Fundamentals of the Electron Transport Chain
Aerobic cellular respiration, the process powering most life on Earth, relies on three key stages: glycolysis, the citric acid cycle (Krebs cycle), and oxidative phosphorylation. Glycolysis initiates glucose breakdown into pyruvate, yielding a small amount of ATP and NADH. Pyruvate is then transformed into acetyl CoA, generating more NADH and releasing carbon dioxide (CO2). Acetyl CoA fuels the citric acid cycle, a series of reactions producing CO2, NADH, FADH2, and a little ATP. Finally, oxidative phosphorylation, the culmination of these processes, utilizes NADH and FADH2 to generate water and a significant amount of ATP.
Oxidative phosphorylation is a two-part system encompassing the electron transport chain (ETC) and chemiosmosis. The ETC is an intricate assembly of proteins and organic molecules embedded within the inner mitochondrial membrane. Through a series of redox reactions, electrons traverse this chain, releasing energy. This released energy is harnessed to establish a proton gradient across the membrane. Chemiosmosis then leverages this proton gradient to drive ATP synthase, a remarkable protein complex that produces the bulk of cellular ATP.
Photosynthesis, the process by which plants and other organisms convert light energy into chemical energy, also employs an electron transport chain. In the light-dependent reactions of photosynthesis, light energy and water are used to generate ATP, NADPH, and oxygen (O2). Similar to cellular respiration, a proton gradient, formed by an electron transport chain within chloroplasts, is crucial for ATP production. The ATP and NADPH generated in these reactions are then used in the light-independent reactions (Calvin cycle) to synthesize sugars.
The Electron Transport Chain at the Cellular Level
Within the electron transport chain (ETC), electrons are passed along a sequence of protein complexes, each with progressively higher reduction potential. This stepwise transfer of electrons releases energy. A significant portion of this energy is dissipated as heat, while the remainder is skillfully used to pump protons (H+) from the mitochondrial matrix into the intermembrane space, creating a proton gradient. This gradient results in a higher concentration of protons (increased acidity) in the intermembrane space and an electrochemical difference, with a positive charge outside and a negative charge inside the mitochondrial matrix. The major protein complexes of the ETC, in their functional order, are Complex I, Complex II, Coenzyme Q, Complex III, Cytochrome C, and Complex IV.
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Complex II, also known as succinate dehydrogenase, serves as a secondary entry point for electrons into the ETC, accepting them from succinate, an intermediate of the citric acid cycle. As succinate is oxidized to fumarate, two electrons are transferred to FAD within Complex II. FAD then passes these electrons to iron-sulfur (Fe-S) clusters and subsequently to coenzyme Q, a process similar to that in Complex I. It’s important to note that unlike Complex I, Complex II does not directly pump protons across the mitochondrial membrane. Consequently, the pathway involving Complex II results in the production of less ATP.
Succinate + FAD → Fumarate + 2 H+(matrix) + FADH2 FADH2 + CoQ → FAD + CoQH2 -
Coenzyme Q, also called ubiquinone (CoQ), is a mobile electron carrier composed of a quinone group and a hydrophobic tail. Its primary role is to transport electrons from Complexes I and II to Complex III. Coenzyme Q undergoes reduction in a two-step process via the Q cycle, transitioning through a semiquinone intermediate (partially reduced, radical form CoQH-) to ubiquinol (fully reduced CoQH2). The intricacies of the Q cycle are further explained in the Complex III section.
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ATP synthase, also known as Complex V, is the molecular machine that harnesses the proton gradient generated by the ETC to synthesize ATP. ATP synthase is composed of two main subunits: F0 and F1, acting as a rotary motor system. The hydrophobic F0 subunit is embedded in the inner mitochondrial membrane and contains a channel for protons. As protons flow down the electrochemical gradient from the intermembrane space back into the matrix, they pass through this channel, causing F0 to rotate. This rotation mechanically alters the conformation of the hydrophilic F1 subunit, which protrudes into the mitochondrial matrix. These conformational changes in F1 catalyze the synthesis of ATP from ADP and inorganic phosphate (Pi). It is estimated that approximately 4 protons are required to flow through ATP synthase for the production of one ATP molecule. Interestingly, ATP synthase can also operate in reverse under certain conditions, consuming ATP to pump protons and establish a hydrogen gradient, a function observed in some bacteria.
Electron Transport Chain at the Molecular Level
Nicotinamide adenine dinucleotide exists in two forms: NAD+ (oxidized) and NADH (reduced). It’s a dinucleotide, composed of two nucleotides linked by phosphate groups. One nucleotide contains an adenine base, and the other contains nicotinamide. In metabolic redox reactions, NAD+ acts as an electron acceptor, becoming reduced to NADH, as shown in Reaction 1.
- Reaction 1: RH2 + NAD+ → R + H+ + NADH
Here, RH2 represents a reactant molecule, such as a sugar, being oxidized.
NADH delivers its electrons to Complex I of the ETC. As electrons move through the chain from Complex I to Complex IV, a total of 10 protons are pumped across the inner mitochondrial membrane (4 by Complex I, 4 by Complex III, and 2 by Complex IV). Given that ATP synthase synthesizes approximately 1 ATP for every 4 protons that pass through it, the oxidation of one NADH molecule is estimated to yield around 2.5 ATP molecules (some sources round this value up or down slightly). When NADH is oxidized in Complex I, it releases a proton, NAD+, and two electrons, as shown in Reaction 2.
