Kinetic mechanism of the exchanges catalysed by the adenine-nucleotide carrier

Should the standard model of cellular energy metabolism be reconsidered? Possible coupling between the pentose phosphate pathway, glycolysis and extra-mitochondrial oxidative phosphorylation

The process of cellular respiration occurs for energy production through catabolic reactions, generally with glucose as the first process step. In the present work, we introduce a novel concept for understanding this process, based on our conclusion that glucose metabolism is coupled to the pentose phosphate pathway (PPP) and extra-mitochondrial oxidative phosphorylation in a closed-loop process. According to the current standard model of glycolysis, glucose is first converted to glucose 6-phosphate (glucose 6-P) and then to fructose 6-phosphate, glyceraldehyde 3-phosphate and pyruvate, which then enters the Krebs cycle in the mitochondria. However, it is more likely that the pyruvate will be converted to lactate. In the PPP, glucose 6-P is branched off from glycolysis and used to produce NADPH and ribulose 5-phosphate (ribulose 5-P). Ribulose 5-P can be converted to fructose 6-P and glyceraldehyde 3-P. In our view, a circular process can take place in which the ribulose 5-P produced by the PPP enters the glycolysis pathway and is then retrogradely converted to glucose 6-P. This process is repeated several times until the complete degradation of glucose 6-P. The role of mitochondria in this process is to degrade lipids by beta-oxidation and produce acetyl-CoA; the function of producing ATP appears to be only secondary. This proposed new concept of cellular bioenergetics allows the resolution of some previously unresolved controversies related to cellular respiration and provides a deeper understanding of metabolic processes in the cell, including new insights into the Warburg effect.

Mitochondrial morphology governs ATP production rate

Life is based on energy conversion. In particular, in the nervous system, significant amounts of energy are needed to maintain synaptic transmission and homeostasis. To a large extent, neurons depend on oxidative phosphorylation in mitochondria to meet their high energy demand. For a comprehensive understanding of the metabolic demands in neuronal signaling, accurate models of ATP production in mitochondria are required. Here, we present a thermodynamically consistent model of ATP production in mitochondria based on previous work. The significant improvement of the model is that the reaction rate constants are set such that detailed balance is satisfied. Moreover, using thermodynamic considerations, the dependence of the reaction rate constants on membrane potential, pH, and substrate concentrations are explicitly provided. These constraints assure that the model is physically plausible. Furthermore, we explore different parameter regimes to understand in which conditions ATP production or its export are the limiting steps in making ATP available in the cytosol. The outcomes reveal that, under the conditions used in our simulations, ATP production is the limiting step and not its export. Finally, we performed spatial simulations with nine 3-D realistic mitochondrial reconstructions and linked the ATP production rate in the cytosol with morphological features of the organelles.

Crosstalk between adenine nucleotide transporter and mitochondrial swelling: experimental and computational approaches

Mitochondrial metabolism and function are modulated by changes in matrix Ca²⁺. Small increases in the matrix Ca²⁺ stimulate mitochondrial bioenergetics, whereas excessive Ca²⁺ leads to cell death by causing massive matrix swelling and impairing the structural and functional integrity of mitochondria. Sustained opening of the non-selective mitochondrial permeability transition pores (PTP) is the main mechanism responsible for mitochondrial Ca²⁺ overload that leads to mitochondrial dysfunction and cell death. Recent studies suggest the existence of two or more types of PTP, and adenine nucleotide translocator (ANT) and F_OF₁-ATP synthase were proposed to form the PTP independent of each other. Here, we elucidated the role of ANT in PTP opening by applying both experimental and computational approaches. We first developed and corroborated a detailed model of the ANT transport mechanism including the matrix (ANT_M), cytosolic (ANT_C), and pore (ANT_P) states of the transporter. Then, the ANT model was incorporated into a simple, yet effective, empirical model of mitochondrial bioenergetics to ascertain the point when Ca²⁺ overload initiates PTP opening via an ANT switch-like mechanism activated by matrix Ca²⁺ and is inhibited by extra-mitochondrial ADP. We found that encoding a heterogeneous Ca²⁺ response of at least three types of PTPs, weakly, moderately, and strongly sensitive to Ca²⁺, enabled the model to simulate Ca²⁺ release dynamics observed after large boluses were administered to a population of energized cardiac mitochondria. Thus, this study demonstrates the potential role of ANT in PTP gating and proposes a novel mechanism governing the cryptic nature of the PTP phenomenon.

Efficient extra-mitochondrial aerobic ATP synthesis in neuronal membrane systems

The nervous system displays high energy consumption, apparently not fulfilled by mitochondria, which are underrepresented therein. The oxidative phosphorylation (OxPhos) activity, a mitochondrial process that aerobically provides ATP, has also been reported also in the myelin sheath and the rod outer segment (OS) disks. Thus, commonalities and differences between the extra-mitochondrial and mitochondrial aerobic metabolism were evaluated in bovine isolated myelin (IM), rod OS, and mitochondria-enriched fractions (MIT). The subcellular fraction quality and the absence of contamination fractions have been estimated by western blot analysis. Oxygen consumption and ATP synthesis were stimulated by conventional (pyruvate + malate or succinate) and unconventional (NADH) substrates, observing that oxygen consumption and ATP synthesis by IM and rod OS are more efficient than by MIT, in the presence of both kinds of respiratory substrates. Mitochondria did not utilize NADH as a respiring substrate. When ATP synthesis by either sample was assayed in the presence of 10-100 µM ATP in the assay medium, only in IM and OS it was not inhibited, suggesting that the ATP exportation by the mitochondria is limited by extravesicular ATP concentration. Interestingly, IM and OS but not mitochondria appear able to synthesize ATP at a later time with respect to exposure to respiratory substrates, supporting the hypothesis that the proton gradient produced by the electron transport chain is buffered by membrane phospholipids. The putative transfer mode of the OxPhos molecular machinery from mitochondria to the extra-mitochondrial structures is also discussed, opening new perspectives in the field of neurophysiology.

