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Alberts B, Johnson A, Lewis J, et al. Molecular Biology of the Cell. 4th edition. New York: Garland Science; 2002.

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Molecular Biology of the Cell. 4th edition.

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The Mitochondrion

Mitochondria occupy a substantial portion of the cytoplasmic volume of eucaryotic cells, and they have been essential for the evolution of complex animals. Without mitochondria, present-day animal cells would be dependent on anaerobic glycolysis for all of their ATP. When glucose is converted to pyruvate by glycolysis, only a very small fraction of the total free energy potentially available from the glucose is released. In mitochondria, the metabolism of sugars is completed: the pyruvate is imported into the mitochondrion and oxidized by O2 to CO2 and H2O. This allows 15 times more ATP to be made than that produced by glycolysis alone.

Mitochondria are usually depicted as stiff, elongated cylinders with a diameter of 0.5–1 μm, resembling bacteria. Time-lapse microcinematography of living cells, however, shows that mitochondria are remarkably mobile and plastic organelles, constantly changing their shape (Figure 14-4) and even fusing with one another and then separating again. As they move about in the cytoplasm, they often seem to be associated with microtubules (Figure 14-5), which can determine the unique orientation and distribution of mitochondria in different types of cells. Thus, the mitochondria in some cells form long moving filaments or chains. In others they remain fixed in one position where they provide ATP directly to a site of unusually high ATP consumption—packed between adjacent myofibrils in a cardiac muscle cell, for example, or wrapped tightly around the flagellum in a sperm (Figure 14-6).

Figure 14-4. Mitochondrial plasticity.

Figure 14-4

Mitochondrial plasticity. Rapid changes of shape are observed when an individual mitochondrion is followed in a living cell.

Figure 14-5. The relationship between mitochondria and microtubules.

Figure 14-5

The relationship between mitochondria and microtubules. (A) A light micrograph of chains of elongated mitochondria in a living mammalian cell in culture. The cell was stained with a fluorescent dye (rhodamine 123) that specifically labels mitochondria (more...)

Figure 14-6. Localization of mitochondria near sites of high ATP utilization in cardiac muscle and a sperm tail.

Figure 14-6

Localization of mitochondria near sites of high ATP utilization in cardiac muscle and a sperm tail. During the development of the flagellum of the sperm tail, microtubules wind helically around the axoneme, where they are thought to help localize the (more...)

Mitochondria are large enough to be seen in the light microscope, and they were first identified during the nineteenth century. Real progress in understanding their function, however, depended on procedures developed in 1948 for isolating intact mitochondria. For technical reasons, many of these biochemical studies have been performed with mitochondria purified from liver; each liver cell contains 1000–2000 mitochondria, which in total occupy about one-fifth of the cell volume.

The Mitochondrion Contains an Outer Membrane, an Inner Membrane, and Two Internal Compartments

Each mitochondrion is bounded by two highly specialized membranes, which have very different functions. Together they create two separate mitochondrial compartments: the internal matrix and a much narrower intermembrane space. If purified mitochondria are gently disrupted and then fractionated into separate components (Figure 14-7), the biochemical composition of each of the two membranes and of the spaces enclosed by them can be determined. As described in Figure 14-8, each contains a unique collection of proteins.

Figure 14-7. Biochemical fractionation of purified mitochondria into separate components.

Figure 14-7

Biochemical fractionation of purified mitochondria into separate components. These techniques have made it possible to study the different proteins in each mitochondrial compartment. The method shown allows the processing of large numbers of mitochondria (more...)

Figure 14-8. The general organization of a mitochondrion.

Figure 14-8

The general organization of a mitochondrion. In the liver, an estimated 67% of the total mitochondrial protein is located in the matrix, 21% is located in the inner membrane, 6% in the outer membrane, and 6% in the intermembrane space. As indicated below, (more...)

The outer membrane contains many copies of a transport protein called porin (discussed in Chapter 11), which forms large aqueous channels through the lipid bilayer. This membrane thus resembles a sieve that is permeable to all molecules of 5000 daltons or less, including small proteins. Such molecules can enter the intermembrane space, but most of them cannot pass the impermeable inner membrane. Thus, whereas the intermembrane space is chemically equivalent to the cytosol with respect to the small molecules it contains, the matrix contains a highly selected set of these molecules.

