Atp

Figure 6.17 Electron Transport Energy is released as electrons are passed along carriers of the electron transport chain.

water

Figure 6.17 Electron Transport Energy is released as electrons are passed along carriers of the electron transport chain.

these processes is that protons are pumped to the other side of the membrane, establishing the concentration gradient across the membrane.

Most carriers of the electron transport chain are grouped into several large protein complexes that function as proton pumps; other carriers shuttle electrons from one complex to the next.

The Electron Transport Chain of Mitochondria

Mitochondria have four different protein complexes, three of which function as proton pumps (complexes I, III, and IV). In addition, two electron carriers (coenzyme Q and cytochrome c) shuttle electrons between the complexes. The electron transport chain of mitochondria consists of these components (figure 6.18):

■ Complex I (also called NADH dehydrogenase complex). This complex accepts electrons from NADH, ultimately transferring them to the electron carrier coenzyme Q; in the process, 4 protons are pumped across the membrane.

■ Complex II (also called succinate dehydrogenase complex). This complex accepts electrons from the TCA cycle, when FADH2 is formed during the oxidation of succinate (see figure 6.16, step 6). Electrons are then transferred to coenzyme Q.

■ Coenzyme Q (also called ubiquinone). This lipid soluble carrier accepts electrons from either complex I or complex II and then shuttles them to complex III. Note that the electrons carried by FADH2 have entered the electron transport chain "downstream" of those carried by NADH. Because of this, a pair of electrons carried by NAHD result in more protons being expelled than does a pair carried by FADH2.

water

148 Chapter 6 Metabolism: Fueling Cell Growth

Eukaryotic cell

148 Chapter 6 Metabolism: Fueling Cell Growth

Eukaryotic cell

Intermembrane space

Figure 6.18 The Electron Transport Chain of Mitochondria The electrons carried by NADH are passed to complex I.They are then passed to coenzyme Q, which transfers them to the complex III. Cytochrome c then transfers electrons to complex IV. From there, they are passed to O2. Unlike the electrons carried by NADH, those carried by FADH2 are passed to complex II, which then passes them to coenzyme Q; from there, the electrons follow the same path as the ones donated by NADH. Protons are shuttled from the mitochondrial matrix to the intermembrane space by complexes I, III and IV, creating the proton motive force. ATP synthase allows protons to reenter the mitochondrial matrix, using the energy released to drive ATP synthesis.

Intermembrane space

Figure 6.18 The Electron Transport Chain of Mitochondria The electrons carried by NADH are passed to complex I.They are then passed to coenzyme Q, which transfers them to the complex III. Cytochrome c then transfers electrons to complex IV. From there, they are passed to O2. Unlike the electrons carried by NADH, those carried by FADH2 are passed to complex II, which then passes them to coenzyme Q; from there, the electrons follow the same path as the ones donated by NADH. Protons are shuttled from the mitochondrial matrix to the intermembrane space by complexes I, III and IV, creating the proton motive force. ATP synthase allows protons to reenter the mitochondrial matrix, using the energy released to drive ATP synthesis.

■ Complex III (also called cytochrome bc1 complex). This complex accepts electrons from coenzyme Q, ultimately transferring them to the electron carrier cytochrome c; in the process, 4 protons are pumped across the membrane.

■ Cytochrome c. This carrier accepts electrons from complex III and then shuttles them to complex IV.

■ Complex IV (also called cytochrome c oxidase complex). This complex accepts electrons from cytochrome c, ultimately transferring them to oxygen (O2), forming H2O. In the process 2 protons are pumped across the membrane. Complex IV is a terminal oxidoreductase, meaning that it transfers the electrons to the terminal electron acceptor, which, in this case, is O2.

The Electron Transport Chains of Prokaryotes

Considering the flexibility and diversity of prokaryotes, it is not surprising that they vary with respect to the types and arrange ment of their electron transport components. In fact, a single species may have several alternative carriers so that the system as a whole can function optimally under changeable growth conditions. In the laboratory, the different electron system components provide a mechanism to distinguish between certain types of bacteria. For example, the activity of cytochrome c oxidase, which is found in species of Pseudomonas, Campylobacter, and certain other genera, is detected using the rapid biochemical test called the oxidase test and is important in the identification scheme of these organisms (see table 10.4).