A 20/20 view of ANT function in mitochondrial biology and necrotic cell death

The adenosine nucleotide translocase (ANT) family of proteins are inner mitochondrial membrane proteins involved in energy homeostasis and cell death. The primary function of ANT proteins is to exchange cytosolic ADP with matrix ATP, facilitating the export of newly synthesized ATP to the cell while providing new ADP substrate to the mitochondria. As such, the ANT proteins are central to maintaining energy homeostasis in all eukaryotic cells. Evidence also suggests that the ANTs constitute a pore-forming component of the mitochondrial permeability transition pore (MPTP), a structure that forms in the inner mitochondrial membrane that is thought to underlie regulated necrotic cell death. Additionally, emerging studies suggest that ANT proteins are also critical for mitochondrial uncoupling and for promoting mitophagy. Thus, the ANTs are multifunctional proteins that are poised to participate in several aspects of mitochondrial biology and the greater regulation of cell death, which will be discussed here.

Mitochondrial morphology provides a mechanism for energy buffering at synapses

Mitochondria as the main energy suppliers of eukaryotic cells are highly dynamic organelles that fuse, divide and are transported along the cytoskeleton to ensure cellular energy homeostasis. While these processes are well established, substantial evidence indicates that the internal structure is also highly variable in dependence on metabolic conditions. However, a quantitative mechanistic understanding of how mitochondrial morphology affects energetic states is still elusive. To address this question, we here present an agent-based multiscale model that integrates three-dimensional morphologies from electron microscopy tomography with the molecular dynamics of the main ATP producing components. We apply our modeling approach to mitochondria at the synapse which is the largest energy consumer within the brain. Interestingly, comparing the spatiotemporal simulations with a corresponding space-independent approach, we find minor spatial effects when the system relaxes toward equilibrium but a qualitative difference in fluctuating environments. These results suggest that internal mitochondrial morphology is not only optimized for ATP production but also provides a mechanism for energy buffering and may represent a mechanism for cellular robustness.

The transport mechanism of the mitochondrial ADP/ATP carrier

The mitochondrial ADP/ATP carrier imports ADP from the cytosol and exports ATP from the mitochondrial matrix, which are key transport steps for oxidative phosphorylation in eukaryotic organisms. The transport protein belongs to the mitochondrial carrier family, a large transporter family in the inner membrane of mitochondria. It is one of the best studied members of the family and serves as a paradigm for the molecular mechanism of mitochondrial carriers. Structurally, the carrier consists of three homologous domains, each composed of two transmembrane α-helices linked with a loop and short α-helix on the matrix side. The transporter cycles between a cytoplasmic and matrix state in which a central substrate binding site is alternately accessible to these compartments for binding of ADP or ATP. On both the cytoplasmic and matrix side of the carrier are networks consisting of three salt bridges each. In the cytoplasmic state, the matrix salt bridge network is formed and the cytoplasmic network is disrupted, opening the central substrate binding site to the intermembrane space and cytosol, whereas the converse occurs in the matrix state. In the transport cycle, tighter substrate binding in the intermediate states allows the interconversion of conformations by lowering the energy barrier for disruption and formation of these networks, opening and closing the carrier to either side of the membrane in an alternating way. Conversion between cytoplasmic and matrix states might require the simultaneous rotation of three domains around a central translocation pathway, constituting a unique mechanism among transport proteins. This article is part of a Special Issue entitled: Mitochondrial Channels edited by Pierre Sonveaux, Pierre Maechler and Jean-Claude Martinou.

Mapping multiple potential ATP binding sites on the matrix side of the bovine ADP/ATP carrier by the combined use of MD simulation and docking

The mitochondrial adenosine diphosphate/adenosine triphosphate (ADP/ATP) carrier-AAC-was crystallized in complex with its specific inhibitor carboxyatractyloside (CATR). The protein consists of a six-transmembrane helix bundle that defines the nucleotide translocation pathway, which is closed towards the matrix side due to sharp kinks in the odd-numbered helices. In this paper, we describe the interaction between the matrix side of the AAC transporter and the ATP(4-) molecule using carrier structures obtained through classical molecular dynamics simulation (MD) and a protein-ligand docking procedure. Fifteen structures were extracted from a previously published MD trajectory through clustering analysis, and 50 docking runs were carried out for each carrier conformation, for a total of 750 runs ("MD docking"). The results were compared to those from 750 docking runs performed on the X-ray structure ("X docking"). The docking procedure indicated the presence of a single interaction site in the X-ray structure that was conserved in the structures extracted from the MD trajectory. MD docking showed the presence of a second binding site that was not found in the X docking. The interaction strategy between the AAC transporter and the ATP(4-) molecule was analyzed by investigating the composition and 3D arrangement of the interaction pockets, together with the orientations of the substrate inside them. A relationship between sequence repeats and the ATP(4-) binding sites in the AAC carrier structure is proposed.

