ETC

The electron transport chain (ETC) is a series of protein complexes and mobile electron carriers that transfer electrons from NADH and FADH2 to molecular oxygen, ultimately producing ATP through oxidative phosphorylation.

Key Concepts and Components:

• Location: The ETC is located in the inner mitochondrial membrane in eukaryotes and the plasma membrane of prokaryotes.

• Purpose: The ETC is the final common pathway where electrons derived from various metabolic fuels are transferred to oxygen. This process drives the production of ATP, the main energy currency of the cell.

• Electron Flow: Electrons flow through the ETC in a series of steps, moving from carriers with relatively low reduction potential to those with higher reduction potential. This ensures a controlled and energetically useful transfer.

• Complexes: The ETC is organized into four protein complexes (I-IV):

  • Complex I (NADH-CoQ reductase): Accepts electrons from NADH and passes them to CoQ. This process releases energy and pumps protons across the inner mitochondrial membrane.

  • Complex II (Succinate-CoQ reductase): Accepts electrons from FADH2 and passes them to CoQ.

  • Complex III (CoQ-cytochrome c reductase): Transfers electrons from CoQ to cytochrome c, also pumping protons across the membrane. The Q cycle is part of this complex.

  • Complex IV (Cytochrome c oxidase): Transfers electrons from cytochrome c to molecular oxygen, reducing it to water. This is the terminal step of the ETC, and this complex also pumps protons.

• Mobile Carriers:

  • Coenzyme Q (CoQ): A lipid-soluble quinone that moves electrons between complexes I and II to complex III.

  • Cytochrome c: A soluble protein that transfers electrons from complex III to complex IV.

• Proton Pumping: As electrons move through complexes I, III, and IV, protons (H+) are pumped from the mitochondrial matrix to the intermembrane space. This creates an electrochemical gradient called the proton-motive force.

• Final Electron Acceptor: Molecular oxygen (O2) is the final electron acceptor in the ETC, combining with electrons and protons to form water (H2O).

• Redox Potential: The ETC components are arranged in order of increasing redox potential, which means the electron affinity increases along the chain. This facilitates the directional flow of electrons from NADH or FADH2 to oxygen.

ATP Synthesis and Oxidative Phosphorylation:

• Chemiosmotic Coupling: The proton gradient generated by the ETC is used to drive the synthesis of ATP via chemiosmotic coupling.

• ATP Synthase: Protons flow down the electrochemical gradient through a membrane-bound enzyme called ATP synthase. This flow of protons provides the energy for ATP synthase to synthesize ATP from ADP and inorganic phosphate.

• ATP Yield: The transfer of electrons from one molecule of NADH through the ETC results in the production of approximately 2.5 ATP, while the transfer of electrons from one molecule of FADH2 yields about 1.5 ATP.

• Overall Process: The process of electron transport coupled with ATP synthesis is known as oxidative phosphorylation.

Clinical Significance:

• Inhibitors: Certain substances like cyanide can inhibit the ETC, blocking electron flow and ATP production.

• Uncouplers: Uncouplers like 2,4-dinitrophenol disrupt the proton gradient, leading to increased oxygen consumption without ATP production, as the energy is released as heat.

• Mitochondrial Function: The structure of the mitochondria, especially the inner membrane with its cristae, is vital for the ETC and ATP production.

• Metabolic Importance: The ETC and oxidative phosphorylation are critical for aerobic metabolism, enabling efficient energy extraction from nutrients.

Additional Notes

• The components of the ETC, with the exception of CoQ, are proteins that may function as enzymes or contain metal ions like iron or copper.

• The precise amount of ATP generated per NADH has not been definitively established, but current research suggests that 2.5 ATP are made per NADH.

• The movement of protons from the mitochondrial matrix to the cytosolic side occurs at three steps of the ETC: the NADH dehydrogenase reaction, the oxidation of cytochrome b, and the reaction of cytochrome oxidase with oxygen.

• The mitochondrial outer membrane is permeable to small molecules, whereas the inner membrane has selective permeability with specific transport systems.

• The enzymes of the citric acid cycle, fatty acid oxidation, and the electron transport chain are found in the mitochondria.

• The transfer of electrons from NADH to oxygen releases a large amount of energy, which is used for ATP synthesis.

This overview covers the main aspects of the electron transport chain.

The electron transport chain (ETC) is a crucial component of cellular respiration, responsible for generating the majority of ATP in aerobic organisms. The ETC is a series of protein complexes and mobile electron carriers that facilitate the transfer of electrons and the pumping of protons, ultimately leading to ATP synthesis.

