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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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Electron-Transport Chains and Their Proton Pumps

Having considered in general terms how a mitochondrion uses electron transport to create an electrochemical proton gradient, we need to examine the mechanisms that underlie this membrane-based energy-conversion process. In doing so, we also accomplish a larger purpose. As emphasized at the beginning of this chapter, very similar chemiosmotic mechanisms are used by mitochondria, chloroplasts, archea, and bacteria. In fact, these mechanisms underlie the function of nearly all living organisms—including anaerobes that derive all their energy from electron transfers between two inorganic molecules. It is therefore rather humbling for scientists to remind themselves that the existence of chemiosmosis has been recognized for only about 40 years.

We begin with a look at some of the principles that underlie the electron-transport process, with the aim of explaining how it can pump protons across a membrane.

Protons Are Unusually Easy to Move

Although protons resemble other positive ions such as Na+ and K+ in their movement across membranes, in some respects they are unique. Hydrogen atoms are by far the most abundant type of atom in living organisms; they are plentiful not only in all carbon-containing biological molecules, but also in the water molecules that surround them. The protons in water are highly mobile, flickering through the hydrogen-bonded network of water molecules by rapidly dissociating from one water molecule to associate with its neighbor, as illustrated in Figure 14-20A. Protons are thought to move across a protein pump embedded in a lipid bilayer in a similar way: they transfer from one amino acid side chain to another, following a special channel through the protein.

Figure 14-20. How protons behave in water.

Figure 14-20

How protons behave in water. (A) Protons move very rapidly along a chain of hydrogen-bonded water molecules. In this diagram, proton jumps are indicated by blue arrows, and hydronium ions are indicated by green shading. As discussed in Chapter 2, naked (more...)

Protons are also special with respect to electron transport. Whenever a molecule is reduced by acquiring an electron, the electron (e -) brings with it a negative charge. In many cases, this charge is rapidly neutralized by the addition of a proton (H+) from water, so that the net effect of the reduction is to transfer an entire hydrogen atom, H+ + e - (Figure 14-20B). Similarly, when a molecule is oxidized, a hydrogen atom removed from it can be readily dissociated into its constituent electron and proton—allowing the electron to be transferred separately to a molecule that accepts electrons, while the proton is passed to the water. Therefore, in a membrane in which electrons are being passed along an electron-transport chain, pumping protons from one side of the membrane to another can be relatively simple. The electron carrier merely needs to be arranged in the membrane in a way that causes it to pick up a proton from one side of the membrane when it accepts an electron, and to release the proton on the other side of the membrane as the electron is passed to the next carrier molecule in the chain (Figure 14-21).

Figure 14-21. How protons can be pumped across membranes.

Figure 14-21

How protons can be pumped across membranes. As an electron passes along an electron-transport chain embedded in a lipid-bilayer membrane, it can bind and release a proton at each step. In this diagram, electron carrier B picks up a proton (H+) from one (more...)

The Redox Potential Is a Measure of Electron Affinities

In biochemical reactions, any electrons removed from one molecule are always passed to another, so that whenever one molecule is oxidized, another is reduced. Like any other chemical reaction, the tendency of such oxidation-reduction reactions, or redox reactions, to proceed spontaneously depends on the free-energy change (ΔG) for the electron transfer, which in turn depends on the relative affinities of the two molecules for electrons.

Because electron transfers provide most of the energy for living things, it is worth spending the time to understand them. Many readers are already familiar with acids and bases, which donate and accept protons (see Panel 2-2, pp. 112–113). Acids and bases exist in conjugate acid-base pairs, in which the acid is readily converted into the base by the loss of a proton. For example, acetic acid (CH3COOH) is converted into its conjugate base (CH3COO-) in the reaction:

Image ch14e3.jpg

In exactly the same way, pairs of compounds such as NADH and NAD+ are called redox pairs, since NADH is converted to NAD+ by the loss of electrons in the reaction:

Image ch14e4.jpg

NADH is a strong electron donor: because its electrons are held in a high-energy linkage, the free-energy change for passing its electrons to many other molecules is favorable (see Figure 14-9). It is difficult to form a high-energy linkage. Therefore its redox partner, NAD+, is of necessity a weak electron acceptor.