Mitochondrial carriers function as monomers

Mitochondrial carriers link biochemical pathways in the mitochondrial matrix and cytosol by transporting metabolites, inorganic ions, nucleotides and cofactors across the mitochondrial inner membrane. Uncoupling proteins that dissipate the proton electrochemical gradient also belong to this protein family. For almost 35 years the general consensus has been that mitochondrial carriers are dimeric in structure and function. This view was based on data from inhibitor binding studies, small-angle neutron scattering, electron microscopy, differential tagging/affinity chromatography, size-exclusion chromatography, analytical ultracentrifugation, native gel electrophoresis, cross-linking experiments, tandem-fusions, negative dominance studies and mutagenesis. However, the structural folds of the ADP/ATP carriers were found to be monomeric, lacking obvious dimerisation interfaces. Subsequently, the yeast ADP/ATP carrier was demonstrated to function as a monomer. Here, we revisit the data that have been published in support of a dimeric state of mitochondrial carriers. Our analysis shows that when critical factors are taken into account, the monomer is the only plausible functional form of mitochondrial carriers. We propose a transport model based on the monomer, in which access to a single substrate binding site is controlled by two flanking salt bridge networks, explaining uniport and strict exchange of substrates.

Modeling mitochondrial bioenergetics with integrated volume dynamics

Mathematical models of mitochondrial bioenergetics provide powerful analytical tools to help interpret experimental data and facilitate experimental design for elucidating the supporting biochemical and physical processes. As a next step towards constructing a complete physiologically faithful mitochondrial bioenergetics model, a mathematical model was developed targeting the cardiac mitochondrial bioenergetic based upon previous efforts, and corroborated using both transient and steady state data. The model consists of several modified rate functions of mitochondrial bioenergetics, integrated calcium dynamics and a detailed description of the K⁺-cycle and its effect on mitochondrial bioenergetics and matrix volume regulation. Model simulations were used to fit 42 adjustable parameters to four independent experimental data sets consisting of 32 data curves. During the model development, a certain network topology had to be in place and some assumptions about uncertain or unobserved experimental factors and conditions were explicitly constrained in order to faithfully reproduce all the data sets. These realizations are discussed, and their necessity helps contribute to the collective understanding of the mitochondrial bioenergetics.

Mathematically modeling biological systems challenges our current understanding of the physical and biochemical events contributing to the observed dynamics. It requires careful consideration of hypothesized mechanisms, model development assumptions and details regarding the experimental conditions. We have adopted a modeling approach to translate these factors that explicitly considers the thermodynamic constraints, biochemical states and reaction mechanisms during model development. Such models have numerous constant parameters that must be determined. Integrating thermodynamics and detailed mechanistic representation of the principal phenomena help constrain these parameter values; therefore, only a handful of the total number of model parameters (∼10%) must be adjusted during parameter estimation through model simulations. Additionally, all models must undergo some form of corroboration prior to application. In practice, this corroboration should challenge all possible dynamics of the model, but it is recognized that in this data rich world, we are surprisingly data poor. Eventually such developed and corroborated models are capable of supporting current hypotheses, guiding experimental designs and contributing to the overall knowledge base of biological processes.

Magnitude and control of mitochondrial sensitivity to ADP

The transduction function for ADP stimulation of mitochondrial ATP synthesis in skeletal muscle was reconstructed in vivo and in silico to investigate the magnitude and origin of mitochondrial sensitivity to cytoplasmic ADP concentration changes. Dynamic in vivo measurements of human leg muscle phosphocreatine (PCr) content during metabolic recovery from contractions were performed by (31)P-NMR spectroscopy. The cytoplasmic ADP concentration ([ADP]) and rate of oxidative ATP synthesis (Jp) at each time point were calculated from creatine kinase equilibrium and the derivative of a monoexponential fit to the PCr recovery data, respectively. Reconstructed [ADP]-Jp relations for individual muscles containing more than 100 data points were kinetically characterized by nonlinear curve fitting yielding an apparent kinetic order and ADP affinity of 1.9 +/- 0.2 and 0.022 +/- 0.003 mM, respectively (means +/- SD; n = 6). Next, in silico [ADP]-Jp relations for skeletal muscle were generated using a computational model of muscle oxidative ATP metabolism whereby model parameters corresponding to mitochondrial enzymes were randomly changed by 50-150% to determine control of mitochondrial ADP sensitivity. The multiparametric sensitivity analysis showed that mitochondrial ADP ultrasensitivity is an emergent property of the integrated mitochondrial enzyme network controlled primarily by kinetic properties of the adenine nucleotide translocator.

A novel kinetic assay of mitochondrial ATP-ADP exchange rate mediated by the ANT

A novel method exploiting the differential affinity of ADP and ATP to Mg(2+) was developed to measure mitochondrial ADP-ATP exchange rate. The rate of ATP appearing in the medium after addition of ADP to energized mitochondria, is calculated from the measured rate of change in free extramitochondrial [Mg(2+)] reported by the membrane-impermeable 5K(+) salt of the Mg(2+)-sensitive fluorescent indicator, Magnesium Green, using standard binding equations. The assay is designed such that the adenine nucleotide translocase (ANT) is the sole mediator of changes in [Mg(2+)] in the extramitochondrial volume, as a result of ADP-ATP exchange. We also provide data on the dependence of ATP efflux rate within the 6.8-7.8 matrix pH range as a function of membrane potential. Finally, by comparing the ATP-ADP steady-state exchange rate to the amount of the ANT in rat brain synaptic, brain nonsynaptic, heart and liver mitochondria, we provide molecular turnover numbers for the known ANT isotypes.