As we explain in detail later, the major working part of the mitochondrion is the matrix and the inner membrane that surrounds it. The inner membrane is highly specialized. Its lipid bilayer contains a high proportion of the “double” phospholipid cardiolipin, which has four fatty acids rather than two and may help to make the membrane especially impermeable to ions. This membrane also contains a variety of transport proteins that make it selectively permeable to those small molecules that are metabolized or required by the many mitochondrial enzymes concentrated in the matrix. The matrix enzymes include those that metabolize pyruvate and fatty acids to produce acetyl CoA and those that oxidize acetyl CoA in the citric acid cycle. The principal end-products of this oxidation are CO2, which is released from the cell as waste, and NADH, which is the main source of electrons for transport along the respiratory chain—the name given to the electron-transport chain in mitochondria. The enzymes of the respiratory chain are embedded in the inner mitochondrial membrane, and they are essential to the process of oxidative phosphorylation, which generates most of the animal cell's ATP.

The inner membrane is usually highly convoluted, forming a series of infoldings, known as cristae, that project into the matrix. These convolutions greatly increase the area of the inner membrane, so that in a liver cell, for example, it constitutes about one-third of the total cell membrane. The number of cristae is three times greater in the mitochondrion of a cardiac muscle cell than in the mitochondrion of a liver cell, presumably because of the greater demand for ATP in heart cells. There are also substantial differences in the mitochondrial enzymes of different cell types. In this chapter, we largely ignore these differences and focus instead on the enzymes and properties that are common to all mitochondria.

High-Energy Electrons Are Generated via the Citric Acid Cycle

As previously mentioned, without mitochondria present-day eucaryotes would be dependent on the relatively inefficient process of glycolysis (described in Chapter 2) for all of their ATP production, and it seems unlikely that complex multicellular organisms could have been supported in this way. When glucose is converted to pyruvate by glycolysis, less than 10% of the total free energy potentially available from the glucose is released. In the mitochondria, the metabolism of sugars is completed, and the energy released is harnessed so efficiently that about 30 molecules of ATP are produced for each molecule of glucose oxidized. By contrast, only 2 molecules of ATP are produced per glucose molecule by glycolysis alone.

Mitochondria can use both pyruvate and fatty acids as fuel. Pyruvate comes from glucose and other sugars, whereas fatty acids come from fats. Both of these fuel molecules are transported across the inner mitochondrial membrane and then converted to the crucial metabolic intermediate acetyl CoA by enzymes located in the mitochondrial matrix. The acetyl groups in acetyl CoA are then oxidized in the matrix via the citric acid cycle, described in Chapter 2. The cycle converts the carbon atoms in acetyl CoA to CO2, which is released from the cell as a waste product. Most importantly, the cycle generates high-energy electrons, carried by the activated carrier molecules NADH and FADH2 (Figure 14-9). These high-energy electrons are then transferred to the inner mitochondrial membrane, where they enter the electron-transport chain; the loss of electrons from NADH and FADH2 also regenerates the NAD+ and FAD that is needed for continued oxidative metabolism. The entire sequence of reactions is outlined in Figure 14-10.

Figure 14-9. How electrons are donated by NADH.

Figure 14-9

How electrons are donated by NADH. In this diagram, the high-energy electrons are shown as two red dots on a yellow hydrogen atom. A hydride ion (H- a hydrogen atom and an extra electron) is removed from NADH and is converted into a proton and two high-energy (more...)

Figure 14-10. A summary of energy-generating metabolism in mitochondria.

Figure 14-10

A summary of energy-generating metabolism in mitochondria. Pyruvate and fatty acids enter the mitochondrion (bottom) and are broken down to acetyl CoA. The acetyl CoA is then metabolized by the citric acid cycle, which reduces NAD+ to NADH (and FAD to (more...)

