CONCEPT MAP: AMINO ACIDS AND PROTEINS
20.9 The Electron-Transport Chain and ATP Production
Keep in mind that in some ways catabolism is just like burning petroleum or natural gas. In both cases, the goal is to produce useful energy and the reaction products are water and carbon dioxide. The difference is that in catabolism the products are not released all at once and not all of the energy is released as heat.
At the conclusion of the citric acid cycle, the reduced coenzymes formed during the cycle are ready to donate their energy to making additional ATP. The energy is released in a series of oxidation–reduction reactions that move electrons from one electron car- rier to the next as each carrier is reduced (gains an electron from the preceding car- rier) and then oxidized (loses an electron by passing it along to the next carrier). Each reaction in the series is favorable; that is, it is exergonic. You can think of each reaction as a step along the way down our waterfall. The sequence of reactions that move the electrons along is known as the electron-transport chain (also called the respiratory chain). The enzymes and coenzymes of the chain and ATP synthesis are embedded in the inner membrane of the mitochondrion (Figure 20.10).
In the last step of the chain, the electrons combine with the oxygen that we breathe and with hydrogen ions from their surroundings to produce water:
O2 + 4 e- + 4 H+ h 2H2O
This reaction is fundamentally the combination of hydrogen and oxygen gases.
Carried out all at once with the gases themselves, the reaction is explosive. What hap- pens to all that energy during electron transport?
As electrons move down the electron-transport pathway, the energy released is used to move hydrogen ions out of the mitochondrial matrix (across the inner membrane) and into the intermembrane space. Because the inner membrane is otherwise imper- meable to the H+ ion, the result is a higher H+ concentration in the intermembrane space than in the mitochondrial matrix. Moving ions from a region of lower concentra- tion to one of higher concentration opposes the natural tendency for random motion to equalize concentrations throughout a mixture and therefore requires energy to make it happen. This energy is recaptured for use in ATP synthesis.
Electron Transport
Electron transport proceeds via four enzyme complexes held in fixed positions within the inner membrane of mitochondria, along with two electron carriers that move through the membrane from one complex to another. The complexes and mobile electron carriers are organized in the sequence of their ability to pick up electrons, as illustrated in Figure 20.10. The four fixed complexes are very large assemblages of
Electron-transport chain The series of biochemical reactions that passes electrons from reduced coenzymes to oxygen and is coupled to ATP formation.
FOOD
PROTEINS CARBOHYDRATES
LIPIDS
Pyruvate
Acetyl-CoA
CO2 ATP
O2
H2O ATP REDUCED COENZYMES
Amino acid catabolism Fatty acid
oxidation
Electron transport chain Glycolysis
Citric acid cycle ATP
polypeptides and electron acceptors. The most important electron acceptors are of three types: (1) various cytochromes, which are proteins that contain heme groups (Figure 20.11 a) in which the iron cycles between Fe2+ and Fe3+; (2) proteins contain- ing iron–sulfur groups in which the iron also cycles between Fe2+ and Fe3+; and (3) coenzyme Q (CoQ), often known as ubiquinone because of its ubiquitous (widespread) occurrence and because its ring structure with the two ketone groups is a quinone:
CH3O CH3 O
O
CH3O (CH2CH CCH2)nH CH3
(CH2CH CCH2)nH CH3 CH3O CH3
OH
OH CH3O
Reduced coenzyme Q Oxidized coenzyme Q
CH3
CH3
CH3
H3C
N N Fe N
N
(a)A heme group (b)A representative cytochrome protein Outer mitochondrial membrane
Inner mitochondrial membrane Intermembrane space
Mitochondrial matrix Cytosol
H+
H+
I
CoQ
Cyt c
IV
II FADH2
NADH
Succinate Fumarate III
H+
H+ H+
H+
H+
H+
H+ H+
H+
H+
NAD+
H2O ATP synthase H+ ion channel
ADP + HOPO32−
ATP O2
▲ Figure 20.10
The mitochondrial electron-transport chain and ATP synthase.
The red line shows the path of electrons, and the black lines show the paths of hydrogen ions. The movement of hydrogen ions across the inner membrane at complexes I, III, and IV creates a higher concentration on the intermembrane side of the inner membrane than on the matrix side. The energy released by hydrogen ions returning to the matrix through ATP synthase provides the energy needed for ATP synthesis.
▶ Figure 20.11
A heme group and a cytochrome (a) Heme groups, in which the substit- uents at the bonds marked in red vary, are iron-containing coenzymes in the cytochromes of the electron-transport chain. They are also the oxygen carri- ers in hemoglobin in red blood cells.
(b) In the cytochrome shown here, the coiled blue ribbon is the amino acid chain and the heme group is in red.
S E C T I O N 2 0 . 9 The Electron-Transport Chain and ATP Production 645 The details of the reactions that move electrons in the electron-transport chain are
not important to us here. We need only focus on the following essential features of the pathway (Figure 20.12; refer also to Figure 20.10).
