Energetics and stoichiometry of oxidative phosphorylation from NADH to cytochrome c in isolated rat liver mitochondria

Low Cancer Incidence in Naked Mole-Rats May Be Related to Their Inability to Express the Warburg Effect

Metabolic flexibility in mammals enables stressed tissues to generate additional ATP by converting large amounts of glucose into lactic acid; however, this process can cause transient local or systemic acidosis. Certain mammals are adapted to extreme environments and are capable of enhanced metabolic flexibility as a specialized adaptation to challenging habitat niches. For example, naked mole-rats (NMRs) are a fossorial and hypoxia-tolerant mammal whose metabolic responses to environmental stressors markedly differ from most other mammals. When exposed to hypoxia, NMRs exhibit robust hypometabolism but develop minimal acidosis. Furthermore, and despite a very long lifespan relative to other rodents, NMRs have a remarkably low cancer incidence. Most advanced cancers in mammals display increased production of lactic acid from glucose, irrespective of oxygen availability. This hallmark of cancer is known as the Warburg effect (WE). Most malignancies acquire this metabolic phenotype during their somatic evolution, as the WE benefits tumor growth in several ways. We propose that the peculiar metabolism of the NMR makes development of the WE inherently difficult, which might contribute to the extraordinarily low cancer rate in NMRs. Such an adaptation of NMRs to their subterranean environment may have been facilitated by modified biochemical responses with a stronger inhibition of the production of CO₂ and lactic acid by a decreased extracellular pH. Since this pH-inhibition could be deeply hard-wired in their metabolic make-up, it may be difficult for malignant cells in NMRs to acquire the WE-phenotype that facilitates cancer growth in other mammals. In the present commentary, we discuss this idea and propose experimental tests of our hypothesis.

The thermodynamic basis of glucose-stimulated insulin release: a model of the core mechanism

A model for glucose sensing by pancreatic β-cells is developed and compared with the available experimental data. The model brings together mathematical representations for the activities of the glucose sensor, glucokinase, and oxidative phosphorylation. Glucokinase produces glucose 6-phosphate (G-6-P) in an irreversible reaction that determines glycolytic flux. The primary products of glycolysis are NADH and pyruvate. The NADH is reoxidized and the reducing equivalents transferred to oxidative phosphorylation by the glycerol phosphate shuttle, and some of the pyruvate is oxidized by pyruvate dehydrogenase and enters the citric acid cycle. These reactions are irreversible and result in a glucose concentration-dependent reduction of the intramitochondrial NAD pool. This increases the electrochemical energy coupled to ATP synthesis and thereby the cellular energy state ([ATP]/[ADP][Pi]). ATP and Pi are 10-100 times greater than ADP, so the increase in energy state is primarily through decrease in ADP The decrease in ADP is considered responsible for altering ion channel conductance and releasing insulin. Applied to the reported glucose concentration-dependent release of insulin by perifused islet preparations (Doliba et al. 2012), the model predicts that the dependence of insulin release on ADP is strongly cooperative with a threshold of about 30 μmol/L and a negative Hill coefficient near -5.5. The predicted cellular energy state, ADP, creatine phosphate/creatine ratio, and cytochrome c reduction, including their dependence on glucose concentration, are consistent with experimental data. The ability of the model to predict behavior consistent with experiment is an invaluable resource for understanding glucose sensing and planning experiments.