Here is an expansion of the key concepts and components of the electron transport chain:

• Location: The ETC is embedded within the inner mitochondrial membrane in eukaryotic cells. In prokaryotic cells, the ETC is located in the plasma membrane. The inner mitochondrial membrane is folded into cristae, which increase the surface area available for electron transport and ATP synthesis. The structure of the mitochondria, especially the inner membrane with its cristae, is vital for the ETC and ATP production. The mitochondrial outer membrane is permeable to small molecules, while the inner membrane has selective permeability with specific transport systems.

• Purpose: The primary purpose of the ETC is to facilitate the transfer of electrons derived from metabolic fuels, such as NADH and FADH2, to molecular oxygen (O2). This transfer of electrons is coupled with the pumping of protons (H+) across the inner mitochondrial membrane, generating an electrochemical gradient, also known as the proton-motive force. This process ultimately drives the synthesis of ATP, the primary energy currency of the cell, through a process called oxidative phosphorylation. The ETC allows for the efficient extraction of energy from nutrients.

• Electron Flow: Electrons move through the ETC in a series of controlled steps. They flow from carriers with a lower reduction potential to those with a higher reduction potential, ensuring that the energy is released in a controlled manner and can be used to pump protons. The ETC components are arranged in order of increasing redox potential, which means the electron affinity increases along the chain. This facilitates the directional flow of electrons from NADH or FADH2 to oxygen.

• Complexes: The ETC consists of four major protein complexes, each with unique roles in electron transfer and proton pumping:

  • Complex I (NADH-CoQ reductase): This complex is the entry point for electrons from NADH, which is generated in the citric acid cycle. Complex I accepts electrons from NADH and transfers them to Coenzyme Q (CoQ or ubiquinone). This transfer of electrons releases energy that is used to pump protons from the mitochondrial matrix into the intermembrane space. Complex I is also known as NADH dehydrogenase.

  • Complex II (Succinate-CoQ reductase): Complex II receives electrons from FADH2, which is also generated in the citric acid cycle. FADH2 transfers electrons to CoQ, but unlike Complex I, Complex II does not directly pump protons.

  • Complex III (CoQ-cytochrome c reductase): This complex accepts electrons from the reduced CoQ (CoQH2) and transfers them to cytochrome c. The Q cycle, a mechanism involving the cyclical oxidation and reduction of CoQ, is part of the function of complex III. As electrons move through this complex, protons are pumped from the mitochondrial matrix to the intermembrane space.

  • Complex IV (Cytochrome c oxidase): Complex IV receives electrons from cytochrome c and transfers them to molecular oxygen (O2), which is reduced to water (H2O). This is the final step of the ETC. This complex also functions as a proton pump, further contributing to the proton gradient.

• Mobile Carriers: These are molecules that transport electrons between the protein complexes.

  • Coenzyme Q (CoQ or Ubiquinone): A small, lipid-soluble molecule that moves within the inner mitochondrial membrane. It carries electrons from Complex I and II to Complex III.

  • Cytochrome c: A soluble protein located in the intermembrane space of the mitochondria. It carries electrons from Complex III to Complex IV.

• Proton Pumping: Complexes I, III, and IV of the ETC act as proton pumps. As electrons move through these complexes, protons are actively transported from the mitochondrial matrix to the intermembrane space. This creates an electrochemical gradient, with a higher concentration of protons in the intermembrane space and a lower concentration in the matrix. This gradient, also called the proton-motive force, stores potential energy that will be used by ATP synthase to make ATP.

• Final Electron Acceptor: The final step of the ETC involves the reduction of molecular oxygen (O2) to water (H2O) by Complex IV. Oxygen is the terminal electron acceptor, which combines with electrons and protons to form water.

• Redox Potential: The arrangement of the ETC components is based on their increasing redox potential. The redox potential measures the tendency of a chemical species to acquire electrons and be reduced. This increasing electron affinity ensures a unidirectional flow of electrons to oxygen.

• Enzymes and Metal Ions: With the exception of CoQ, the components of the ETC are primarily proteins. These proteins may function as enzymes or contain metal ions, such as iron or copper, which play a vital role in electron transfer.

The precise amount of ATP generated per NADH has not been definitively established, but current research suggests that 2.5 ATP are made per NADH and 1.5 ATP per FADH2. The movement of protons from the mitochondrial matrix to the cytosolic side occurs at three steps of the ETC: the NADH dehydrogenase reaction, the oxidation of cytochrome b, and the reaction of cytochrome oxidase with oxygen. The enzymes of the citric acid cycle, fatty acid oxidation, and the electron transport chain are found in the mitochondria.