• Hydrogen ions and electrons from NADH and FADH2 enter the electron- transport chain at enzyme complexes I and II, respectively. (In this case, the complexes func- tion independently and not necessarily in numerical order.) The enzyme for Step 6 of the citric acid cycle is part of complex II, where FADH2 is produced when that step of the cycle occurs. FADH2 does not leave complex II. It is immediately oxi- dized there by reaction with mobile coenzyme Q, forming QH2. Following forma- tion of reduced mobile coenzyme QH2, hydrogen ions no longer participate directly in the reductions of electron carriers. Instead, electrons are transferred directly, one by one from carrier to carrier.
• Electrons are passed from weaker to increasingly stronger oxidizing agents, with energy released at each transfer. Much of this energy is conserved during the transfer; however, some energy is used to pump protons across the inner mitochon- drial membrane, and some is lost as heat at each electron transfer.
• Hydrogen ions are released for transport through the inner mitochondrial mem- brane to the intermembrane space at complexes I, III, and IV, creating an H+ gradi- ent, with the intermembrane space becoming acidic and the matrix alkaline due to changes in H+ concentration. Some of these ions come from the reduced coenzymes and some from the matrix—exactly how the hydrogen ions are transported to the intermembrane space is not yet fully understood, although the process appears to be via an energy-requiring pump.
• The H+ concentration difference creates a potential energy difference across the two sides of the inner membrane (like the energy difference between water at the top and bottom of a waterfall). The maintenance of this concentration gradi- ent across the membrane is crucial—it is the mechanism by which energy for ATP formation is made available.
Plant cells, like animal cells, contain mitochondria and carry out oxidative phosphor- ylation. In addition, plant cells also contain chloroplasts, organelles that are similar to mitochondria but instead carry out photosynthesis, a series of reactions that also involve electron and hydrogen ion transfer through a series of enzyme complexes arranged in an electron transport chain. See Chemistry in Action: Plants and Photo- synthesis on p. 649 for more information.
ATP Synthesis
The reactions of the electron-transport chain are tightly coupled to oxidative phosphorylation, the conversion of ADP to ATP, by a reaction that is both an oxida- tion and a phosphorylation. Hydrogen ions can return to the matrix only by passing through a channel that is part of the ATP synthase enzyme complex (black pathway at the right in Figure 20.10). In doing so, they release the potential energy gained as they were moved against the concentration gradient at the enzyme complexes of the electron-transport chain. This energy release drives the phosphorylation of ADP by reaction with hydrogen phosphate ion 1HOPO32-2:
ADP + HOPO32- h ATP + H2O
ATP synthase has knob-tipped stalks that protrude into the matrix and are clearly visible in electron micrographs, as seen in the accompanying drawing based on struc- tural studies. ADP and HOPO32- are attracted into the knob portion. As hydrogen ions flow through the complex, ATP is produced and released back into the matrix.
The reaction is facilitated by changes in the shape of the enzyme complex that are induced by the flow of hydrogen ions.
How much ATP energy is produced from a molecule of NADH or a molecule of FADH2 by oxidative phosphorylation? The electrons from molecules of NADH enter
Oxidative phosphorylation The syn- thesis of ATP from ADP using energy released in the electron-transport chain.
ATP synthase The enzyme complex in the inner mitochondrial membrane at which hydrogen ions cross the mem- brane and ATP is synthesized from ADP.
FMN Fe-S proteins
FAD Fe-S proteins A cytochrome H+
H+
O2 H2O Coenzyme Q
Cytochromes Fe-S protein
Cytochromes Cu ions
ATP Synthase Cytochrome c I
II
III
IV Energy
▲ Figure 20.12
Pathway of electrons in electron transport.
Each of the enzyme complexes I–IV contains several electron carriers.
(FMN in complex I is similar in structure to FAD.) Hydrogen ions and electrons move through the components of the electron-transport pathway in the direction of the arrow.
Energy is transferred, with some loss, at each complex; each succeeding com- plex is at a lower energy level than the preceding, as indicated by the color change.
646 C H A P T E R 2 0 The Generation of Biochemical Energy
the electron-transport chain at complex I, while those from FADH2 enter at complex II.
These different entry points into the electron-transport chain result in different yields of ATP molecules. Recent research suggests that each NADH molecule yields about 2.5 molecules of ATP and that each FADH2 molecule yields approximately 1.5 molecules of ATP. In this book, we round these numbers up and use the older yields of 3 ATP mol- ecules generated for every NADH molecule and 2 ATP molecules generated from every FADH2 molecule during oxidative phosphorylation.
PROBLEM 20.19
Within the mitochondrion, is the pH higher in the intermembrane space or in the mitochondrial matrix? Why?
PROBLEM 20.20
Plants carry out both photosynthesis and oxidative phosphorylation (see Chemistry in Action: Plants and Photosynthesis on p. 649). Photosynthesis occurs in chloroplasts, while oxidative phosphorylation occurs in mitochondria. Name some similarities and some differences between photosynthesis and oxidative phosphorylation.
