How does oxidative phosphorylation result in the production of atp




















As long as the potential for the overall reaction is positive the reaction is spontaneous. Hence, from Table 2 below, we see that cytochrome c 1 part of the cytochrome reductase complex, 3 in Figure 9 can spontaneously transfer an electron to cytochrome c 4 in Figure 9.

The net reaction is given by Equation 16, below. We can also see from Table 2 that cytochrome c 1 cannot spontaneously transfer an electron to cytochrome b Equation 19 :. Table 2 lists the reduction potentials for each of the cytochrome proteins i. Note that each electron transfer is to a cytochrome with a higher reduction potential than the previous cytochrome. As described in the box above and seen in Equations , an increase in potential leads to a decrease in D G Equation 13 , and thus the transfer of electrons through the chain is spontaneous.

To view the cytochrome molecules interactively using RASMOL, please click on the name of the complex to download the pdb file. As we shall see below, this huge concentration gradient leads to the production of ATP. We have seen that the electron-transport chain generates a large proton gradient across the inner mitochondrial membrane.

But recall that the ultimate goal of oxidative phosphorylation is to generate ATP to supply readily-available free energy for the body. How does this occur? In addition to the electron-carrier proteins embedded in the inner mitochondrial membrane, a special protein called ATP synthetase Figure 9, the red-colored protein is also embedded in this membrane. ATP synthetase uses the proton gradient created by the electron-transport chain to drive the phosphorylation reaction that generates ATP Figure 7c.

ATP synthetase is a protein consisting of two important segments: a transmembrane proton channel, and a catalytic component located inside the matrix. Recall from the Kidney Dialysis tutorial that particles spontaneously diffuse from areas of high concentration to areas of low concentration.

Thus, since the diffusion of protons through the channel component of ATP synthetase is spontaneous, this process is accompanied by a negative change in free energy i. Then, using the free energy released by the spontaneous diffusion of protons through the channel segment, a bond is formed between the ADP and a free phosphate group, creating an ATP molecule.

A scientist has created a phospholipid-bilayer membrane containing ATP-synthetase proteins. Briefly, explain your answer. What effect do you expect these toxins to have on the production of ATP? Summary In this tutorial, we have learned that the ability of the body to perform daily activities is dependent on thermodynamic, equilibrium, and electrochemical concepts. These activities, which are typically based on nonspontaneous chemical reactions, are performed by using free-energy currency.

The common free-energy currency is ATP, which is a molecule that easily dephosphorylates loses a phosphate group and releases a large amount of free energy. As the coupled reactions occur i. As seen in Figure 4, the breakdown of glucose glycolysis obtained from the food we eat cannot by itself generate the large amount of ATP that is needed for metabolic energy by the body.

These redox molecules are used in an oxidative-phosphorylation process to produce the majority of the ATP that the body uses. Oxidative phosphorylation occurs in the mitochondria, and the two reactions oxidation of NADH or FADH 2 and phosphorylation to generate ATP are coupled by a proton gradient across the inner membrane of the mitochondria Figure 9.

As seen in Figures 7 and 9, the oxidation of NADH occurs by electron transport through a series of protein complexes located in the inner membrane of the mitochondria. This electron transport is very spontaneous and creates the proton gradient that is necessary to then drive the phosphorylation reaction that generates the ATP. Hence, oxidative-phosphorylation demonstrates that free energy can be easily transferred by proton gradients.

Oxidative-phosphorylation is the primary means of generating free-energy currency for aerobic organisms, and as such is one of the most important subjects in the study of bioenergetics the study of energy and its chemical changes in the biological world.

Alberts, B. In Molecular Biology of the Cell, 3rd ed. Becker, W. In The World of the Cell, 2nd ed. Fasman, G. In Handbook of Biochemistry and Molecular Biology, 3rd ed. I Physical and Chemical Data , pp. Guex, N. Electrophoresis, , 18, Moa, C. Biochemical and Biophysical Reaearch Communications. Stryer, L. In Biochemistry, 4th. Freeman and Co. Louis for many helpful suggestions in the writing of this tutorial.

Louis, MO Figure 3 This flowchart shows that the energy used by the body for its many activities ultimately comes from the chemical energy in our food. Pyruvate Note: Carbon atoms from glucose are shown in red.

Acetyl CoA Note: Carbon atoms from glucose are shown in red. NADH Note: The part of the molecule that participates in oxidation-reduction reactions is shown in blue. FADH 2 Note: The part of the molecule that participates in oxidation-reduction reactions is shown in blue. To start, two electrons are carried to the first complex aboard NADH. FMN, which is derived from vitamin B 2 also called riboflavin , is one of several prosthetic groups or co-factors in the electron transport chain.