Oxidative phosphorylation: regulation and role in cellular and tissue metabolism

Oxidative phosphorylation provides most of the ATP that higher animals and plants use to support life and is responsible for setting and maintaining metabolic homeostasis. The pathway incorporates three consecutive near equilibrium steps for moving reducing equivalents between the intramitochondrial [NAD+ ]/[NADH] pool to molecular oxygen, with irreversible reduction of oxygen to bound peroxide at cytochrome c oxidase determining the net flux. Net flux (oxygen consumption rate) is determined by demand for ATP, with feedback by the energy state ([ATP]/[ADP][Pi ]) regulating the pathway. This feedback affects the reversible steps equally and independently, resulting in the rate being coupled to ([ATP]/[ADP][Pi ])3 . With increasing energy state, oxygen consumption decreases rapidly until a threshold is reached, above which there is little further decrease. In most cells, [ATP] and [Pi ] are much higher than [ADP] and change in [ADP] is primarily responsible for the change in energy state. As a result, the rate of ATP synthesis, plotted against [ADP], remains low until [ADP] reaches about 30 μm and then increases rapidly with further increase in [ADP]. The dependencies on energy state and [ADP] near the threshold can be fitted by the Hill equation with a Hill coefficients of about -2.6 and 4.2, respectively. The homeostatic set point for metabolism is determined by the threshold, which can be modulated by the PO2 and intramitochondrial [NAD+ ]/[NADH]. The ability of oxidative phosphorylation to precisely set and maintain metabolic homeostasis is consistent with it being permissive of, and essential to, development of higher plants and animals.

Oxidative phosphorylation: unique regulatory mechanism and role in metabolic homeostasis

Oxidative phosphorylation is the primary source of metabolic energy, in the form of ATP, in higher plants and animals, but its regulation in vivo is not well understood. A model has been developed for oxidative phosphorylation in vivo that predicts behavior patterns that are both distinctive and consistent with experimental measurements of metabolism in intact cells and tissues. A major regulatory parameter is the energy state ([ATP]/[ADP][P_i], where brackets denote concentration). Under physiological conditions, the [ATP] and [P_i] are ~100 times that of [ADP], and most of the change in energy state is through change in [ADP]. The rate of oxidative phosphorylation (y-axis) increases slowly with increasing [ADP] until a threshold is reached and then increases very rapidly and linearly with further increase in [ADP]. The dependence on [ADP] can be characterized by a threshold [ADP] (T) and control strength (CS), the normalized slope above threshold (Δy/(Δx/T). For normoxic cells without creatine kinase, T is ~30 µM and CS is ~10 s^-1 Myocytes and cells with larger ranges of rates of ATP utilization, however, have the same [ADP]- and [AMP]-dependent mechanisms regulating metabolism and gene expression. To compensate, these cells have creatine kinase, and hydrolysis/synthesis of creatine phosphate increases the change in [P_i] and thereby CS. Cells with creatine kinase have [ADP] and [AMP], which are similar to cells without creatine kinase, despite the large differences in metabolic rate. ³¹P measurements in human muscles during work-to-rest and rest-to-work transitions are consistent with predictions of the model.NEW & NOTEWORTHY A model developed for oxidative phosphorylation in vivo is shown to predict behavior patterns that are both novel and consistent with experimental measurements of metabolism in working muscle and other cells. The dependence of the rate on ADP concentration shows a pronounced threshold with a steep, nearly linear increase above the threshold. The threshold determines the homeostatic set point, and the slope above threshold determines how much metabolism changes in response to varied energy demand.

Regulation of metabolism: the work-to-rest transition in skeletal muscle

The behavior of oxidative phosphorylation predicted by a model for the mechanism and kinetics of cytochrome c oxidase is compared with the experimentally observed behavior during the work-to-rest transition in skeletal muscle. For both experiment and model, when work stops, the increase in creatine phosphate and decrease in creatine and inorganic phosphate concentrations ([CrP], [Cr], and [Pi]) begin immediately. The rate of change for each is maximal and then progressively slows as the increasing energy state ([ATP]/[ADP][Pi]) suppresses the rate of oxidative phosphorylation. The time courses can be reasonably fitted to single exponential curves with similar time constants. The energy state in the working and resting steady states at constant Po₂ are dependent on the intramitochondrial [NAD⁺]/[NADH], mitochondrial content, and size of the creatine pool ([CrP] + [Cr]). The rate of change in [CrP] is linearly correlated with [CrP] and with [Pi] and [Cr]. The time constant for [CrP] increase in the resting and working steady states, and the rate of decrease in oxygen consumption are similarly dependent on the Po₂ in the inspired gas (experimental) or tissue Po₂ (model). Myoglobin strongly buffers intracellular Po₂ below ∼15 torr, truncating the low end of the oxygen distribution in the tissue and suppressing intra- and intermyocyte oxygen gradients. The predictions of the model are consistent with the experimental data throughout the work/rest transition, providing valuable insights into the regulation of cellular and tissue metabolism.