Complex I, also known as NADH-CoQ reductase or NADH dehydrogenase, is the first protein complex in the electron transport chain (ETC) and plays a vital role in cellular respiration. It is a large, multi-subunit enzyme that catalyzes the transfer of electrons from NADH to coenzyme Q (CoQ), also known as ubiquinone, coupled with the translocation of protons across the inner mitochondrial membrane.

Here are key aspects of Complex I:

• Function: Complex I’s primary role is to accept electrons from NADH, which is produced during metabolic processes like the citric acid cycle, and pass them to CoQ. This transfer is a key step in initiating the electron flow through the ETC.

• Structure: Complex I is a large, L-shaped protein complex. The mammalian complex is composed of 46 subunits, with a molecular mass of approximately 1,000 kDa. Some of the genes encoding the proteins within Complex I are located in the cell nucleus, while others are found in the mitochondrial genome. The complex includes the flavoprotein (FP), iron-sulfur protein (IP), and hydrophobic protein (HP).

  • The flavoprotein (FP) contains three peptides, with masses of 51, 24, and 10 kD, and contains FMN and two Fe-S centers.

  • The iron-sulfur protein (IP) contains six peptides and at least three Fe-S centers.

  • The hydrophobic protein (HP) contains at least seven peptides and one Fe-S center.

• Electron Transfer:

  • NADH binds to Complex I, donating two electrons.

  • The electrons are transferred through a series of redox centers, including flavin mononucleotide (FMN) and iron-sulfur (Fe-S) clusters, within the complex.

  • The electrons are then passed to coenzyme Q (CoQ), reducing it to CoQH2 (ubiquinol).

• Proton Pumping: The transfer of electrons through Complex I is coupled with the translocation of protons (H+) from the mitochondrial matrix to the intermembrane space. This process contributes to the formation of the electrochemical proton gradient across the inner mitochondrial membrane, which is critical for ATP synthesis.

• Redox Centers: Complex I contains several redox centers that facilitate electron transfer:

  • Flavin Mononucleotide (FMN): Accepts electrons from NADH.

  • Iron-Sulfur (Fe-S) clusters: A series of Fe-S centers that facilitate electron transfer through the complex.

• Location: Complex I is located in the inner mitochondrial membrane. It is positioned so that it can accept electrons from NADH in the matrix and pass them to CoQ in the inner membrane.

• Importance: Complex I is a crucial entry point for electrons into the ETC. By oxidizing NADH and reducing CoQ, it initiates the electron flow that drives ATP production through oxidative phosphorylation. Additionally, its role in proton pumping directly contributes to the proton-motive force.

• Inhibition: Complex I can be inhibited by specific compounds, such as rotenone, which blocks electron transfer, thereby reducing ATP production.

• Structural Studies: Structural analysis of Complex I, including the L-shape of the complex, has been determined using the E. coli version of this complex.

• Evolutionary Conservation: Complex I is highly conserved across species, reflecting its vital role in energy metabolism. The genes that encode the individual proteins are contained in both the cell nucleus and the mitochondrial genome.

In summary, Complex I is a critical enzyme in cellular respiration. It oxidizes NADH, transfers electrons to CoQ, and pumps protons, thereby playing essential roles in both electron transport and the generation of the proton-motive force. The structure of Complex I includes a flavoprotein (FP), iron-sulfur protein (IP) and hydrophobic protein (HP).

Complex II, also known as succinate-CoQ reductase or succinate dehydrogenase, is the second protein complex in the electron transport chain (ETC). It plays a role in both the citric acid cycle and the ETC, and it is responsible for the transfer of electrons from succinate to coenzyme Q (CoQ).

Here are some key aspects of Complex II:

• Function: The primary function of Complex II is to oxidize succinate, a key intermediate in the citric acid cycle, and transfer the electrons from succinate to coenzyme Q (CoQ), also known as ubiquinone. This reaction is part of the citric acid cycle and also contributes electrons to the ETC.

• Structure:

  • Complex II is a smaller complex compared to Complex I, consisting of four subunits.

  • The complex is anchored in the inner mitochondrial membrane with its head projecting into the mitochondrial matrix.

  • It has a trimeric structure with three identical multisubunit enzymes.

  • Each of the three succinate dehydrogenase enzymes that form the trimer have two subunits forming the head, and one or two subunits forming the membrane-bound stalk.