The electron transport chain of E. coli provides an excellent example of the diversity found even in a single organism. This organism preferentially uses aerobic respiration, but when molecular oxygen is not available, it can switch to anaerobic respiration provided that a suitable terminal electron acceptor such as nitrate is available. The E. coli electron transport chain serves as a model for both aerobic and anaerobic respiration.

Aerobic Respiration When growing aerobically in a glucose-containing medium, E. coli can use two different NADH dehydro-

6.4 Respiration 149

Prokaryotic cell

Cytoplasmic membrane

NADH dehydrogenase H+ (0 or 4)

Active transport (one mechanism)

Rotation of flagella substance 1200 H+ / / Outside of / l cytoplasmic membrane

Cytoplasmic membrane

NADH dehydrogenase H+ (0 or 4)

Active transport (one mechanism)

Rotation of flagella

Inside of cytoplasmic membrane

Figure 6.19 The Electron Transport Chain of E. coli Growing Aerobically in a Glucose-Containing Medium The electrons carried by NADH are passed to one of two different NADH dehydrogenases.They are then passed to ubiquinone, which transfers them to one of two ubiquinol oxidases. From there they are passed to O2. Unlike the electrons carried by NADH , those carried by FADH2 are passed to succinate dehydrogenase, which then transfers them to ubiquinone; from there, the electrons follow the same path as the ones donated by NADH. Protons are ejected by one of the two NADH dehydrogenases and both ubiquinol oxidases, creating the proton motive force. ATP synthase allows protons to reenter the cell, using the energy released to drive ATP synthesis.The proton motive force is also used to drive one form of active transport and to power the rotation of flagella. E. coli has other components of the electron transport chain that function under different growth conditions.

Inside of cytoplasmic membrane

Figure 6.19 The Electron Transport Chain of E. coli Growing Aerobically in a Glucose-Containing Medium The electrons carried by NADH are passed to one of two different NADH dehydrogenases.They are then passed to ubiquinone, which transfers them to one of two ubiquinol oxidases. From there they are passed to O2. Unlike the electrons carried by NADH , those carried by FADH2 are passed to succinate dehydrogenase, which then transfers them to ubiquinone; from there, the electrons follow the same path as the ones donated by NADH. Protons are ejected by one of the two NADH dehydrogenases and both ubiquinol oxidases, creating the proton motive force. ATP synthase allows protons to reenter the cell, using the energy released to drive ATP synthesis.The proton motive force is also used to drive one form of active transport and to power the rotation of flagella. E. coli has other components of the electron transport chain that function under different growth conditions.

genases (figure 6.19). One is a proton pump functionally equivalent to complex I of the mitochondrion. E. coli also has a succinate dehydrogenase that is functionally equivalent to complex II of the mitochondrion. In addition to these enzyme complexes, E. coli can produce several alternatives, enabling the organism to optimally use a variety of different energy sources, including hydrogen gas. E. coli does not have the equivalent of complex III or cytochrome c; instead quinones, including ubiquinone, shuttle the electrons directly to a terminal oxidore-ductase. When O2 is available to serve as a terminal electron acceptor, one of two variations of a terminal oxidoreductase called ubiquinone oxidase is used. One form functions optimally only in high O2 conditions and results in the expulsion of 4 protons. The other results in the ejection of only 2 protons, but it can more effectively scavenge O2 and thus is particularly useful when the supply of O2 is limited.

Anaerobic Respiration Anaerobic respiration is a less efficient form of energy transformation than is aerobic respiration. This is partly due to the lesser amount of energy released in reactions that involve the reduction of inorganic compounds other than molecular oxygen. Alternative electron carriers are used in the electron transport chain during anaerobic respiration.

When oxygen is absent and nitrate is available, E. coli responds by synthesizing a terminal oxidoreductase that uses nitrate as a terminal electron acceptor, producing nitrite. The organism then converts nitrite to ammonia, presumably to avoid the toxic effects of nitrite. Other bacteria can reduce nitrate further than E. coli can, forming compounds such as nitrous oxide (N2O), and nitrogen gas (N2) (figure 6.20). The quinone that bacteria use during anaerobic respiration, menaquinone, provides humans and other mammals with a source of the nutrient called vitamin K. This vitamin is required for the proper coagulation of blood, and mammals are able to obtain at least part of their requirement by absorbing menaquinone produced by bacteria growing in the intestinal tract.