Regulation of metabolism: the rest-to-work transition in skeletal muscle

Mitochondrial oxidative phosphorylation is programmed to set and maintain metabolic homeostasis, and understanding that program is essential for an integrated view of cellular and tissue metabolism. The behavior predicted by a mechanism-based model for oxidative phosphorylation is compared with that experimentally measured for skeletal muscle when work is initiated. For the model, initiation of work is simulated by imposing a rate of ATP utilization of either 0.6 (equivalent of 13.4 ml O2·100 g tissue(-1)·min(-1) or 6 μmol O2·g tissue(-1)·min(-1)) or 0.3 mM ATP/s. Creatine phosphate ([CrP]) decrease, both experimentally measured and predicted by the model, can be fit to a single exponential. Increase in ATP synthesis begins immediately but can show a "lag period," during which the rate accelerates. The length of the lag period is similar for both experiment and model; in the model, the lag depends on intramitochondrial [NAD(+)]/[NADH], mitochondrial content, and size of the creatine pool ([CrP] + [Cr]) as well as the resting [CrP]/[Cr]. For in vivo conditions, increase in oxygen consumption may be linearly correlated with a decrease in [CrP] and an increase in inorganic phosphate ([Pi]) and [Cr]. The decrease in [CrP], resting and working steady state [CrP], and the increase in oxygen consumption are dependent on the Po2 in the inspired gas (experimental) or tissue Po2 (model). The metabolic behavior predicted by the model is consistent with available experimental measurements in muscle upon initiation of work, with the model providing valuable insight into how metabolic homeostasis is set and maintained.

Programming and regulation of metabolic homeostasis

Evidence is presented that the rate and equilibrium constants in mitochondrial oxidative phosphorylation set and maintain metabolic homeostasis in eukaryotic cells. These internal constants determine the energy state ([ATP]/[ADP][Pi]), and the energy state maintains homeostasis through a bidirectional sensory/signaling control network that reaches every aspect of cellular metabolism. The energy state is maintained with high precision (to ∼1 part in 10(10)), and the control system can respond to transient changes in energy demand (ATP utilization) of more than 100 times the resting rate. Epigenetic and environmental factors are able to "fine-tune" the programmed set point over a narrow range to meet the special needs associated with cell differentiation and chronic changes in metabolic requirements. The result is robust across-platform control of metabolism, which is essential to cellular differentiation and the evolution of complex organisms. A model of oxidative phosphorylation is presented, for which the steady-state rate expression has been derived and computer programmed. The behavior of oxidative phosphorylation predicted by the model is shown to fit the experimental data available for isolated mitochondria as well as for cells and tissues. This includes measurements from several different mammalian tissues as well as from insect flight muscle and plants. The respiratory chain and oxidative phosphorylation is remarkably similar for all higher plants and animals. This is consistent with the efficient synthesis of ATP and precise control of metabolic homeostasis provided by oxidative phosphorylation being a key to cellular differentiation and the evolution of structures with specialized function.

Metabolic requirements for the maintenance of self-renewing stem cells

A distinctive feature of stem cells is their capacity to self-renew to maintain pluripotency. Studies of genetically-engineered mouse models and recent advances in metabolomic analysis, particularly in haematopoietic stem cells, have deepened our understanding of the contribution made by metabolic cues to the regulation of stem cell self-renewal. Many types of stem cells heavily rely on anaerobic glycolysis, and stem cell function is also regulated by bioenergetic signalling, the AKT-mTOR pathway, Gln metabolism and fatty acid metabolism. As maintenance of a stem cell pool requires a finely-tuned balance between self-renewal and differentiation, investigations into the molecular mechanisms and metabolic pathways underlying these decisions hold great therapeutic promise.

