We now come to the energy chain, which is the final “processing mill” for electrons and protons. Overall, the electron transport system (ETS) consists of a chain of special carriers that receive electrons from reduced carriers (NADH, FADH2) generated by glycolysis and the Krebs cycle. The ETS shuttles the electrons through a series of redox transfers (figure 1). The flow of electrons down this chain is highly energetic and ultimately drives the synthesis of ATP. The step that finalizes the transport process is the acceptance of electrons and hydrogen ions by oxygen, with the production of water.
Fig1. Electron transport, oxidative phosphorylation, the proton motive force, chemiosmosis, and ATP synthesis in the mitochondrion.
Some variability exists from one organism to another, but the principal compounds that carry out these complex reactions are NADH dehydrogenase, iron-sulfide complexes (FeS), coenzyme Q (ubiquinone), and several cytochromes. The cytochromes contain a tightly bound metal atom at their center that is actively involved in accepting electrons and donating them to the next carrier in the series. The highly compartmentalized structure of the respiratory chain is an important factor in its function. Note in figure 1 that the electron transport carriers and enzymes are associated in complexes in the inner mitochondrial membranes in eukaryotes. The cell membrane is the equivalent structure for housing them in bacteria (figure2).
Fig2. Chemiosmosis in bacteria. Enlarged view of bacterial cell envelope to show the relationship of electron transport and ATP synthesis. Bacteria have the ETS carriers and ATP synthase stationed in the cell membrane. As the ETS shuttles electrons delivered by NADH and FADH2, some of the ETS carriers also transport protons into the periplasmic space between the membrane and the cell wall, which sets up a gradient similar to mitochondria. Bacteria will vary in the exact nature of the carriers in the ETS.
The Carriers of Electron Transport: The Energy Cascade in the Mitochondria
The principal questions about the electron transport system are these: How are the electrons passed from one carrier to another in the series? How is this progression coupled to ATP synthesis? Where and how is oxygen utilized? Although the biochemical de tails of this process are somewhat complicated, the basic reactions consist of a number of redox reactions now familiar to us. In general, the carrier compounds and their associated enzymes are arranged in linear sequence and are reduced and oxidized while shuttling the electrons along to their final acceptor (figure 1, part 2).
The electron carrier complexes and associated molecules present in aerobic organisms are as follows:
1. Complex I consists of a huge multienzyme cluster termed NADH dehydrogenase (reductase), which receives the NADH from glycolysis and the Krebs cycle.
2. Complex II is made up of a series of iron-sulfur (FeS) proteins that receive electrons from FADH2 produced in the sixth step of the Krebs cycle.
3. Coenzyme Q, or ubiquinone, is a mobile carrier not embedded in the membrane that can pick up electrons from both complexes I and II and donate them to complex III.
4. Complex III is composed of cytochromes b and c1, which deliver electrons to cytochrome c.
5. Cytochrome c is another mobile carrier that shuttles electrons between complexes III and IV.
6. Complex IV catalyzes the reaction between electrons, H+, and oxygen, yielding H2O and completing the electron trans fer process.
In addition to the roles outlined here, complexes I, III, and IV have the function of pumping protons from the matrix into the inter membrane space to establish a charge gradient (discussed under the next heading). With each redox exchange, the energy level of the reactants is lessened. This free energy is captured and processed by ATP synthase complexes stationed in the cristae in close association with the ETS carriers. Each NADH that enters electron transport can potentially give rise to a maximum of 3 ATPs. Because Quick Search FADH2 from the Krebs cycle enters the cycle after complex I, its reactions result in a maximum yield of about 2 ATPs. This coupling of ATP synthesis to electron transport is termed oxidative phosphorylation.
production of ATP? According to a widely accepted concept called chemiosmosis, during electron transport, some of the carriers actively transport protons across the cristae membrane and into the intermembrane compartment of the mitochondrion. This process sets up a concentration gradient of hydrogen ions called the proton motive force (PMF). The PMF consists of a difference in charge between the intermembrane compartment (+) and the matrix compartment (−) (figure 1, part 3).
