Close mobile search navigation Article navigation. Volume 41, Issue 1. Next Article. All Issues. Cover Image Cover Image. Article Navigation. Conference Article January 29 Walker John E. Walker 1. This Site. Google Scholar. Biochem Soc Trans 41 1 : 1— Article history Received:. Get Permissions. You do not currently have access to this content. These proteins operate electrogenically, and the adenylate translocator exchanges free adenylates, while the phosphate translocator exchanges free phosphate in the symport with proton or in the antiport with OH —.
The electrical currents measured with the reconstituted adenylate translocator demonstrate electrogenic translocation of adenylates and charge shift of reorienting carrier sites Klingenberg, The mitochondrial phosphate transporter makes it possible for a very rapid transport of most of the Pi used in ATP synthesis Ferreira and Pedersen, Since the inner membrane of mitochondria possesses electrical potential difference depending on the rate of proton pumping by electron transport, the adenylate transporter and other charge-moving processes, this affects the transport of adenylates and their equilibration by AK Igamberdiev and Kleczkowski, , In the absence of a membrane potential, the equilibrium concentrations of total adenylates will correspond to equimolar concentrations of free adenylates inside and outside mitochondria.
The involvement of AK in respiration is likely supported by apyrase, an Mg-dependent enzyme, which is ubiquitously distributed in different tissues and exists in several subcellular compartments, including a cytosol and IMS-confined isozymes Flores-Herrera et al. Other sources of AMP include reactions leading to the formation of CoA-derivatives, activation of amino acids for protein synthesis, or nucleotide pyrophosphatase Igamberdiev and Kleczkowski, Thus the bioenergetic function of mitochondria is controlled from the outside cytosol , whereas chloroplast appears as a more autonomous system supporting its ATP-generating function via the ratio of adenylates in its stroma.
The dynamic environment of ATP synthase in chloroplasts is established in a different and in most aspects opposite way as compared to mitochondria. ATP synthase receives protons from the thylakoid lumen Figure 2 , which has smaller volume as compared to the mitochondrial IMS, and its pH dropping to the values below 5 Oja et al. The size of granal thylakoids was determined for Arabidopsis as 4 nm stacking repeat distance to 5 nm diameter in darkness, increasing to 19 nm in width and to 9—10 nm in diameter in the light Kirchhoff et al.
Although two chloroplast adenylate transporters were identified Mohlmann et al. Thus, it is quite certain that the stromal pool of adenylates is the sole source for AK-equilibrium governed delivery of ADP for ATP synthase reaction in chloroplasts. Figure 2. Abbreviations are the same as in Figure 1.
TM, thylakoid membrane; TPT, triose phosphate transporter. There is no AK in thylakoid lumen, and the entire chloroplastic AK activity is confined to chloroplast stroma. Lange et al. Whereas silencing of the gene for one of the chloroplastic AK had no effect on plant phenotype, the second chloroplast AK was essential for proper growth and development. Although to-date the importance of the first chloroplast AK isoform is not clear, the crucial role of the second is evident in providing proper chloroplast functioning and integrity.
Several key metabolic processes are strongly affected by AK, e. Thus, the AK reaction prevents over-accumulation of ATP, resulting in the balance of anabolic Calvin cycle, starch synthesis, lipid biosynthesis, etc. Both in rice and Arabidopsis , SnRK1 critically influences stress-inducible gene expression and the induction of stress tolerance, and its activity modulates plant developmental processes from early seedling development through late senescence Cho et al.
Plants carrying out C 4 metabolism e. This duality underscores different functions of chloroplastic ATP synthase in those cells. Whereas bundle sheath chloroplasts carry out the Calvin cycle and accumulate starch in the light, the mesophyll chloroplasts do not have Rubisco, and starch accumulation there ceases in mature leaves Weise et al.
Thus, in the mesophyll, the ATP formed by ATP-synthase must be linked to entirely different processes than in bundle sheath cells, and this occurs prominently by coupling to AK reaction. In C 4 plants, the activity of AK from mesophyll cell chloroplasts is many-fold higher than in bundle sheath cells Kleczkowski and Randall, and it is coupled to regeneration of phosphoenolpyruvate PEP , the primary CO 2 acceptor in C 4 photosynthesis Hatch, A special function of AK and pyruvate, Pi-dikinase is evident also in C 3 plants under anoxic conditions, where the joint operation of these enzymes provides an efficient use of PPi in addition to ATP as energy currency, thus avoiding drastic depletion of energy when mitochondrial respiration is suppressed by the lack of oxygen Igamberdiev and Kleczkowski, a , b.
Figure 3 shows how the interactions between chloroplasts and mitochondria involving also cytosol are optimized by operation of ATP synthase in the two compartments and by AK present in the chloroplast stroma and mitochondrial IMS. In mitochondria, on the other hand, equilibration of adenylates takes place in the IMS, i. The depots of magnesium stored in vacuoles and mitochondria contribute via corresponding transporters Shaul et al.