A Chemiosmotic Process Converts Oxidation Energy into ATP

Although the citric acid cycle is considered to be part of aerobic metabolism, it does not itself use the oxygen. Only in the final catabolic reactions that take place on the inner mitochondrial membrane is molecular oxygen (O2) directly consumed. Nearly all the energy available from burning carbohydrates, fats, and other foodstuffs in the earlier stages of their oxidation is initially saved in the form of high-energy electrons removed from substrates by NAD+ and FAD. These electrons, carried by NADH and FADH2, are then combined with O2 by means of the respiratory chain embedded in the inner mitochondrial membrane. The large amount of energy released is harnessed by the inner membrane to drive the conversion of ADP + Pi to ATP. For this reason, the term oxidative phosphorylation is used to describe this last series of reactions (Figure 14-11).

Figure 14-11. The major net energy conversion catalyzed by the mitochondrion.

Figure 14-11

The major net energy conversion catalyzed by the mitochondrion. In this process of oxidative phosphorylation, the inner mitochondrial membrane serves as a device that changes one form of chemical bond energy to another, converting a major part of the (more...)

As previously mentioned, the generation of ATP by oxidative phosphorylation via the respiratory chain depends on a chemiosmotic process. When it was first proposed in 1961, this mechanism explained a long-standing puzzle in cell biology. Nonetheless, the idea was so novel that it was some years before enough supporting evidence accumulated to make it generally accepted.

In the remainder of this section we shall briefly outline the type of reactions that make oxidative phosphorylation possible, saving the details of the respiratory chain for later.

Electrons Are Transferred from NADH to Oxygen Through Three Large Respiratory Enzyme Complexes

Although the mechanism by which energy is harvested by the respiratory chain differs from that in other catabolic reactions, the principle is the same. The energetically favorable reaction H2 + ½O2 → H2O is made to occur in many small steps, so that most of the energy released can be stored instead of being lost to the environment as heat. The hydrogen atoms are first separated into protons and electrons. The electrons pass through a series of electron carriers in the inner mitochondrial membrane. At several steps along the way, protons and electrons are transiently recombined. But only when the electrons reach the end of the electron-transport chain are the protons returned permanently, when they are used to neutralize the negative charges created by the final addition of the electrons to the oxygen molecule (Figure 14-12).

Figure 14-12. A comparison of biological oxidations with combustion.

Figure 14-12

A comparison of biological oxidations with combustion. (A) Most of the energy would be released as heat if hydrogen were simply burned. (B) In biological oxidation by contrast, most of the released energy is stored in a form useful to the cell by means (more...)

The process of electron transport begins when the hydride ion is removed from NADH (to regenerate NAD+) and is converted into a proton and two electrons (H-H+ + 2e -). The two electrons are passed to the first of the more than 15 different electron carriers in the respiratory chain. The electrons start with very high energy and gradually lose it as they pass along the chain. For the most part, the electrons pass from one metal ion to another, each of these ions being tightly bound to a protein molecule that alters the electron affinity of the metal ion (discussed in detail later). Most of the proteins involved are grouped into three large respiratory enzyme complexes, each containing transmembrane proteins that hold the complex firmly in the inner mitochondrial membrane. Each complex in the chain has a greater affinity for electrons than its predecessor, and electrons pass sequentially from one complex to another until they are finally transferred to oxygen, which has the greatest affinity of all for electrons.

As Electrons Move Along the Respiratory Chain, Energy Is Stored as an Electrochemical Proton Gradient Across the Inner Membrane

Oxidative phosphorylation is made possible by the close association of the electron carriers with protein molecules. The proteins guide the electrons along the respiratory chain so that the electrons move sequentially from one enzyme complex to another—with no short circuits. Most importantly, the transfer of electrons is coupled to oriented H+ uptake and release, as well as to allosteric changes in energy-converting protein pumps. The net result is the pumping of H+ across the inner membrane—from the matrix to the intermembrane space, driven by the energetically favorable flow of electrons. This movement of H+ has two major consequences:

1.

It generates a pH gradient across the inner mitochondrial membrane, with the pH higher in the matrix than in the cytosol, where the pH is generally close to 7. (Since small molecules equilibrate freely across the outer membrane of the mitochondrion, the pH in the intermembrane space is the same as in the cytosol.)

2.

It generates a voltage gradient (membrane potential) across the inner mitochondrial membrane, with the inside negative and the outside positive (as a result of the net outflow of positive ions).

