Cellular respiration, the fundamental process that fuels life, hinges on a remarkable sequence of protein complexes known as the electron transport chain (ETC). This intricate system, located within the mitochondria of cells, orchestrates a series of redox reactions, ultimately leading to the generation of the majority of cellular energy in the form of ATP. While prominently featured in cellular respiration, an analogous electron transport chain also plays a vital role in photosynthesis within chloroplasts. In cellular respiration, the ETC harnesses energy from the breakdown of organic molecules, whereas in photosynthesis, it captures light energy to drive carbohydrate synthesis.
Fundamentals of Cellular Respiration and the Electron Transport Chain
Aerobic cellular respiration unfolds in three interconnected stages: glycolysis, the citric acid cycle (also known as the Krebs cycle), and oxidative phosphorylation. Glycolysis, the initial step, occurs in the cytoplasm and breaks down glucose into two pyruvate molecules, yielding a small amount of ATP and NADH. Pyruvate then transitions into the mitochondria, where it undergoes oxidation to acetyl-CoA, producing additional NADH and carbon dioxide (CO2). Acetyl-CoA enters the citric acid cycle, a cyclical series of reactions within the mitochondrial matrix. This cycle further oxidizes carbon fuels, releasing CO2, NADH, FADH2, and a modest amount of ATP.
The culmination of these preparatory stages is oxidative phosphorylation, which accounts for the vast majority of ATP generated during cellular respiration. Oxidative phosphorylation comprises two tightly coupled components: the electron transport chain (ETC) and chemiosmosis. The ETC, embedded within the inner mitochondrial membrane, is a series of protein complexes and organic molecules that facilitate electron transfer through redox reactions, releasing energy in a controlled manner. This released energy is not directly used to make ATP; instead, it is harnessed to establish a proton gradient across the inner mitochondrial membrane. Chemiosmosis then utilizes the potential energy stored in this proton gradient to drive ATP synthesis by the enzyme ATP synthase.
Photosynthesis, the process by which plants and other organisms convert light energy into chemical energy, also employs an electron transport chain in its light-dependent reactions. Here, light energy excites electrons, initiating their journey through the photosynthetic ETC. Similar to cellular respiration, the energy released during electron transport in photosynthesis is used to create a proton gradient, which then powers ATP synthesis. The ATP and NADPH generated in the light-dependent reactions are subsequently used in the light-independent reactions (Calvin cycle) to fix carbon dioxide and produce sugars.
The Electron Transport Chain at the Cellular Level: A Step-by-Step Journey
Within the inner mitochondrial membrane, the electron transport chain is organized into four major protein complexes (Complex I to IV) and mobile electron carriers. As electrons move through these complexes, they progress to states of increasingly higher reduction potential, meaning they become progressively more stable and less likely to donate electrons. This downhill flow of electrons is coupled to the pumping of protons (H+) from the mitochondrial matrix to the intermembrane space, creating an electrochemical gradient. This gradient manifests as a higher concentration of protons and a more positive charge in the intermembrane space compared to the matrix. The complexes of the ETC are arranged in a specific sequence: Complex I, Complex II, Coenzyme Q, Complex III, Cytochrome C, and Complex IV.
Complex I: NADH-CoQ Reductase
Complex I, also known as NADH dehydrogenase or NADH-CoQ reductase, serves as the entry point for electrons derived from NADH, a key electron carrier generated in glycolysis and the citric acid cycle. NADH donates two electrons to Complex I, which then transfers them to coenzyme Q (ubiquinone). This electron transfer is coupled to the translocation of four protons from the matrix to the intermembrane space, contributing to the proton gradient.
Complex II: Succinate-CoQ Reductase
Complex II, also called succinate dehydrogenase, provides an alternative entry point for electrons into the ETC. It accepts electrons from succinate, a citric acid cycle intermediate, oxidizing it to fumarate. In this process, FADH2, another electron carrier, is generated within Complex II. FADH2 then donates its electrons to iron-sulfur (Fe-S) clusters within the complex, which subsequently transfer them to coenzyme Q. Notably, unlike Complex I, Complex II does not pump protons across the membrane. This difference explains why FADH2 yields less ATP than NADH during oxidative phosphorylation.
