H+-Adenosine triphosphatase and membrane energy coupling

Phosphorus Chemistry at the Roots of Bioenergetics: Ligand Permutation as the Molecular Basis of the Mechanism of ATP Synthesis/Hydrolysis by FOF1-ATP Synthase

The integration of phosphorus chemistry with the mechanism of ATP synthesis/hydrolysis requires dynamical information during ATP turnover and catalysis. Oxygen exchange reactions occurring at β-catalytic sites of the FOF1-ATP synthase/F1-ATPase imprint a unique record of molecular events during the catalytic cycle of ATP synthesis/hydrolysis. They have been shown to provide valuable time-resolved information on enzyme catalysis during ATP synthesis and ATP hydrolysis. The present work conducts new experiments on oxygen exchange catalyzed by submitochondrial particles designed to (i) measure the relative rates of Pi-ATP, Pi-HOH, and ATP-HOH isotope exchanges; (ii) probe the effect of ADP removal on the extent of inhibition of the exchanges, and (iii) test their uncoupler sensitivity/resistance. The objectives have been realized based on new experiments on submitochondrial particles, which show that both the Pi-HOH and ATP-HOH exchanges occur at a considerably higher rate relative to the Pi-ATP exchange, an observation that cannot be explained by previous mechanisms. A unifying explanation of the kinetic data that rationalizes these observations is given. The experimental results in (ii) show that ADP removal does not inhibit the intermediate Pi-HOH exchange when ATP and submitochondrial particles are incubated, and that the nucleotide requirement of the intermediate Pi-HOH exchange is adequately met by ATP, but not by ADP. These results contradicts the central postulate in Boyer's binding change mechanism of reversible catalysis at a F1 catalytic site with Keq~1 that predicts an absolute requirement of ADP for the occurrence of the Pi-HOH exchange. The prominent intermediate Pi-HOH exchange occurring under hydrolytic conditions is shown to be best explained by Nath's torsional mechanism of energy transduction and ATP synthesis/hydrolysis, which postulates an essentially irreversible cleavage of ATP by mitochondria/particles, independent from a reversible formation of ATP from ADP and Pi. The explanation within the torsional mechanism is also shown to rationalize the relative insensitivity of the intermediate Pi-HOH exchange to uncouplers observed in the experiments in (iii) compared to the Pi-ATP and ATP-HOH exchanges. This is shown to lead to new concepts and perspectives based on ligand displacement/substitution and ligand permutation for the elucidation of the oxygen exchange reactions within the framework of fundamental phosphorus chemistry. Fast mechanisms that realize the rotation/twist, tilt, permutation and switch of ligands, as well as inversion at the γ-phosphorus synchronously and simultaneously and in a concerted manner, have been proposed, and their stereochemical consequences have been analyzed. These considerations take us beyond the binding change mechanism of ATP synthesis/hydrolysis in bioenergetics.

A molecular piston mechanism of pumping protons by bacteriorhodopsin

In this review the proton-pumping mechanism proposed recently for bacteriorhodopsin [Chou, K. C. (1993) Journal of Protein Chemistry, 12: 337-350] is illustrated in terms of a phenomenological model. According to the model, theβ-ionone of the retinal chromophore in bacteriorhodopsin can be phenomenologically imagined as a molecular "piston". The photon capture by bacteriorhodopsin would "pull" it up while the spontaneous decrease in potential energy would "push" it down so that it would be up and down alternately during the photocycle process. When it is pulled up, the gate of pore is open and the water channel for the proton translocation is through; when it is pushed down, the gate of pore is closed and the water channel is shut up. Such a model not only is quite consistent with experimental observations, but also provides useful insights and a different view to elucidate the protonpumping mechanism of bacteriorhodopsin. The essence of the model might be useful in investigating the mechanism of ion-channels of other membrane proteins.

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.

Mitochondrial ATPase

Considerable progress has been made in recent years in our understanding of the phosphorylating apparatus in mitochondria, chloroplasts, and bacteria. It has become clear that the structure and the function of the ATP synthesizing apparatus in these widely divergent organisms is similar if not virtually identical. The subunit composition of F1, its molecular architecture, the location and function of substrate binding sites, as well as putative control sites, understanding of the component parts of the oligomycin-sensitive ATPase complex, and the role of these components in the function of the complex all are under active investigation in many laboratories. The developing information and the new insights provided have begun to permit experimental approaches, at the molecular level, to the mode of action of the ATPase in electron-transport-coupled ATP synthesis.

