Regulation of oxidative phoshorylation

Regulation of Oxidative Phosphorylation

Submitted By: Farheen ShaikhTo: Dr. Ahmad Ali

MSc Part -I (Semester- I)Paper-II

University of Mumbai, Department of life sciences, vidyanagri, santacruz (East),Mumbai-400098.

REGULATION OF OXIDATIVE PHOSPHORYLATIONIntroduction: Oxidative phosphorylation is the culmination of energy yielding metabolism in aerobic organisms. All oxidative steps in the degradation of carbohydrates, fats, and amino acids converge at this final stage of cellular respiration, in which the energy of oxidation drives the synthesis of ATP. In eukaryotes, oxidative phosphorylation occurs in mitochondria. Oxidative phosphorylation involves the reduction of O2 to H2O with electrons donated by NADH and FADH2; it occurs equally well in light or darkness.The mechanism for extracting the energy from the reduced cofactors was a matter of considerable debate. The Chemiosmotic hypothesis proposed by Peter Mitchell in 1961 has the most experimental support, and is probably correct in its essential points. In essence, Mitchell proposed that the electron transport pathway conserves the energy from the electrons being transported by creating a proton gradient across the mitochondrial membrane, and that this proton gradient is then used to provide the energy required for ATP synthesis. How these processes work has been the subject of considerable research.Definition: The process of synthesizing ATP from ADP and Pi coupled with the electron transport chain is known as oxidative phosphorylation.Purpose: Oxidative phosphorylation uses the proton gradient established by the electron transport chain in mitochondria to power the synthesis of adenosine triphosphate (ATP) from adenosine diphosphate (ADP) and inorganic phosphate (Pi). Oxidative phosphorylation produces much more ATP than glycolysis - about 28 molecules. This ATP can then be hydrolysed by water to release free energy. Oxidative phosphorylation is the main form of ATP production in aerobically respiring organisms.Where it takes Place: Oxidative phosphorylation takes place in the mitochondria of eukaryotic cells, specifically in the cytochrome of the inner mitochondrial membrane, matrix, and intermembrane space. In prokaryotic cells, it occurs in the cytosol. Mitochondrial structure:

In order to understand how the pathways for electron transport and oxidative phosphorylation work, we need to look at the general structure of a mitochondrion.

1. A mitochondrion contains two membranes: an outer membrane, which appears to largely be responsible for maintaining the shape of the organelle, and a much less permeable inner membrane. The outer membrane contains porin, a protein that forms pores large enough allow molecules less than ~10 kDa to diffuse freely across the membrane.

2. The region between the membranes is called the inter-membrane space. The intermembrane space is occupied by soluble proteins large enough that they cannot pass through porin. For small molecules, the cytoplasm and inter-membrane space are essentially contiguous regions.

3. The inner membrane acts as a barrier to prevent the movement of most molecules. A few molecules have specific transporters that allow them to enter or exit the mitochondrion. The inner membrane contains cristae, which are involutions in inner membrane. The function of the cristae is to increase the surface area of the inner membrane. The mitochondrial inner membrane may have a larger surface area than the cell plasma membrane, due to the involutions in the membrane.

4. Finally, within the inner membrane is the matrix. The matrix is a very dense protein solution (~50% protein by weight). The TCA cycle enzymes are located in the matrix, as are the enzymes for several other metabolic pathways. Mitochondria contain a small genome (~16,500 bp). The genome contains 22 transfer RNA genes, 2 ribosomal RNA genes, and 13 polypeptide genes; the polypeptides are all involved in the electron transport pathway or oxidative phosphorylation pathway.

5. The TCA cycle enzymes (including succinate dehydrogenase) are all produced from nuclear genes; the multi subunit complexes of the electron transport pathway and ATP synthase (with the exception of succinate dehydrogenase) are made up of proteins derived from both nuclear and mitochondrial genes.