Succinate + FAD -> Fumarate + FADH2
FADH2 + CoQ -> FAD + CoQH2Coenzyme Q (Ubiquinone)
Coenzyme Q (CoQ), also known as ubiquinone, is a small, mobile, lipid-soluble molecule residing within the inner mitochondrial membrane. Its primary function is to act as an electron carrier, shuttling electrons from both Complex I and Complex II to Complex III. CoQ undergoes a cyclical reduction and oxidation process known as the Q cycle. It can exist in three redox states: ubiquinone (oxidized), semiquinone (partially reduced radical form), and ubiquinol (fully reduced).
Complex III: CoQ-Cytochrome c Reductase
Complex III, or CoQ-cytochrome c reductase (also known as cytochrome bc1 complex), accepts electrons from ubiquinol (CoQH2). Through the Q cycle within Complex III, electrons are passed to cytochrome c, another mobile electron carrier, and four protons are pumped across the inner mitochondrial membrane. The Q cycle is a complex process that ensures efficient electron transfer and proton pumping.
Cytochrome c
Cytochrome c is a small, soluble protein located in the intermembrane space. It acts as a mobile electron carrier, transporting electrons from Complex III to Complex IV. Cytochrome c can only carry one electron at a time.
Complex IV: Cytochrome c Oxidase
Complex IV, cytochrome c oxidase, is the final protein complex in the electron transport chain. It accepts electrons from cytochrome c and catalyzes the reduction of molecular oxygen (O2) to water (H2O). This crucial step is the terminal electron acceptor in aerobic respiration. For every four electrons passed through Complex IV, four protons are pumped across the membrane, and one molecule of oxygen is reduced to two molecules of water.
ATP Synthase: Complex V and Chemiosmosis
The proton gradient established by Complexes I, III, and IV represents a form of potential energy. ATP synthase, also known as Complex V, harnesses this energy to synthesize ATP. ATP synthase is a remarkable molecular machine composed of two main subunits: F0 and F1. The F0 subunit is embedded in the inner mitochondrial membrane and forms a channel through which protons can flow down their electrochemical gradient, from the intermembrane space back to the matrix. This proton flow drives the rotation of the F0 subunit, which in turn causes conformational changes in the F1 subunit. The F1 subunit, located in the mitochondrial matrix, contains the catalytic sites for ATP synthesis. For every approximately four protons that pass through ATP synthase, one molecule of ATP is generated from ADP and inorganic phosphate (Pi). This process of ATP synthesis driven by the proton gradient is known as chemiosmosis.
Figure: Diagram illustrating the electron transport chain in cellular respiration, showing the complexes within the inner mitochondrial membrane, intermembrane space, and mitochondrial matrix.
Molecular Players: NADH and FADH2 in Redox Reactions
NADH (nicotinamide adenine dinucleotide) and FADH2 (flavin adenine dinucleotide) are crucial electron carriers in cellular respiration. They are derived from B vitamins and play essential roles in redox reactions, carrying electrons to the electron transport chain.
Nicotinamide Adenine Dinucleotide (NADH)
NADH exists in two forms: NAD+ (oxidized) and NADH (reduced). It is a dinucleotide composed of two nucleotides linked by phosphate groups, one containing adenine and the other nicotinamide. In metabolic redox reactions, NADH acts as an electron donor, as shown in Reaction 1:
Reaction 1: RH2 + NAD+ -> R + H+ + NADHWhere RH2 represents a reduced substrate (e.g., a sugar molecule). NADH enters the electron transport chain at Complex I, donating its electrons and ultimately contributing to the pumping of approximately 10 protons across the membrane (4 from Complex I, 4 from Complex III, and 2 from Complex IV). Given that approximately 4 protons are required to synthesize 1 ATP by ATP synthase, NADH theoretically yields around 2.5 ATP molecules (some sources round this to 3 ATP). When NADH is oxidized within Complex I, it is converted back to NAD+, releasing a proton and two electrons (Reaction 2):
Reaction 2: NADH -> H+ + NAD+ + 2 e-Flavin Adenine Dinucleotide (FADH2)
FADH2, like NADH, is a crucial electron carrier, but it enters the ETC at Complex II. FADH2 is derived from flavin adenine dinucleotide (FAD), which can exist in multiple redox states: FAD (quinone, fully oxidized), FADH- (semiquinone, partially reduced), and FADH2 (hydroquinone, fully reduced). FAD consists of an adenine nucleotide and flavin mononucleotide (FMN) linked by phosphate groups, with FMN derived from vitamin B2 (riboflavin). FAD’s aromatic ring structure provides stability, while FADH2 lacks this aromaticity. Oxidation of FADH2 to FAD (Reaction 3) releases energy, making FAD a potent oxidizing agent with a higher reduction potential than NAD+.
