🔬biological chemistry i review
8.2 Electron transport chain and oxidative phosphorylation
Last Updated on August 7, 2024
The electron transport chain and oxidative phosphorylation are the final stages of cellular respiration. These processes harness energy from electrons to create a proton gradient, ultimately producing ATP, the cell's energy currency.
In this section, we'll explore the complexes involved in electron transport, key electron carriers, and how the proton gradient drives ATP synthesis. Understanding these processes is crucial for grasping how cells efficiently generate energy from nutrients.
Electron Transport Chain Complexes
Complex Structure and Function
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- Complex I (NADH dehydrogenase) oxidizes NADH, transferring electrons to ubiquinone and pumping protons across the inner mitochondrial membrane
- Complex II (Succinate dehydrogenase) oxidizes succinate to fumarate, reducing ubiquinone and does not transport protons
- Complex III (Cytochrome bc1 complex) transfers electrons from ubiquinol to cytochrome c, pumping protons across the inner mitochondrial membrane (Q cycle)
- Complex IV (Cytochrome c oxidase) transfers electrons from cytochrome c to oxygen, the final electron acceptor, pumping protons across the inner mitochondrial membrane
Electron Flow and Energy Release
- Electrons flow through the complexes in order of increasing reduction potential, releasing energy at each step
- Energy released from electron transfer is used to pump protons across the inner mitochondrial membrane, generating a proton gradient
- Electron transport chain is the major site of ATP production in cellular respiration (oxidative phosphorylation)
- Inhibitors of electron transport chain complexes (rotenone, antimycin A, cyanide) can disrupt ATP production and lead to cell death
Electron Carriers
Coenzyme Q (Ubiquinone)
- Lipid-soluble electron carrier that shuttles electrons between Complex I, II, and III
- Exists in oxidized form (ubiquinone) and reduced form (ubiquinol)
- Accepts electrons from NADH (via Complex I) and FADH2 (via Complex II), becoming reduced to ubiquinol
- Donates electrons to Complex III, becoming oxidized back to ubiquinone
Cytochrome c
- Water-soluble electron carrier that shuttles electrons from Complex III to Complex IV
- Heme-containing protein that alternates between reduced (ferrous, Fe2+) and oxidized (ferric, Fe3+) states
- Accepts electrons from Complex III (cytochrome c1) and donates them to Complex IV
- Cytochrome c release from mitochondria can trigger apoptosis (programmed cell death)
Proton Gradient and Membrane
Proton Gradient Formation and Function
- Proton gradient is formed by the pumping of protons (H+) from the mitochondrial matrix to the intermembrane space
- Complexes I, III, and IV contribute to the proton gradient by coupling electron transfer to proton pumping
- Proton gradient is used to drive ATP synthesis through ATP synthase (chemiosmotic coupling)
- Proton gradient also powers other processes (mitochondrial protein import, metabolite transport)
Redox Reactions and Electron Transport
- Redox reactions involve the transfer of electrons between molecules, with one molecule being oxidized (losing electrons) and the other reduced (gaining electrons)
- Electron transport chain involves a series of redox reactions, with electrons being transferred from NADH and FADH2 to oxygen
- Redox potential difference between electron donors (NADH, FADH2) and acceptors (ubiquinone, cytochrome c, oxygen) drives electron flow
- Electron transport is coupled to proton pumping, converting redox energy into a proton gradient
Inner Mitochondrial Membrane Structure and Function
- Inner mitochondrial membrane is highly folded, forming cristae that increase surface area for electron transport and ATP synthesis
- Electron transport chain complexes and ATP synthase are embedded in the inner mitochondrial membrane
- Inner mitochondrial membrane is impermeable to protons, allowing the formation and maintenance of the proton gradient
- Cardiolipin, a unique phospholipid found in the inner mitochondrial membrane, is essential for the function of electron transport chain complexes and ATP synthase
Key Terms to Review (20)
Mitochondrial disease: Mitochondrial disease refers to a group of disorders caused by dysfunctional mitochondria, the energy-producing structures in cells. These diseases can affect multiple systems in the body because mitochondria are crucial for producing ATP through processes like the electron transport chain and oxidative phosphorylation. As a result, patients often experience symptoms related to energy deficiency in various organs, highlighting the importance of mitochondrial function in overall health.
FCCP: FCCP, or carbonyl cyanide-p-trifluoromethoxyphenylhydrazone, is a chemical compound used as a protonophore that uncouples oxidative phosphorylation from the electron transport chain. It functions by facilitating the transport of protons across the mitochondrial membrane, disrupting the proton gradient required for ATP synthesis. By doing so, FCCP allows for electron transport to continue while preventing ATP production, thereby impacting cellular respiration and energy metabolism.
Cyanide: Cyanide is a highly toxic compound that can exist in various forms, such as hydrogen cyanide (HCN) or cyanide salts like sodium cyanide. It is known for its ability to inhibit cellular respiration by blocking the electron transport chain, specifically targeting cytochrome c oxidase, which is essential for oxidative phosphorylation. This inhibition leads to a decrease in ATP production, causing severe metabolic disturbances and potentially resulting in cellular death.
Electron carriers: Electron carriers are molecules that transport electrons during cellular respiration and photosynthesis, playing a critical role in energy production. These carriers facilitate the transfer of electrons from one molecule to another, allowing energy to be released in a controlled manner. They are essential for generating a proton gradient that drives ATP synthesis through oxidative phosphorylation.