  • One of the head subunits contains the substrate binding site and a covalently bound flavin adenine dinucleotide (FAD).

  • The other head subunit contains three Fe-S clusters.

• Electron Transfer:

  • Succinate is oxidized to fumarate, and the electrons from this reaction are transferred to FAD, which is reduced to FADH2.

  • The electrons from FADH2 are then passed through a series of iron-sulfur (Fe-S) clusters within the complex.

  • Finally, the electrons are transferred to coenzyme Q (CoQ), reducing it to CoQH2 (ubiquinol).

• Proton Pumping: Unlike Complexes I, III, and IV, Complex II does not directly pump protons across the inner mitochondrial membrane. Therefore, Complex II contributes to the electron flow but does not directly contribute to the creation of the proton-motive force.

• Redox Centers: Complex II includes specific redox centers that are involved in the electron transfer reactions:

  • Flavin Adenine Dinucleotide (FAD): Accepts electrons from succinate.

  • Iron-Sulfur (Fe-S) clusters: A series of Fe-S centers that facilitate electron transfer through the complex.

• Location: Complex II is embedded within the inner mitochondrial membrane. Its position allows it to receive succinate from the mitochondrial matrix and transfer electrons to CoQ within the inner membrane.

• Importance: Complex II plays a crucial dual role in cellular respiration:

  • It participates in the citric acid cycle by oxidizing succinate to fumarate.

  • It contributes electrons to the ETC, thus facilitating ATP production via oxidative phosphorylation.

• Unique Characteristics:

  • Complex II is the only enzyme of the citric acid cycle that is an integral part of the inner mitochondrial membrane and the electron transport chain.

  • It does not directly pump protons, which distinguishes it from Complexes I, III, and IV.

• Inhibition: Complex II can be inhibited by compounds such as malonate, which is a competitive inhibitor of the succinate dehydrogenase active site.

In summary, Complex II, or succinate-CoQ reductase, serves as a vital link between the citric acid cycle and the electron transport chain. It oxidizes succinate, reduces CoQ, and contains FAD and Fe-S clusters, but it does not directly contribute to the proton gradient.

Complex III, also known as CoQ-cytochrome c reductase or cytochrome bc1 complex, is the third protein complex in the electron transport chain (ETC). It plays a crucial role in cellular respiration by transferring electrons from coenzyme Q (CoQH2) to cytochrome c, coupled with the translocation of protons across the inner mitochondrial membrane.

Here are some key aspects of Complex III:

• Function: The main function of Complex III is to facilitate the transfer of electrons from reduced coenzyme Q (CoQH2) to cytochrome c. This transfer is coupled with the pumping of protons from the mitochondrial matrix to the intermembrane space.

• Structure:

  • Complex III is a dimer, with each monomer containing multiple subunits.

  • Each monomer contains 11 subunits, including cytochrome b (cyt bL and cyt bH), cytochrome c1 (cyt c1), and an iron-sulfur (Fe-S) cluster.

  • The complex has a pear-shaped structure with its widest part extending into the mitochondrial matrix.

  • The transmembrane portion of the complex consists of several transmembrane helices.

  • It has two channels through which H+ ions are pumped.

• Electron Transfer:

  • Complex III accepts electrons from reduced coenzyme Q (CoQH2).

  • Electrons are passed through a series of redox centers, including cytochrome b, cytochrome c1, and an iron-sulfur cluster.

  • The electrons are then transferred to cytochrome c, a mobile electron carrier.

  • Because each cytochrome c can only pick up one electron, two cytochrome c units are required.

• Proton Pumping:

  • As electrons are transferred, Complex III pumps protons (H+) from the mitochondrial matrix to the intermembrane space.

  • Four protons are pumped across the inner mitochondrial membrane for every pair of electrons transferred to cytochrome c.

• Redox Centers: Complex III contains several key redox centers:

  • Cytochrome b: Includes two b-type hemes, b562 (or bH) and b566 (or bL).

  • Cytochrome c1: Contains a c-type heme.

  • Iron-sulfur (Fe-S) cluster: A Rieske [2Fe–2S] center, where one of the Fe atoms is coordinated by two histidine residues rather than two cysteine residues.

• Location: Complex III is an integral membrane protein embedded in the inner mitochondrial membrane. It is positioned to accept electrons from CoQH2 and transfer them to cytochrome c.

• Importance:

  • Complex III is essential for efficient electron transport and ATP production.

