On the localized coupling of respiration and phosphorylation in mitochondria

OXPHOS Organization and Activity in Mitochondria of Plants with Different Life Strategies

The study of the supramolecular organization of the mitochondrial oxidative phosphorylation system (OXPHOS) in various eukaryotes has led to the accumulation of a considerable amount of data on the composition, stoichiometry, and architecture of its constituent superstructures. However, the link between the features of system arrangement and the biological characteristics of the studied organisms has been poorly explored. Here, we report a comparative investigation into supramolecular and functional OXPHOS organization in the mitochondria of etiolated shoots of winter wheat (Triticum aestivum L.), maize (Zea mays L.), and pea (Pisum sativum L.). Investigations based on BN-PAGE, in-gel activity assays, and densitometric analysis revealed both similarities and specific OXPHOS features apparently related to the life strategies of each species. Frost-resistant winter wheat was distinguished by highly stable basic I1III2IVa/b respirasomes and V2 dimers, highly active complex I, and labile complex IV, which were probably essential for effective OXPHOS adaptation during hypothermia. Maize, a C4 plant, had the highly stable dimers IV2 and V2, less active complex I, and active alternative NAD(P)H dehydrogenases. The latter fact could contribute to successful chloroplast-mitochondrial cooperation, which is essential for highly efficient photosynthesis in this species. The pea OXPHOS contained detergent-resistant high-molecular respirasomes I1-2III2IVn, highly active complexes IV and V, and stable succinate dehydrogenase, suggesting an active energy metabolism in organelles of this plant. The results and conclusions are in good agreement with the literature data on the respiratory activity of mitochondria from these species and are summarized in a proposed scheme of organization of OXPHOS fragments.

Infrared fingerprints of water collective dynamics indicate proton transport in biological systems

Recent publications on spectroscopy of water layers in water bridge structures revealed a significant enhancement of the proton mobility and the dielectric contribution of translational vibrations of water molecules in the interfacial layers compared to bulk water. Herewith, the results of long-term studies of proton dynamics in solid-state acids have shown that proton mobility increases significantly with the predominance of hydronium, but not Zundel, cations in the aqueous phase. In the present work, in the light of these data, we reanalyzed our previously published results on broadband dielectric spectroscopy of bovine heart cytochrome c, bovine serum albumin, and the extracellular matrix and filaments of Shewanella oneidensis MR-1. We revealed that, just as in water bridges, an increase in electrical conductivity in these systems correlates with an increase in the dielectric contribution of water molecular translational vibrations. In addition, the appearance of spectral signatures of the hydronium cations was observed only in those cases when the system revealed noticeable electrical conductivity due to delocalized charge carriers.

VJ21.089The subcompartmented oxphosomic model of the phosphorylating system organization in mitochondria

The oxidative phosphorylation (OXPHOS) system of mitochondria supports all the vitally important energy-consuming processes in eukaryotic cells, providing them with energy in the form of ATP. OXPHOS enzymes (complexes I-V) are located in the inner mitochondrial membrane, mainly in the cristae subcompartment. At present, there is a large body of data evidencing that the respiratory complexes I, III2 and IV under in vivo conditions can physically interact with each other in diverse stoichiometry, thereby forming supercomplexes. Despite active accumulation of knowledge about the structure of the main supercomplexes of the OXPHOS system, its physical and functional organization in vivo remains unclear. Contemporary models of the OXPHOS system's organization in the inner membrane of mitochondria are contradictory and presume the existence of either highly organized respiratory strings, or, by contrast, a set of randomly dispersed respiratory supercomplexes and complexes. Furthermore, it is assumed that ATP-synthase (complex V) does not form associations with respiratory enzymes and operates autonomously. Our latest data obtained on mitochondria of etiolated shoots of pea evidence the possibility of physical association between the respiratory supercomplexes and dimeric ATP-synthase. These data have allowed us to reconsider the contemporary concept of the phosphorylation system organization and propose a new subcompartmented oxphosomic model. According to this model, a substantial number of the OXPHOS complexes form oxphosomes, which in a def inite stoichiometry include complexes I-V and are located predominantly in the cristae subcompartment of mitochondria in the form of highly organized strings or patches. These suprastructures represent "mini-factories" for ATP production. It is assumed that such an organization (1) contributes to increasing the eff iciency of the OXPHOS system operation, (2) involves new levels of activity regulation, and (3) may determine the inner membrane morphology to some extent. The review discusses the proposed model in detail. For a better understanding of the matter, the history of development of concepts concerning the OXPHOS organization with the emphasis on recent contemporary models is brief ly considered. The principal experimental data accumulated over the past 40 years, which conf irm the validity of the oxphosomic hypothesis, are also provided.