The tendency to transfer electrons from any redox pair can be measured experimentally. All that is required is the formation of an electrical circuit linking a 1:1 (equimolar) mixture of the redox pair to a second redox pair that has been arbitrarily selected as a reference standard, so the voltage difference can be measured between them (Panel 14-1, p. 784). This voltage difference is defined as the redox potential; as defined, electrons move spontaneously from a redox pair like NADH/NAD+ with a low redox potential (a low affinity for electrons) to a redox pair like O2/H2O with a high redox potential (a high affinity for electrons). Thus, NADH is a good molecule for donating electrons to the respiratory chain, while O2 is well suited to act as the “sink” for electrons at the end of the pathway. As explained in Panel 14-1, the difference in redox potential, ΔE0′, is a direct measure of the standard free-energy change (ΔG°) for the transfer of an electron from one molecule to another.

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Panel 14-1

Redox Potentials.

Electron Transfers Release Large Amounts of Energy

As just discussed, those pairs of compounds that have the most negative redox potentials have the weakest affinity for electrons and therefore contain carriers with the strongest tendency to donate electrons. Conversely, those pairs that have the most positive redox potentials have the strongest affinity for electrons and therefore contain carriers with the strongest tendency to accept electrons. A 1:1 mixture of NADH and NAD+ has a redox potential of -320 mV, indicating that NADH has a strong tendency to donate electrons; a 1:1 mixture of H2O and ½O2 has a redox potential of +820 mV, indicating that O2 has a strong tendency to accept electrons. The difference in redox potential is 1.14 volts (1140 mV), which means that the transfer of each electron from NADH to O2 under these standard conditions is enormously favorable, where ΔG° = -26.2 kcal/mole (-52.4 kcal/mole for the two electrons transferred per NADH molecule; see Panel 14-1). If we compare this free-energy change with that for the formation of the phosphoanhydride bonds in ATP (ΔG° = -7.3 kcal/mole, see Figure 2-75), we see that more than enough energy is released by the oxidization of one NADH molecule to synthesize several molecules of ATP from ADP and Pi.

Living systems could certainly have evolved enzymes that would allow NADH to donate electrons directly to O2 to make water in the reaction:

Image ch14e5.jpg

But because of the huge free-energy drop, this reaction would proceed with almost explosive force and nearly all of the energy would be released as heat. Cells do perform this reaction, but they make it proceed much more gradually by passing the high-energy electrons from NADH to O2 via the many electron carriers in the electron-transport chain. Since each successive carrier in the chain holds its electrons more tightly, the highly energetically favorable reaction 2H+ + 2e - + ½O2 → H2O is made to occur in many small steps. This enables nearly half of the released energy to be stored, instead of being lost to the environment as heat.

Spectroscopic Methods Have Been Used to Identify Many Electron Carriers in the Respiratory Chain

Many of the electron carriers in the respiratory chain absorb visible light and change color when they are oxidized or reduced. In general, each has an absorption spectrum and reactivity that are distinct enough to allow its behavior to be traced spectroscopically, even in crude mixtures. It was therefore possible to purify these components long before their exact functions were known. Thus, the cytochromes were discovered in 1925 as compounds that undergo rapid oxidation and reduction in living organisms as disparate as bacteria, yeasts, and insects. By observing cells and tissues with a spectroscope, three types of cytochromes were identified by their distinctive absorption spectra and designated cytochromes a, b, and c. This nomenclature has survived, even though cells are now known to contain several cytochromes of each type and the classification into types is not functionally important.

The cytochromes constitute a family of colored proteins that are related by the presence of a bound heme group, whose iron atom changes from the ferric oxidation state (Fe3+) to the ferrous oxidation state (Fe2+) whenever it accepts an electron. The heme group consists of a porphyrin ring with a tightly bound iron atom held by four nitrogen atoms at the corners of a square (Figure 14-22). A similar porphyrin ring is responsible for the red color of blood and for the green color of leaves, being bound to iron in hemoglobin and to magnesium in chlorophyll, respectively.