Mammalian complex I pumps 4 protons per 2 electrons at high and physiological proton motive force in living cells

Mitochondrial complex I couples electron transfer between matrix NADH and inner-membrane ubiquinone to the pumping of protons against a proton motive force. The accepted proton pumping stoichiometry was 4 protons per 2 electrons transferred (4H(+)/2e(-)) but it has been suggested that stoichiometry may be 3H(+)/2e(-) based on the identification of only 3 proton pumping units in the crystal structure and a revision of the previous experimental data. Measurement of proton pumping stoichiometry is challenging because, even in isolated mitochondria, it is difficult to measure the proton motive force while simultaneously measuring the redox potentials of the NADH/NAD(+) and ubiquinol/ubiquinone pools. Here we employ a new method to quantify the proton motive force in living cells from the redox poise of the bc(1) complex measured using multiwavelength cell spectroscopy and show that the correct stoichiometry for complex I is 4H(+)/2e(-) in mouse and human cells at high and physiological proton motive force.

The ROS production induced by a reverse-electron flux at respiratory-chain complex 1 is hampered by metformin

Mitochondrial reactive oxygen species (ROS) production was investigated in mitochondria extracted from liver of rats treated with or without metformin, a mild inhibitor of respiratory chain complex 1 used in type 2 diabetes. A high rate of ROS production, fully suppressed by rotenone, was evidenced in non-phosphorylating mitochondria in the presence of succinate as a single complex 2 substrate. This ROS production was substantially lowered by metformin pretreatment and by any decrease in membrane potential (Delta Phi(m)), redox potential (NADH/NAD), or phosphate potential, as induced by malonate, 2,4-dinitrophenol, or ATP synthesis, respectively. ROS production in the presence of glutamate-malate plus succinate was lower than in the presence of succinate alone, but higher than in the presence of glutamate-malate. Moreover, while rotenone both increased and decreased ROS production at complex 1 depending on forward (glutamate-malate) or reverse (succinate) electron flux, no ROS overproduction was evidenced in the forward direction with metformin. Therefore, we propose that reverse electron flux through complex 1 is an alternative pathway, which leads to a specific metformin-sensitive ROS production.

A possible restriction of ferro- and ferricyanide oxidoreductase activities of rat liver mitochondria by the outer membrane

In this work, various ferro-ferricyanide oxidoreductase activities of rat liver mitochondria were studied to find conditions under which the outer membrane might restrict the flux of these highly charged non-biological anions. When the isotonic low ionic strength medium was supplemented with 25mM KCl, a several-fold increase in the succinate-ferricyanide reductase activity of mitochondria and in the rate of external NADH oxidation in the presence of ferrocyanide was observed. Mitochondrial respiration with 5mM ferrocyanide was almost completely inhibited after consumption of 3.8-18.5% of the dissolved oxygen, depending on the medium and the presence of 2,4-dinitrophenol. These and other experimental data together with mathematical modeling of the redox-state equilibrium suggest that the measured activities might be restricted by two factors: first, the permeability of the outer mitochondrial membrane and second, a strong influence of the ionic strength of incubation media on the intermembrane space redox reactions.

P/O ratios of mitochondrial oxidative phosphorylation

Mitochondrial mechanistic P/O ratios are still in question. The major studies since 1937 are summarized and various systematic errors are discussed. Values of about 2.5 with NADH-linked substrates and 1.5 with succinate are consistent with most reports after apparent contradictions are explained. Variability of coupling may occur under some conditions but is generally not significant. The fractional values result from the coupling ratios of proton transport. An additional revision of P/O ratios may be required because of a report of the structure of ATP synthase (D. Stock, A.G.W. Leslie, J.E. Walker, Science 286 (1999) 1700-1705) which suggests that the H+/ATP ratio is 10/3, rather than 3, consistent with P/O ratios of 2.3 with NADH and 1.4 with succinate, values that are also possible.