  • Its role in proton pumping contributes to the generation of the electrochemical proton gradient, which is essential for ATP synthesis.

• Unique Characteristics:

  • The cytochromes within Complex III have a heme prosthetic group, similar to those found in hemoglobin and myoglobin.

  • Electrons are transferred by cytochromes one at a time, associated with a reversible change in the oxidation state of a heme iron (between Fe2+ and Fe3+).

  • The two ISPs of the dimeric complex are intertwined so that the [2Fe–2S] cluster in the ISP of one protomer interacts with the cytochrome b and cytochrome c1 subunits of the other protomer.

In summary, Complex III is a critical component of the electron transport chain. It transfers electrons from CoQH2 to cytochrome c, pumps protons across the inner mitochondrial membrane, and has important redox centers including cytochromes and an iron-sulfur cluster.

Complex IV, also known as cytochrome c oxidase, is the final protein complex in the electron transport chain (ETC). It plays a critical role in cellular respiration by catalyzing the transfer of electrons from cytochrome c to molecular oxygen, reducing the oxygen to water. This process is coupled with the pumping of protons across the inner mitochondrial membrane, contributing to the electrochemical gradient that drives ATP synthesis.

Here are key aspects of Complex IV:

• Function: The primary function of Complex IV is to catalyze the four-electron reduction of molecular oxygen (O2) to water (H2O). This is the final step in the ETC, and it is essential for the efficient generation of ATP.

• Structure:

  • Complex IV is a large, multi-subunit protein complex. The mammalian enzyme is a homodimer with each monomer containing 13 or 14 subunits, depending on the species.

  • It is a transmembrane protein complex embedded within the inner mitochondrial membrane.

  • The core of Complex IV consists of its three largest subunits, I, II, and III. Subunits I and II contain the redox-active groups.

  • Subunit I contains two heme groups, a and a3, as well as the CuB center.

  • Subunit II contains the CuA center.

  • Subunit III appears to be primarily structural.

• Electron Transfer:

  • Complex IV accepts electrons from cytochrome c.

  • Electrons are transferred through a series of redox centers: CuA, cytochrome a, and the binuclear a3-CuB center.

  • Finally, the electrons are passed to molecular oxygen (O2), which is reduced to water (H2O).

  • Four cytochrome c molecules are sequentially oxidized.

• Proton Pumping:

  • Complex IV pumps protons (H+) from the mitochondrial matrix to the intermembrane space.

  • Four protons are translocated across the membrane for each molecule of oxygen reduced to water.

  • Additionally, four protons from the matrix are consumed in the reduction of oxygen to water.

• Redox Centers: Complex IV has multiple redox centers that facilitate electron transfer:

  • Cytochrome a: A heme group that accepts electrons from the CuA center.

  • Cytochrome a3: Forms a binuclear Fe-Cu center with CuB and is the site of oxygen reduction.

  • CuA: A binuclear copper center that accepts electrons from cytochrome c.

  • CuB: A copper ion that forms a binuclear center with heme a3.

• Location: Complex IV is located in the inner mitochondrial membrane. It is positioned to receive electrons from cytochrome c in the intermembrane space and to interact with oxygen in the matrix.

• Importance:

  • Complex IV is the terminal enzyme in the electron transport chain, playing an essential role in cellular respiration.

  • It ensures the efficient reduction of oxygen to water, which is necessary for continuous electron flow.

  • It contributes to the proton gradient, which drives ATP synthesis.

  • It prevents the leakage of electrons to oxygen, which could result in the formation of damaging reactive oxygen species.

• Unique Characteristics:

  • Complex IV is regulated by ATP, reversible phosphorylation/dephosphorylation, and thyroid hormone (T2, or diiodothyronine).

  • The mammalian enzyme has an extremely complicated structure containing 13 subunits, two heme groups, and three copper ions, as well as magnesium and zinc.

  • Unlike other electron transport complexes, electrons do not leak out of complex IV.

• Subunits:

  • In mammals, subunits I through III are encoded by mitochondrial DNA.

  • The remaining subunits are encoded by nuclear DNA.

• Inhibition: Complex IV can be inhibited by compounds such as cyanide and carbon monoxide. These inhibitors block the binding of oxygen, preventing the electron flow and ATP production.

• Clinical Significance: A deficiency in cytochrome oxidase can result in conditions such as MELAS (Mitochondrial Encephalopathy, Lactic Acidosis, Stroke).

In summary, Complex IV, or cytochrome c oxidase, is the terminal complex of the electron transport chain. It accepts electrons from cytochrome c, reduces oxygen to water, pumps protons, and contains redox-active centers including cytochromes and copper ions.