The pH gradient (ΔpH) drives H+ back into the matrix and OH- out of the matrix, thereby reinforcing the effect of the membrane potentialV), which acts to attract any positive ion into the matrix and to push any negative ion out. Together, the ΔpH and the ΔV are said to constitute an electrochemical proton gradient (Figure 14-13).

Figure 14-13. The two components of the electrochemical proton gradient.

Figure 14-13

The two components of the electrochemical proton gradient. The total proton-motive force across the inner mitochondrial membrane consists of a large force due to the membrane potential (traditionally designated ΔΨ by experts, but designated (more...)

The electrochemical proton gradient exerts a proton-motive force, which can be measured in units of millivolts (mV). Since each ΔpH of 1 pH unit has an effect equivalent to a membrane potential of about 60 mV, the total proton-motive force equals ΔV - 60(ΔpH). In a typical cell, the proton-motive force across the inner membrane of a respiring mitochondrion is about 200 mV and is made up of a membrane potential of about 140 mV and a pH gradient of about -1 pH unit.

How the Proton Gradient Drives ATP Synthesis

The electrochemical proton gradient across the inner mitochondrial membrane is used to drive ATP synthesis in the critical process of oxidative phosphorylation (Figure 14-14). This is made possible by the membrane-bound enzyme ATP synthase, mentioned previously. This enzyme creates a hydrophilic pathway across the inner mitochondrial membrane that allows protons to flow down their electrochemical gradient. As these ions thread their way through the ATP synthase, they are used to drive the energetically unfavorable reaction between ADP and Pi that makes ATP (see Figure 2-27). The ATP synthase is of ancient origin; the same enzyme occurs in the mitochondria of animal cells, the chloroplasts of plants and algae, and in the plasma membrane of bacteria and archea.

Figure 14-14. The general mechanism of oxidative phosphorylation.

Figure 14-14

The general mechanism of oxidative phosphorylation. (A) As a high-energy electron is passed along the electron-transport chain, some of the energy released is used to drive the three respiratory enzyme complexes that pump H+ out of the matrix. The resulting (more...)

The structure of ATP synthase is shown in Figure 14-15. Also called the F0F1 ATPase, it is a multisubunit protein with a mass of more than 500,000 daltons. A large enzymatic portion, shaped like a lollipop head and composed of a ring of 6 subunits, projects on the matrix side of the inner mitochondrial membrane. This head is held in place by an elongated arm that binds to the head, tying it to a group of transmembrane proteins that produce a “stator” in the membrane. This stator is in contact with a “rotor” that is formed by a ring of 10 to 14 identical transmembrane protein subunits. As protons pass through a narrow channel formed at the stator-rotor contact, their movement causes the rotor ring to spin. This spinning also turns a stalk attached to the rotor (blue in Figure 14-15B), which is thereby made to turn rapidly inside the lollipop head. As a result, the energy of proton flow down a gradient has been converted into the mechanical energy of two sets of proteins rubbing against each other: rotating stalk proteins pushing against a stationary ring of head proteins.

Figure 14-15. ATP synthase.

Figure 14-15

ATP synthase. (A) The enzyme is composed of a head portion, called the F1 ATPase, and a transmembrane H+ carrier, called F0. Both F1 and F0 are formed from multiple subunits, as indicated. A rotating stalk turns with a rotor formed by a ring of 10 to (more...)

Three of the six subunits in the head contain binding sites for ADP and inorganic phosphate. These are driven to form ATP as mechanical energy is converted into chemical bond energy through the repeated changes in protein conformation that the rotating stalk creates. In this way, the ATP synthase is able to produce more than 100 molecules of ATP per second. Three or four protons need to pass through this marvelous device to make each molecule of ATP.

How the Proton Gradient Drives Coupled Transport Across the Inner Membrane

The synthesis of ATP is not the only process driven by the electrochemical proton gradient. In mitochondria, many charged small molecules, such as pyruvate, ADP, and Pi, are pumped into the matrix from the cytosol, while others, such as ATP, must be moved in the opposite direction. Carrier proteins that bind these molecules can couple their transport to the energetically favorable flow of H+ into the mitochondrial matrix. Thus, for example, pyruvate and inorganic phosphate (Pi) are co-transported inward with H+ as the H+ moves into the matrix.