Contraction transitions of F1-F0 ATPase during catalytic turnover

Strong acoustic pressure was applied to submitochondrial particles (SMP) from bovine heart in order to drive ATP synthesis by F1-F0 complex for the account of sound waves. We observed a net ATP production at two narrow frequency ranges, about 170 Hz and about 340 Hz, that corresponds to the resonance oscillations of experimental cuvette when the acoustic pressure had a magnitude of 100 kPa. The results can be explained quantitatively by contractive conformational changes of F1-F0 complex during catalytic turnover. Negative staining electron microscopy of SMP preparations was used to visualize the ADP(Mg2+)-induced conformational changes of F1-F0 complex. In the particles with high ATPase activity in the presence of phosphate the factors F1 and F0 formed a congregated domain plunged into the membrane without any observable stalk in between. The presence of ADP(Mg2+) caused a structural rearrangement of F1-F0 to the essentially different conformation: the domains F1 and F0 were dislodged distinctly from each other and connected by a long thin stalk. The latter conformation resembled well the usual bipartite profile of ATPase. The data indicate that besides rotation, the catalytic turnover of ATP synthase is also accompanied by stretch transitions of F1-F0 complex.

Modulation of oxidative phosphorylation by Mg2+ in rat heart mitochondria

The effect of varying the Mg2+ concentration on the 2-oxoglutarate dehydrogenase (2-OGDH) activity and the rate of oxidative phosphorylation of rat heart mitochondria was studied. The ionophore A23187 was used to modify the mitochondrial free Mg2+ concentration. Half-maximal stimulation (K0.5) of ATP synthesis by Mg2+ was obtained with 0.13 +/- 0.02 mM (n = 7) with succinate (+rotenone) and 0.48 +/- 0.13 mM (n = 6) with 2-oxoglutarate (2-OG) as substrates. Similar K0.5 values were found for NAD(P)H formation, generation of membrane potential, and state 4 respiration with 2-OG. In the presence of ADP, an increase in Pi concentration promoted a decrease in the K0.5 values of ATP synthesis, membrane potential formation and state 4 respiration for Mg2+ with 2-OG, but not with succinate. These results indicate that 2-OGDH is the main step of oxidative phosphorylation modulated by Mg2+ when 2-OG is the oxidizable substrate; with succinate, the ATP synthase is the Mg2+-sensitive step. Replacement of Pi by acetate, which promotes changes on intramitochondrial pH abolished Mg2+ activation of 2-OGDH. Thus, the modulation of the 2-OGDH activity by Mg2+ has an essential requirement for Pi (and ADP) in intact mitochondria which is not associated to variations in matrix pH.

Reinstatement of the ATP high energy paradigm

The paradigm that the hydrolysis of ATP releases high Gibbs energy able to perform work has increasingly been questioned over the last two decades. Results from theoretical and experimental studies have been interpreted to indicate that the synthesis of ATP from ADP and P(i) does not require energy supply and that binding of ATP per se can transmit utilizable energy to an enzyme. As has recently been concluded, all this has led to a change of the ATP high energy paradigm in bioenergetics. Starting from this challenge, the present review singles out the striking sources of the apparent dichotomy in bioenergetics, and endeavours to eliminate the apparent contradictions by the application of the prior knowledge on both the participation of the enzyme protein in energy exchange processes and the particular reactivities of phosphorus that make it an outstanding element for functionally variable work assignments in enzymatic systems.

Photoaffinity cross-linking of F1ATPase from spinach chloroplasts by 3'-arylazido-beta-alanyl-8-azido ATP

UV irradiation of the ATPase (CF1) from spinach chloroplasts in the presence of 3'-arylazido-beta-alanyl-8-azido ATP (8,3'-DiN3ATP) results in a nucleotide-dependent inactivation of the enzyme and in a nucleotide-dependent formation of alpha-beta cross-links. The results demonstrate an interfacial localization of the nucleotide binding sites on CF1.

