BIOCHEMICAL STRUCTURE OF MITOCHONDRIA

A Walk in the Memory, from the First Functional Approach up to Its Regulatory Role of Mitochondrial Bioenergetic Flow in Health and Disease: Focus on the Adenine Nucleotide Translocator

The mitochondrial adenine nucleotide translocator (ANT) plays the fundamental role of gatekeeper of cellular energy flow, carrying out the reversible exchange of ADP for ATP across the inner mitochondrial membrane. ADP enters the mitochondria where, through the oxidative phosphorylation process, it is the substrate of Fo-F1 ATP synthase, producing ATP that is dispatched from the mitochondrion to the cytoplasm of the host cell, where it can be used as energy currency for the metabolic needs of the cell that require energy. Long ago, we performed a method that allowed us to monitor the activity of ANT by continuously detecting the ATP gradually produced inside the mitochondria and exported in the extramitochondrial phase in exchange with externally added ADP, under conditions quite close to a physiological state, i.e., when oxidative phosphorylation takes place. More than 30 years after the development of the method, here we aim to put the spotlight on it and to emphasize its versatile applicability in the most varied pathophysiological conditions, reviewing all the studies, in which we were able to observe what really happened in the cell thanks to the use of the "ATP detecting system" allowing the functional activity of the ANT-mediated ADP/ATP exchange to be measured.

Cell biology. Metabolic control of cell death

Beyond their contribution to basic metabolism, the major cellular organelles, in particular mitochondria, can determine whether cells respond to stress in an adaptive or suicidal manner. Thus, mitochondria can continuously adapt their shape to changing bioenergetic demands as they are subjected to quality control by autophagy, or they can undergo a lethal permeabilization process that initiates apoptosis. Along similar lines, multiple proteins involved in metabolic circuitries, including oxidative phosphorylation and transport of metabolites across membranes, may participate in the regulated or catastrophic dismantling of organelles. Many factors that were initially characterized as cell death regulators are now known to physically or functionally interact with metabolic enzymes. Thus, several metabolic cues regulate the propensity of cells to activate self-destructive programs, in part by acting on nutrient sensors. This suggests the existence of "metabolic checkpoints" that dictate cell fate in response to metabolic fluctuations. Here, we discuss recent insights into the intersection between metabolism and cell death regulation that have major implications for the comprehension and manipulation of unwarranted cell loss.

Wanderings in bioenergetics and biomembranes

Having worked for 55 years in the center and at the fringe of bioenergetics, my major research stations are reviewed in the following wanderings: from microsomes to mitochondria, from NAD to CoQ, from reversed electron transport to reversed oxidative phosphorylation, from mitochondrial hydrogen transfer to phosphate transfer pathways, from endogenous nucleotides to mitochondrial compartmentation, from transport to mechanism, from carrier to structure, from coupling by AAC to uncoupling by UCP, and from specific to general transport laws. These wanderings are recalled with varying emphasis paid to the covered science stations.

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.

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.

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.

Spectroscopic evidence of metabolic control

The Pasteur and Crabtree effects demonstrate that changes at the beginning of the metabolic sequence for glucose metabolism give rise to effects at the end, and vice versa. We have presented here three additional responses of the ascites tumor cell suspensions, and presumably more will be uncovered. Each one of these responses is a manifestation of factors in the underlying mechanism that are in the nature of chemical feedback of a linear or nonlinear nature. The metabolic reactions are sufficiently complex that it is unlikely that any single component or step need control metabolism in different types of cells or under all conditions for a particular cell. However, it is due to a favorable circumstance that, in an appropriate type of cell and with the use of a direct intracellular indicator for changes in ADP concentration, we can state that the respiratory metabolism of the ascites tumor cell suspension, as freshly withdrawn from the mouse abdomen, is limited by the intracellular ADP concentration, and that this is why these cells show a predominance of glycolytic over respiratory activity. The response of the metabolism to small and large additions of glucose illustrates aspects of the metabolic mechanism which involve control of endogenous metabolism and compartmentalization of ATP formed in oxidative phosphorylation, the net result being a depression of the respiratory activity. The results of this approach emphasize the importance of chemical assays of localized portions of the living cell in its physiological state (61).