Figure 14-22. The structure of the heme group attached covalently to cytochrome c.

Figure 14-22

The structure of the heme group attached covalently to cytochrome c. The porphyrin ring is shown in blue. There are five different cytochromes in the respiratory chain. Because the hemes in different cytochromes have slightly different structures and (more...)

Iron-sulfur proteins are a second major family of electron carriers. In these proteins, either two or four iron atoms are bound to an equal number of sulfur atoms and to cysteine side chains, forming an iron-sulfur center on the protein (Figure 14-23). There are more iron-sulfur centers than cytochromes in the respiratory chain. But their spectroscopic detection requires electron spin resonance (ESR) spectroscopy, and they are less completely characterized. Like the cytochromes, these centers carry one electron at a time.

Figure 14-23. The structures of two types of iron-sulfur centers.

Figure 14-23

The structures of two types of iron-sulfur centers. (A) A center of the 2Fe2S type. (B) A center of the 4Fe4S type. Although they contain multiple iron atoms, each iron-sulfur center can carry only one electron at a time. There are more than seven different (more...)

The simplest of the electron carriers in the respiratory chain—and the only one that is not part of a protein—is a small hydrophobic molecule that is freely mobile in the lipid bilayer known as ubiquinone, or coenzyme Q. A quinone (Q) can pick up or donate either one or two electrons; upon reduction, it picks up a proton from the medium along with each electron it carries (Figure 14-24).

Figure 14-24. Quinone electron carriers.

Figure 14-24

Quinone electron carriers. Ubiquinone in the respiratory chain picks up one H+ from the aqueous environment for every electron it accepts, and it can carry either one or two electrons as part of a hydrogen atom (yellow). When reduced ubiquinone donates (more...)

In addition to six different hemes linked to cytochromes, more than seven iron-sulfur centers, and ubiquinone, there are also two copper atoms and a flavin serving as electron carriers tightly bound to respiratory-chain proteins in the pathway from NADH to oxygen. This pathway involves more than 60 different proteins in all.

As one would expect, the electron carriers have higher and higher affinities for electrons (greater redox potentials) as one moves along the respiratory chain. The redox potentials have been fine-tuned during evolution by the binding of each electron carrier in a particular protein context, which can alter its normal affinity for electrons. However, because iron-sulfur centers have a relatively low affinity for electrons, they predominate in the early part of the respiratory chain; in contrast, the cytochromes predominate further down the chain, where a higher affinity for electrons is required.

The order of the individual electron carriers in the chain was determined by sophisticated spectroscopic measurements (Figure 14-25), and many of the proteins were initially isolated and characterized as individual polypeptides. A major advance in understanding the respiratory chain, however, was the later realization that most of the proteins are organized into three large enzyme complexes.

Figure 14-25. The general methods used to determine the path of electrons along an electron-transport chain.

Figure 14-25

The general methods used to determine the path of electrons along an electron-transport chain. The extent of oxidation of electron carriers a, b, c, and d is continuously monitored by following their distinct spectra, which differ in their oxidized and (more...)

The Respiratory Chain Includes Three Large Enzyme Complexes Embedded in the Inner Membrane

Membrane proteins are difficult to purify as intact complexes because they are insoluble in aqueous solutions, and some of the detergents required to solubilize them can destroy normal protein-protein interactions. In the early 1960s, however, it was found that relatively mild ionic detergents, such as deoxycholate, can solubilize selected components of the inner mitochondrial membrane in their native form. This permitted the identification and purification of the three major membrane-bound respiratory enzyme complexes in the pathway from NADH to oxygen (Figure 14-26). As we shall see in this section, each of these complexes acts as an electron-transport-driven H+ pump; however, they were initially characterized in terms of the electron carriers that they interact with and contain:

Figure 14-26. The path of electrons through the three respiratory enzyme complexes.

Figure 14-26

The path of electrons through the three respiratory enzyme complexes. The relative size and shape of each complex are shown. During the transfer of electrons from NADH to oxygen (red lines), ubiquinone and cytochrome c serve as mobile carriers that ferry (more...)