Figure 3. When it is not necessary for mitochondrial ATP synthase to further support ATP synthesis the non-coupled pathways of respiration become operative. The shifts in balance between the reactions of load and consumption that are beyond the buffering capacity of AK and related mechanisms can be adjusted via irreversible exergonic reactions that are not coupled to ATP synthesis.
They correspond to a slippage occurring when an enzyme passes a proton without ATP synthesis, e. This slippage decreases the efficiency of energy utilization but enables controlling and regulating metabolic demands. Other non-coupled systems include the uncoupling proteins UCPs; Vercesi et al.
It prevents excessive proton pumping and thus buffers proton concentration for providing the optimal performance of the ATP synthase. In chloroplasts, there are many alternative sinks for electrons, and several of them are non-coupled with proton gradient Ivanov et al.
However, these pathways are important mainly in preventing overreduction of chloroplast ETC and their capacity is insufficient for fine-tuning of redox and energy balance in the whole cell. We have presented evidence in this paper that the steady fluxes of adenylates, magnesium, hydrogen ions and phosphate established via thermodynamic buffering and regulated uncoupling support optimal load and consumption of ATP synthase and provide its stable catalytic cycle.
AK equilibrium represents an essential bioenergetic regulatory principle for the maintenance of steady regimes of ATP synthesis in mitochondria and chloroplasts and its utilization in metabolic processes. Despite of all similarities and differences in molecular regulation of ATP synthases in both mitochondria and chloroplasts, even though the topology is totally different, and despite the different location of AK in chloroplasts and mitochondria, in both cases the activities of ATP synthases are finely optimized.
This optimization provides a dynamically stable homeostatic state essential for the maintenance of photosynthesis and for support of metabolic processes in plant cells and tissues. The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. Blair, J. Magnesium, potassium, and the adenylate kinase equilibrium. Magnesium as a feedback signal from the adenine nucleotide pool. Blum, D.
Biochemistry 51, — Blumenfeld, L. Physics of Bioenergetic Processes. Berlin: Springer. Google Scholar. Boyer, P. A perspective of the binding change mechanism for ATP synthesis. The ATP synthase—a splendid molecular machine. Catalytic site forms and controls in ATP synthase catalysis. Acta , — Buchachenko, A. Magnesium isotope effects in enzymatic phosphorylation. B , — Bykova, N. The function of glycine decarboxylase complex is optimized to maintain high photorespiratory flux via buffering of its reaction products.
Mitochondrion 19, — Cho, Y. Regulatory functions of SnRK1 in stress-responsive gene expression and in plant growth and development. Plant Physiol. Dahnke, T. Mechanism of adenylate kinase.
The conserved aspartates and are important for transition state stabilization instead of substrate-induced conformational changes.
Ferreira, G. Phosphate transport in mitochondria: past accomplishments, present problems, and future challenges. Flores-Herrera, O. A novel ATP-diphosphohydrolase from human term placental mitochondria. Placenta 20, — Fragoso, S. Gaballo, A. Structures and interactions of proteins involved in the coupling function of the protonmotive FoF1-ATP synthase. Gilli, R. Thermodynamic analysis of calcium and magnesium binding to calmodulin. Biochemistry 37, — They also divide independently of the cell in which they reside, meaning mitochondrial replication is not coupled to cell division.
Some of these features are holdovers from the ancient ancestors of mitochondria, which were likely free-living prokaryotes. What Is the Origin of Mitochondria? Figure 1: A mitochondrion. What Is the Purpose of a Mitochondrial Membranes? Figure 2: The electrochemical proton gradient and ATP synthase. At the inner mitochondrial membrane, a high energy electron is passed along an electron transport chain. Is the Mitochondrial Genome Still Functional? Figure 3: Protein import into a mitochondrion.
A signal sequence at the tip of a protein blue recognizes a receptor protein pink on the outer mitochondrial membrane and sticks to it. Logically, mitochondria multiply when a the energy needs of a cell increase. Therefore, power-hungry cells have more mitochondria than cells with lower energy needs. For example, repeatedly stimulating a muscle cell will spur the production of more mitochondria in that cell, to keep up with energy demand. Mitochondria, the so-called "powerhouses" of cells, are unusual organelles in that they are surrounded by a double membrane and retain their own small genome.
They also divide independently of the cell cycle by simple fission. Mitochondrial division is stimulated by energy demand, so cells with an increased need for energy contain greater numbers of these organelles than cells with lower energy needs. Topic rooms within Cell Biology Close. No topic rooms are there. Or Browse Visually. Student Voices. Creature Cast. Simply Science. Green Screen.
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