A prosthetic group is a non-protein molecule required for the activity of a protein. Prosthetic groups can be organic or inorganic and are non-peptide molecules bound to a protein that facilitate its function.

Prosthetic groups include co-enzymes, which are the prosthetic groups of enzymes. Complex I can pump four hydrogen ions across the membrane from the matrix into the intermembrane space; it is in this way that the hydrogen ion gradient is established and maintained between the two compartments separated by the inner mitochondrial membrane. The compound connecting the first and second complexes to the third is ubiquinone Q. The Q molecule is lipid soluble and freely moves through the hydrophobic core of the membrane.

Once it is reduced to QH 2 , ubiquinone delivers its electrons to the next complex in the electron transport chain. This enzyme and FADH 2 form a small complex that delivers electrons directly to the electron transport chain, bypassing the first complex. Since these electrons bypass, and thus do not energize, the proton pump in the first complex, fewer ATP molecules are made from the FADH 2 electrons.

The number of ATP molecules ultimately obtained is directly proportional to the number of protons pumped across the inner mitochondrial membrane.

The third complex is composed of cytochrome b, another Fe-S protein, Rieske center 2Fe-2S center , and cytochrome c proteins; this complex is also called cytochrome oxidoreductase. Cytochrome proteins have a prosthetic heme group.

The heme molecule is similar to the heme in hemoglobin, but it carries electrons, not oxygen. The heme molecules in the cytochromes have slightly different characteristics due to the effects of the different proteins binding them, which makes each complex. Complex III pumps protons through the membrane and passes its electrons to cytochrome c for transport to the fourth complex of proteins and enzymes.

Cytochrome c is the acceptor of electrons from Q; however, whereas Q carries pairs of electrons, cytochrome c can accept only one at a time.

The fourth complex is composed of cytochrome proteins c, a, and a 3. This complex contains two heme groups one in each of the cytochromes a and a 3 and three copper ions a pair of Cu A and one Cu B in cytochrome a 3.

The cytochromes hold an oxygen molecule very tightly between the iron and copper ions until the oxygen is completely reduced. The reduced oxygen then picks up two hydrogen ions from the surrounding medium to produce water H 2 O. The removal of the hydrogen ions from the system also contributes to the ion gradient used in the process of chemiosmosis.

Trends in molecular medicine. Biophysical chemistry. Journal of the history of biology. Physical chemistry chemical physics : PCCP. International journal of molecular sciences. Comprehensive Physiology. Frontiers in physiology. Canadian journal of physiology and pharmacology.

Letts JA,Sazanov LA, Clarifying the supercomplex: the higher-order organization of the mitochondrial electron transport chain. Biomolecular concepts. Scientific reports. BMC neurology. Biochemistry, Oxidative Phosphorylation. Free Review Questions. Introduction Oxidative phosphorylation is a cellular process that harnesses the reduction of oxygen to generate high-energy phosphate bonds in the form of adenosine triphosphate ATP.

Electronegativity is the ability of an elemental atom to attract a bonding pair of electrons. Elements with a high electronegativity can attract electrons to their atomic nuclei more easily. Fluorine is considered the most electronegative element; however, oxygen is also highly electronegative and has a low molecular mass.

Given its greater availability in the atmosphere, elemental oxygen is used as the final electron acceptor in oxidative phosphorylation.

They get reduced in several reactions, principally including glycolysis, the tricarboxylic acid citric acid cycle, and beta-oxidation of fatty acids. The mitochondrion consists of inner and outer membranes, both of which are composed of phospholipid bilayers and integral membrane proteins involved in enzymatic action and molecular transport. The inner membrane has inward-facing fold-like projections known as cristae that vastly increase the surface area of the membrane to maximize the amount of energy production.

The requirement for CoQ increases with increasing energy needs of cells, so the highest concentrations of CoQ in the body are found in tissues that are the most metabolically active - heart, liver, and kidney. CoQ is useful because of its ability to carry and donate electrons and particularly because it can exist in forms with two extra electrons fully reduced - ubiquinol , one extra electron semi-reduced - ubisemiquinone , or no extra electrons fully oxidized - ubiquinone.

This ability allows CoQ to provide transition between the first part of the electron transport system that moves electrons in pairs and the last part of the system that moves electrons one at a time.

Complex III also known as coenzyme Q : cytochrome c — oxidoreductase or the cytochrome bc1 complex - Figure 5. It is a transmembrane protein with multiple subunits present in the mitochondria of all aerobic eukaryotic organisms and and the cell membrane of almost all bacteria.