1.

The NADH dehydrogenase complex (generally known as complex I) is the largest of the respiratory enzyme complexes, containing more than 40 polypeptide chains. It accepts electrons from NADH and passes them through a flavin and at least seven iron-sulfur centers to ubiquinone. Ubiquinone then transfers its electrons to a second respiratory enzyme complex, the cytochrome b-c1 complex.

2.

The cytochrome b-c1 complex contains at least 11 different polypeptide chains and functions as a dimer. Each monomer contains three hemes bound to cytochromes and an iron-sulfur protein. The complex accepts electrons from ubiquinone and passes them on to cytochrome c, which carries its electron to the cytochrome oxidase complex.

3.

The cytochrome oxidase complex also functions as a dimer; each monomer contains 13 different polypeptide chains, including two cytochromes and two copper atoms. The complex accepts one electron at a time from cytochrome c and passes them four at a time to oxygen.

The cytochromes, iron-sulfur centers, and copper atoms can carry only one electron at a time. Yet each NADH donates two electrons, and each O2 molecule must receive four electrons to produce water. There are several electron-collecting and electron-dispersing points along the electron-transport chain where these changes in electron number are accommodated. The most obvious of these is cytochrome oxidase.

An Iron-Copper Center in Cytochrome Oxidase Catalyzes Efficient O2 Reduction

Because oxygen has a high affinity for electrons, it releases a large amount of free energy when it is reduced to form water. Thus, the evolution of cellular respiration, in which O2 is converted to water, enabled organisms to harness much more energy than can be derived from anaerobic metabolism. This is presumably why all higher organisms respire. The ability of biological systems to use O2 in this way, however, requires a very sophisticated chemistry. We can tolerate O2 in the air we breathe because it has trouble picking up its first electron; this fact allows its initial reaction in cells to be controlled closely by enzymatic catalysis. But once a molecule of O2 has picked up one electron to form a superoxide radical (O2 -), it becomes dangerously reactive and rapidly takes up an additional three electrons wherever it can find them. The cell can use O2 for respiration only because cytochrome oxidase holds onto oxygen at a special bimetallic center, where it remains clamped between a heme-linked iron atom and a copper atom until it has picked up a total of four electrons. Only then can the two oxygen atoms of the oxygen molecule be safely released as two molecules of water (Figure 14-27).

Figure 14-27. The reaction of O2 with electrons in cytochrome oxidase.

Figure 14-27

The reaction of O2 with electrons in cytochrome oxidase. As indicated, the iron atom in heme a serves as an electron queuing point; this heme feeds four electrons into an O2 molecule held at the bimetallic center active site, which is formed by the other (more...)

The cytochrome oxidase reaction is estimated to account for 90% of the total oxygen uptake in most cells. This protein complex is therefore crucial for all aerobic life. Cyanide and azide are extremely toxic because they bind tightly to the cell's cytochrome oxidase complexes to stop electron transport, thereby greatly reducing ATP production.

Although the cytochrome oxidase in mammals contains 13 different protein subunits, most of these seem to have a subsidiary role, helping to regulate either the activity or the assembly of the three subunits that form the core of the enzyme. The complete structure of this large enzyme complex has recently been determined by x-ray crystallography, as illustrated in Figure 14-28. The atomic resolution structures, combined with mechanistic studies of the effect of precisely tailored mutations introduced into the enzyme by genetic engineering of the yeast and bacterial proteins, are revealing the detailed mechanisms of this finely tuned protein machine.

Figure 14-28. The molecular structure of cytochrome oxidase.

Figure 14-28

The molecular structure of cytochrome oxidase. This protein is a dimer formed from a monomer with 13 different protein subunits (monomer mass of 204,000 daltons). The three colored subunits are encoded by the mitochondrial genome, and they form the functional (more...)