In contrast, ADP is co-transported with ATP in opposite directions by a single carrier protein. Since an ATP molecule has one more negative charge than ADP, each nucleotide exchange results in a total of one negative charge being moved out of the mitochondrion. This ADP-ATP co-transport is thereby driven by the voltage difference across the membrane (Figure 14-16).

Figure 14-16. Some of the active transport processes driven by the electrochemical proton gradient across the inner mitochondrial membrane.

Figure 14-16

Some of the active transport processes driven by the electrochemical proton gradient across the inner mitochondrial membrane. Pyruvate, inorganic phosphate (Pi), and ADP are moved into the matrix, while ATP is pumped out. The charge on each of the transported (more...)

In eucaryotic cells, the proton gradient is thus used to drive both the formation of ATP and the transport of certain metabolites across the inner mitochondrial membrane. In bacteria, the proton gradient across the bacterial plasma membrane is harnessed for both types of functions. And in the plasma membrane of motile bacteria, the gradient also drives the rapid rotation of the bacterial flagellum, which propels the bacterium along (Figure 14-17).

Figure 14-17. The rotation of the bacterial flagellum driven by H+ flow.

Figure 14-17

The rotation of the bacterial flagellum driven by H+ flow. The flagellum is attached to a series of protein rings (orange), which are embedded in the outer and inner membranes and rotate with the flagellum. The rotation is driven by a flow of protons (more...)

Proton Gradients Produce Most of the Cell's ATP

As stated previously, glycolysis alone produces a net yield of 2 molecules of ATP for every molecule of glucose that is metabolized, and this is the total energy yield for the fermentation processes that occur in the absence of O2 (discussed in Chapter 2). During oxidative phosphorylation, each pair of electrons donated by the NADH produced in mitochondria is thought to provide energy for the formation of about 2.5 molecules of ATP, after subtracting the energy needed for transporting this ATP to the cytosol. Oxidative phosphorylation also produces 1.5 ATP molecules per electron pair from FADH2, or from the NADH molecules produced by glycolysis in the cytosol. From the product yields of glycolysis and the citric acid cycle summarized in Table 14-1A, one can calculate that the complete oxidation of one molecule of glucose—starting with glycolysis and ending with oxidative phosphorylation—gives a net yield of about 30 ATPs.

Table 14-1. Product Yields from the Oxidation of Sugars and Fats.

Table 14-1

Product Yields from the Oxidation of Sugars and Fats.

In conclusion, the vast majority of the ATP produced from the oxidation of glucose in an animal cell is produced by chemiosmotic mechanisms in the mitochondrial membrane. Oxidative phosphorylation in the mitochondrion also produces a large amount of ATP from the NADH and the FADH2 that is derived from the oxidation of fats (Table 14-1B; see also Figure 2-78).

Mitochondria Maintain a High ATP:ADP Ratio in Cells

Because of the carrier protein in the inner mitochondrial membrane that exchanges ATP for ADP, the ADP molecules produced by ATP hydrolysis in the cytosol rapidly enter mitochondria for recharging, while the ATP molecules formed in the mitochondrial matrix by oxidative phosphorylation are rapidly pumped into the cytosol, where they are needed. A typical ATP molecule in the human body shuttles out of a mitochondrion and back into it (as ADP) for recharging more than once per minute, keeping the concentration of ATP in the cell about 10 times higher than that of ADP.

As discussed in Chapter 2, biosynthetic enzymes often drive energetically unfavorable reactions by coupling them to the energetically favorable hydrolysis of ATP (see Figure 2-56). The ATP pool is therefore used to drive cellular processes in much the same way that a battery can be used to drive electric engines. If the activity of the mitochondria is blocked, ATP levels fall and the cell's battery runs down; eventually, energetically unfavorable reactions are no longer driven, and the cell dies. The poison cyanide, which blocks electron transport in the inner mitochondrial membrane, causes death in exactly this way.

It might seem that cellular processes would stop only when the concentration of ATP reaches zero; but, in fact, life is more demanding: it depends on cells maintaining a concentration of ATP that is high compared to the concentrations of ADP and Pi. To explain why, we must consider some elementary principles of thermodynamics.