Reaction 3: FADH2 -> FAD + 2 H+ + 2 e-FADH2 enters the ETC at Complex II and leads to the pumping of approximately 6 protons (4 from Complex III and 2 from Complex IV). Consequently, FADH2 yields approximately 1.5 ATP molecules (some sources round this to 2 ATP). Beyond its role in the ETC, FAD participates in various metabolic pathways, including fatty acid beta-oxidation, DNA repair, and coenzyme synthesis.
Clinical Significance: Disruptions and Interventions in the Electron Transport Chain
The electron transport chain’s critical role in energy production makes it a target for various pharmacological and toxicological agents. Disruptions to the ETC can have severe clinical consequences.
Uncoupling Agents: Disrupting the Proton Gradient
Uncoupling agents are substances that dissociate the electron transport chain from ATP synthesis by ATP synthase. They disrupt the tight coupling between electron transport and phosphorylation, preventing efficient ATP production. These agents often compromise the phospholipid bilayer of the inner mitochondrial membrane, increasing its permeability to protons. This proton leak diminishes the electrochemical gradient, as protons can re-enter the matrix without passing through ATP synthase. As a result, the energy released by electron transport is dissipated as heat rather than being used to synthesize ATP.
Cells treated with uncoupling agents become ATP-depleted. To compensate, the ETC becomes hyperactive, attempting to restore the proton gradient and drive ATP synthesis. However, this futile cycle leads to increased electron transport and heat generation, potentially causing hyperthermia. Furthermore, the cellular energy deficit can shift metabolism towards anaerobic pathways like fermentation, potentially leading to lactic acidosis.
Examples of Uncoupling Agents:
- Aspirin (Salicylic Acid): In high doses, aspirin can act as an uncoupling agent.
- Thermogenin: This protein, naturally present in brown adipose tissue, is a physiological uncoupling agent, facilitating heat generation for non-shivering thermogenesis.
Oxidative Phosphorylation Inhibitors: Blocking Electron Flow
Oxidative phosphorylation inhibitors are toxins and drugs that directly block specific components of the electron transport chain or ATP synthase, halting ATP production.
Examples of Inhibitors:
- Rotenone: Inhibits Complex I by blocking the coenzyme Q binding site.
- Carboxin: Similar to rotenone, carboxin inhibits Complex II at the coenzyme Q binding site. Carboxin, a fungicide, is less commonly used now.
- Antimycin A: Inhibits Complex III by binding to the Qi binding site of cytochrome c reductase, preventing ubiquinone binding and disrupting the Q cycle. Antimycin A is used as a piscicide.
- Cyanide (CN-) and Carbon Monoxide (CO): Potent inhibitors of Complex IV (cytochrome c oxidase). They bind to the heme iron in cytochrome oxidase, blocking oxygen reduction. Cyanide poisoning can result from house fires, industrial exposures, and certain fruit seeds. Carbon monoxide poisoning is common in smoke inhalation and from car exhaust. Cyanide poisoning can be treated with nitrites to induce methemoglobinemia, which binds cyanide, and subsequently with methylene blue to regenerate hemoglobin. Hydroxocobalamin and thiosulfate are alternative treatments.
- Oligomycin: Inhibits ATP synthase (Complex V) by blocking the proton channel (F0 subunit), directly preventing ATP synthesis.
Understanding the electron transport chain and its vulnerabilities is crucial for comprehending cellular energy metabolism, disease pathogenesis, and pharmacological interventions. By meticulously orchestrating electron flow and proton pumping, this remarkable system powers life at the cellular level.