Electron Transfers Are Mediated by Random Collisions in the Inner Mitochondrial Membrane

The two components that carry electrons between the three major enzyme complexes of the respiratory chain—ubiquinone and cytochrome c—diffuse rapidly in the plane of the inner mitochondrial membrane. The expected rate of random collisions between these mobile carriers and the more slowly diffusing enzyme complexes can account for the observed rates of electron transfer (each complex donates and receives an electron about once every 5–20 milliseconds). Thus, there is no need to postulate a structurally ordered chain of electron-transfer proteins in the lipid bilayer; indeed, the three enzyme complexes seem to exist as independent entities in the plane of the inner membrane, being present in different ratios in different mitochondria.

The ordered transfer of electrons along the respiratory chain is due entirely to the specificity of the functional interactions between the components of the chain: each electron carrier is able to interact only with the carrier adjacent to it in the sequence shown in Figure 14-26, with no short circuits.

Electrons move between the molecules that carry them in biological systems not only by moving along covalent bonds within a molecule, but also by jumping across a gap as large as 2 nm. The jumps occur by electron “tunneling,” a quantum-mechanical property that is critical for the processes we are discussing. Insulation is needed to prevent short circuits that would otherwise occur when an electron carrier with a low redox potential collides with a carrier with a high redox potential. This insulation seems to be provided by carrying an electron deep enough inside a protein to prevent its tunneling interactions with an inappropriate partner.

How the changes in redox potential from one electron carrier to the next are harnessed to pump protons out of the mitochondrial matrix is the topic we discuss next.

A Large Drop in Redox Potential Across Each of the Three Respiratory Enzyme Complexes Provides the Energy for H+ Pumping

We have previously discussed how the redox potential reflects electron affinities (see p. 783). An outline of the redox potentials measured along the respiratory chain is shown in Figure 14-29. These potentials drop in three large steps, one across each major respiratory complex. The change in redox potential between any two electron carriers is directly proportional to the free energy released when an electron transfers between them. Each enzyme complex acts as an energy-conversion device by harnessing some of this free-energy change to pump H+ across the inner membrane, thereby creating an electrochemical proton gradient as electrons pass through that complex. This conversion can be demonstrated by purifying each respiratory enzyme complex and incorporating it separately into liposomes: when an appropriate electron donor and acceptor are added so that electrons can pass through the complex, H+ is translocated across the liposome membrane.

Figure 14-29. Redox potential changes along the mitochondrial electron-transport chain.

Figure 14-29

Redox potential changes along the mitochondrial electron-transport chain. The redox potential (designated E0) increases as electrons flow down the respiratory chain to oxygen. The standard free-energy change, Δ, for the transfer (more...)

The Mechanism of H+ Pumping Will Soon Be Understood in Atomic Detail

Some respiratory enzyme complexes pump one H+ per electron across the inner mitochondrial membrane, whereas others pump two. The detailed mechanism by which electron transport is coupled to H+ pumping is different for the three different enzyme complexes. In the cytochrome b-c1 complex, the quinones clearly have a role. As mentioned previously, a quinone picks up a H+ from the aqueous medium along with each electron it carries and liberates it when it releases the electron (see Figure 14-24). Since ubiquinone is freely mobile in the lipid bilayer, it could accept electrons near the inside surface of the membrane and donate them to the cytochrome b-c1 complex near the outside surface, thereby transferring one H+ across the bilayer for every electron transported. Two protons are pumped per electron in the cytochrome b-c1 complex, however, and there is good evidence for a so-called Q-cycle, in which ubiquinone is recycled through the complex in an ordered way that makes this two-for-one transfer possible. Exactly how this occurs can now be worked out at the atomic level, because the complete structure of the cytochrome b-c1 complex has been determined by x-ray crystallography (Figure 14-30).

Figure 14-30. The atomic structure of cytochrome b-c 1.

Figure 14-30

The atomic structure of cytochrome b-c 1. This protein is a dimer. The 240,000-dalton monomer is composed of 11 different protein molecules in mammals. The three colored proteins form the functional core of the enzyme: cytochrome b (green), cytochrome (more...)