OXIDATIVE PHOSPHORYLATION STAGES:

A. Glycolysis: oxidation of glucose to pyruvic acid with some ATP and NADH produced.

During glycolysis, glucose (C6) is broken down to two molecules of pyruvate (C3).There are ten steps in glycolysis and each one is catalysed by a specific enzyme. The two 3-carbon molecules are oxidized to generate two 3- carbon pyruvic acid molecules. At the same time two NAD+ molecules are reduced to two NADH molecules and four ATP molecules are produced by substrate level phosphorylation.Summary of glycolysis:Glucose + 2 NAD+ + 2 ADP + 2 P 2 Pyruvic acid + 2 NADH + 2 H+ + 2 ATP

B. Citric Acid Cycle: oxidation of acetyl to carbon dioxide with some ATP, NADH and FADH2 produced:

Summary of decarboxylation:2Pyruvic acid + 2 NAD+ + 2 CoA 2 Acetyl CoA + 2 CO2 + 2 NADHDecarboxylation i.e. formation of Acetyl CoA: Pyruvate produced by glycolysis enters themitochondrionby active transport and is converted toacetyl CoA. The remainder of the reactions of cellular respiration occur in themitochondrion. A carbon atom is removed from each of the pyruvate molecules forming a two-carbon compound and CO2. Each of the two-carbon compounds are oxidized forming NADH from NAD+. Coenzyme A is attached to each of the two-carbon compounds producing two acetyl CoA molecules.Citric acid (TCA) Cycle:

Summary of the Citric Acid Cycle:2 Acetyl Co A + 6 NAD+ + 2 FAD + 2 ADP + 2 P + 4 H2O 2 CoA + 4 CO2 + 6 NADH + 4 H+ + 2 FADH2 + 2 ATPThe cycle occurs twice, once for each acetyl CoA. Coenzyme A is removed when the two-carbon compound is attached to a four-carbon compound producing a six-carbon compound (citrate).Each citrate molecule undergoes a series of reactions that removes 2 carbon atoms which are released as CO2. In addition, 3 NADH, 1 ATP, and 1 FADH2are produced. In addition, the four-carbon compound that began the cycle is regenerated. C.ELECTRON TRANSPORT CHAIN:

This is the truly aerobic part of the aerobic metabolism of glucose as this is where the oxygen is utilized. Oxidative phosphorylation occurs on a membrane, the mitochondrial cristae, to generate most of the ATP produced from glucose. Coenzymes from the previous reactions pass electrons to a series of electron carrier molecules, which carry out redox reactions resulting in the chemiosmotic generation of ATP.NADH and FADH2 are oxidized providing electrons for redox reactions ultimately reduce oxygen to generate ATP. The majority of the ATP is produced at this step. Electron Transport in the mitochondria occurs in four steps at four different sites embedded in the inner membrane, protein complexes I-IV.. Complexes I-IV has a variety of prosthetic groups including metal ions, iron-sulphur centres, hemes, and flavins.There are three classes of carrier molecules:1. FMN (flavin mononucleotide): protein + flavin coenzyme2. CoEnzyme Q: nonprotein3. Cytochromes: protein + an iron group (most common)