A protet-based, protonic charge transfer model of energy coupling in oxidative and photosynthetic phosphorylation

Textbooks of biochemistry will explain that the otherwise endergonic reactions of ATP synthesis can be driven by the exergonic reactions of respiratory electron transport, and that these two half-reactions are catalyzed by protein complexes embedded in the same, closed membrane. These views are correct. The textbooks also state that, according to the chemiosmotic coupling hypothesis, a (or the) kinetically and thermodynamically competent intermediate linking the two half-reactions is the electrochemical difference of protons that is in equilibrium with that between the two bulk phases that the coupling membrane serves to separate. This gradient consists of a membrane potential term Δψ and a pH gradient term ΔpH, and is known colloquially as the protonmotive force or pmf. Artificial imposition of a pmf can drive phosphorylation, but only if the pmf exceeds some 150-170mV; to achieve in vivo rates the imposed pmf must reach 200mV. The key question then is 'does the pmf generated by electron transport exceed 200mV, or even 170mV?' The possibly surprising answer, from a great many kinds of experiment and sources of evidence, including direct measurements with microelectrodes, indicates it that it does not. Observable pH changes driven by electron transport are real, and they control various processes; however, compensating ion movements restrict the Δψ component to low values. A protet-based model, that I outline here, can account for all the necessary observations, including all of those inconsistent with chemiosmotic coupling, and provides for a variety of testable hypotheses by which it might be refined.

New insights into the organisation of the oxidative phosphorylation system in the example of pea shoot mitochondria

The physical and functional organisation of the OXPHOS system in mitochondria in vivo remains elusive. At present, different models of OXPHOS arrangement, representing either highly ordered respiratory strings or, vice versa, a set of randomly dispersed supercomplexes and respiratory complexes, have been suggested. In the present study, we examined a supramolecular arrangement of the OXPHOS system in pea shoot mitochondria using digitonin solubilisation of its constituents, which were further analysed by classical BN-related techniques and a multidimensional gel electrophoresis system when required. As a result, in addition to supercomplexes I1III2, I1III2IVn and III2IV1-2, dimer V2, and individual complexes I-V previously detected in plant mitochondria, new OXPHOS structures were also revealed. Of them, (1) a megacomplex (IIxIIIyIVz)n including complex II, (2) respirasomes I2III4IVn with two copies of complex I and dimeric complex III2, (3) a minor new supercomplex IV1Va2 comigrating with I1III2, and (4) a second minor form of ATP synthase, Va, were found. The activity of singular complexes I, IV, and V was higher than the activity of the associated forms. The detection of new supercomplex IV1Va2, along with assemblies I1III2 and I1-2III2-4IVn, prompted us to suggest the occurrence of in vivo oxphosomes comprising complexes I, III2, IV, and V. The putative oxphosome's stoichiometry, historical background, assumed functional significance, and subcompartmental location are discussed herein.

A Comparative Study of the Action of Protonophore Uncouplers and Decoupling Agents as Inducers of Free Respiration in Mitochondria in States 3 and 4: Theoretical and Experimental Approaches

Theoretical and experimental studies have revealed that that in the liver mitochondria an increase in the rate of free respiration in state 3 induced by protonophore uncouplers 2,4-dinitrophenol and сarbonyl cyanide 4-(trifluoromethoxy)phenylhydrazone is equal to or slightly greater than the increase in respiration rate in state 4 induced by these uncouplers. In contrast to these protonophore uncouplers, the decoupler α,ω-tetradecanedioic acid, increasing the rate of respiration in state 4, does not significantly affect the rate of free respiration in state 3. We have proposed quantitative indicators that allow determining the constituent part of the rate of respiration in state 4, associated with the decoupling effect of the uncoupler. Using the example of palmitic acid, we have found out the fundamental possibility of the simultaneous functioning of uncouplers by two mechanisms: as protonophores and as decouplers. The data obtained contradict the delocalized version of Mitchell's chemiosmotic theory, but are in complete agreement with its local version. It can be assumed that the F₀F₁-ATP synthase and nearby respiratory chain complexes form a local zone of coupled respiration and oxidative ATP synthesis (zones of oxidative phosphorylation). The uncoupler-induced stimulation of mitochondrial free respiration of mitochondria in state 3 is mainly due to the return of protons to the matrix in local zones, where the generation of a proton motive force (Δр) by respiratory chain complexes is associated with various transport processes, but not with ATP synthesis (zones of protonophore uncoupling). In contrast, respiratory stimulation in state 4 by decouplers is realized in local zones of oxidative phosphorylation by switching the respiratory chain complexes to the idle mode of operation in the absence of ATP synthesis.