The complex contains 11 subunits, a 2-iron ferredoxin, cytochromes b and c1 and belongs to the family of oxidoreductase enzymes. It accepts electrons from coenzyme Q in electron transport and passes them off to cytochrome c. In this cycle, known as the Q cycle, electrons arrive from CoQ in pairs, but get passed to cytochrome c individually.

In the overall process, two protons are consumed from the matrix and four protons are pumped into the intermembrane space.

Movement of electrons through the complex can be inhibited by antimycin A, myxothiazol, and stigmatellin. Complex III is also implicated in creation of superoxide a reactive oxygen species when electrons from it leak out of the chain of transfer.

The phenomenon is more pronounced when antimycin A is present. In the Q-cycle, electrons are passed from ubiquinol QH2 to cytochrome c using Complex III as an intermediary docking station for the transfer.

The other pair is donated singly to two different cytochrome c molecules. The Q-cycle happens in a two step process. Ubiquinol transfers two electrons to Complex III. One electron goes to a docked cytochrome c, reducing it and it exits replaced by an oxidized cytochrome c. The other goes to the docked uniquinone to create the semi-reduced semiubiquinone CoQ. This is the end of step 1. The gap left behind by the ubiquinone Q that departed is replaced by another ubiquinol QH2.

It too donates two electrons to Complex III, which splits them. One goes to the newly docked oxidized cytochrome c, which is reduced and exits. The other goes to the ubisemiquinone. Two protons from the matrix combine with it to make another ubiquinol.

It and the ubiquinone created by the electron donation exit Complex III and the process starts again. Cytochrome c Figure 5. Cytochrome c also plays an important role in apoptosis in higher organisms. Damage to the mitochondrion that results in release of cytochrome c can stimulate assembly of the apoptosome and activation of the caspase cascade that leads to programmed cell death.

Complex IV, also known as cytochrome c oxidase is a 14 subunit integral membrane protein at the end of the electron transport chain Figure 5. It is responsible for accepting one electron each from four cytochrome c proteins and adding them to molecular oxygen O2 along with four protons from the mitochondrial matrix to make two molecules of water.

Four protons from the matrix are also pumped into the intermembrane space in the process. The complex has two molecules of heme, two cytochromes a and a3 , and two copper centers called CuA ad CuB. Cytochrome c docks near the CuA and donates an electron to it.

The reduced CuA passes the electron to cytochrome a, which turns it over to the a3-CuB center where the oxygen is reduced. The four electrons are thought to pass through the complex rapidly resulting in complete reduction of the oxygen-oxygen molecule without formation of a peroxide intermediate or superoxide, in contrast to previous predictions.

There has been speculation for many years that a supercomplex of electron carriers in the inner membrane of the mitochondrion may exist in cells with individual carriers making physical contact with each other. This would make for more efficient transfer reactions, minimize the production of reactive oxygen species and be similar to metabolons of metabolic pathway enzymes, for which there is some evidence. Now, evidence appears to be accumulating that complexes I, III, and IV form a supercomplex, which has been dubbed the respirasome1.

The process of oxidative phosphorylation uses the energy of the proton gradient established by the electron transport system as a means of phosphorylating ADP to make ATP.

The establishment of the proton gradient is dependent upon electron transport. Central to its function is the movement of protons through it from the intermembrane space back into the matrix. Protons will only provide energy to make ATP if their concentration is greater in the intermembrane space than in the matrix and if ADP is available.

It is possible, in some cases, for the concentration of protons to be greater inside the matrix than outside of it. This is usually not a desirable circumstance and there are some controls to reduce its occurrence. Normally, ATP concentration will be higher inside of the mitochondrion and ADP concentration be higher outside the mitochondrion.

This may happen, for example, during periods of rest. It has the overall effect of reducing transport and thus lowering the concentration of ADP inside the matrix. Another important consideration is that when ATP is made in oxidative phosphorylation, it is released into the mitochondrial matrix, but must be transported into the cytosol to meet the energy needs of the rest of the cell. This is accomplished by action of the adenine nucleotide translocase, an antiport that moves ATP out of the matrix in exchange for ADP moving into the matrix.

This is accomplished by action of the phosphate translocase, which is a symport that moves phosphate into the mitochondrial matrix along with a proton. There is evidence that the two translocases and ATP synthase may exist in a complex, which has been dubbed the ATP synthasome. In summary, the electron transport system charges the battery for oxidative phosphorylation by pumping protons out of the mitochondrion.