1. NADH dehydrogenase (Complex I):

Complex1 also called ubiquinone oxidoreductase or NADH dehydrogenase is a large enzyme composed of 42 different polypeptide chains, including An FMN-containing flavoprotein and at least six iron sulphur centres. High-resolution electron microscopy shows Complex I to be L-shaped, with one arm of the L in the membrane and the other extending into the matrix. In the inner mitochondrial membrane,Nicotinamide adenine dinucleotide (NADH) produced by glycolysis is oxidized (removes electrons) by the enzyme NADH dehydrogenase. The enzyme removes two electrons from NADH and attaches them to an electron carrier, ubiquinone (Q). The transfer of these electrons reduces (adds electrons) ubiquinone into ubiquinol. While this, redox reaction is occurring, four hydrogen atoms (protons) are pumped across the inner membrane to the intermembrane space. This creates a proton gradient, which basically means there is a higher concentration of protons outside the inner membrane (in the intermembrane space) than inside the membrane (in the mitochondrial matrix). 1. NADH + H+ +Q NAD+ +QH22. NADH +5H+N +Q NAD+ + QH2 +4H+PNADH binds to Flavin mononucleotide, reducing NADH to NAD+ and reducing Flavin mononucleotide to FMNH2. Notice that NADH is losing its negative hydrogen atom, resulting in the positive charge of NAD+. The two electrons and two hydrogens taken from NADH are carried by FMNH2 (which is now called an "electron carrier") to two Iron (Fe) atoms in Iron-Sulfur (Fe-S) centers located within the complex. The hydrogens then act as protons and are pumped back into the mitochondrial matrix, not the intermembrane space. The electrons in the two irons are accompanied by two protons and transfered to ubiquinone, which is also called "coenzyme Q." Ubiquinone then passes the electrons to a new Fe-S center, releasing the two protons into the matrix. A new ubiquinone is given the electrons and rests within the inner membrane, again pushing two protons to the matrix.

Ubiquinone (Coenzyme Q):

Coenzyme Q is a non-protein electron carrier located in the inner mitochondrial membrane. Mammals use Q10 in mammals, (the compound has ten isoprene units, while some other species use versions with 6 or 8 isoprene units). Note that Coenzyme Q can transfer one or two electrons. Coenzyme Q can accept electrons from Complex I and II (and from other proteins); and it donates the electrons to Complex III.2. Succinate Dehydrogenase (Complex II): Two electrons from the citric acid cycle are transfered to complex II, powering the oxidation of the enzyme succinate (also from the citric acid cycle) into fumarate. Fumarate then passes the two electrons to coenzyme FAD, which moves the electrons to a Fe-S complex and then to ubiquinone. Complex II does not produce a proton gradient because there is not enough free energy to pump protons into the intermembrane space. Other electron donors such as fatty acids and glycerol 3-phosphate also funnel electrons into Q (via FAD).3. Coenzyme Q-dependent cytochrome c reductase (Complex III): Complex III receives two electrons from the reduced ubiquinone from complexes I and II. Complex III contains several heme prosthetic groups. Different heme domains have different absorbance spectra. The different spectral species are sometimes referred to as cytochrome b and cytochrome c1; these are all part of the same protein complex. The electrons are passed through a Fe-S complex to cytochrome C. Cytochrome C is soluble heme containing electron carrier protein, pumping four protons into the intermembrane space, two from ubiquinone and two from cytochrome C. This creates another proton gradient.4. Cytochrome c oxidase (Complex IV): Cytochrome c oxidase, as the name implies, accepts electrons from cytochrome c. Cytochrome c oxidase is sometimes referred to as the cytochrome a-a3 complex. The complex contains a total of four hemes as well as copper and magnesium ions. Cytochrome C, which operates in the inter-membrane space, transports one electron at a time to complex IV. Complex IV is the terminal part of the electron chain and transfers electrons directly to oxygen. These electrons provide the energy needed to reduce molecular oxygen to two molecules of water. Complex IV creates a proton gradient. Like Complexes I and III, Complex IV is a proton pump.