The electron carriers and include:

• Ubiquinone (CoQ): A small, hydrophobic molecule that diffuses within the inner mitochondrial membrane, accepting electrons from Complexes I and II, and transferring them to Complex III.

• Cytochrome c: A small, soluble protein located in the intermembrane space of the mitochondria. It accepts electrons from Complex III and carries them to Complex IV.

In this context, these molecules are mobile in that they diffuse within the mitochondrial membrane or intermembrane space, allowing for the sequential transfer of electrons, and can be considered as "carriers" of electrons.

Here's a breakdown of their roles in the ETC, based on information in the sources:

• Ubiquinone (CoQ):

  • It is a lipid-soluble molecule that can move freely within the inner mitochondrial membrane.

  • It accepts electrons from Complex I and Complex II.

  • It carries the electrons to Complex III.

  • Ubiquinone is reduced to ubiquinol (CoQH2) when it accepts electrons and protons and can carry two electrons.

• Cytochrome c:

  • It is a small protein that can move in the intermembrane space.

  • It carries a single electron at a time and accepts electrons from Complex III.

  • It carries the electrons to Complex IV.

These mobile carriers are essential for the functioning of the ETC. They ensure the continuous transfer of electrons from one complex to the next, which is crucial for the generation of the proton gradient that drives ATP synthesis.

Complex V, also known as ATP synthase, is a crucial enzyme complex in the inner mitochondrial membrane responsible for the synthesis of ATP using the proton gradient generated by the electron transport chain. It's also found in chloroplasts and bacterial membranes. This complex is sometimes referred to as F0F1-ATPase due to its ability to also catalyze the reverse reaction, the hydrolysis of ATP.

Here are the key aspects of Complex V, as described in the sources:

• Structure: ATP synthase is composed of two main structural components:

  • F0 complex: This is the membrane-spanning domain, which forms a proton channel through the inner mitochondrial membrane. It is also referred to as the 'base' or 'stalk' of the mushroom shaped structure.

    • The F0 complex consists of three types of subunits: a, b, and c.

    • The c subunits form a ring, which rotates in response to proton flow. There are 10-14 c subunits. The c-ring is composed of hydrophobic polypeptides, each with two transmembrane helices.

    • The a and b subunits are also part of the membrane-spanning domain of F0.

  • F1 complex: This is the extramembranous domain that projects into the mitochondrial matrix. Also referred to as the 'head' or 'cap' of the mushroom shaped structure.

    • The F1 complex contains five different polypeptide chains: α, β, γ, δ, and ε. They are typically present in the ratio α3β3γδε.

    • The α and β subunits bind nucleotides, but only the β subunits are involved in catalysis of ATP synthesis.

    • The γ subunit forms a central stalk or "bent axle" that connects to the c-ring of the F0 complex. This stalk rotates within the α3β3 subunits.

    • The δ subunit helps anchor the F1 complex to the F0 complex.

    • The ε subunit is also part of the stalk.

• Mechanism:

  • Proton Flow: The enzyme utilizes the energy stored in a proton gradient across the inner mitochondrial membrane. Protons flow through the F0 channel from the intermembrane space to the mitochondrial matrix.

  • Rotation: The flow of protons through the F0 channel drives the rotation of the c-ring, and with it, the attached γ and ε subunits. The rotation of the F0 part of the complex is driven by the flow of hydrogen ions across the membrane.

  • ATP Synthesis: The rotating γ subunit induces conformational changes in the β subunits of F1, allowing them to bind ADP and Pi, then to phosphorylate ADP to ATP, and finally release the ATP. The binding of ADP and Pi to the catalytic center of ATP synthetase, occurs without any energy input. However, energy is needed for the release of ATP. One complete rotation of the c-ring generates approximately three ATP molecules.

  • The enzyme reaction is reversible. The enzyme is also capable of hydrolyzing ATP, and using the energy from ATP to pump protons into vacuoles and other locations.

• Location: ATP synthase is located in the inner mitochondrial membrane. In electron microscopy images, ATP synthase dimers appear to be arranged in rows along the highly curved mitochondrial cristae edges. It is present across the evolutionary spectrum. In chloroplasts, the ATP synthase is embedded in the thylakoid membrane with the F1 portion projecting into the stroma. In bacteria, the F1 component projects into the cytoplasm.

• Function:

  • ATP synthase uses the energy of the proton gradient to synthesize ATP from ADP and inorganic phosphate (Pi).