The intact inner membrane of the mitochondrion keeps the protons out, except for those that re-enter through ATP Synthase. The F1 head contains the catalytic ability to make ATP. Each of these forms has a function. The Open form releases the ATP into the mitochondrial matrix.

When a mitochondrion has an intact inner membrane and protons can only return to the matrix by passing through the ATP synthase, the processes of electron transport and oxidative phosphorylation are said to be tightly coupled. In simple terms, tight coupling means that the processes of electron transport and oxidative phosphorylation are interdependent. Without electron transport going on in the cell, oxidative phosphorylation will soon stop. The reverse is also true, because if oxidative phosphorylation stops, the proton gradient will not be dissipated as it is being built by the electron transport system and will grow larger and larger.

The greater the gradient, the greater the energy needed to pump protons out of the mitochondrion. Eventually, if nothing relieves the gradient, it becomes too large and the energy of electron transport is insufficient to perform the pumping. When pumping stops, so too does electron transport. In the absence of ADP, the ATP synthase stops functioning and when it stops, so too does movement of protons back into the mitochondrion.

With this information, it is possible to understand the link between energy usage and metabolism. The root of this, as noted, is respiratory control. To illustrate these links, let us first consider a person, initially at rest, who then suddenly jumps up and runs away. ATP synthase begins working and protons begin to come back into the mitochondrial matrix.

The proton gradient decreases, so electron transport re-starts. Electron transport needs an electron acceptor, so oxygen use increases and when oxygen use increases, the person starts breathing more heavily to supply it. With little or no proton movement, electron transport stops because the proton gradient is too large. When electron transport stops, oxygen use decreases and the rate of breathing slows down.

The really interesting links to metabolism occur relative to whether or not electron transport is occurring. From the examples, we can see that electron transport will be relatively slowed when not exercising and more rapid when exercise or other ATP usage is occurring.

If one does not have the proper amount of exercise, reduced carriers remain high in concentration for long periods of time. This means we have an excess of energy and then anabolic pathways, particularly fatty acid synthesis, are favored, so we get fatter. One might suspect that altering respiratory control could have some very dire consequences and that would be correct.

These alterations can be achieved using compounds with specific effects on particular components of the system. All of the chemicals described here are laboratory tools and should never be used by people. The first group for discussion are the inhibitors. In tightly coupled mitochondria, inhibiting either electron transport or oxidative phosphorylation has the effect of inhibiting the other one as well. Common inhibitors of electron transport include rotenone and amytal, which stop movement of electrons past Complex I, malonate, malate, and oxaloacetate, which inhibit movement of electrons through Complex II, antimycin A which stops movement of electrons past Complex III, and cyanide, carbon monoxide, azide, and hydrogen sulfide, which inhibit electron movement through Complex IV Figure 5.

All of these compounds can stop electron transport directly no movement of electrons and oxidative phosphorylation indirectly proton gradient will dissipate.

While some of these compounds are not commonly known, almost everyone is aware of the hazards of carbon monoxide and cyanide, both of which can be lethal. It is also possible to use an inhibitor of ATP synthase to stop oxidative phosphorylation directly no ATP production and electron transport indirectly proton gradient not relieved so it becomes increasingly difficult to pump protons out of matrix.

Oligomycin A Figure 5. Rotenone, which is a plant product, is used as a natural insecticide that is permitted for organic farming. When mitochondria are treated with this, electron transport will stop at Complex I and so, too, will the pumping of protons out of the matrix.

When this occurs, the proton gradient rapidly dissipates, stopping oxidative phosphorylation as a consequence. There are other entry points for electrons than Complex I, so this type of inhibition is not as serious as using inhibitors of Complex IV, since no alternative route for electrons is available.

It is for this reason that cyanide, for example, is so poisonous. Imagine a dam holding back water with a turbine generating electricity through which water must flow. When all water flows through the turbine, the maximum amount of electricity can be generated. If one pokes a hole in the dam, though, water will flow through the hole and less electricity will be created. The generation of electricity will thus be uncoupled from the flow of water. If the hole is big enough, the water will all drain out through the hole and no electricity will be made.

Imagine, now, that the proton gradient is the equivalent of the water, the inner membrane is the equivalent of the dam and the ATP synthase is the turbine. It is important to recognize, though, that uncoupling by 2,4 DNP works differently from the electron transport inhibitors or the ATP synthase inhibitor.

In those situations, stopping oxidative phosphorylation resulted in indirectly stopping electron transport, since the two processes were coupled and the inhibitors did not uncouple them.

Similarly, stopping electron transport indirectly stopped oxidative phosphorylation for the same reason.



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