B. OXIDATIVE PHOSPHORYLATION:5. F1F0-ATPase = ATP Synthase (Complex V):

ATP synthase is a huge molecular complex (>500,000daltons) embedded in the inner membrane of mitochondria. Its function is to convert the energy of protons (H+) moving down their concentration gradient into the synthesis ofATP. 3 to 4 protons moving through this machine is enough to convert a molecule of ADP and Pi(inorganic phosphate) into a molecule of ATP. One ATP synthase complex can generate >100 molecules of ATP each second.ATP synthase can be separated into 2 parts: Fo- the portion embedded in the inner mitochondrial membrane and F1-ATPase the portion projecting into the matrix of the mitochondrion.This is why the intact ATP synthase is also called the FoF1-ATPase.When the F1-ATPase is isolated in vitro, it catalyses the hydrolysis of ATP to ADP and Pi (which is why it is called the F1-ATPase). While it is doing so, the central portion of Foattached to the stalk rotates rapidly in a counter-clockwise direction (as viewed from above).In the intact mitochondrion, the protons that have accumulated in the intermembrane space enter the Focomplex and exit from it into the matrix. The energy they give up as they travel down their concentration gradient rotates Foand its stalk (at ~6000 rpm) in a clockwise direction. As it does so, it induces repeating conformational changes in the head proteins that enable them to convert ADP and Piinto ATP. (In the figure, two of the three dimers that make up the head proteins have been pulled aside to reveal the stalk inserted in their center.)In both these cases, the machine is converting chemical energy from the hydrolysis of ATP in the in vitro case and The flow of protons down their concentration gradient in the intact mitochondrion into mechanical energy the turning of the motor.Summary of Electron Transport:2 NADH from Glycolysis + 2 NADH from Decarboxylation + 6 NADH from Citric Acid Cycle+ 2 FADH2 from Citric Acid Cycle + 6 O2 + 32 ADP + 32P 12 H2O + 32 ATP + 10 NAD+ + 2 FADFinal Summary for Aerobic Respiration:C6H12O6 + 6 O2 + 36 ADP + 36 P 6 CO2 + 6 H2O + 36 ATPHOW OXIDATIVE PHOSPHORYLATION IS REGULATED?Aerobic oxidative pathways that result in electron transfer to O2 accompanied by oxidativephosphorylation therefore account for the vast majority of the ATP produced in catabolism, so the regulation of ATP production by oxidative phosphorylation to match the cells fluctuating needs for ATP is absolutely essential.There are five levels of oxidative phosphorylation regulation: direct modulation of electron transport chain kinetic parameters; regulation of intrinsic efficiency of oxidative phosphorylation (by changes in proton conductance, in the measure of oxidative phosphorylation or in the channelling of electron transport chain intermediate substrates); mitochondrial network dynamics (fusion, fission, motility, membrane lipid composition, swelling); mitochondrial biogenesis and degradation; cellular and mitochondrialmicroenvironment.The synthesis of ATP by electron transport and oxidative phosphorylation appearsto be regulated essentially exclusively by substrate availability. The pathway cannot proceed without ADP+Pi or NADH; if both are available, then the pathwaywill result in ATP synthesis.

Energetics of the TCA cycle and glucose oxidation:The number of ATP produced per NADH oxidized depends on the number of protons pumped by each complex, and on the number of protons required by the ATP synthase. Some research indicates ~3 ATP/NADH, while other studies suggest somewhat fewer (~2.5). Possible causes of discrepancies include:1) The number of protons pumped at each stage may vary somewhat.2) The mitochondrial inner membrane is not a perfect barrier: a few protons leak through the membrane, partially uncoupling the system3) The proton gradient is used to pump other molecules (e.g., protons drive the pumping of pyruvate into the mitochondria).4) Many of the measurements (for example, of the exact proton gradient existing in cells, and of the ADP concentration in the mitochondrion) are difficult to perform.

For the purposes of the following discussion under optimum conditions, NADH can be converted to about three ATP, because Complex I, III, and IV are each thought to pump four protons, and ATP synthesis is thought to require four protons. Electrons from FADH2 enter at the level of Complex II, which does not pump protons, but instead hands them to Complex III; this suggests that FADH2 results in formation of about two ATP. Using these estimates of the number of ATP produced, and the net reaction for TCA cycle suggests that the TCA cycle can result in the production of 12 ATP: three ATP for each of the three NADH, two ATP for the FADH2, and one substrate level phosphorylation.