▲A color enhanced model of ATP synthase. The blue portion is embedded in the inner mitochondrial membrane facing the mitochondrial matrix. Hⴙ travels through the blue stalk and into the red knob, as oxida- tive phosphorylation occurs.
KEY CONCEPT PROBLEM 20.21
The reduced coenzymes NADH and FADH2 are oxidized in the electron-transport system. What is the final electron acceptor of the electron-transport system? What is the function of the H+ ion in ATP synthesis?
Blockers and Uncouplers of Oxidative Phosphorylation
The order of the components of the electron-transport chain was elucidated through the use of compounds that block the passage of electrons at different points in the chain. Subsequent research showed that there are also compounds that uncouple—
disconnect—oxidative phosphorylation from the generation of ATP.
Cyanide and barbiturates such as amytal have long been known to be so dangerous—
even fatal—that mystery writers often use these substances in their books as murder weapons. What makes them so dangerous? They are among a group of substances that block respiration (oxidative phosphorylation) at one of the electron transfer stages, result- ing in blockage of electron flow through the electron-transport system and cessation of ATP production. Continuous production of ATP at tightly regulated levels is crucial to an organism’s survival. ATP is the energy link between the oxidation of fuels and energy- requiring processes. Without continuous ATP production, the organism will die.
The blockers act at the cytochromes in the electron-transport chain, with differ- ent blockers acting on different cytochromes. Rotenone, derived from plants and used to kill both fish and insects, and barbiturates like amytal inhibit complex I proteins in the electron-transport system, so that electrons are not transferred to coenzyme Q from complex I. (See Figure 20.12.) The antibiotic antimycin A inhibits some of the cytochromes and proteins of complex III from transferring electrons to cytochrome c. The third blockage point in the electron-transport system occurs between complex IV and oxygen. Here cyanide and hydrogen sulfide bind tightly to the iron and cop- per in the enzymes involved, preventing the conversion of oxygen to water. Cyanide is released from bitter almonds, cassava, and the seeds of apples, peaches, and apricots (used to make amygdalin, an alternative cancer drug) and is used in various indus- trial processes. Cyanide is no longer available as a pest-control agent due to its toxicity.
The mode of action of each inhibitor is similar—binding tightly to the complex in the chain, interrupting the flow of electrons. For example, cyanide binds nearly irrevers- ibly to the iron center in the heme group that is part of complex IV. No matter what the
S E C T I O N 2 0 . 1 0 Harmful Oxygen By-Products and Antioxidant Vitamins 647 inhibitor is, once electron movement has ceased, no ATP can be produced and the cell
soon runs out of energy from its best source.
Just as the production of ATP can be blocked, some substances, such as dicumarol, an anticoagulant, allow electron transport to occur but prevent the conversion of ADP to ATP by ATP synthase. If this happens, the rate of oxygen use increases as the proton gradient between the mitochondrial matrix and the intermembrane space dissipates, with the simultaneous formation of water. When ATP production is thus severed from energy use, it is said that ATP production is uncoupled from the energy of the proton gradient. One chemical that has this effect, once used as a weight-reducing agent, is 2,4-dinitrophenol. Uncoupling electron transport does result in weight loss; however, the toxic dose is too close to the therapeutic dose, so 2,4-dinitrophenol is no longer used as a reducing agent.
The body does have a tissue with the capacity to uncouple oxidative phosphoryla- tion intentionally. It comes into play when the environment is cold. Brown fat, rich in mitochondria, can uncouple oxidative phosphorylation in order to generate heat through dissipation of the proton gradient. This is accomplished by the presence of a special uncoupling protein, thermogenin, in the inner membrane of the mitochondrion in these cells. Human infants and other newborn mammals have deposits of brown fat in order to keep warm. This type of fat disappears in most humans unless they have an occupation that routinely puts them in a cold environment for an extended period.
Pearl divers, who spend several hours daily diving in the ocean, tend to have deposits of brown fat as do mammals that hibernate.
Worked Example 20.6 Determining Phosphorylation Type
(a) Succinyl@SCoA + Pi + GDP m Succinate + GTP + CoA (b) ADP + HOPO32- 999: ATP + H2O
ANALYSIS Both substrate-level phosphorylation and oxidative phosphorylation involve the transfer of a phosphate group and its energy to another molecule. Both may result in the production of ATP. The key difference is that substrate-level phosphorylation involves the transfer of a phosphate group from one molecule to another, whereas oxidative phosphorlyation adds a phosphate ion directly to ADP with the aid of ATP synthase.
SOLUTION
Reaction (a), involving the formation of GTP coupled with the conversion of succinyl-CoA to succinate, is an example of substrate-level phosphorylation.
Reaction (b), involving the direct addition of phosphate to ADP by ATP synthase, is oxidative phosphorylation.
ATP synthase