken, salmonids and Xenopus. The mutation was detected in semen samples collected from the same man 9 months apart and in peripheral blood lymphocytes. Single sperm cell analyses did not find this mutation in the mature sperm, but the mutation was detected in 7% of immature spermatids. Our finding suggests that immature spermatids with this mutation fail to develop fully.

F0 complex of the Escherichia coli ATP synthase. Not all monomers of the subunit c oligomer are involved in F1 interaction

The antigenic determinants of mAbs against subunit c of the Escherichia coli ATP synthase were mapped by ELISA using overlapping synthetic heptapeptides. All epitopes recognized are located in the hydrophilic loop region and are as follows: 31-LGGKFLE-37, 35-FLEGAAR-41, 36-LEGAAR-41 and 36-LEGAARQ-42. Binding studies with membrane vesicles of different orientation revealed that all mAbs bind to everted membrane vesicles independent of the presence or absence of the F1 part. Although the hydrophilic region of subunit c and particularly the highly conserved residues A40, R41, Q42 and P43 are known to interact with subunits gamma and epsilon of the F1 part, the mAb molecules have no effect on the function of F0. Furthermore, it could be demonstrated that the F1 part and the mAb molecule(s) are bound simultaneously to the F0 complex suggesting that not all c subunits are involved in F1 interaction. From the results obtained, it can be concluded that this interaction is fixed, which means that subunits gamma and epsilon do not switch between the c subunits during catalysis and furthermore, a complete rotation of the subunit c oligomer modified with mAb(s) along the stator of the F1F0 complex, proposed to be composed of at least subunits b and delta, seems to be unlikely.

Glutamate residues at positions 219 and 252 in the a-subunit of the Escherichia coli ATP synthase are not functionally equivalent

The role of glutamate-219 in the a-subunit of the Escherichia coli F0F1-ATPase was examined using site-directed mutagenesis. The replacement of Glu-219 by lysine, alanine or glycine resulted in a partially functional F0F1-ATPase. Combining any of these mutations with the substitution of glutamate for Gln-252 did not result in any increase in function. These findings rule out a proposal that glutamate at position 252 can functionally replace glutamate at position 219 [S.B. Vik, B.J. Antonio, J. Biol. Chem. 269 (1994) 30364-30369]. All the single and double mutants grew better at 25 degrees C than at 37 degrees C, suggesting a role for Glu-219 in maintaining the structure of the F0.

Function of the COOH-terminal domain of Vph1p in activity and assembly of the yeast V-ATPase

We have previously shown that mutations in buried charged residues in the last two transmembrane helices of Vph1p (the 100-kDa subunit of the yeast V-ATPase) inhibit proton transport and ATPase activity (Leng, X. H., Manolson, M., Liu, Q., and Forgac, M. (1996) J. Biol. Chem. 271, 22487-22493). In this report we have further explored the function of this region of Vph1p (residues 721-840) using a combination of site-directed and random mutagenesis. Effects of mutations on stability of Vph1p, assembly of the V-ATPase complex, 9-amino-6-chloro-2-methoxyacridine quenching (as a measure of proton transport), and ATPase activity were assessed. Additional mutations were analyzed to test the importance of Glu-789 in TM7 and His-743 in TM6. Although substitution of Asp for Glu at position 789 led to a 50% decrease in 9-amino-6-chloro-2-methoxyacridine quenching, substitution of Ala at this position gave a mutant with 40% quenching relative to wild type, suggesting that a negative charge at this position is not absolutely essential for proton transport. Similarly, a positive charge is not essential at position His-743, since the H743Y and H743A mutants retain 20 and 60% of wild-type quenching, respectively. Interestingly, H743A approaches wild-type ATPase activity at elevated pH while the E789D mutant shows a slightly lower pH optimum than wild type, suggesting that these residues are in a location to influence V-ATPase activity. The low pumping activity of the double mutant (E789H/H743E) suggests that these residues do not form a simple ion pair. Random mutagenesis identified a number of additional mutations both inside the membrane (L739S and L746S) as well as external to the membrane (H729R and V803D) which also significantly inhibited proton pumping and ATPase activity. By contrast, a cluster of five mutations were identified between residues 800 and 814 in the soluble segment just COOH-terminal to TM7 which affected either assembly or stability of the V-ATPase complex. Two mutations (F809L and G814D) may also affect targeting of the 100-kDa subunit. These results suggest that this segment of Vph1p plays a crucial role in organization of the V-ATPase complex.