Conformational change during photocycle of bacteriorhodopsin and its proton-pumping mechanism

Based on the recent finding on the structural difference of seven helix bundles in the all-trans and 13-cis bacteriorhodopsins, the distances among the key groups performing the function of proton translocation as well as their microenvironments have been investigated. Consequently, a pore-gated model was proposed for the light-driven proton-pumping mechanism of bacteriorhodopsin. According to this model, the five double-bounded polyene chain in retinal chromophore can be phenomenologically likened to a molecular "lever," whose one end links to a "piston" (the beta-ionone ring) and the other end to a pump "relay station" (the Schiff base). During the photocycle of bacteriorhodopsin, the molecular "lever" is moving up and down as marked by the position change of the "piston," so as to trigger the gate of pore to open and close alternately. When the "piston" is up, the pore-controlled gate is open so that the water channel from Asp-96 to the Schiff base and that from the Schiff base to Asp-85 is established; when the "piston" is down, the pore-controlled gate is closed and the water channels for proton transportation in both the cytoplasmic half and extracellular half are blocked. The current model allows a consistent interpretation of a great deal of experimental data and also provides a useful basis for further investigating the mechanism of proton pumping by bacteriorhodopsin.

Transfer of tightly-bound tritium from the chloroplast membranes to CF1 is activated by the photophosphorylation process

Isolated thylakoids were incubated for 14-16 h in the buffer containing 3-6 mCi T2O/ml and then resedimented and suspended in non-radioactive medium. It was found that illumination of thylakoids induced an increase in radioactivity level in CF1 isolated from these thylakoids. Such effect was observed only if photophosphorylation substrates (ADP and phosphate or arsenate) were added to the medium during illumination. The light-induced ADP and arsenate-dependent incorporation of tritium into CF1 was suppressed by DCCD and inhibited by low gramicidin concentrations to the same extent as photophosphorylation.

The effect of inorganic pyrophosphate on the activity and Pi-binding properties of mitochondrial F1-ATPase

Interaction of F1-ATPase from beef heart mitochondria with PPi has been investigated. The presence of PPi in the ATPase assay medium does not affect the initial rate of ATP hydrolysis by F1-ATPase, but slows down the decrease of enzyme activity in the course of ATP hydrolysis and increases the steady-state rate of ATP hydrolysis. Being present in the ATPase assay medium, PPi accelerates the ATP-dependent reactivation of an inactive complex formed by F1-ATPase and ADP. This inactive complex is also reactivated after preincubation with PPi. F1-ATPase, preincubated with PPi, is inactivated by azide much more slowly than is the non-preincubated enzyme. PPi stimulates the binding of Pi to F1-ATPase by decreasing mainly the Kd for Pi and only slightly raising the stoichiometry of high-affinity Pi binding. It follows from the results obtained that PPi interacts with the non-catalytic site(s) of F1-ATPase.

Role of energy in oxidative phosphorylation

This article reviews the current status of information regarding the role of energy in the process of oxidative phosphorylation by mitochondria. The available data suggest that in submitochondrial particles (SMP) energy is utilized for the binding of ADP and Pi and for the release of ATP bound at the catalytic sites of F1-ATPase. The process of ATP synthesis on the surface of F1 from F1-bound ADP and Pi appears to be associated with negligible free energy change. The rate of energy production by the respiratory chain modulates the kinetics of ATP synthesis between a low Km (for ADP and Pi)-low Vmax mode and a high Km-high Vmax mode. The Km extremes for ADP are 2-3 microM and 120-150 microM, and Vmax for ATP synthesis at high rates of energy production by bovine-heart SMP is about 440 S-1 (mole F1)-1 at 30 degrees C, which corresponds to 11 mumol ATP (min.mg of protein)-1. The interaction of dicyclohexylcarbodiimide (DCCD) or oligomycin at the proteolipid (subunit c) of the membrane sector (F0) of the ATP synthase complex alters the mode of ATP binding at the catalytic sites of F1, probably to one of lower affinity. It has been suggested that protonic energy might be conveyed to the catalytic sites of F1 in an analogous manner, i.e., via conformation changes in the ATP synthase complex initiated by proton-induced alterations in the structure of the DCCD-binding proteolipid. Finally, the relationship between the steady-state membrane potential (delta psi) and the rates of electron transfer and ATP synthesis has been discussed. It has been shown, in agreement with the delocalized chemiosmotic mechanism, that under appropriate conditions delta psi is exquisitely sensitive to changes in the rates of energy production and consumption.