Oxygen dependency of cerebral oxidative phosphorylation in newborn piglets

Changes in hemoglobin oxygenation and oxidation state of the CuA centre of cytochrome oxidase were measured with full spectral near infrared spectroscopy simultaneously with phosphorus metabolites using nuclear magnetic resonance 31P spectroscopy at high time resolution (10 seconds) during transient anoxia (FiO2 = 0.0 for 105 seconds) in the newborn piglet brain. During the onset of anoxia, there was no change in either phosphocreatine (PCr) concentration or the oxidation state of the CuA centre of cytochrome oxidase until there was a substantial fall in cerebral hemoglobin oxygenation, at which point the CuA centre reduced simultaneously with the decline in PCr. At a later time during the anoxia, intracellular pH decreased rapidly, consistent with a fall in cerebral metabolic rate for O2 and reduced flux through the tricarboxylic acid cycle. The simultaneous reduction of CuA and decline in PCr can be explained in terms of the effects of the falling mitochondrial electrochemical potential. From these observations, it is concluded that, at normoxia, oxidative phosphorylation and the oxidation state of the components of the electron transport chain are independent of cerebral oxygenation and that the reduction in the CuA signal occurs when oxygen tension limits the capacity of oxidative phosphorylation to maintain the phosphorylation potential.

The oxygen dependence of the mitochondrial respiration rate in ascites tumor cells

The effect of the oxygen concentration on the rate of oxygen consumption by 786 and TA3 ascites tumor cell lines has been determined under steady-flow conditions with a membraneless fast-responding O2 electrode and using ascorbate and N,N,N',N'-tetramethyl-p-phenylenediamine as electron donors. The reaction was initiated by rapid injection of O2 into anaerobically incubated test system. The time-dependence of the intact cell respiration showed three distinct phases; an early very fast but short duration phase, a subsequent slow phase that prevailed for most of the reaction period and a third phase which preceded the reestablishment of anaerobiosis. Kinetic analysis of the reaction indicated a linkage between the catalytic efficiency and the transmembrane electrochemical potential. The rates of O2 uptake, obtained in the presence of both protonophores and ionophores, were monotonic and pseudo-first order over 90% of the course of O2 consumption. Extrapolation of the observed rates to zero time, at which zero delta mu H+ and thus constant flow prevails, was used to calculate the oxygen concentration for the half-maximal respiratory rate, which was found to be in the range 1.55-2.10 microM O2. No noticeable variation in the value of this kinetic parameter was found between the two cell lines used. Possible reasons for discrepancies in published reports on the oxygen dependence of the cytochrome c oxidase activity in various mitochondrial and reconstituted systems are discussed.

Role of intramitochondrial pH in the energetics and regulation of mitochondrial oxidative phosphorylation

The dependence of ATP synthesis coupled to electron transfer from 3-hydroxy-butyrate (3-OH-B) to cytochrome c on the intramitochondrial pH (pHi) was investigated. Suspensions of isolated rat liver mitochondria were incubated at constant extramitochondrial pH (pHe) with ATP, ADP, Pi, 3-OH-B, and acetoacetate (acac) (the last two were varied to maintain [3-OH-B]/[acac] constant), with or without sodium propionate to change the intramitochondrial pH. Measurements were made of the steady-state water volume of the mitochondrial matrix, transmembrane pH difference, level of cytochrome c reduction, concentration of metabolites and rate of oxygen consumption. For each experiment, conditions were used for which transmembrane pH was near maximal and minimal values and the measured extramitochondrial [ATP], [ADP], and [Pi] were used to calculate log[ATP]/[ADP][Pi]. When [3-OH-B]/[acac] and [cyt c2+]/[cyt c3+] were constant, and pHi was decreased from approx. 7.7 to 7.2, log [ATP]/[ADP][Pi] at high pHi was significantly (P less than 0.02) greater than at low pHi. The mean slope (delta log [ATP]/[ADP][Pi] divided by the change in pHi) was 1.08 +/- 0.15 (mean +/- S.E.). This agrees with the slope of 1.0 predicted if the energy available for ATP synthesis is dependent upon the pH at which 3-hydroxybutyrate dehydrogenase operates, that is, on the pH of the matrix space. The steady-state respiratory rate and reduction of cytochrome c were measured at different pHi and pHe values. Plots of respiratory rate vs.% cytochrome c reduction at different intra- and extramitochondrial pH values indicated that the respiratory rate is dependent upon pHi and not on pHe. This implies that the matrix space is the source of protons involved in the reduction of oxygen to water in coupled mitochondria.