A group of obligate anaerobes called the sulfate-reducers use sulfate (SO42:) as a terminal electron acceptor, producing hydrogen sulfide as an end product. The diversity ecology of the sulfate reducers will be discussed in chapter 11. ■ sulfate-reducers, p. 274

150 Chapter 6 Metabolism: Fueling Cell Growth

150 Chapter 6 Metabolism: Fueling Cell Growth

Figure 6.20 Anaeroblc Respiration Anaerobic respiration employs an inorganic molecule other than oxygen as a terminal electron acceptor.

ATP Synthase—Harvesting the Proton Motive Force to Synthesize ATP

Just as energy is required to establish a concentration gradient, energy is released when a gradient is eased. The enzyme ATP synthase uses that energy to synthesize ATP. It permits protons to flow back into the bacterial cell (or matrix of the mitochondrion) in a controlled manner, harvesting the energy that is released to fuel the addition of a phosphate group to ADP. It appears that one molecule of ATP is formed from the entry of approximately 3 protons. The precise mechanism of how this occurs is not well understood.

Theoretical ATP Yield of Oxidative Phosphorylation

The complexity of oxidative phosphorylation makes it exceedingly difficult to determine the actual maximum yield of ATP. Unlike the yield of substrate-level phosphorylation, which can be calculated based on the stoichiometry of relatively simple chemical reactions, oxidative phosphorylation involves processes that have many variables. This is particularly true for prokary-otic cells because they use proton motive force to drive processes other than ATP synthesis, including flagella rotation and membrane transport. In addition, as a group, they use different carriers in their electron transport chain, and these may vary in the number of protons that are ejected per pair of electrons passed. Another complicating factor in energy yield calculations is that the number of ATP generated per reduced electron carrier is not necessarily a whole number.

For each pair of electrons transferred to the electron transport chain by NADH, between 2 and 3 ATP may be generated; for each pair transferred by FADH2, the maximum yield is between 1 and 2 ATP. Although experimental studies using rat mitochondria indicate that the yield is approximately 2.5 ATP/NADH and 1.5 ATP/FADH2, for simplicity we will use whole numbers (3 ATP/NADH and 2 ATP/FADH2) to calculate the maximum ATP gain of oxidative phosphorylation in a prokaryotic cell. Note, however, that these numbers are only theoretical and serve primarily as a means of comparing the relative energy gains of respiration and fermentation.

The ATP gain as a result of oxidative phosphorylation will be at least slightly different in eukaryotic cells than in prokaryot-ic cells because of the fate of the reducing power (NADH) generated during glycolysis. Recall that in eukaryotic cells, glycolysis takes place in the cytoplasm, whereas the electron transport chain is located in the mitochondria. Consequently, the electrons carried by cytoplasmic NADH must be translocated across the mito-chondrial membrane before they can enter the electron transport chain. This requires an expenditure of approximately 2 ATP.

The maximum theoretical energy yield for oxidative phos-phorylation in a prokaryotic cell that uses an electron transport chain similar to that of mitochondria is:

■ From glycolysis:

2 NADH d 6 ATP (assuming 3 for each NADH)

■ From the transition step:

2 NADH d 6 ATP (assuming 3 for each NADH)

6 NADH d 18 ATP (assuming 3 for each NADH)

2 FADH,

4 ATP (assuming 2 for each FADH2)

ATP Yield of Aerobic Respiration in Prokaryotes

Now that the ATP-yielding components of the central metabolic pathways have been considered, we can calculate the theoretical maximum ATP yield of aerobic respiration in prokary-otes. This yield is illustrated in figure 6.21.

■ Substrate-level phosphorylation:

2 ATP (from glycolysis; net gain)

2 ATP (from the TCA cycle)_

4 ATP (total; substrate-level phosphorylation)

■ Oxidative phosphorylation:

6 ATP (from the reducing power gained in glycolysis) 6 ATP (from the reducing power gained in the transition step) 22 ATP (from the reducing power gained in the

TCA cycle)_

34 (total; oxidative phosphorylation)

■ Total ATP gain (theoretical maximum) =38

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