Thermal inactivation of electron-transport functions and F0F1-ATPase activities

Bovine heart submitochondrial particles in suspension were heated at a designated temperature for 3 min, then cooled for biochemical assays at 30 degrees C. By enzyme activity measurements and polarographic assay of oxygen consumption, it is shown that the thermal denaturation of the respiratory chain takes place in at least four stages and each stage is irreversible. The first stage occurs at 51.0 +/- 1.0 degrees C, with the inactivation of NADH-linked respiration, ATP-driven reverse electron transport, F0F1 catalyzed ATP/Pi exchange, NADH and succinate-driven ATP synthesis. The second stage occurs at 56.0 +/- 1.0 degrees C, with the inactivation of succinate-linked proton pumping and respiration. The third stage occurs at 59.0 +/- 1.0 degrees C, with the inactivation of electron transfer from cytochrome c to cytochrome oxidase and ATP-dependent proton pumping. The ATP hydrolysis activity of F0F1 persists to 61.0 +/- 1.0 degrees C. An additional transition, detectable by differential scanning calorimetry, occurring around 70.0 +/- 2.0 degrees C, is probably associated with thermal denaturation of cytochrome c and other stable membrane proteins. In the presence of either mitochondrial matrix fluid or 2 mM mercaptoethanol, all five stages give rise to endothermic effects, with the absorption of approx. 25 J/g protein. Under aerobic conditions, however, the first four transitions become strongly exothermic, and release a total of approx. 105 J/g protein. Solubilized and reconstituted F0F1 vesicles also exhibit different inactivation temperatures for the ATP/Pi exchange, proton pumping and ATP hydrolysis activities. The first two activities are abolished at 49.0 +/- 1.0 degrees C, but the latter at 58.0 +/- 2.0 degrees C. Differential scanning calorimetry also detects biphasic transitions of F0F1, with similar temperatures of denaturation (49.0 and 54.0 degrees C). From these and other results presented in this communication, the following is concluded. (1) A selective inactivation, by the temperature treatment, of various functions of the electron-transport chain and of the F0F1 complex can be done. (2) The ATP synthesis activity of the F0F1 complex involves either a catalytic or a regulation subunit(s) which is not essential for ATP hydrolysis and the proton translocation. This subunit is 10 degrees C less stable than the hydrolytic site. Micromolar ADP stabilizes it from thermal denaturation by 4-5 degrees C, although ADP up to millimolar concentration does not protect the hydrolytic site and the proton-translocation site.(ABSTRACT TRUNCATED AT 400 WORDS)

Electron-microscopic studies on location of SH-groups in mitochondrial F1-ATPase using a ferritin label

A new approach has been suggested for electron-microscopic study of the structure of mitochondrial F1-ATPase based on ferritin labeling. By means of sequential treatment with 2-iminothiolane and Nbs2 we obtained a modified ferritin (NbsSPrCNH-Ft) able to react with SH-groups of proteins and to form conjugates in which the protein and ferritin are bound by disulfide bonds. An electron-microscopic investigation of the negatively stained preparations of mitochondrial F1-ATPase, preincubated with modified ferritin, revealed such enzyme-ferritin conjugates. In case of modified ferritin, containing 360 mol SH-groups per mol protein, and F1-ATPase, pretreated with N-ethylmaleimide and then with dithiothreitol, conjugates were obtained in which ferritin molecules are bound to several (as many as four) of the six protein masses, comprising a bilayer molecule of the enzyme. Taking into consideration the biochemical data on the location of accessible SH-groups (only in alpha, gamma or epsilon subunits), it is inferred from the results obtained that one of the protein masses is a complex between beta subunit and at least one of the minor subunits located partially on the molecule's external side. This indicates the nonequivalence of different copies of the major subunits. Averaged images of the particles of the F1-F0 complex from bovine heart mitochondria and bacteria Micrococcus lysodeicticus were obtained. It was found that F0 component is bound to two adjacent protein masses of the F1-ATPase molecule. It is suggested that this binding may be due the nonequivalency of single-type major subunits.