The mitochondrial ADP/ATP carrier: functional and structural studies in the route of elucidating pathophysiological aspects

The mitochondrial ADP/ATP carrier plays a central role in aerobic cell energetics by providing to the cytosol the ATP generated by oxidative phosphorylation. Though discovered around 40 years ago owing to the existence of unique inhibitors and in spite of numerous experimental approaches, this carrier, which stands as a model of the mitochondrial solute carriers keeps some long-standing mystery. There are still open challenging questions among them the precise ADP/ATP transport mechanism, the functional oligomeric state of the carrier and relationships between human ADP/ATP carrier dysfunctioning and pathologies. Deciphering the 3D structure of this carrier afforded a considerable progress of the knowledge but requires now additional data focused on molecular dynamics from this static picture. State of the art in this topic is reviewed and debated in this paper in view of better comprehending origin of the discrepancies in these questions and, finally, the multiple physiological roles of this carrier in eukaryotic cell economy.

The ADP and ATP transport in mitochondria and its carrier

Different from some more specialised short reviews, here a general although not encyclopaedic survey of the function, metabolic role, structure and mechanism of the ADP/ATP transport in mitochondria is presented. The obvious need for an "old fashioned" review comes from the gateway role in metabolism of the ATP transfer to the cytosol from mitochondria. Amidst the labours, 40 or more years ago, of unravelling the role of mitochondrial compartments and of the two membranes, the sequence of steps of how ATP arrives in the cytosol became a major issue. When the dust settled, a picture emerged where ATP is exported across the inner membrane in a 1:1 exchange against ADP and where the selection of ATP versus ADP is controlled by the high membrane potential at the inner membrane, thus uplifting the free energy of ATP in the cytosol over the mitochondrial matrix. Thus the disparate energy and redox states of the two major compartments are bridged by two membrane potential responsive carriers to enable their symbiosis in the eukaryotic cell. The advance to the molecular level by studying the binding of nucleotides and inhibitors was facilitated by the high level of carrier (AAC) binding sites in the mitochondrial membrane. A striking flexibility of nucleotide binding uncovered the reorientation of carrier sites between outer and inner face, assisted by the side specific high affinity inhibitors. The evidence of a single carrier site versus separate sites for substrate and inhibitors was expounded. In an ideal setting principles of transport catalysis were elucidated. The isolation of intact AAC as a first for any transporter enabled the reconstitution of transport for unravelling, independently of mitochondrial complications, the factors controlling the ADP/ATP exchange. Electrical currents measured with the reconstituted AAC demonstrated electrogenic translocation and charge shift of reorienting carrier sites. Aberrant or vital para-functions of AAC in basal uncoupling and in the mitochondrial pore transition were demonstrated in mitochondria and by patch clamp with reconstituted AAC. The first amino acid sequence of AAC and of any eukaryotic carrier furnished a 6-transmembrane helix folding model, and was the basis for mapping the structure by access studies with various probes, and for demonstrating the strong conformation changes demanded by the reorientation mechanism. Mutations served to elucidate the function of residues, including the particular sensitivity of ATP versus ADP transport to deletion of critical positive charge in AAC. After resisting for decades, at last the atomic crystal structure of the stabilised CAT-AAC complex emerged supporting the predicted principle fold of the AAC but showing unexpected features relevant to mechanism. Being a snapshot of an extreme abortive "c-state" the actual mechanism still remains a conjecture.

Leishmania mexicana amazonensis: plasma membrane adenine nucleotide translocator and chemotaxis

Leishmania cannot synthesize purines de novo and rely on their host to furnish these compounds. To accomplish this, they possess multiple purine nucleoside and nucleobase transporters. Subcellular fractionation, immunohistochemical localization with anti-adenine nucleotide translocator (ANT) antibodies and surface biotinylation show that the mitochondrial ANT is also present in the plasma membrane of both promastigotes and amastigotes. Leishmania, however, do not appear to rely on this transporter to supplement their purine or energy requirements via preformed ATP from its host. Rather, Leishmania appear to use the plasma membrane ANT as part of a chemotaxis response. ATP is a chemorepellant for Leishmania and cells treated with atractyloside, an inhibitor of ANT, no longer exhibit negative chemotaxis for this compound.

The creatine kinase phosphotransfer network: thermodynamic and kinetic considerations, the impact of the mitochondrial outer membrane and modelling approaches

In this review, we summarize the main structural and functional data on the role of the phosphocreatine (PCr)--creatine kinase (CK) pathway for compartmentalized energy transfer in cardiac cells. Mitochondrial creatine kinase, MtCK, fixed by cardiolipin molecules in the vicinity of the adenine nucleotide translocator, is a key enzyme in this pathway. Direct transfer of ATP and ADP between these proteins has been revealed both in experimental studies on the kinetics of the regulation of mitochondrial respiration and by mathematical modelling as a main mechanism of functional coupling of PCr production to oxidative phosphorylation. In cells in vivo or in permeabilized cells in situ, this coupling is reinforced by limited permeability of the outer membrane of the mitochondria for adenine nucleotides due to the contacts with cytoskeletal proteins. Due to these mechanisms, at least 80% of total energy is exported from mitochondria by PCr molecules. Mathematical modelling of intracellular diffusion and energy transfer shows that the main function of the PCr-CK pathway is to connect different pools (compartments) of ATP and, by this way, to overcome the local restrictions and diffusion limitation of adenine nucleotides due to the high degree of structural organization of cardiac cells.

Relations between structure and function of the mitochondrial ADP/ATP carrier

Import and export of metabolites through mitochondrial membranes are vital processes that are highly controlled and regulated at the level of the inner membrane. Proteins of the mitochondrial carrier family ( MCF ) are embedded in this membrane, and each member of the family achieves the selective transport of a specific metabolite. Among these, the ADP/ATP carrier transports ADP into the mitochondrial matrix and exports ATP toward the cytosol after its synthesis. Because of its natural abundance, the ADP/ATP carrier is the best characterized within MCF, and a high-resolution structure of one conformation is known. The overall structure is basket shaped and formed by six transmembrane helices that are not only tilted with respect to the membrane, but three of them are also kinked at the level of prolines. The functional mechanisms, nucleotide recognition, and conformational changes for the transport, suggested from the structure, are discussed along with the large body of biochemical and functional results.