A Large Negative Value of ΔG for ATP Hydrolysis Makes ATP Useful to the Cell

In Chapter 2, the concept of free energy (G) was discussed. The free energy change for a reaction, ΔG, determines whether this reaction will occur in a cell. We showed on p. 80 that the ΔG for a given reaction can be written as the sum of two parts: the first, called the standard free-energy change, ΔG°, depends on the intrinsic characters of the reacting molecules; the second depends on their concentrations. For the simple reaction A → B,

Image ch14e1.jpg

where [A] and [B] denote the concentrations of A and B, and ln is the natural logarithm. ΔG° is therefore only a reference value that equals the value of ΔG when the molar concentrations of A and B are equal (ln 1 = 0).

In Chapter 2, ATP was described as the major “activated carrier molecule” in cells. The large, favorable free-energy change (large negative ΔG) for its hydrolysis is used, through coupled reactions, to drive other chemical reactions that would otherwise not occur (see pp. 82–85). The ATP hydrolysis reaction produces two products, ADP and inorganic phosphate (Pi); it is therefore of the type A → B + C, where, as described in Figure 14-18,

Image ch14e2.jpg

Figure 14-18. The basic relationship between free-energy changes and equilibrium in the ATP hydrolysis reaction.

Figure 14-18

The basic relationship between free-energy changes and equilibrium in the ATP hydrolysis reaction. The rate constants in boxes 1 and 2 are determined from experiments in which product accumulation is measured as a function of time. The equilibrium constant (more...)

When ATP is hydrolyzed to ADP and Pi under the conditions that normally exist in a cell, the free-energy change is roughly -11 to -13 kcal/mole. This extremely favorable ΔG depends on having a high concentration of ATP in the cell compared with the concentration of ADP and Pi. When ATP, ADP, and Pi are all present at the same concentration of 1 mole/liter (so-called standard conditions), the ΔG for ATP hydrolysis is the standard free-energy change (ΔG°), which is only -7.3 kcal/mole. At much lower concentrations of ATP relative to ADP and Pi, ΔG becomes zero. At this point, the rate at which ADP and Pi will join to form ATP will be equal to the rate at which ATP hydrolyzes to form ADP and Pi. In other words, when ΔG = 0, the reaction is at equilibrium (see Figure 14-18).

It is ΔG, not ΔG°, that indicates how far a reaction is from equilibrium and determines whether it can be used to drive other reactions. Because the efficient conversion of ADP to ATP in mitochondria maintains such a high concentration of ATP relative to ADP and Pi, the ATP-hydrolysis reaction in cells is kept very far from equilibrium and ΔG is correspondingly very negative. Without this large disequilibrium, ATP hydrolysis could not be used to direct the reactions of the cell; for example, many biosynthetic reactions would run backward rather than forward at low ATP concentrations.

ATP Synthase Can Also Function in Reverse to Hydrolyze ATP and Pump H+

In addition to harnessing the flow of H+ down an electrochemical proton gradient to make ATP, the ATP synthase can work in reverse: it can use the energy of ATP hydrolysis to pump H+ across the inner mitochondrial membrane (Figure 14-19). It thus acts as a reversible coupling device, interconverting electrochemical proton gradient and chemical bond energies. The direction of action at any instant depends on the balance between the steepness of the electrochemical proton gradient and the local ΔG for ATP hydrolysis, as we now explain.

Figure 14-19. The ATP synthase is a reversible coupling device that can convert the energy of the electrochemical proton gradient into chemical-bond energy, or vice versa.

Figure 14-19

The ATP synthase is a reversible coupling device that can convert the energy of the electrochemical proton gradient into chemical-bond energy, or vice versa. The ATP synthase can either (A) synthesize ATP by harnessing the proton-motive force or (B) pump (more...)