During the conversion of pyruvate to acetyl-CoA, one NADH is generated, resulting in an additional three ATP; each complete oxidation of pyruvate to carbon dioxide therefore results in 15 ATP. Glucose conversion to pyruvate produces 2 NADH and 2 ATP, adding a total of 8 ATP, and therefore resulting in 38 ATP (under optimum conditions) for complete aerobic glycolysis. (Note once again, that this represents the maximal amount of ATP obtainable from complete oxidation of glucose; the value is more for the purposes of comparison than for claiming that each molecule of glucose will always result in 38 molecules of ATP).

1. Regulation of mitochondrial oxidative phosphorylation by second messenger-mediated signal transduction mechanisms:The mitochondrial oxidative phosphorylation system is responsible for providing the bulk of cellular ATP molecules. There is a growing body of information regarding the regulation of this process by a number of second messenger-mediated signal transduction mechanisms, although direct studies aimed at elucidating this regulation are limited. The main second messengers affecting mitochondrial signal transduction are cAMP and calcium. Other second messengers include ceramide and reactive oxygen species as well as nitric oxide and reactive nitrogen species. This review focuses on available data on the regulation of the mitochondrial oxidative phosphorylation system by signal transduction mechanisms and is organised according to the second messengers involved, because of their pivotal role in mitochondrial function. Future perspectives for further investigations regarding these mechanisms in the regulation of the oxidative phosphorylation system are formulated.

2. Regulation of Oxidative Phosphorylation Efficiency and Respiratory States:Oxidative phosphorylation efficiency is dependent on delivery of reducing equivalents into electron transport chain and on activities of participating enzymes or enzyme complexes. The optimal efficiency and flow ratios are determined by control of complex I (reflects integrated cellular pathway) and complex II (the predominantly tricarboxylic acid cycle pathway) Depletion of tricarboxylic acid cycle intermediates plays an important role in the oxidative phosphorylation flux control. In respirometric assays, supplies of complex I as well as complex II are required. Convergent electron input and reconstitution of the tricarboxylic acid are needed to achieve maximal respiration. It is controlled also by the availability of adenosine-5-diphosphate for the adenine nucleotide transporter in the inner mitochondrial membrane. Complex I is suggested to be responsible for adaptive changes and physiological set up of oxidative phosphorylation efficiency. The stoichiometric efficiency of oxidative phosphorylation is defined by the phosphorylation, or the amount of inorganic phosphate (Pi) incorporated into adenosine-5-triphosphate per amount of consumed oxygen.

3. Regulation of Oxidative Phosphorylation by Mitochondrial Calcium:Stimulation of mitochondrial oxidative metabolism by Ca2+is now generally recognised as important for the control of cellular ATP homeostasis. Here, we review the mechanisms through which Ca2+regulates mitochondrial ATP synthesis.Calcium is believed to regulate mitochondrial oxidative phosphorylation, thereby contributing to the maintenance of cellular energy homeostasis. Skeletal muscle, with an energy conversion dynamic range of up to 100-fold, is an extreme case for evaluating the cellular balance of ATP production and consumption. This study examined the role of Ca2+in the entire oxidative phosphorylation reaction network in isolated skeletal muscle mitochondria and attempted to extrapolate these results back to the muscle, in vivo. Kinetic analysis was conducted to evaluate the dose-response effect of Ca2+ on the maximal velocity of oxidative phosphorylation [V (maxO)] and the ADP affinity. Force-flow analysis evaluated the interplay between energetic driving forces and flux to determine the conductance, or effective activity, of individual steps within oxidative phosphorylation. Force-flow analysis revealed that Ca2+ activation of [V (maxO)] was distributed throughout the oxidative phosphorylation reaction sequence. Specifically, Ca2+ increased the conductance of Complex IV (2.3-fold), Complexes I and III (2.2-fold), ATP production/transport (2.4-fold), and fuel transport/dehydrogenases (1.7-fold). These data support the notion that Ca2+ activates the entire muscle oxidative phosphorylation cascade, while extrapolation of these data to the exercising muscle predicts a significant role of Ca2+ in maintaining cellular energy homeostasis.