On Local Coupling of Electron Transport and ATP-Synthesis System in Mitochondria. Theory and Experiment

A brief description of the principal directions for searching and investigating the model of local coupling between respiration and phosphorylation proposed by R. Williams is given in this paper. We found conditions where it was possible to reveal typical functional special features of the mitochondrial phosphorylating system. According to the theory, such special features should be observed experimentally if the mitochondrial phosphorylating system operated in the state of a supercomplex. It was proved that the phosphorylating system is able to operate in two states: P. Mitchell state and R. Williams state. It was demonstrated that in the ATP synthesis reaction, ATP-synthase (F1F0) was able to use thermodynamic potential of Bronsted acids as a source of energy. It was shown using a double-inhibitor titration technique that when the phosphorylating system operated in the supercomplex state, the electron transfer system and ATP-synthesis system were docked rigidly. A model system of chemical synthesis of membrane-bound proton fraction (Bronsted acids), carrying a free energy excess, was developed on the model of bilayer lipid membrane. Catalysts selectively accelerating proton detachment of this fraction were also found. The formation of a Bronsted acids fraction carrying free energy excess was recorded during the operation of proton pumps on mitochondrial and mitoplastic membranes. In the experimental part of the work, a brief description is given of studies on new uncouplers that transfer the phosphorylation system from the local coupling state to the state of transmembrane proton transfer. Thus, they accelerated the respiration of mitochondria and decreased the ADP/O parameter.

Biophysical significance of the inner mitochondrial membrane structure on the electrochemical potential of mitochondria

The available literature supports the hypothesis that the morphology of the inner mitochondrial membrane is regulated by different energy states, that the three-dimensional morphology of cristae is dynamic, and that both are related to biochemical function. Examination of the correlation between the inner mitochondrial membrane (IMM) structure and mitochondrial energetic function is critical to an understanding of the links between mesoscale morphology and function in progressive mitochondrial dysfunction such as aging, neurodegeneration, and disease. To investigate this relationship, we develop a model to examine the effects of three-dimensional IMM morphology on the electrochemical potential of mitochondria. The two-dimensional axisymmetric finite element method is used to simulate mitochondrial electric potential and proton concentration distribution. This simulation model demonstrates that the proton motive force (Δp) produced on the membranes of cristae can be higher than that on the inner boundary membrane. The model also shows that high proton concentration in cristae can be induced by the morphology-dependent electric potential gradient along the outer side of the IMM. Furthermore, simulation results show that a high Δp is induced by the large surface-to-volume ratio of an individual crista, whereas a high capacity for ATP synthesis can primarily be achieved by increasing the surface area of an individual crista. The mathematical model presented here provides compelling support for the idea that morphology at the mesoscale is a significant driver of mitochondrial function.

Supramolecular organization of the respiratory chain in Neurospora crassa mitochondria

The existence of specific respiratory supercomplexes in mitochondria of most organisms has gained much momentum. However, its functional significance is still poorly understood. The availability of many deletion mutants in complex I (NADH:ubiquinone oxidoreductase) of Neurospora crassa, distinctly affected in the assembly process, offers unique opportunities to analyze the biogenesis of respiratory supercomplexes. Herein, we describe the role of complex I in assembly of respiratory complexes and supercomplexes as suggested by blue and colorless native polyacrylamide gel electrophoresis and mass spectrometry analyses of mildly solubilized mitochondria from the wild type and eight deletion mutants. As an important refinement of the fungal respirasome model, we found that the standard respiratory chain of N. crassa comprises putative complex I dimers in addition to I-III-IV and III-IV supercomplexes. Three Neurospora mutants able to assemble a complete complex I, lacking only the disrupted subunit, have respiratory supercomplexes, in particular I-III-IV supercomplexes and complex I dimers, like the wild-type strain. Furthermore, we were able to detect the I-III-IV supercomplexes in the nuo51 mutant with no overall enzymatic activity, representing the first example of inactive respirasomes. In addition, III-IV supercomplexes were also present in strains lacking an assembled complex I, namely, in four membrane arm subunit mutants as well as in the peripheral arm nuo30.4 mutant. In membrane arm mutants, high-molecular-mass species of the 30.4-kDa peripheral arm subunit comigrating with III-IV supercomplexes and/or the prohibitin complex were detected. The data presented herein suggest that the biogenesis of complex I is linked with its assembly into supercomplexes.