The mitochondrial ADP/ATP carrier: structural, physiological and pathological aspects

Under the conditions of oxidative phosphorylation, the mitochondrial ADP/ATP carrier catalyses the one to one exchange of cytosolic ADP against matrix ATP across the inner mitochondrial membrane. The ADP/ATP transport system can be blocked very specifically by two families of inhibitors: atractyloside (ATR) and carboxyatractyloside (CATR) on one hand, and bongkrekic acid (BA) and isobongkrekic acid (isoBA) on the other hand. It is well established that these inhibitors recognise two different conformations of the carrier protein, the CATR- and BA-conformations, which exhibit different chemical, immunochemical and enzymatic reactivities. The reversible transition of the ADP/ATP carrier between the two conformations was studied by fluorometric techniques. This transconversion, which is only triggered by transportable nucleotides, is probably the same as that which occurs during the functioning of ADP/ATP transport system. The fluorometric approach, using the tryptophanyl residues of the yeast carrier as intrinsic fluorescence probes, was combined to a mutagenesis approach to elucidate the ADP/ATP transport mechanism at the molecular level. Finally, recent reports that myopathies might result from defect in ADP/ATP transport led us to develop a method to quantify the carrier protein in muscular biopsies.

An examination of the in vivo distribution of brain hexokinase between the cytosol and the outer mitochondrial membrane

These studies addressed the question of the in vivo distribution of rat brain hexokinase (HK), and whether physiologically relevant changes in the glycolytic rate are accompanied by changes in the distribution of HK. Homogenates of fresh tissue showed only 11-15% of the overt (assayable without added detergent) HK to be soluble (found in high-speed centrifugation supernatant fractions) when homogenization was begun within 15-20 s of sacrifice. Freeze-blown rat brain tissue also was used, coupled with a new technique wherein it was homogenized as it thawed in a buffered sucrose solution containing 1 mM EDTA. In tissue sampled 15 min (anesthetized) or 60 min (waking) after ip Nembutal injection (40 mg/kg), 23% of the overt HK and 79% of the total lactate dehydrogenase were soluble. The average phosphocreatine content of these and similar homogenates had decreased only 23% from in vivo levels, while ATP had decreased by 65%, due to the combined effects of a high level of endogenous ATPase, chelation of Mg2+ by EDTA, and the greater stability of Mg-ATP2- relative to Mg-ADP1-. These data indicated that the tissue experienced, at most, the equivalent of 6 s of complete ischemia prior to the completion of homogenization. Synaptosomes derived from rat and chicken cerebra were incubated at 37 degrees C in a physiological salt solution containing 10 mM glucose. Addition of veratridine has been shown to stimulate glycolysis and oxidative phosphorylation two- to threefold (H. T. Kyriazi and R. E. Basford (1986) J. Neurochem., in press), but did not alter the HK distribution, as 21% was found in the supernatant fractions of both control and veratridine-stimulated synaptosomes treated with digitonin. These results indicate that in brain tissue, large net movements of HK on and off the outer mitochondrial membrane do not occur, and thus play no role in the regulation of glycolysis.

ITP pyrophosphohydrolase and IDP phosphohydrolase in rat tissue

The existence of a nucleoside triphosphate pyrophosphohydrolase specific for ITP has been demonstrated in the cytosol fraction of a variety of rat tissues. The enzyme, stable to moderate heat treatment, was present in erythrocytes as well as brain, heart, kidney, liver, lung, muscle, ovaries, spleen, testes and thymus. The specific activity of the enzyme ranges from 26 to 150 mumoles/min/g protein. In addition, evidence is given for a heat labile nucleoside diphosphate (IDP) phosphohydrolase present in most rat tissues, and particularly high in the adrenal (137 mumoles/min/g protein). An "ITP-IMP cycle" is proposed as a rgulating mechanism for intracellular levels of ATP.

BIOCHEMICAL CHANGES DURING ACUTE PHYSIOLOGICAL FAILURE IN THE RAT: II. THE BEHAVIOR OF ADENINE AND PYRIDINE NUCLEOTIDES OF THE LIVER DURING SHOCK

The acute hypoxia, caused by severe blood loss, gives rise to the rapid breakdown of glycogen in the liver and concurrent increase in the concentration of ATP in the mitochondria. The increase in adenosine triphosphate (ATP) continues until the onset of the reversible phase of shock. The glycogen reserve approaches depletion during the late reversible phase. Simultaneously, the generation of ATP in the mitochondria ceases and the concentration begins to fall. It would appear that at this time the adenylate kinase mechanism in the mitochondrial membrane comes into play to convert the adenosine diphosphate (ADP) into ATP and adenosine monophosphate (AMP). As the condition becomes irreversible the residual ATP and phosphorylated intermediates of the Embden–Meyerhof system undergo rapid hydrolysis with liberation of AMP and inorganic phosphate in the cytoplasm.The concentration of the pyridine nucleotides undergoes no change in any of the liver cell components until the onset of the irreversible phase of failure. Thereafter, these nucleotides undergo a progressive conversion to the reduced form.