References
1.Lencina AM, Franza T, Sullivan MJ, Ulett GC, Ipe DS, Gaudu P, Gennis RB, Schurig-Briccio LA. Type 2 NADH Dehydrogenase Is the Only Point of Entry for Electrons into the Streptococcus agalactiae Respiratory Chain and Is a Potential Drug Target. mBio. 2018 Jul 03;9(4) [PMC free article: PMC6030563] [PubMed: 29970468]
2.Hirst J. Towards the molecular mechanism of respiratory complex I. Biochem J. 2009 Dec 23;425(2):327-39. [PubMed: 20025615]
3.Sazanov LA, Hinchliffe P. Structure of the hydrophilic domain of respiratory complex I from Thermus thermophilus. Science. 2006 Mar 10;311(5766):1430-6. [PubMed: 16469879]
4.Hirst J. Energy transduction by respiratory complex I–an evaluation of current knowledge. Biochem Soc Trans. 2005 Jun;33(Pt 3):525-9. [PubMed: 15916556]
5.Yankovskaya V, Horsefield R, Törnroth S, Luna-Chavez C, Miyoshi H, Léger C, Byrne B, Cecchini G, Iwata S. Architecture of succinate dehydrogenase and reactive oxygen species generation. Science. 2003 Jan 31;299(5607):700-4. [PubMed: 12560550]
6.Horsefield R, Iwata S, Byrne B. Complex II from a structural perspective. Curr Protein Pept Sci. 2004 Apr;5(2):107-18. [PubMed: 15078221]
7.Geertman JM, van Maris AJ, van Dijken JP, Pronk JT. Physiological and genetic engineering of cytosolic redox metabolism in Saccharomyces cerevisiae for improved glycerol production. Metab Eng. 2006 Nov;8(6):532-42. [PubMed: 16891140]
8.Thorpe C, Kim JJ. Structure and mechanism of action of the acyl-CoA dehydrogenases. FASEB J. 1995 Jun;9(9):718-25. [PubMed: 7601336]
9.Sun C, Benlekbir S, Venkatakrishnan P, Wang Y, Hong S, Hosler J, Tajkhorshid E, Rubinstein JL, Gennis RB. Structure of the alternative complex III in a supercomplex with cytochrome oxidase. Nature. 2018 May;557(7703):123-126. [PMC free article: PMC6004266] [PubMed: 29695868]
10.Iwata S, Lee JW, Okada K, Lee JK, Iwata M, Rasmussen B, Link TA, Ramaswamy S, Jap BK. Complete structure of the 11-subunit bovine mitochondrial cytochrome bc1 complex. Science. 1998 Jul 03;281(5373):64-71. [PubMed: 9651245]
11.Trumpower BL. The protonmotive Q cycle. Energy transduction by coupling of proton translocation to electron transfer by the cytochrome bc1 complex. J Biol Chem. 1990 Jul 15;265(20):11409-12. [PubMed: 2164001]
12.Hunte C, Palsdottir H, Trumpower BL. Protonmotive pathways and mechanisms in the cytochrome bc1 complex. FEBS Lett. 2003 Jun 12;545(1):39-46. [PubMed: 12788490]
13.Calhoun MW, Thomas JW, Gennis RB. The cytochrome oxidase superfamily of redox-driven proton pumps. Trends Biochem Sci. 1994 Aug;19(8):325-30. [PubMed: 7940677]
14.Schmidt-Rohr K. Oxygen Is the High-Energy Molecule Powering Complex Multicellular Life: Fundamental Corrections to Traditional Bioenergetics. ACS Omega. 2020 Feb 11;5(5):2221-2233. [PMC free article: PMC7016920] [PubMed: 32064383]
15.Lovero D, Giordano L, Marsano RM, Sanchez-Martinez A, Boukhatmi H, Drechsler M, Oliva M, Whitworth AJ, Porcelli D, Caggese C. Characterization of Drosophila ATPsynC mutants as a new model of mitochondrial ATP synthase disorders. PLoS One. 2018;13(8):e0201811. [PMC free article: PMC6086398] [PubMed: 30096161]
16.Okuno D, Iino R, Noji H. Rotation and structure of FoF1-ATP synthase. J Biochem. 2011 Jun;149(6):655-64. [PubMed: 21524994]