Nutrient-gene interactions determine mitochondrial function: effect of dietary fat

The effect on mitochondrial respiration of feeding hydrogenated coconut oil, corn oil, or menhaden oil (MO) to diabetes-prone BHE/cdb rats and normal Sprague Dawley (SD) rats was studied. Both fat source and strain affected the temperature dependence of succinate-supported respiration. The transition temperature was greater in BHE/cdb rats than in the SD rats. The efficiency of ATP synthesis as reflected by the ADP:O ratio was decreased in the BHE/cdb rats compared to SD rats, with the exception of the comparison made at 37 degrees C with the MO-fed rats; at this temperature, the ADP:O ratios were similar. The diet and strain differences suggest a dietary lipid-gene interaction with respect to the mobility of subunit 6 of the F1F0ATPase. This subunit has two errors in its gene: one that affects the proton channel and another that likely affects its mobility within the inner mitochondrial membrane.

Dimerization interactions of the b subunit of the Escherichia coli F1F0-ATPase

Site-directed mutagenesis and N-terminal truncations were used to examine dimerization interactions in the b subunit of Escherichia coli F1F0-ATPase. Individual cysteine residues were incorporated into bsyn, a soluble form of the protein lacking the membrane-spanning N-terminal domain, in two main areas: the heptad repeat region and the hydrophobic region which begins at residue Val-124. The tendencies of these cysteine residues to form disulfide bonds with the corresponding cysteine in the bsyn dimer were tested using disulfide exchange by glutathione and air oxidation catalyzed by Cu2+. Within the heptad repeat region, only cysteines at residues 59 and 60, which occupy the b and c positions of the heptad repeat, showed significant tendencies to form disulfides, a result inconsistent with a coiled-coil model for bsyn. Mixed disulfide formation most readily occurred with the S60C + L65C and A61C + L65C pairs. Cysteines at positions 124, 128, 132, and 139 showed strong tendencies to form disulfides with their mates in the dimer, suggesting a parallel alpha-helical interaction between the subunits in this region. Deletion of residues N-terminal to either Glu-34 or Asp-53 had no apparent effect on dimerization as determined by sedimentation equilibrium, while deletion of all residues N-terminal to Lys-67 produced a monomeric form. These results imply that residues 53-66 but not 24-52 are essential for bsyn dimerization. Taken together the results are consistent with a model in which the two b subunits interact in more than one region, including a parallel alignment of helices containing residues 124-139.

Mitochondrial disease associated with the T8993G mutation of the mitochondrial ATPase 6 gene: a clinical, biochemical, and molecular study in six families

AIM

To contribute to the establishment of a rational clinical, neuroradiological, and molecular approach to neurogenic muscle weakness, ataxia, and retinitis pigmentosa (NARP) and maternally inherited Leigh's syndrome (MILS).

METHODS AND RESULTS

The T8993G mutation in the mitochondrial genome was found in several maternal members of six pedigrees, whose clinical status ranged from no symptoms to severe infantile subacute necrotising encephalomyelopathy (Leigh's disease). In one case a MELAS-like syndrome was documented both clinically and neuroradiologically. Relevant genetic features of the series were anticipation of symptoms through subsequent generations, and the presence of several cases in whom the mutation apparently occurred recently or was new. A uniform distribution of the mutation in many tissues was shown in one patient subjected to necropsy. In general, a good correlation was found between clinical severity and mutation heteroplasmy in readily accessible tissues, such as lymphocytes or fibroblasts. By contrast, a consistent reduction of the mitochondrial ATPase activity, to about half of the normal values, was found in most of the clinically affected cases, irrespective of the amount of mutant mitochondrial DNA.

CONCLUSIONS

Although the measurement of ATP hydrolysis in cultured fibroblasts was a reliable, and sometimes instrumental, means to identify T8993G positive patients, the relation between the mutation and the oxidative phosphorylation defect is probably very complex, and its understanding requires more complex biochemical analysis.