Yeast mitochondrial ADP/ATP carriers are monomeric in detergents

Mitochondrial carriers are believed widely to be homodimers both in the inner membrane of the organelle and in detergents. The dimensions and molecular masses of the detergent and protein-detergent micelles were measured for yeast ADP/ATP carriers in a range of different detergents. The radius of the carrier at the midpoint of the membrane, its average radius, its Stokes' radius, its molecular mass, and its excluded volume were determined. These parameters are consistent with the known structural model of the bovine ADP/ATP carrier and they demonstrate that the yeast mitochondrial ADP/ATP carriers are monomeric in detergents. Therefore, models of substrate transport have to be considered in which the carrier operates as a monomer rather than as a dimer.

Compartmentalization of a unique ADP/ATP carrier protein SFEC (Sperm Flagellar Energy Carrier, AAC4) with glycolytic enzymes in the fibrous sheath of the human sperm flagellar principal piece

The longest part of the sperm flagellum, the principal piece, contains the fibrous sheath, a cytoskeletal element unique to spermiogenesis. We performed mass spectrometry proteomics on isolated human fibrous sheaths identifying a unique ADP/ATP carrier protein, SFEC [AAC4], seven glycolytic enzymes previously unreported in the human sperm fibrous sheath, and sorbitol dehydrogenase. SFEC, pyruvate kinase and aldolase were co-localized by immunofluorescence to the principal piece. A homology model constructed for SFEC predicted unique residues at the entrance to the nucleotide binding pocket of SFEC that are absent in other human ADP/ATP carriers, suggesting opportunities for selective drug targeting. This study provides the first evidence of a role for an ADP/ATP carrier family member in glycolysis. The co-localization of SFEC and glycolytic enzymes in the fibrous sheath supports a growing literature that the principal piece of the flagellum is capable of generating and regulating ATP independently from mitochondrial oxidation in the mid-piece. A model is proposed that the fibrous sheath represents a highly ordered complex, analogous to the electron transport chain, in which adjacent enzymes in the glycolytic pathway are assembled to permit efficient flux of energy substrates and products with SFEC serving to mediate energy generating and energy consuming processes in the distal flagellum, possibly as a nucleotide shuttle between flagellar glycolysis, protein phosphorylation and mechanisms of motility.

Molecular, functional, and pathological aspects of the mitochondrial ADP/ATP carrier

In providing the cell with ATP generated by oxidative phosphorylation, the mitochondrial ADP/ATP carrier plays a central role in aerobic eukaryotic cells. Combining biochemical, genetic, and structural approaches contributes to understanding the molecular mechanism of this essential transport system, the dysfunction of which is implicated in neuromuscular diseases.

Mathematical modeling of mitochondrial adenine nucleotide translocase

We have developed a mathematical model of adenine nucleotide translocase (ANT) function on the basis of the structural and kinetic properties of the transporter. The model takes into account the effect of membrane potential, pH, and magnesium concentration on ATP and ADP exchange velocity. The parameters of the model have been estimated from experimental data. A satisfactory model should take into account the influence of the electric potential difference on both ternary complex formation and translocation processes. To describe the dependence of translocation constants on electric potential we have supposed that ANT molecules carry charged groups. These groups are shifted during the translocation. Using the model we have evaluated the translocator efficiency and predicted the behavior of ANT under physiological conditions.

Analysis of functional coupling: mitochondrial creatine kinase and adenine nucleotide translocase

The mechanism of functional coupling between mitochondrial creatine kinase (MiCK) and adenine nucleotide translocase (ANT) in isolated heart mitochondria is analyzed. Two alternative mechanisms are studied: 1), dynamic compartmentation of ATP and ADP, which assumes the differences in concentrations of the substrates between intermembrane space and surrounding solution due to some diffusion restriction and 2), direct transfer of the substrates between MiCK and ANT. The mathematical models based on these possible mechanisms were composed and simulation results were compared with the available experimental data. The first model, based on a dynamic compartmentation mechanism, was not sufficient to reproduce the measured values of apparent dissociation constants of MiCK reaction coupled to oxidative phosphorylation. The second model, which assumes the direct transfer of substrates between MiCK and ANT, is shown to be in good agreement with experiments--i.e., the second model reproduced the measured constants and the estimated ADP flux, entering mitochondria after the MiCK reaction. This model is thermodynamically consistent, utilizing the free energy profiles of reactions. The analysis revealed the minimal changes in the free energy profile of the MiCK-ANT interaction required to reproduce the experimental data. A possible free energy profile of the coupled MiCK-ANT system is presented.

Structure of mitochondrial ADP/ATP carrier in complex with carboxyatractyloside

ATP, the principal energy currency of the cell, fuels most biosynthetic reactions in the cytoplasm by its hydrolysis into ADP and inorganic phosphate. Because resynthesis of ATP occurs in the mitochondrial matrix, ATP is exported into the cytoplasm while ADP is imported into the matrix. The exchange is accomplished by a single protein, the ADP/ATP carrier. Here we have solved the bovine carrier structure at a resolution of 2.2 A by X-ray crystallography in complex with an inhibitor, carboxyatractyloside. Six alpha-helices form a compact transmembrane domain, which, at the surface towards the space between inner and outer mitochondrial membranes, reveals a deep depression. At its bottom, a hexapeptide carrying the signature of nucleotide carriers (RRRMMM) is located. Our structure, together with earlier biochemical results, suggests that transport substrates bind to the bottom of the cavity and that translocation results from a transient transition from a 'pit' to a 'channel' conformation.