Investigation of adenylate energy charge, phosphorylation potential, and ATP concentration in cells stressed with starvation and heat

We have attempted to determine the appropriate parameter of energy status associated with the survival of CHO fibroblasts under starvation conditions. Survival correlated well with adenylate energy charge (EC) but not so well with the phosphorylation potential or ATP concentration. Starved cells exhibited the capacity to resist (transiently) decreases in both EC and survival. A fall in EC was associated with decreased survival. Using this correlation, we subsequently investigated whether killing after thermal stress occurred by a mechanism analogous to starvation, perhaps due to inhibition of energy yielding pathways. This hypothesis proved to be false; over 99% of cells were killed before a decrease was observed in any of the parameters of energy status. Cells were, however, sensitized to heat under nutritionally deprived conditions, a finding which may be significant for tumor treatment by heat in vivo.

Energetics of ATP-driven reverse electron transfer from cytochrome c to fumarate and from succinate to NAD in submitochondrial particles

The maximum Gibbs free energies of reverse electron transfer from succinate to NAD+ and from cytochrome c to fumarate driven by ATP hydrolysis in submitochondrial particles from beef heart were measured as a function of the Gibbs free energy of ATP hydrolysis. The ratio of the energies delta G'redox/delta G'ATP was 1.40 from succinate to NAD+ and 0.89 from cytochrome c to succinate. The ratio, equivalent to a thermodynamic P/2e-ratio, was dependent on whether the electrochemical proton gradient was primarily a membrane potential or a pH gradient for the cytochrome c to fumarate reaction. The results are consistent with H+/ATP = 3 for F1 ATPase, H+/2e- = 4 for NADH-CoQ reductase, and H+(matrix)/2e- = 2 for succinate-cytochrome c reductase.

Interactions in cytochrome oxidase: functions and structure

Mitochondrial cytochrome c oxidase is an exceedingly complex multistructural and multifunctional membranous enzyme. In this review, we will provide an overview of the many interactions of cytochrome oxidase, stressing developments not covered by the excellent monograph of Wikström, Krab, and Saraste (1981), and continuing into early 1983. First we describe its functions (both in the nominal sense, as a transporter of electrons between cytochrome c and oxygen, and in its role in energy transduction). Then we describe its structure, emphasizing the protein (its structure as a whole, the number and stoichiometry of its subunits, their biosynthetic origin, and their interactions with each other, with other components of the enzyme complex, and with the membrane as a whole). Finally, we present a model in which the protein conformation serves as the focus for the dynamic interaction of its two major functions.

Ferricyanide can replace pyruvate to stimulate growth and attachment of serum restricted human melanoma cells

Addition of potassium ferricyanide to RPMI 1640 medium can stimulate cell attachment and replication, in a closely correlated fashion, of a human melanoma line when serum is a limiting growth factor. Ferricyanide is more effective than pyruvate on a molar basis but toxic effects at concentrations greater than 0.03mM limit its full potential. Since ferricyanide cannot itself provide nutrients for the cell and is extracellular but may be involved in transmembrane electron flow, it is suggested that its mechanism of action may be to provide energy for cell surface processes concerned with attachment and thus secondarily for replication.