Model of the c-subunit oligomer in the membrane domain of F-ATPases

A model is described of a dodecameric complex consisting of the integral membrane component subunit c of the H+-transporting Fo domain of Escherichia coli F-ATPase. A high-resolution partial structure of monomeric subunit c resulting from 1H-NMR studies [1] was used for constructing the model. The validity of the proposed arrangement of protomers in the dodecameric complex was tested by amino acid substitution analysis and chemical, biochemical and genetic data on subunit c.

Protonophoric activity of NADH coenzyme Q reductase and ATP synthase in coupled submitochondrial particles from horse platelets

A method to prepare coupled submitochondrial particles from horse platelets is described. The method allowed us to study the protonophoric activities of both complex I and complex V following the fluorescence quenching of the monoamine 9-amino-6-chloro-2 methoxyacridine (ACMA), a probe highly sensitive to the generation of a transmembrane delta pH. We carried out a kinetic analysis of each enzyme complex studying the proton translocation and the electron transfer activities of complex I as well as the proton translocation and the ATP hydrolytic activities of complex V. A micromethod to prepare coupled submitochondrial particles from platelets might be useful to investigate cell bioenergetic damage occurring in mitochondrial diseases and ageing.

Catalytic mechanism of F1-ATPase

The structure of the core catalytic unit of ATP synthase, alpha 3 beta 3 gamma, has been determined by X-ray crystallography, revealing a roughly symmetrical arrangement of alternating alpha and beta subunits around a central cavity in which helical portions of gamma are found. A low-resolution structural model of F0, based on electron spectroscopic imaging, locates subunit a and the two copies of subunit b outside of a subunit c oligomer. The structures of individual subunits epsilon and c (largely) have been solved by NMR spectroscopy, but the oligomeric structure of c is still unknown. The structures of subunits a and delta remain undefined, that of b has not yet been defined but biochemical evidence indicates a credible model. Subunits gamma, epsilon, b, and delta are at the interface between F1 and F0; gamma epsilon complex forms one element of the stalk, interacting with c at the base and alpha and beta at the top. The locations of b and delta are less clear. Elucidation of the structure F0, of the stalk, and of the entire F1F0 remains a challenging goal.

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.

Localization of a conformational energy-coupling determinant near the C terminus of the beta subunit of the F1F0-ATPase

Escherichia coli mutants in the beta subunit of the F1F0-ATPase can be complemented with the beta subunit from the obligate aerobe Bacillus megaterium. It has been shown that cells carrying such hybrid ATPases have an unusual energy-coupling phenotype. Although they are able to grow on minimal succinate medium, and therefore carry a functional ATP synthase, they are defective in the ability to grow anaerobically, indicating some defect in ATP-driven proton pumping (Scarpetta, M., Hawthorne, C. A., and Brusilow, W. S. A. (1991) J. Biol. Chem. 266, 18567-18572). In this study, chimeric beta subunits were constructed consisting of the E. coli or the B. megaterium beta subunit carrying the C-terminal 18% of the other's beta subunit. The phenotypes of an E. coli beta mutant complemented with these chimeric subunits showed that the energy-coupling defect was located in this C-terminal region. The E. coli beta subunit carrying the B. megaterium C-terminal region displayed the energy-coupling defect, while the B. megaterium beta subunit carrying the E. coli C-terminal region did not. In ATP-dependent fluorescence quenching assays, membranes isolated from cells displaying the energy-coupling defect also pumped protons less well than membranes isolated from cells that were able to grow anaerobically. These results demonstrate that the C terminus of the beta subunit is involved in the conformational coupling pathway, which, through the polypeptide backbone of the beta subunit, physically links ATP synthesis or hydrolysis to the energy of proton translocation.

Topographical structure of membrane-bound Escherichia coli F1F0 ATP synthase in aqueous buffer

Scanning force microscope images of membrane-bound Escherichia coli ATP synthase F0 complexes have been obtained in aqueous solution. The images show a consistent set of internal features: a ring structure which surrounds a central dimple and contains an asymmetric lateral mass. Images of trypsin-treated F0 complexes, which have lost part of their b subunits, show a reduced asymmetric mass, while images of c-subunit oligomers, which lack both the a and b subunits, show a ring and dimple but do not have an asymmetric mass. These results support models in which the F0 complex contains a ring of 9-12 c subunits with the b subunits located outside this ring, and show that scanning force microscopy is able to provide structural information on membrane proteins of molecular mass less than 200 000 Da.