Allosteric changes in protein conformations driven by electron transport can also pump H+, just as H+ is pumped when ATP is hydrolyzed by the ATP synthase running in reverse. For both the NADH dehydrogenase complex and the cytochrome oxidase complex, it seems likely that electron transport drives sequential allosteric changes in protein conformation that cause a portion of the protein to pump H+ across the mitochondrial inner membrane. A general mechanism for this type of H+ pumping is presented in Figure 14-31.

Figure 14-31. A general model for H+ pumping.

Figure 14-31

A general model for H+ pumping. This model for H+ pumping by a transmembrane protein is based on mechanisms that are thought to be used by both cytochrome oxidase and the light-driven procaryotic proton pump, bacteriorhodopsin. The protein is driven through (more...)

H+ Ionophores Uncouple Electron Transport from ATP Synthesis

Since the 1940s, several substances—such as 2,4-dinitrophenol—have been known to act as uncoupling agents, uncoupling electron transport from ATP synthesis. The addition of these low-molecular-weight organic compounds to cells stops ATP synthesis by mitochondria without blocking their uptake of oxygen. In the presence of an uncoupling agent, electron transport and H+ pumping continue at a rapid rate, but no H+ gradient is generated. The explanation for this effect is both simple and elegant: uncoupling agents are lipid-soluble weak acids that act as H+ carriers (H+ ionophores), and they provide a pathway for the flow of H+ across the inner mitochondrial membrane that bypasses the ATP synthase. As a result of this short-circuiting, the proton-motive force is dissipated completely, and ATP can no longer be made.

Respiratory Control Normally Restrains Electron Flow Through the Chain

When an uncoupler such as dinitrophenol is added to cells, mitochondria increase their oxygen uptake substantially because of an increased rate of electron transport. This increase reflects the existence of respiratory control. The control is thought to act via a direct inhibitory influence of the electrochemical proton gradient on the rate of electron transport. When the gradient is collapsed by an uncoupler, electron transport is free to run unchecked at the maximal rate. As the gradient increases, electron transport becomes more difficult, and the process slows. Moreover, if an artificially large electrochemical proton gradient is experimentally created across the inner membrane, normal electron transport stops completely, and a reverse electron flow can be detected in some sections of the respiratory chain. This observation suggests that respiratory control reflects a simple balance between the free-energy change for electron-transport-linked proton pumping and the free-energy change for electron transport—that is, the magnitude of the electrochemical proton gradient affects both the rate and the direction of electron transport, just as it affects the directionality of the ATP synthase (see Figure 14-19).

Respiratory control is just one part of an elaborate interlocking system of feedback controls that coordinate the rates of glycolysis, fatty acid breakdown, the citric acid cycle, and electron transport. The rates of all of these processes are adjusted to the ATP:ADP ratio, increasing whenever an increased utilization of ATP causes the ratio to fall. The ATP synthase in the inner mitochondrial membrane, for example, works faster as the concentrations of its substrates ADP and Pi increase. As it speeds up, the enzyme lets more H+ flow into the matrix and thereby dissipates the electrochemical proton gradient more rapidly. The falling gradient, in turn, enhances the rate of electron transport.

Similar controls, including feedback inhibition of several key enzymes by ATP, act to adjust the rates of NADH production to the rate of NADH utilization by the respiratory chain, and so on. As a result of these many control mechanisms, the body oxidizes fats and sugars 5–10 times more rapidly during a period of strenuous exercise than during a period of rest.

Natural Uncouplers Convert the Mitochondria in Brown Fat into Heat-generating Machines

In some specialized fat cells, mitochondrial respiration is normally uncoupled from ATP synthesis. In these cells, known as brown fat cells, most of the energy of oxidation is dissipated as heat rather than being converted into ATP. The inner membranes of the large mitochondria in these cells contain a special transport protein that allows protons to move down their electrochemical gradient, by-passing ATP synthase. As a result, the cells oxidize their fat stores at a rapid rate and produce more heat than ATP. Tissues containing brown fat serve as “heating pads,” helping to revive hibernating animals and to protect sensitive areas of newborn human babies from the cold.