Chimers of two fused ADP/ATP carrier monomers indicate a single channel for ADP/ATP transport

The mitochondrial ADP/ATP carrier (AAC) is generally believed to function as a homodimer (Wt. Wt). It remains unknown whether the two monomers possess two independent but fully anticooperative channels or they form a single central channel for nucleotide transport. Here we generated fusion proteins consisting of two tandem covalent-linked AAC monomers and studied the kinetics of ADP/ATP transport in reconstituted proteoliposomes. Functional 64-kDa fusion proteins Wt-Wt and Wt-R294A (wild-type AAC linked to a mutant having low ATP transport activity) were expressed in mitochondria of yeast transformants. Compared to homodimer Wt. Wt, the fusion protein Wt-Wt retained the transport activity and selectivity of ADP versus ATP. The strongly divergent selectivities of Wt and R294A were partially propagated in the Wt-R294A fusion protein, suggesting a limited cooperativity during solute translocation. The rates of ADP or ATP transport were significantly higher than those predicted by the two-channel model. Fusion proteins for Wt-R204L (Wt linked to an inactive mutant) and R204L-Wt were not expressed in aerobically grown yeast cells, which contained plasmid rearrangements that regenerated the fully active 32-kDa homodimer Wt. Wt, suggesting that these fusion proteins are inactive in ADP/ATP transport. These results favor a single binding center gated pore model [Klingenberg, M. (1991) in A Study of Enzymes, Vol. 2: pp. 367-388] in which two AAC subunits cooperate for a coordinated ADP/ATP exchange through a single channel.

A covalent tandem dimer of the mitochondrial ADP/ATP carrier is functional in vivo

The adenine nucleotide carrier, or Ancp, is an integral protein of the inner mitochondrial membrane. It is established that the inactive Ancp bound to one of its inhibitors (CATR or BA) is a dimer, but different contradictory models were proposed over the past years to describe the organization of the active Ancp. In order to decide in favor of a single model, it is necessary to establish the orientations of the N- and C-termini and thus the parity of the Ancp transmembrane segments (TMS). According to this, we have constructed a gene encoding a covalent tandem dimer of the Saccharomyces cerevisiae Anc2p and we demonstrate that it is stable and active in vivo as well as in vitro. The properties of the isolated dimer are strongly similar to those of the native Anc2p, as seen from nucleotide exchange and inhibitor binding experiments. We can therefore conclude that the native Anc2p has an even number of TMS and that the N- and C-terminal regions are exposed to the same cellular compartment. Furthermore, our results support the idea of a minimal dimeric functional organization of the Ancp in the mitochondrial membrane and we can suggest that TMS 1 of one monomer and TMS 6 of the other monomer in the native dimer are very close to each other.

Periodate-oxidized ATP stimulates the permeability transition of rat liver mitochondria

Periodate-oxidized ADP (oADP)2 and periodate-oxidized ATP (oATP) stimulate the permeability transition in energized rat liver mitochondria measured as the Ca2+-efflux induced by Ca2+ and Pi. In the presence of Mg2+ and Pi, mitochondria lose intramitochondrial adenine nucleotides at a slow rate. oATP induces a strong decrease of the matrix adenine nucleotides which is inhibited by carboxyatractyloside. Under these conditions, Mg2+ prevents the opening of the permeability transition pore. EGTA prevents the Pi-induced slow efflux of adenine nucleotides, but is without effect on the oATP-induced strong decrease of adenine nucleotides. This oATP-induced strong adenine nucleotide efflux is inhibited by ADP. oATP reduces the increase of matrix adenine nucleotides occurring when the mitochondria are incubated with Mg2+ and ATP. This effect of oATP is also prevented by carboxyatractyloside. oATP is not taken up by the mitochondria. It is suggested that oATP induces a strong efflux of matrix adenine nucleotides by the interaction with the ADP/ATP carrier from the cytosolic side. The induction of the mitochondrial permeability transition by oADP and oATP is attributed to two mechanisms-a strong decrease in the intramitochondrial adenine nucleotide content, especially that of ADP, and a stabilization of the c-conformation of the ADP/ATP carrier.

The signal transduction function for oxidative phosphorylation is at least second order in ADP

To maintain ATP constant in the cell, mitochondria must sense cellular ATP utilization and transduce this demand to F0-F1-ATPase. In spite of a considerable research effort over the past three decades, no combination of signal(s) and kinetic function has emerged with the power to explain ATP homeostasis in all mammalian cells. We studied this signal transduction problem in intact human muscle using 31P NMR spectroscopy. We find that the apparent kinetic order of the transduction function of the signal cytosolic ADP concentration ([ADP]) is at least second order and not first order as has been assumed. We show that amplified mitochondrial sensitivity to cytosolic [ADP] harmonizes with in vitro kinetics of [ADP] stimulation of respiration and explains ATP homeostasis also in mouse liver and canine heart. This result may well be generalizable to all mammalian cells.