Regulation of the mitochondrial ATP synthase/ATPase complex

The mitochondrial ATP synthase/ATPase (F0F1 ATPase) is perhaps the most complex enzyme known. In animal systems it consists of a minimum of 11 different polypeptide chains, 10 (or more) of which appear to be essential for function, and 1 called the "ATPase inhibitor peptide" which is involved in regulation. Recent studies from a variety of laboratories indicate that the ATP synthase/ATPase complex is regulated by several interrelated factors including the thermodynamic poise of the proton gradient across the inner mitochondrial membrane; the ATPase inhibitor peptide; ADP (and/or ADP and Pi); divalent cations; and perhaps the redox state of SH groups on the F1 molecule. The central focus of this review is the ATPase inhibitor peptide. A model involving four distinct conformational states of F1 seems essential to account for the inhibitor's mode of action. The model depicts the ATPase inhibitor protein as acting at the asymmetric center of the F1 moiety. In addition, it accounts for the "unidirectional" role of the inhibitor peptide as a "down regulator" of ATP hydrolysis and for its binding/debinding dependence on the proton motive force and other regulatory factors. Finally, it is suggested that during any physiological process, where there is an energy demand followed by a resting phase, the F1 molecule may follow a "cyclic" path involving the four distinct conformational states of the enzyme.

Kinetics of ATP hydrolysis catalyzed by isolated TF1 and reconstituted TF0F1 ATPase

The rate of ATP hydrolysis catalyzed by isolated TF1 and reconstituted TF0F1 was measured as a function of the ATP concentration in the presence of inhibitors [ADP, Pi and 3'-O-(1-naphthoyl)ATP]. ATP hydrolysis can be described by Michaelis-Menten kinetics with Km(TF1) = 390 microM and Km (TF0F1) = 180 microM. The inhibition constants are for ADP Ki(TF1) = 20 microM and Ki(TF0F1) = 100 microM, for 3'-O-(1-naphthoyl)ATP Ki(TF1) = 150 microM and Ki(TF0F1) = 3 microM, and for Pi Ki(TF1) = 60 mM. From these results it is concluded that upon binding of TF0 to TF1 the mechanism of ATP hydrolysis catalyzed by TF1 is not changed qualitatively; however, the kinetic constants differ quantitatively.

A chemically explicit model for the molecular mechanism of the F1F0 H+-ATPase/ATP synthases

A general hypothesis for the molecular mechanism of membrane transport based on current knowledge of protein structure and the nature of ligand-induced protein conformational changes has recently been proposed [Scarborough, G. A. (1985) Microbiol. Rev. 49, 214-231]. According to this hypothesis, the essential reaction undergone by all proteinaceous transport catalysts is a ligand-induced hinge-bending-type conformational change that results in the transposition of binding-site residues from access on one side of the membrane to access on the other side. Subsequent release and/or alteration of the ligand or ligands that induce the conformational change facilitates the converse conformational change, which returns the binding-site residues to their original position. With this simple cyclic ligand-dependent gating process as a central feature, biochemically orthodox mechanisms for virtually all known transporters are readily conceived. In this article, a chemically explicit model for the molecular mechanism of the F1F0 H+-ATPase/ATP synthases of mitochondria, bacteria, and chloroplasts, formulated within the guidelines of this general transport paradigm, is presented. At least three points of potential interest arise from this exercise. First, with the aid of the model, it is possible to visualize how energy transduction catalyzed by these enzymes might proceed, with no major events left unspecified. Second, explicit possibilities as to the molecular nature of electric field effects on the transport process are raised. And finally, it is shown that enzyme conformational changes, energy-dependent binding-affinity changes, and several other related phenomena as well, need not be taken as evidence of "action at a distance" or indirect energy coupling mechanisms, as is sometimes assumed, because such events are also integral features of the mechanism presented, even though all of the key reactions proposed for both ATP-driven proton translocation and proton translocation-driven ATP synthesis occur at the enzyme active site.