Separating the charge has an effect similar to a battery, which can temporarily store potential energy. This charge will be maintained by the impermeability of the cristae membranes to protons (H+). The only sites where protons can diffuse into the matrix compartment is at the ATP synthase complexes, which sets the stage for ATP synthesis.
ATP synthase is one of the most remarkable enzymes in the molecular world. Its structure includes two motors, an ion pump, and an enzyme that can capture energy and store it as ATP. These synthases are located throughout the membranes of the cristae in high numbers (one estimate is around 10,000 per mitochondrion). Each ATP synthase complex consists of two large units, F0 and F1 (figure 1, part 3). The F0 portion is embedded in the membrane and can rotate like a motor to pull in protons. As protons flow through the F0 portion by diffusion, the F1 compartments pull in ADP and Pi. Rotation induces a change in the three-dimensional structure of ATP synthase. This adjustment creates a high-energy bond between ADP and Pi. The ATP that forms is released into the matrix, where it will serve as a constant supply for cellular reactions (figure 1, part 3). The enzyme is then rotated back to the start position for the next round of ATP synthesis.
An important issue is just how the ATP is delivered out of the mitochondrion and into the cytoplasm for use in cell synthesis and other activities. The mitochondrial membranes are impregnated with a number of specialized active transporters called ATP-ADP translocases that constantly transport ATP out of, and ADP into, the mitochondrion.
Bacterial ATP synthesis occurs by means of this same overall process. However, bacteria have the ETS stationed in the cell mem brane, and the direction of the proton movement is from the cytoplasm to the periplasmic space between the membrane and cell wall. Bacteria may also show variations in the number and types of electron carriers (figure 2). As we will see, these differences will affect the amount of ATP produced. In both cell types, the chemiosmotic theory has been supported by tests showing that oxidative phosphorylation is blocked if the mitochondrial or bacterial cell membranes are disrupted.
The Terminal Step of Electron Transport
The terminal step, during which oxygen accepts the electrons, is catalyzed by cytochrome a/a3, also called cytochrome oxidase. This large enzyme complex is specifically adapted to receive electrons from cytochrome c, pick up hydrogen ions from solution, and react with oxygen to form a molecule of water (figure 1, part 3). This reaction, though in actuality more complex, is summarized as follows:
2H+ + 2e− + 1/2O2 → H2O
Most eukaryotic aerobes have a fully functioning cytochrome system, but bacteria exhibit wide-ranging variations in this part of the system. Some species lack one or more of the redox steps; others have several alternative electron transport schemes. Because many bacteria lack cytochrome oxidase, this variation can be used to differentiate among certain genera of bacteria. An oxidase detection test can be used to help identify members of the genera Neisseria and Pseudomonas and some species of Bacillus. Another variation in the cytochrome system is evident in certain bacteria (Klebsiella, Enterobacter) that can grow even in the presence of cyanide be cause they lack cytochrome oxidase. Cyanide will cause rapid death in humans and other eukaryotes because it blocks cytochrome oxidase, thereby completely terminating aerobic respiration, but it is harmless to these bacteria.
A potential side reaction of the respiratory chain in aerobic organisms is the incomplete reduction of oxygen to superoxide ion (O2−) and hydrogen peroxide (H2O2). As mentioned in chapter 7, these toxic oxygen products can be very damaging to cells. Aerobes have neutralizing enzymes to deal with these products, including superoxide dismutase and catalase. One exception is the genus Streptococcus, which can grow well in oxygen yet lacks both cytochromes and catalase. The tolerance of these organisms to oxygen can be explained by the neutralizing effects of a special peroxidase. The lack of cytochromes, catalase, and peroxidases in strict anaerobes as a rule limits their ability to process free oxygen and contributes to its toxic effects on them.