Functional properties of purified and reconstituted mitochondrial metabolite carriers

Eight mitochondrial carrier proteins were solubilized and purified in the authors' laboratories using variations of a general procedure based on hydroxyapatite and Celite chromatography. The molecular mass of all the carriers ranges between 28 and 34 kDa on SDS-PAGE. The purified carrier proteins were reconstituted into liposomes mainly by using a method of detergent removal by hydrophobic chromatography on polystyrene beads. The various carriers were identified in the reconstituted state by their kinetic properties . A complete set of basic kinetic data including substrate specificity, affinity, interaction with inhibitors, and activation energy was obtained. These data closely resemble those of intact mitochondria, as far as they are available from the intact organelle. Mainly on the basis of kinetic data, the asymmetric orientation of most of the reconstituted carrier proteins were established. Several of their functional properties are significantly affected by the type of phospholipids used for reconstitution. All carriers which have been investigated in proteoliposomes function according to a simultaneous (sequential) mechanism of transport; i.e., a ternary complex, made up of two substrates and the carrier protein, is involved in the catalytic cycle. The only exception was the carnitine carrier, where a ping-pong mechanism of transport was found. By reaction of particular cysteine residues with mercurial reagents, several carriers could be reversibly converted to a functional state different from the various physiological transport modes. This "unphysiological" transport mode is characterized by a combination of channel-type and carrier-type properties.

Chemical, immunological, enzymatic, and genetic approaches to studying the arrangement of the peptide chain of the ADP/ATP carrier in the mitochondrial membrane

In the process of oxidative phosphorylation, the exchange of cytosolic ADP3- against mitochondrial ATP4- across the inner mitochondrial membrane is mediated by a specific carrier protein. Two different conformations for this carrier have been demonstrated on the basis of interactions with specific inhibitors, namely carboxyatractyloside (CATR) and bongkrekic acid (BA). The two conformations, referred to as CATR and BA conformations, are interconvertible, provided that ADP or ATP are present. The functional ADP/ATP carrier is probably organized as a tetramer. In the presence of CATR or BA the tetramer is split into two dimers combined with either of the two inhibitors. The amino acid sequence of the beef heart carrier monomer (297 residues) contains three repeats of about 100 residues each. Experimental results obtained through different approaches, including photolabeling, immunochemistry, and limited proteolysis, can be interpreted on the basis of a model with five or six transmembrane alpha helices per carrier monomer. Two mobile regions involved in the binding of nucleotides and accessible to proteolytic enzymes have been identified. Each of them may be visualized as consisting of two pairs of short amphipathic alpha helices, which can be juxtaposed to form hydrophilic channels facilitating the nucleotide transport. Mutagenesis in yeast is currently being used to detect strategic amino acids in ADP/ATP transport.

An extended dynamic model of oxidative phosphorylation

The presented model based on an earlier one (Korzeniewski, B. and Froncisz, W. (1989) Studia Biophys. 132, 173-187) simulates concentration changes in time of chemical compounds and thermodynamic forces during respiration of cell suspension in a closed chamber. A set of differential equations solved numerically describes the utilization of oxygen up to anaerobiosis and the behaviour of the system after a sudden pulse of oxygen. Flux control coefficients for most important reactions (enzymes) of oxidative phosphorylation were calculated. A good qualitative and (when a direct comparison is possible) quantitative agreement with experimental results can be observed. The following conclusions can be drawn from the simulation: (1) Wilson's steady state model is not in contradiction with sharing of the control over the respiration between some steps and displacement of the ATP/ADP carrier from equilibrium. (2) The overshoot characteristics of the delta microH+ time-course after reoxygenation can be explained without using the lag-phase kinetics of ATP-synthetase. (3) A 'hot region' (sharp changes of many parameters) can be distinguished when the oxygen concentration approaches zero; only cytochrome oxidase is clearly sensitive on oxygen concentration in all its range. (4) Control over oxidative phosphorylation is shared mainly between inputs of the system (ATP utilization and substrate dehydrogenation) and the proton leak.

Reaction mechanism of the reconstituted aspartate/glutamate carrier from bovine heart mitochondria

A functional model for the aspartate/glutamate carrier of the inner mitochondrial membrane was established based on a kinetic evaluation of this transporter. Antiport kinetics were measured in proteoliposomes that contained partially purified carrier protein of definite transmembrane orientation (Dierks, T. and Krämer, R. (1988) Biochim. Biophys. Acta 937, 122-126). Bireactant initial velocity analyses of the counterexchange reaction were carried out varying substrate concentrations both in the internal and the external compartment. The kinetic patterns obtained were inconsistent with a pong-pong mechanism; rather they demonstrated the formation of a ternary complex as a consequence of sequential binding of one internal and one external substrate molecule to the carrier. Studies on transport activity in the presence of aspartate and glutamate in the same compartment (formally treated as substrate inhibition) clearly indicated that during exchange only one form of the carrier at either membrane surface exposes its binding sites, for which the two different substrates compete. In the deenergized state (pH 6.5) both substrates were translocated at about the same rate. Aspartate/glutamate antiport became asymmetric if a membrane potential was imposed, due to the electrogenic nature of the heteroexchange resulting from proton cotransport together with glutamate. Investigation of the electrical properties of aspartate/aspartate homoexchange led to the conclusion that the translocating carrier-substrate intermediate exhibits a transmembrane symmetry with respect to the (negative) charge, which again only is conceivable assuming a ternary complex. Thus, an antiport model is outlined that shows the functional complex of the carrier with two substrate molecules bound, one at either side of the membrane. The conformational change associated with the transition of both substrate molecules across the membrane then occurs in a single step. Furthermore the model implicates a distinct proton binding site, which is derived from the different influence of H+ concentration observed on transport affinity and transport velocity, respectively, when glutamate is used as a substrate.