  • It couples the re-entry of protons into the matrix with the formation of ATP.

  • It is essential for cellular energy production and is found in all forms of life.

• Additional Details:

  • The ATP synthase complex is considered the smallest molecular motor present in the human body.

  • The F1 portion is the site of ATP synthesis.

  • The F0 portion is a proton-driven motor.

  • The enzyme complex can also hydrolyze ATP (hence the name ATPase), but its primary role is ATP synthesis.

In summary, Complex V, or ATP synthase, is a molecular machine that uses the energy of a proton gradient to synthesize ATP. It consists of the F0 transmembrane proton channel and the F1 catalytic head, which work together through a rotary mechanism to produce ATP.

ATP synthesis and oxidative phosphorylation are vital processes for energy production in cells, particularly in the mitochondria of eukaryotes. These processes involve the electron transport chain (ETC) and the enzyme complex ATP synthase.

Oxidative Phosphorylation:

• Oxidative phosphorylation is the process where ATP is synthesized using the energy released during the transfer of electrons through the ETC and the subsequent movement of protons across the inner mitochondrial membrane.

• It is a key mechanism for generating ATP, the primary energy currency of the cell.

• This process is coupled with the biological oxidation that occurs in the respiratory chain.

• The electron transport chain (ETC), located in the inner mitochondrial membrane, is a series of protein complexes that facilitate the transfer of electrons from NADH and FADH2 to molecular oxygen. This electron transfer is coupled with the movement of protons (H+) across the inner mitochondrial membrane from the matrix to the intermembrane space.

• This creates an electrochemical gradient or proton motive force, consisting of a proton gradient and a membrane potential. This gradient stores potential energy that can be used for ATP synthesis.

• The movement of protons back into the matrix through the ATP synthase complex provides the energy for ATP production.

ATP Synthase (Complex V):

• ATP synthase is an enzyme complex, also known as Complex V or F0F1-ATPase, which is present in the inner mitochondrial membrane.

• It facilitates the synthesis of ATP using the proton gradient generated by the ETC.

• Structure: ATP synthase has two main components:

  • F0 complex: This is a membrane-spanning domain that forms a proton channel through the inner mitochondrial membrane [Complex V notes]. It includes subunits a, b, and c. The c subunits form a ring which rotates in response to proton flow [Complex V notes].

  • F1 complex: This is the extramembranous domain that projects into the mitochondrial matrix. It contains five different polypeptide chains: α, β, γ, δ, and ε. The β subunits are involved in the catalytic activity of ATP synthesis [Complex V notes].

• Mechanism:

  • Proton Flow: Protons flow through the F0 channel from the intermembrane space into the mitochondrial matrix [Complex V notes].

  • Rotation: The flow of protons through the F0 channel causes the c-ring, and with it, the attached γ and ε subunits, to rotate. The rotating γ subunit induces conformational changes in the β subunits of F1 [Complex V notes].

  • ATP Synthesis: These conformational changes enable the β subunits to bind ADP and Pi, then to phosphorylate ADP to ATP, and finally release the ATP [Complex V notes].

  • The enzyme reaction is reversible [Complex V notes].

• ATP synthase is considered a molecular motor, using the flow of protons to drive ATP production.

• The enzyme complex can also hydrolyze ATP (hence the name ATPase), but its primary role is ATP synthesis [Complex V notes].

Key Concepts in Oxidative Phosphorylation:

• Chemiosmotic Theory: The proton gradient established during electron transport provides the energy for ATP synthesis. This concept is known as the chemiosmotic theory.

• P/O Ratio: The P/O ratio refers to the number of ATP molecules produced per atom of oxygen reduced. The oxidation of one mole of NADH generates approximately 2.5 moles of ATP, whereas the oxidation of one mole of FADH2 generates approximately 1.5 moles of ATP.

• Respiratory Control: ATP synthesis is coupled with the ETC and respiration and is regulated by the availability of ADP and Pi.

• Uncouplers: Uncouplers, such as dinitrophenol, are molecules that can disrupt the proton gradient, causing oxidation to proceed without phosphorylation, thereby decreasing ATP production.

• Inhibitors: Certain substances can inhibit the ETC at various steps, blocking electron transport and ATP synthesis.

Summary: ATP synthesis and oxidative phosphorylation are essential biochemical processes in cellular respiration. The electron transport chain generates a proton gradient, and ATP synthase uses this gradient to produce ATP. The coordinated actions of these components ensure an efficient production of ATP to meet the energy demands of the cell.