Complex IV: Complex IV, also known as cytochrome c oxidase, is the fourth protein complex in the electron transport chain, crucial for cellular respiration. It plays a key role in the final steps of transferring electrons from cytochrome c to molecular oxygen, facilitating the reduction of oxygen to water. This process is essential for maintaining the proton gradient across the inner mitochondrial membrane, which ultimately drives ATP synthesis during oxidative phosphorylation.
Complex II: Complex II, also known as succinate dehydrogenase, is an essential enzyme in the electron transport chain that plays a crucial role in cellular respiration. It functions as a bridge between the Krebs cycle and the electron transport chain by catalyzing the oxidation of succinate to fumarate while reducing ubiquinone to ubiquinol. This process is vital for ATP production through oxidative phosphorylation.
Complex III: Complex III, also known as cytochrome bc1 complex, is a crucial component of the electron transport chain located in the inner mitochondrial membrane. It plays a key role in transferring electrons from ubiquinol (QH2) to cytochrome c, while simultaneously contributing to the proton gradient that drives ATP synthesis during oxidative phosphorylation. This complex is vital for cellular respiration and energy production.
Chemiosmosis: Chemiosmosis is the process by which ATP is produced in cells through the movement of ions across a selectively permeable membrane, driven by an electrochemical gradient. This process is essential in cellular respiration and photosynthesis, linking the electron transport chain to ATP synthesis, showcasing how energy stored in ion gradients is harnessed to produce usable energy in the form of ATP.
Complex I: Complex I, also known as NADH:ubiquinone oxidoreductase, is the first enzyme in the electron transport chain that plays a crucial role in cellular respiration. It catalyzes the transfer of electrons from NADH to ubiquinone (coenzyme Q), coupled with the translocation of protons across the inner mitochondrial membrane, contributing to the proton gradient essential for ATP synthesis during oxidative phosphorylation.
NADH Dehydrogenase: NADH dehydrogenase is an enzyme that plays a critical role in the electron transport chain by facilitating the transfer of electrons from NADH to ubiquinone (coenzyme Q). This enzyme is essential for the process of oxidative phosphorylation, as it helps to establish the proton gradient across the mitochondrial membrane that drives ATP synthesis. The activity of NADH dehydrogenase not only contributes to energy production but also links metabolic pathways, making it a pivotal component of cellular respiration.
Ubiquinone: Ubiquinone, also known as coenzyme Q10, is a lipid-soluble molecule that plays a crucial role in the electron transport chain, functioning as an electron carrier. It facilitates the transfer of electrons from various dehydrogenases to the cytochrome b-c1 complex, effectively linking different components of cellular respiration and contributing to the generation of ATP through oxidative phosphorylation.
Proton gradient: A proton gradient refers to the difference in proton (H+) concentration across a membrane, creating a potential energy difference that can be used to drive cellular processes. This gradient is essential for the production of ATP during cellular respiration, as it plays a crucial role in generating the energy needed for ATP synthesis through oxidative phosphorylation.
ATP Synthesis: ATP synthesis is the process through which adenosine triphosphate (ATP), the primary energy currency of the cell, is produced. This process primarily occurs in the mitochondria via the electron transport chain and oxidative phosphorylation, where energy derived from electrons transported through membrane proteins is harnessed to convert adenosine diphosphate (ADP) and inorganic phosphate (Pi) into ATP.
Cytochrome c: Cytochrome c is a small heme protein found in the mitochondria of eukaryotic cells, playing a crucial role in the electron transport chain. It serves as an electron carrier, transferring electrons between complex III (cytochrome bc1 complex) and complex IV (cytochrome c oxidase) during cellular respiration, thus facilitating oxidative phosphorylation. This protein is vital for energy production and also participates in apoptosis when released from mitochondria.
Cellular Respiration: Cellular respiration is the biochemical process by which cells convert nutrients, particularly glucose, into energy in the form of adenosine triphosphate (ATP), while releasing waste products such as carbon dioxide and water. This process is essential for maintaining cellular functions and overall homeostasis, as it provides the energy required for various cellular activities and plays a crucial role in regulating metabolic pathways.
Redox reactions: Redox reactions, or reduction-oxidation reactions, are chemical processes where the oxidation state of one or more substances changes due to the transfer of electrons. In these reactions, one substance loses electrons (oxidation) while another gains electrons (reduction), making them crucial for energy transformation in biological systems. They play a fundamental role in metabolic pathways and the conversion of energy, particularly in processes like cellular respiration and photosynthesis.
ADP: Adenosine diphosphate (ADP) is a nucleotide that plays a crucial role in cellular energy transfer. It consists of an adenine base, a ribose sugar, and two phosphate groups. ADP is formed when adenosine triphosphate (ATP) loses one of its phosphate groups, releasing energy that can be used for various metabolic processes. This transformation highlights ADP's key role in energy metabolism and its connection to the mechanisms of oxidative phosphorylation and the electron transport chain.
Oxidative phosphorylation: Oxidative phosphorylation is a metabolic process that produces ATP through the transfer of electrons from NADH and FADH2 to oxygen via the electron transport chain, coupled with the phosphorylation of ADP to ATP. This process is vital for cellular energy production, linking it to other metabolic pathways such as the citric acid cycle and contributing to the overall metabolism and energy balance in biological systems.
FADH2: FADH2 is a coenzyme that plays a critical role in cellular respiration as a carrier of electrons and protons during metabolic reactions. It is produced during the Krebs cycle and is essential for generating energy in the form of ATP through oxidative phosphorylation, linking several important biochemical processes.
ATP: ATP, or adenosine triphosphate, is a nucleotide that serves as the primary energy carrier in all living cells. It plays a crucial role in cellular processes by providing the energy needed for various biochemical reactions, including metabolism, muscle contraction, and the synthesis of macromolecules.