Subunit rotation in F0F1-ATP synthases as a means of coupling proton transport through F0 to the binding changes in F1

The rotation of an asymmetric core of subunits in F0F1-ATP synthases has been proposed as a means of coupling the exergonic transport of protons through F0 to the endergonic conformational changes in F1 required for substrate binding and produce release. Here we review earlier evidence both for and against subunit rotation and then discuss our most recent studies using reversible intersubunit disulfide cross-links to test for rotation. We conclude that the gamma subunit of F1 rotates relative to the surrounding catalytic subunits during catalytic turnover by both soluble F1 and membrane-bound F0F1. Furthermore, the inhibition of this rotation by the modification of F0 with DCCD suggests that rotation in F1 is obligatorily coupled to rotation in F0 as an integral part of the coupling mechanism.

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.

Conformational transmission in ATP synthase during catalysis: search for large structural changes

Escherichia coli ATP synthase has eight subunits and functions through transmission of conformational changes between subunits. Defective mutation at beta Gly-149 was suppressed by the second mutations at the outer surface of the beta subunit, indicating that the defect by the first mutation was suppressed by the second mutation through long range conformation transmission. Extensive mutant/pseudorevertant studies revealed that beta/alpha and beta/gamma subunits interactions are important for the energy coupling between catalysis and H+ translocation. In addition, long range interaction between amino and carboxyl terminal regions of the gamma subunit has a critical role(s) for energy coupling. These results suggest that the dynamic conformation change and its transmission are essential for ATP synthase.

The coupling of the relative movement of the a and c subunits of the F0 to the conformational changes in the F1-ATPase

F0F1-ATPase structural information gained from X-ray crystallography and electron microscopy has activated interest in a rotational mechanism for the F0F1-ATPase. Because of the subunit stoichiometry and the involvement of both a- and c-subunits in the mechanism of proton movement, it is argued that relative movement must occur between the subunits. Various options for the arrangement and structure of the subunits involved are discussed and a mechanism proposed.

Transmembrane topology of Escherichia coli H(+)-ATPase (ATP synthase) subunit a

Escherichia coli H(+)-ATPase subunit a is a hydrophobic F0 subunit. To investigate the topology of the subunit in the membrane, we prepared site-specific polyclonal antibodies against amino-terminal (Ser-3 to Leu-16), middle loop (Lys-167 to Gln-181), and carboxyl-terminal (Thr-259 to His-271) peptide segments. Enzyme-linked immunosorbent assay revealed that these antibodies specifically reacted with subunit a of inside-out membrane vesicles, but not with that of right-side-out spheroplasts. Full reactivity appeared when spheroplasts were disrupted with Triton X-100 (0.5%) or by sonication. These results suggest that at least parts of the three peptide segments of subunit a face the cytoplasm. Based on these observations, we propose a novel transmembrane topology of subunit a.

The essential arginine residue at position 210 in the alpha subunit of the Escherichia coli ATP synthase can be transferred to position 252 with partial retention of activity

The substitution of arginine at position 210 in the alpha subunit of Escherichia coli F0F1-ATPase by either lysine or alanine causes dominance in complementation tests with a chromosomal c subunit mutation. Reversal of dominance was achieved for the alpha R210K mutation but not for the alpha R210A mutation by the presence of an aspartic acid residue at position 50 or at position 252 in the alpha subunit. It was concluded that position 210 in putative helix 4 of a previously proposed model of the alpha subunit is close to position 252 in putative helix 5 and to position 50 in putative helix 1. The juxtaposition of residues 252 and 210 was also indicated by the observation that the double mutant alpha R210Q/Q252R was partially functional. A revertant of the partially functional double mutant, isolated on succinate medium, was found to contain a third mutation resulting in Pro-204 in the alpha subunit being replaced by threonine. That the revertant phenotype was due to the alpha P204T change was confirmed by site-directed mutagenesis. ATP synthesis in the revertant strain was at near normal levels as judged by growth yield experiments, but the revertant strain was unable to pump protons in response to ATP hydrolysis.