Integrated functioning of the chloroplast coupling factor

This review is focused on some functional characteristics of the chloroplast coupling factor. The structure of the enzyme and the putative role of its subunits are recalled. An attempt is made to discriminate the driving force and the activator effects of the electrochemical proton gradient. Respective roles of delta pH, delta phi, external and internal pH are discussed with regard to mechanistic implications. The hypothesis of a functional switch of the enzyme between two states with better efficiency either in ATP synthesis or in ATP hydrolysis is also examined. A brief survey is made on some problems complicating quantitative studies of energy coupling, such as localized chemiosmosis, delta pH and delta phi computations, and scalar ATPases. The main data on the enzyme activation and the energy-dependent release of tightly bound nucleotides are summarized. The arguments for and against the catalytic competence of theses nucleotides are reviewed. Lastly, some prevailing models of the catalytic mechanism are presented. The relevance of nucleotides binding change events in this process is discussed.

A simple nonequilibrium thermodynamic description of some inhibitors of oxidative phosphorylation

We propose a macroscopic description of some inhibitors of oxidative phosphorylation based on a simple modification of the phenomenological coefficients appearing in the constitutive equations of linear irreversible thermodynamics. In this theoretical model, we consider protonophores, some ATPase inactivators and some electron-chain inhibitors, and we provide quantitative expressions for their consequences on the protonmotive force, oxidation flux and phosphorylation flux as well as on heat generation.

Mechanistic investigation on the temperature dependence and inhibition of corn root plasma membrane ATPase

The kinetics of corn root plasma membrane-catalyzed Mg-ATP hydrolysis may be satisfactorily described by a simple Michaelis-Menten scheme. It was found that the Km of the process was relatively insensitive to changes in temperature. This property allowed us to conveniently estimate the activation energy of the enzyme turnover process as approximately 14 kcal mol-1 in the temperature range of 10 to 45 degrees C. The enzyme activity was inhibited by the presence of diethystilbestrol (DES), miconazole, vanadate, and dicyclohexylcarbodiimide (DCCD). The inhibition caused by DES and miconazole was strictly uncompetitive and inhibition by vanadate was noncompetitive. The inhibition by DCCD showed a substrate concentration dependence, i.e., competitive at high and uncompetitive at low concentrations of Mg-ATP. The 1/V vs [I] plots suggested that there were different but unique binding sites for DES, vanadate, and miconazole. However, the modification of the plasma membrane by DCCD exhibited interaction with multiple sites. Unlike yeast plasma membrane ATPase, the enzyme of corn root cells was not affected by the treatment with N-ethylmaleimide. Although the enzyme activity was regulated by ADP, a product of the reaction, the presence of inorganic phosphate showed no inhibition to the hydrolysis of Mg-ATP.

What family of ATPases does the vacuolar H+-ATPase belong to?

Polyacrylamide gel electrophoresis (PAGE) of partially purified ATPase from vacuoles of Saccharomyces carlsbergensis under non-dissociating conditions revealed 3 bands with ATPase activity. Further PAGE in dissociating conditions showed the similarity in composition of these 3 ATPase preparations. They were assumed to contain the same vacuolar ATPase exhibiting, however, different electrophoretic mobility due to the formation of enzyme complexes with different proteins and phospholipids. The ATPase preparation with the highest electrophoretic mobility contained 6 subunits of 75, 62, 16, 14, 12 and 9 kDa. Inhibitors of vacuolar ATPase [14C]DCCD and [14C]NEM bound to a 9 kDa polypeptide, while [14C]DES associated with a polypeptide of 75 kDa. A partially purified preparation of the vacuolar ATPase was not phosphorylated by [gamma-32P]ATP under conditions when plasma membrane ATPase formed a phosphorylated intermediate. Our results show that vacuolar H+-ATPase consists of several polypeptides, does not form the phosphorylated intermediate, and evidently represents a new type of H+-ATPase of yeast.

A plasma membrane Mg2+-ATPase in the cellular slime mold Dictyostelium discoideum

Evidence is presented for the existence of a plasma membrane ATPase in Dictyostelium discoideum. The enzyme is dependent on Mg+2, and is insensitive to both azide and oligomycin. It is, however, sensitive to diethylstilbestrol, dicyclohexylcarbodiimide, vanadate, and thimerosal. Monovalent cations (Na+, K+, and choline) have no effect on enzyme activity, but high concentrations of Ca+2 are highly inhibitory. Vegetative cells express the highest amount of enzyme activity; the activity decreases three- to fourfold during the early stages of differentiation, and then remains constant during the latter stages.