Bacteria Also Exploit Chemiosmotic Mechanisms to Harness Energy

Bacteria use enormously diverse energy sources. Some, like animal cells, are aerobic; they synthesize ATP from sugars they oxidize to CO2 and H2O by glycolysis, the citric acid cycle, and a respiratory chain in their plasma membrane that is similar to the one in the inner mitochondrial membrane. Others are strict anaerobes, deriving their energy either from glycolysis alone (by fermentation) or from an electron-transport chain that employs a molecule other than oxygen as the final electron acceptor. The alternative electron acceptor can be a nitrogen compound (nitrate or nitrite), a sulfur compound (sulfate or sulfite), or a carbon compound (fumarate or carbonate), for example. The electrons are transferred to these acceptors by a series of electron carriers in the plasma membrane that are comparable to those in mitochondrial respiratory chains.

Despite this diversity, the plasma membrane of the vast majority of bacteria contains an ATP synthase that is very similar to the one in mitochondria. In bacteria that use an electron-transport chain to harvest energy, the electron-transport pumps H+ out of the cell and thereby establishes a proton-motive force across the plasma membrane that drives the ATP synthase to make ATP. In other bacteria, the ATP synthase works in reverse, using the ATP produced by glycolysis to pump H+ and establish a proton gradient across the plasma membrane. The ATP used for this process is generated by fermentation processes (discussed in Chapter 2).

Thus, most bacteria, including the strict anaerobes, maintain a proton gradient across their plasma membrane. It can be harnessed to drive a flagellar motor, and it is used to pump Na+ out of the bacterium via a Na+-H+ antiporter that takes the place of the Na+-K+ pump of eucaryotic cells. This gradient is also used for the active inward transport of nutrients, such as most amino acids and many sugars: each nutrient is dragged into the cell along with one or more H+ through a specific symporter (Figure 14-32). In animal cells, by contrast, most inward transport across the plasma membrane is driven by the Na+ gradient that is established by the Na+-K+ pump.

Figure 14-32. The importance of H+-driven transport in bacteria.

Figure 14-32

The importance of H+-driven transport in bacteria. A proton-motive force generated across the plasma membrane pumps nutrients into the cell and expels Na+. (A) In an aerobic bacterium, an electrochemical proton gradient across the plasma membrane is produced (more...)

Some unusual bacteria have adapted to live in a very alkaline environment and yet must maintain their cytoplasm at a physiological pH. For these cells, any attempt to generate an electrochemical H+ gradient would be opposed by a large H+ concentration gradient in the wrong direction (H+ higher inside than outside). Presumably for this reason, some of these bacteria substitute Na+ for H+ in all of their chemiosmotic mechanisms. The respiratory chain pumps Na+ out of the cell, the transport systems and flagellar motor are driven by an inward flux of Na+, and a Na+-driven ATP synthase synthesizes ATP. The existence of such bacteria demonstrates that the principle of chemiosmosis is more fundamental than the proton-motive force on which it is normally based.

Summary

The respiratory chain in the inner mitochondrial membrane contains three respiratory enzyme complexes through which electrons pass on their way from NADH to O2.

Each of these can be purified, inserted into synthetic lipid vesicles, and then shown to pump H+ when electrons are transported through it. In the intact membrane, the mobile electron carriers ubiquinone and cytochrome c complete the electron-transport chain by shuttling between the enzyme complexes. The path of electron flow is NADH → NADH dehydrogenase complex → ubiquinone → cytochrome b-c1 complexcytochrome ccytochrome oxidase complex → molecular oxygen (O2).

The respiratory enzyme complexes couple the energetically favorable transport of electrons to the pumping of H+ out of the matrix. The resulting electrochemical proton gradient is harnessed to make ATP by another transmembrane protein complex, ATP synthase, through which H+ flows back into the matrix. The ATP synthase is a reversible coupling device that normally converts a backflow of H+ into ATP phosphate bond energy by catalyzing the reaction ADP + Pi → ATP, but it can also work in the opposite direction and hydrolyze ATP to pump H+ if the electrochemical proton gradient is sufficiently reduced. Its universal presence in mitochondria, chloroplasts, and procaryotes testifies to the central importance of chemiosmotic mechanisms in cells.

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: NBK26904