Although the exact number of protons needed to make each ATP molecule is not known with certainty, we shall assume that one molecule of ATP is made by the ATP synthase for every 3 protons driven through it. Whether the ATP synthase works in its ATP-synthesizing or its ATP-hydrolyzing direction at any instant depends, in this case, on the exact balance between the favorable free-energy change for moving the three protons across the membrane into the matrix ΔG3H+ (which is less than zero) and the unfavorable free-energy change for ATP synthesis in the matrix ΔGATP synthesis (which is greater than zero). As just discussed, the value of ΔGATP synthesis depends on the exact concentrations of the three reactants ATP, ADP, and Pi in the mitochondrial matrix (see Figure 14-18). The value of ΔG3H+ in contrast, is directly proportional to the value of the proton-motive force across the inner mitochondrial membrane. The following example will help explain how the balance between these two free-energy changes affects the ATP synthase.

As explained in the legend to Figure 14-19, a single H+ moving into the matrix down an electrochemical gradient of 200 mV liberates 4.6 kcal/mole of free energy, while the movement of three protons liberates three times this much free energy (ΔG3H+ = -13.8 kcal/mole). Thus, if the proton-motive force remains constant at 200 mV, the ATP synthase synthesizes ATP until a ratio of ATP to ADP and Pi is reached where ΔGATP synthesis is just equal to +13.8 kcal/mole (here ΔGATP synthesis + ΔG3H+ = 0). At this point there is no further net ATP synthesis or hydrolysis by the ATP synthase.

Suppose a large amount of ATP is suddenly hydrolyzed by energy-requiring reactions in the cytosol, causing the ATP:ADP ratio in the matrix to fall. Now the value of ΔGATP synthesis will decrease (see Figure 14-18), and ATP synthase will begin to synthesize ATP again to restore the original ATP:ADP ratio. Alternatively, if the proton-motive force drops suddenly and is then maintained at a constant 160 mV, ΔG3H+ will change to -11.0 kcal/mole. As a result, ATP synthase will start hydrolyzing some of the ATP in the matrix until a new balance of ATP to ADP and Pi is reached (where ΔGATP synthesis = +11.0 kcal/mole), and so on.

In many bacteria, ATP synthase is routinely reversed in a transition between aerobic and anaerobic metabolism, as we shall see later. The same type of reversibility is shared by other membrane transport proteins that couple the transmembrane movement of an ion to ATP synthesis or hydrolysis. Both the Na+-K+ pump and the Ca2+ pump described in Chapter 11, for example, normally hydrolyze ATP and use the energy released to move their specific ions across a membrane. If either of these pumps is exposed to an abnormally steep gradient of the ions it transports, however, it will act in reverse—synthesizing ATP from ADP and Pi instead of hydrolyzing it. Thus, the ATP synthase is by no means unique in its ability to convert the electrochemical energy stored in a transmembrane ion gradient directly into phosphate-bond energy in ATP.

Summary

The mitochondrion performs most cellular oxidations and produces the bulk of the animal cell's ATP. The mitochondrial matrix contains a large variety of enzymes, including those that convert pyruvate and fatty acids to acetyl CoA and those that oxidize this acetyl CoA to CO2through the citric acid cycle. Large amounts of NADH (and FADH2) are produced by these oxidation reactions.

The energy available from combining molecular oxygen with the reactive electrons carried by NADH and FADH2 is harnessed by an electron-transport chain in the inner mitochondrial membrane called the respiratory chain. The respiratory chain pumps H+ out of the matrix to create a transmembrane electrochemical proton (H+) gradient, which includes contributions from both a membrane potential and a pH difference. The large amount of free energy released when H+ flows back into the matrix (across the inner membrane) provides the basis for ATP production in the matrix by a remarkable protein machine—the ATP synthase. The transmembrane electrochemical gradient is also used to drive the active transport of selected metabolites across the mitochondrial inner membrane, including an efficient ATP-ADP exchange between the mitochondrion and the cytosol that keeps the cell's ATP pool highly charged. The resulting high ratio of ATP to its hydrolysis products makes the free-energy change for ATP hydrolysis extremely favorable, allowing this hydrolysis reaction to drive a large number of the cell's energy-requiring processes.

By agreement with the publisher, this book is accessible by the search feature, but cannot be browsed.

Copyright © 2002, Bruce Alberts, Alexander Johnson, Julian Lewis, Martin Raff, Keith Roberts, and Peter Walter; Copyright © 1983, 1989, 1994, Bruce Alberts, Dennis Bray, Julian Lewis, Martin Raff, Keith Roberts, and James D. Watson .
Bookshelf ID: NBK26894