17.Junge W, Nelson N. ATP synthase. Annu Rev Biochem. 2015;84:631-57. [PubMed: 25839341]
18.Hinkle PC. P/O ratios of mitochondrial oxidative phosphorylation. Biochim Biophys Acta. 2005 Jan 07;1706(1-2):1-11. [PubMed: 15620362]
19.Barrett MA, Zheng S, Roshankar G, Alsop RJ, Belanger RK, Huynh C, Kučerka N, Rheinstädter MC. Interaction of aspirin (acetylsalicylic acid) with lipid membranes. PLoS One. 2012;7(4):e34357. [PMC free article: PMC3328472] [PubMed: 22529913]
20.Warrick BJ, King A, Smolinske S, Thomas R, Aaron C. A 29-year analysis of acute peak salicylate concentrations in fatalities reported to United States poison centers. Clin Toxicol (Phila). 2018 Sep;56(9):846-851. [PubMed: 29431532]
21.Cinti S. The adipose organ. Prostaglandins Leukot Essent Fatty Acids. 2005 Jul;73(1):9-15. [PubMed: 15936182]
22.Enerbäck S. The origins of brown adipose tissue. N Engl J Med. 2009 May 07;360(19):2021-3. [PubMed: 19420373]
23.Zhou W, Faraldo-Gómez JD. Membrane plasticity facilitates recognition of the inhibitor oligomycin by the mitochondrial ATP synthase rotor. Biochim Biophys Acta Bioenerg. 2018 Sep;1859(9):789-796. [PMC free article: PMC6176861] [PubMed: 29630891]
24.Kamalian L, Douglas O, Jolly CE, Snoeys J, Simic D, Monshouwer M, Williams DP, Kevin Park B, Chadwick AE. The utility of HepaRG cells for bioenergetic investigation and detection of drug-induced mitochondrial toxicity. Toxicol In Vitro. 2018 Dec;53:136-147. [PubMed: 30096366]
25.Wood DM, Alsahaf H, Streete P, Dargan PI, Jones AL. Fatality after deliberate ingestion of the pesticide rotenone: a case report. Crit Care. 2005 Jun;9(3):R280-4. [PMC free article: PMC1175899] [PubMed: 15987402]
26.Lupescu A, Jilani K, Zbidah M, Lang F. Induction of apoptotic erythrocyte death by rotenone. Toxicology. 2012 Oct 28;300(3):132-7. [PubMed: 22727881]
27.Wallace KB. Doxorubicin-induced cardiac mitochondrionopathy. Pharmacol Toxicol. 2003 Sep;93(3):105-15. [PubMed: 12969434]
28.Weaver LK. Clinical practice. Carbon monoxide poisoning. N Engl J Med. 2009 Mar 19;360(12):1217-25. [PubMed: 19297574]
29.Sato K, Tamaki K, Hattori H, Moore CM, Tsutsumi H, Okajima H, Katsumata Y. Determination of total hemoglobin in forensic blood samples with special reference to carboxyhemoglobin analysis. Forensic Sci Int. 1990 Nov;48(1):89-96. [PubMed: 2279722]
30.Barker SJ, Tremper KK. The effect of carbon monoxide inhalation on pulse oximetry and transcutaneous PO2. Anesthesiology. 1987 May;66(5):677-9. [PubMed: 3578881]
31.Raub JA, Mathieu-Nolf M, Hampson NB, Thom SR. Carbon monoxide poisoning–a public health perspective. Toxicology. 2000 Apr 07;145(1):1-14. [PubMed: 10771127]
32.Jensen P, Wilson MT, Aasa R, Malmström BG. Cyanide inhibition of cytochrome c oxidase. A rapid-freeze e.p.r. investigation. Biochem J. 1984 Dec 15;224(3):829-37. [PMC free article: PMC1144519] [PubMed: 6098268]
33.Shchepina LA, Pletjushkina OY, Avetisyan AV, Bakeeva LE, Fetisova EK, Izyumov DS, Saprunova VB, Vyssokikh MY, Chernyak BV, Skulachev VP. Oligomycin, inhibitor of the F0 part of H+-ATP-synthase, suppresses the TNF-induced apoptosis. Oncogene. 2002 Nov 21;21(53):8149-57. [PubMed: 12444550]