Energetics of threonine uptake by pod wall tissues of Vicia faba L

Kjeldahl assays showed that the pod wall of Vicia faba fruits behaves as a transitory reservoir of nitrogen. We have studied the properties and energetics of amino-acid uptake during the accumulating stage of pod wall development. A comparative analysis using various inhibitors or activators of the proton pump has been carried out i) on threonine uptake, ii) on the acidifying activity of the tissues, and iii) on the transmembrane potential difference of mesocarp cells. Except for the effect of dicyclohexylcarbodiimide which could not be satisfactorily explained, all other results obtained with ATPase inhibitors, uncouplers and fusicoccin were consistent with the view of a transport energized by the proton-motive force. Adding threonine to a medium containing fragments of pericarp or of endocarp induced a pH change (to-wards more alkaline values) of the medium and a membrane depolarization of the storage cells which depended on the amino-acid concentration added. These data indicate H(+)-threonine cotransport in the pod wall of broad bean. Moreover, because p-chloromercuribenzenesulphonic acid inhibits threonine uptake without affecting the transmembrane potential difference, it is concluded that the threonine carrier possesses a functional SH-group located at the external side of the plasmalemma.

Electron microscopy of beef heart mitochondrial F1-ATPase

The quaternary structure of isolated and membrane-bound F1-ATPase (submitochondrial particles) has been studied by electron microscopy. A model of the molecule has been proposed: six protein masses are arranged in two layers approximately at the vertices of a triangular antiprism. Computer averaging of the images showed that the frontal view of the molecule can be approximately characterized by mirror plane symmetry.

Localization of the tight ADP-binding site on the membrane-bound chloroplast coupling factor one

The photoaffinity analog 2-azido-ADP (2-azidoadenosine 5'-diphosphate) was used as a probe of the spinach chloroplast ATP synthase. The analog acted as a substrate for photophosphorylation. Several observations suggested that 2-azido-ADP and ADP bound to the same class of tight nucleotide binding sites: (a) 2-azido-ADP competitively inhibited ADP tight binding (Ki = 1.4 microM); (b) the concentration giving 50% maximum binding, K0.5 for analog tight binding (1 microM) was similar to that observed for ADP (2 microM); (c) nucleotide tight binding required prior membrane energization and was completely reversed by re-energization; (d) the tight binding of 2-azido-[beta-32P]ADP was completely prevented by ADP; (e) the analog inhibited the light-triggered ATPase activity at micromolar concentrations. Ultraviolet irradiation of washed thylakoid membranes containing tightly bound 2-azido-[beta-32P]ADP resulted in the covalent incorporation of the label into the membranes. Denaturing polyacrylamide gel electrophoresis of the labeled membranes demonstrated that the beta subunit of the coupling factor one complex was the only polypeptide in the thylakoid membranes which was labeled. These results identify the beta subunit of the coupling factor as the location of the tightly bound ADP on the thylakoid membranes.

Evaluation by steady-state enzyme kinetics of the role of tightly bound nucleotides during photophosphorylation

The ATP synthetase of chloroplast membranes binds ADP and ATP with high affinity, and the binding becomes quasi-irreversible under certain conditions. One explanation of the function of these nucleotides is that they are transiently tightly bound during ATP synthesis as part of the catalytic process, and that the release of tightly bound ATP from one catalytic site is promoted when ADP and P(i) bind to a second catalytic site on the enzyme. Alternatively, it is possible that the tightly bound nucleotides are not catalytic, but instead have some regulatory function. We developed steady-state rate equations for both these models for photophosphorylation and tested them with experiments where two alternative substrates, ADP and GDP, were phosphorylated simultaneously. It was impossible to fit the results to the equations that assumed a catalytic role for tightly bound nucleotides, whether we assumed that both ADP and GDP, or only ADP, are phosphorylated by a mechanism involving substrate-induced release of product from another catalytic site. On the other hand, the equations derived from the regulatory-site model that we tested were able to fit all the results relatively well and in an internally consistent manner. We therefore conclude that the tightly bound nucleotides most likely do not derive from catalytic intermediates of ATP synthesis, but that substrate (and possibly also product) probably bind both to catalytic sites and to noncatalytic sites. The latter may modulate the transition of the ATP-synthesizing enzyme complex between its active and inactive states.