4. Oxidative Phosphorylation Is Regulated by Cellular Energy Needs:

Oxidative phosphorylation is regulated by cellular energy demands. The intracellular [ADP] and the mass-action ratio [ATP]/ ([ADP][Pi]) are measures of a cells energy status. The rate of respiration (O2consumption) in mitochondria is under tight regulation; it is generally limited by the availability of ADP as a substrate for phosphorylation. As we saw in Figure 18-13b, the respiration rate in isolated mitochondria is low in the absence of ADP and increases strikingly with the addition of ADP; this phenomenon is part of the definition of coupling of oxidation and phosphorylation. The intracellular concentration of ADP is one measure of the energy status of cells.

Another, related measure is themass-action ratioof the ATP-ADP system: [ATP]/ ([ADP] [Pi]). Normally this ratio is very high, so that the ATP-ADP system is almost fully phosphorylated. When the rate of some energy-requiring process in cells (protein synthesis, for example) increases, there is an increased rate of breakdown of ATP to ADP and Pi, lowering the mass-action ratio. With more ADP available for oxidative phosphorylation, the rate of respiration increases, causing regeneration of ATP. This continues until the mass action ratio returns to its normal high level, at which point respiration slows again. The rate of oxidation of cell fuels is regulated with such sensitivity and precision that the ratio [ATP]/ ([ADP][Pi] fluctuates only slightly in most tissues, even during extreme variations in energy demand. In short, ATP is formed only as fast as it is used in energy requiring cell activities.

5. An Inhibitory Protein Prevents ATP Hydrolysis during Ischemia:

In ischemic (oxygen-deprived) cells, a protein inhibitor blocks ATP hydrolysis by the ATPSynthase operating in reverse, preventing a drastic drop in [ATP]. We have already encountered ATP synthase as an ATP driven proton pump. As in a heart attack or stroke, electron transfer to oxygen ceases, and so does the pumping of), catalysing the reverse of ATP synthesis. When a cell is ischemic (deprived of oxygen), protons the proton-motive force soon collapses. Under these conditions, the ATP synthase could operate in reverse, hydrolysing ATP to pump protons outward and causing a disastrous drop in ATP levels. This is prevented by a small (84 amino acids) protein inhibitor, IF1, which simultaneously binds to two ATP synthase molecules, inhibiting their ATPase activity IF1 is inhibitory only in its dimeric form, which is favoured at pH lower than 6.5.

In a cell starved for oxygen, the main source of ATP becomes glycolysis, and the pyruvic or lactic acid thus formed lowers the pH in the cytosol and the mitochondrial matrix. This favours IF1 dimerization, leading to inhibition of the ATPase activity of ATP synthase, thereby preventing wasteful hydrolysis of ATP. When aerobic metabolism resumes, production of pyruvic acid slows, the pH of the cytosol rises, the IF1 dimer is destabilized, and the inhibition of ATP synthase is lifted.

6. Uncoupled Mitochondria in Brown Fat Produce Heat:

In brown fat, which is specialized for the production of metabolic heat, electron transfer is uncoupled from ATP synthesis and the energy of fatty acid oxidation is dissipated as heat. There is a remarkable and instructive exception to the general rule that respiration slows when the ATP supply is adequate. Most new born mammals, including humans, have a type of adipose tissue called brown fat in which fuel oxidation serves not to produce ATP but to generate heat to keep the new-born warm. This specialized adipose tissue is brown because of the presence of large numbers of mitochondria and thus large amounts of cytochromes, whose heme groups are strong absorbers of visible light.