A mathematical model to study short-term regulation of mitochondrial energy transduction

A mathematical model is presented which includes the following elementary process of mitochondrial energy transduction: hydrogen supply, proton translocation by the respiratory chain, proton-driven ATP synthesis by the F0F1-ATPase, passive back-flow of protons (leak) and carrier-mediated exchange of adenine nucleotides and phosphate. For these processes empirical rate laws are used. The model is applied to calculate time-dependent states of energy transduction in isolated rat liver mitochondria. From the general agreement of the computational results with experimental data (Ogawa, S. and Lee, T.M. (1984) J. Biol. Chem. 259, 10004-10011) the following conclusions can be drawn. (1) The length of the time interval during which mitochondria are able to maintain a relatively high and constant delta pH in the absence of oxygen (anaerobiosis) is limited by the availability of intramitochondrial ATP. (2) The overshoot kinetics of delta pH which appear when reoxigenating mitochondria after a preceeding anaerobiosis might be due to a lag phase kinetics of the F0F1-ATPase. (3) In phosphorylating mitochondria the homeostasis of delta pH is brought about by a high sensitivity of the respiration rate and the rate of the F0F1-ATPase as to changes of delta pH. (4) Analysis of the mean transient times shows that the rate of ATP synthesis in State 3 is controlled to almost the same extent by the hydrogen supply, the respiratory chain, the adenine nucleotide translocator and the proton leak.

Substrate-site interactions in the membrane-bound adenine-nucleotide carrier as disclosed by ADP and ATP analogs

The binding parameters of a number of ADP or ATP analogs to the adenine nucleotide carrier in mitochondria and inside-out submitochondrial particles have been explored by means of two specific inhibitors, carboxyatractyloside and bongkrekic acid. The nucleotides tested fell into two classes depending on the shape of the binding curve. Curvilinear Scatchard plots were obtained for the binding of ADP, ATP, adenosine 5'-triphospho-gamma-1-(5-sulfonic acid)naphthylamidate [gamma-AmNS)ATP) and adenylyl (beta,gamma)-methylenediphosphate (p[CH2]ppA); on the other hand, rectilinear Scatchard plots were obtained in the case of naphthoyl-ADP (N-ADP) and 8-bromo ADP (8Br-ADP) binding. The total number of binding sites for N-ADP and 8Br-ADP could be extrapolated with good accuracy to 1.3-1.5 nmol/mg protein; this value corresponds to the number of carboxyatractyloside-binding sites in heart mitochondria (Block, M.R., Pougeois, R. and Vignais, P.V. (1980) FEBS Lett. 117, 335-340). On the other hand, because of the curvilinearity of the Scatchard plots for the binding of ADP, ATP, (gamma-AmNS)ATP and p[CH2]ppA, the total number of binding sites for these nucleotides could only be approximated to a value higher than 1 nmol/mg protein, the exact value being probably equal to that found for N-ADP and 8Br-ADP binding, i.e. 1.3-1.5 nmol/mg protein. Curvilinearity of Scatchard plots was discussed in terms of negative interactions between nucleotide-binding sites located on the same face of the adenine nucleotide carrier. A possible relationship between the features of the binding plots and the transportable nature of the nucleotide is discussed. Contrary to the enhancing effect of bongkrekic acid on [14C]ADP uptake observed essentially in nucleotide-depleted heart mitochondria (Klingenberg, M., Appel, M., Babel, W. and Aquila, H. (1983) Eur. J. Biochem. 131, 647-654), binding of bongkrekic acid to nondepleted heart mitochondria was found to partially displace previously bound [14C]ADP. These opposite effects of bongkrekic acid may be explained by assuming that bongkrekic acid is able to abolish negative cooperativity between external (cytosolic) ADP-binding sites.

A rapid-filtration technique for membrane fragments or immobilized enzymes: measurements of substrate binding or ion fluxes with a few-millisecond time resolution

The construction and use of a filtration system with milliseconds time resolution is described here. This apparatus allows measurements of substrate binding to immobilized enzyme or ion fluxes through membrane vesicles to be performed over a very large time scale, from 10 ms to seconds. The main advantage of this system compared to the widely used quench-flow technique is that it does not require the use of an inhibitor. Following adsorption of the enzyme in an adequately chosen filter, the reaction is allowed to proceed within the filter during a forced filtration of a buffer containing the reactive substrate (or of a washing solution in the case of efflux measurements). The design allows the duration of filtration and buffer flux to be finely and reproducibly controlled. This paper illustrates the use of this rapid-filtration system for time-resolved measurements of calcium binding and transport by sarcoplasmic reticulum Ca-ATPase and of the initial phase of ADP transport by the ADP/ATP carrier of intact mitochondria.

The role of the adenine nucleotide translocator in oxidative phosphorylation. A theoretical investigation on the basis of a comprehensive rate law of the translocator

A minimum model of adenine nucleotide exchange through the inner membrane of mitochondria is presented. The model is based on a sequential mechanism, which presumes ternary complexes formed by binding of metabolites from both sides of the membrane. The model explains the asymmetric kinetics of ADP-ATP exchange as a consequence of its electrogenic character. In energized mitochondria, a part of the membrane potential suppresses the binding of extramitochondrial ATP in competition with ADP. The remaining part of the potential difference inhibits the back exchange of internal ADP for external ATP. The assumption of particular energy-dependent conformational states of the translocator is not necessary. The model is not only compatible with the kinetic properties reported in the literature about the adenine nucleotide exchange, but it also correctly describes the response of mitochondrial respiration to the extramitochondrial ATP/ADP ratio under different conditions. The model computations reveal that the translocation step requires some loss of free energy as driving force. The size of the driving force depends depends on the flux rate as well as on the extra- and intramitochondrial ATP/ADP quotients. By both quotients the translocator control the export of ATP formed by oxidative phosphorylation in mitochondria.