- Reaction 2: NADH → H+ + NAD+ + 2 e-
Flavin adenine dinucleotide (FAD) is another key electron carrier in the ETC, existing in four redox states, primarily: FAD (quinone, fully oxidized), FADH- (semiquinone, partially reduced), and FADH2 (hydroquinone, fully reduced). FAD is also a dinucleotide, composed of an adenine nucleotide and flavin mononucleotide (FMN) linked by phosphate groups. FMN is derived in part from vitamin B2 (riboflavin). FAD features a stable aromatic ring, while FADH2 lacks this aromaticity. The oxidation of FADH2 to FAD (Reaction 3) results in the formation of an aromatic system and the release of energy. This characteristic makes FAD a potent oxidizing agent, with a reduction potential even more positive than NAD+. FADH2 enters the ETC at Complex II, bypassing Complex I. The electron transfer from FADH2 through the rest of the ETC (Complexes II, III, and IV) results in the pumping of a total of 6 protons (4 by Complex III and 2 by Complex IV). Therefore, the oxidation of one FADH2 molecule is estimated to produce around 1.5 ATP molecules (some sources round this value).
- Reaction 3: FADH2 → FAD + 2 H+ + 2 e-
Beyond its role in the ETC, FAD also participates in various metabolic pathways, including DNA repair (MTHF repair of UV damage), fatty acid beta-oxidation (acyl-CoA dehydrogenase), and the synthesis of coenzymes (CoA, CoQ, heme).
Clinical Significance of Electron Transport Chain Dysfunction
Uncoupling Agents
Uncoupling agents are substances that disrupt the tight coupling between the electron transport chain and ATP synthase. They effectively dissociate the ETC from phosphorylation, preventing or reducing ATP production. Many uncoupling agents work by disrupting the phospholipid bilayer of mitochondrial membranes, increasing their permeability to protons. This proton leak diminishes the electrochemical gradient across the inner mitochondrial membrane. As protons flow back into the mitochondrial matrix through these leaks, rather than exclusively through ATP synthase, the energy of the proton gradient is dissipated as heat instead of being used to synthesize ATP.
This uncoupling leads to a cellular state of ATP starvation. In response, the ETC becomes hyperactive, attempting to restore the proton gradient and drive ATP synthesis. However, because of the uncoupling effect, this increased ETC activity is futile in terms of ATP production. A natural consequence of electron transfer in the ETC is the generation of heat. The overactivity of the ETC in the presence of uncoupling agents results in excessive heat production, potentially leading to a rise in body temperature (hyperthermia). Furthermore, the cell, sensing an energy deficit, may shift towards anaerobic metabolism, such as fermentation, even in the presence of oxygen. This metabolic shift can lead to the accumulation of lactic acid, potentially causing type B lactic acidosis in affected individuals.
Aspirin (Salicylic Acid)
Thermogenin
Oxidative Phosphorylation Inhibitors
Specific toxins and drugs can act as inhibitors of oxidative phosphorylation by targeting different components of the electron transport chain. These inhibitors block electron flow through the ETC, thereby halting ATP production. Examples of such inhibitors include rotenone, carboxin, antimycin A, cyanide, carbon monoxide (CO), sodium azide, and oligomycin. Rotenone blocks Complex I, carboxin inhibits Complex II, antimycin A targets Complex III, and cyanide and CO inhibit Complex IV. Oligomycin specifically inhibits ATP synthase (Complex V).
Rotenone (and some barbiturates) – inhibits Complex I (coenzyme Q binding site)
Carboxin – inhibits Complex II (coenzyme Q binding site)
- Carboxin, previously used as a fungicide, is no longer widely used due to the availability of more effective broad-spectrum agents. Similar to rotenone, carboxin interferes with the binding of ubiquinone at Complex II.
Doxorubicin – coenzyme Q (theoretical)
Antimycin A – inhibits Complex III (cytochrome c reductase)
- Antimycin A, used as a piscicide, binds to Complex III (cytochrome c reductase) at the Qi binding site. This binding prevents ubiquinone from interacting with Complex III and accepting electrons, effectively blocking the Q cycle and thus electron flow through Complex III.
Carbon Monoxide (CO) – inhibits Complex IV (cytochrome c oxidase)
Cyanide (CN) – inhibits Complex IV (cytochrome c oxidase)
- Cyanide also binds to and inhibits cytochrome c oxidase (Complex IV), the terminal enzyme of the ETC. Cyanide poisoning and carbon monoxide poisoning can share similar symptoms related to tissue hypoxia (oxygen deprivation). However, in cyanide poisoning, the hypoxia is often unresponsive to supplemental oxygen administration, and a characteristic almond-like odor on the breath may be present. Common sources of cyanide exposure include smoke inhalation from house fires (due to burning furniture and rugs), jewelry cleaning solutions, plastic and rubber manufacturing, iatrogenic exposure from the drug nitroprusside, and even certain fruit seeds (such as apricot, peach, and apple seeds).
- Treatment for cyanide poisoning may involve the use of nitrites to oxidize hemoglobin iron from Fe2+ to Fe3+, forming methemoglobin. Methemoglobin has a high affinity for cyanide, binding it and preventing it from inhibiting the ETC. However, methemoglobin reduces the oxygen-carrying capacity of the blood, necessitating further treatment with methylene blue to reduce Fe3+ back to Fe2+ in hemoglobin. Another treatment option is hydroxocobalamin, a form of vitamin B12 that can bind cyanide. Thiosulfate can also be used to detoxify cyanide, although it is slower-acting and typically used in combination with nitrites.
Oligomycin – inhibits ATP-synthase (Complex V)
Review Questions
Figure
Electron Transport Chain Diagram. This illustration depicts the electron transport chain, highlighting the inter-membrane space, inner membrane, and matrix of the mitochondria. Illustration by Emma Gregory
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Disclosure: Maria Ahmad declares no relevant financial relationships with ineligible companies.
Disclosure: Adam Wolberg declares no relevant financial relationships with ineligible companies.
Disclosure: Chadi Kahwaji declares no relevant financial relationships with ineligible companies.