The mitochondria of brown fat are like those of other mammalian cells in all respects, except that they have a unique protein in their inner membrane. Thermogenin, also called the uncoupling protein (Table 194), Provides a path for protons to return to the matrix without passing through the FoF1 complex As a result of this short-circuiting of protons, the energy of oxidation is not conserved by ATP formation but is dissipated as heat, which contributes to maintaining the body temperature of the new born. Hibernating animals also depend on uncoupled mitochondria of brown fat to generate heat during their long dormancy

7. ATP-Producing Pathways Are Co-ordinately Regulated:ATP and ADP concentrations set the rate of electron transfer through the respiratory chain via a series of interlocking controls on respiration, glycolysis, and the citric acid cycle.The relative concentrations of ATP and ADP control not only the rates of electron transfer and oxidative phosphorylation but also the rates of the citric acid cycle, pyruvate oxidation, and glycolysis. Whenever ATP consumption increases, the rate of electron transfer and oxidative phosphorylation increases. Simultaneously, the rate of pyruvate oxidation via the citric acid cycle increases, increasing the flow of electrons into the respiratory chain. These events can in turn evoke an increase in the rate of glycolysis, increasing the rate of pyruvate formation. When conversion of ADP to ATP lowers the ADP concentration, acceptor control slows electron transfer and thus oxidative phosphorylation.Glycolysis and the citric acid cycle are also slowed, because ATP is an allosteric inhibitor of the glycolytic enzyme phosphofructokinase-1 and of pyruvate dehydrogenase.

CONCLUSION:Regulation of cellular bioenergetics is crucial in processes of neuroplasticity. Oxidative phosphorylation is the most important source of adenosine-5- triphosphate; its efficacy is determined by different mechanisms. Primary, the supply of substrates implemented by Ca2+ levels, reversible phosphorylation, allosteric inhibition of oxidative phosphorylation subunits, fatty acids and uncoupling protein, and influences of hormones. The system of oxidative phosphorylation does not respond to thermodynamic equilibrium, but embodies a rate of uncoupling. Lower membrane potential (m) can result in hydrolysis of cytoplasmic adenosine-5-triphosphate; high membrane potential (m) leads to proton leak and increased uncoupling. Measurement of both respiration and membrane potential during action of appropriate endogenous and exogenous substances enables the identification of the primary sites of effectors and the distribution of control, allowing deeper quantitative analyses. Better insight into molecular mechanisms of cellular respiration, control of oxidative phosphorylation and its roles in neuroplasticity likely better understand function, physiology as well as pathophysiology of various diseases.

Current Research: Recent experimental results indicate that oxidative phosphorylation in mitochondria is not only regulated by respiratory control, i.e., inhibition of respiration at low ATP utilization via the electrochemical proton gradient across the inner mitochondrial membrane, but in addition by reversible phosphorylation of respiratory chain complexes and of ATP synthase. Thus the formation of ATP and the generation of heat by mitochondria is also controlled by second messenger-mediated signal transduction mechanisms. The second messengers include cAMP, calcium, and ROS leading to activation of mitochondrial protein kinases and phosphatases. Some protein kinases (e.g., PKB = Akt, PKC) have been demonstrated to be translocated into mitochondria after activation (phosphorylation) outside of mitochondria. Subunit phosphorylation has been described for complexes I (NADH dehydrogenase), II (succinate dehydrogenase), III (cytochrome c reductase), IV (cytochrome c oxidase) and V (ATP synthase). Of particular interest is the phosphorylation of complex IV leading to an allosteric ATP-inhibition of cytochrome c oxidase, representing a second mechanism of respiratory control.

BIBLIOGRAPHY:BOOKS: Principles of Biochemistry; Lehninger, AL. Nelson, David L, Cox Micheal M. (3rd edition. 2000, Worth pub.) Textbook of Biochemistry; U.Satayanarayana and U.Chakrapani, (3rd Edition 2006, Arunabha Sen pub.) Biochemistry by Lubert Stryer, New York, W.H.Freeman, (6th Edition 1995) WEBSITES: http://www.sciencedirect.com/science/article/pii/S0167488907002364 http://www.biologyreference.com/Oc-Ph/Oxidative-Phosphorylation.html http://www.sparknotes.com/biology/cellrespiration/oxidativephosphorylation/section3.rhml http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/A/ATPsynthase.html https://www.rpi.edu/dept/bcbp/molbiochem/MBWeb/mb1/part2/redox.htm

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