1. Proposer: Peter Mitchell (Nobel Prize winner).

2. Core Concept: This theory explains how the energy released from electron transport is coupled to the synthesis of ATP (Oxidative Phosphorylation). It proposes that this coupling occurs indirectly via a proton (H+) gradient across the inner mitochondrial membrane (IMM).

3. Key Steps & Principles:

   • Electron Transport & Energy Release: As electrons (from NADH and FADH2) flow through the Electron Transport Chain (ETC) complexes (I, III, IV) embedded in the IMM, they move from lower to higher redox potentials, releasing free energy. 

   • Proton Pumping: Specific ETC complexes act as proton pumps. Complexes I, III, and IV use the energy released during electron transport to actively pump protons (H+) from the mitochondrial matrix into the intermembrane space (IMS). 

   • Electrochemical Gradient (Proton Motive Force - PMF):

     • This pumping action creates an electrochemical gradient across the IMM.

     • This gradient has two components:

       • Chemical Gradient (ΔpH): Higher concentration of H+ (lower pH) in the IMS compared to the matrix.

       • Electrical Gradient (Δψ): Accumulation of positive charges (H+) in the IMS makes it electrically positive relative to the negatively charged matrix.

     • This combined gradient is termed the Proton Motive Force (PMF), which stores potential energy.)

   • IMM Impermeability: The inner mitochondrial membrane is largely impermeable to protons, preventing them from leaking back into the matrix and thus maintaining the gradient. 

   • ATP Synthase (Complex V):

     • Protons can flow back down their electrochemical gradient (from IMS to matrix) only through a specific protein channel: ATP Synthase (also called Complex V). 

     • ATP Synthase has two main parts: F0 (embedded in the IMM, forms the proton channel) and F1 (projects into the matrix, contains the catalytic sites for ATP synthesis). 

     • The flow of protons through the F0 channel drives the rotation of parts of the enzyme (specifically the gamma subunit stalk).

     • This rotation causes conformational changes in the catalytic beta subunits of the F1 particle (Binding Change Mechanism: Open, Loose, Tight states).

     • These conformational changes drive the synthesis of ATP from ADP and inorganic phosphate (Pi) within the matrix. 

4. Coupling: The proton gradient is the essential link coupling the energy-releasing process of electron transport (oxidation) to the energy-conserving process of ATP synthesis (phosphorylation). 

5. Evidence/Support: The action of uncouplers (e.g., DNP, thermogenin) supports the theory. Uncouplers dissipate the proton gradient by making the IMM permeable to protons, allowing electron transport to continue but preventing ATP synthesis (energy is released as heat). 

The P/O ratio (or P:O ratio) is a crucial concept in understanding the efficiency of oxidative phosphorylation, the process by which ATP is generated in the mitochondria. It represents the number of ATP molecules produced per atom of oxygen reduced during the electron transport chain (ETC). This ratio is not a fixed number but is influenced by several factors.

Here's a detailed breakdown of the P/O ratio with examples:

Definition and Significance:

• The P/O ratio is a measure of the efficiency of ATP production linked to electron transport. It quantifies how many ATP molecules are synthesized for every two electrons (or one oxygen atom) that pass through the ETC to reduce oxygen to water.

• A higher P/O ratio indicates a more efficient process of ATP production.

• The ratio is not a whole number because the transfer of electrons through the ETC is linked to proton pumping, and the number of protons required to synthesize one ATP is not a fixed whole number.

• It is an approximate number and can be influenced by a variety of factors.

Theoretical P/O Ratios for Different Substrates:

• NADH: When electrons are passed from NADH through the ETC, approximately 2.5 ATP molecules are generated per atom of oxygen reduced. This is because NADH enters the ETC at Complex I, leading to the pumping of more protons across the inner mitochondrial membrane.

• FADH2: When electrons are passed from FADH2 through the ETC, approximately 1.5 ATP molecules are generated per atom of oxygen reduced. This is because FADH2 enters the ETC at Complex II, bypassing Complex I, resulting in fewer protons being pumped across the membrane.

Factors Affecting the P/O Ratio:

• Proton Leakage: The inner mitochondrial membrane can have some permeability to protons, and some protons may leak back into the matrix without passing through ATP synthase. This proton leakage reduces the efficiency of ATP production.

• Shuttle Systems: Electrons from cytosolic NADH must be transported into the mitochondrial matrix to enter the ETC. Different shuttle systems, such as the malate-aspartate shuttle and the glycerol-3-phosphate shuttle, are used for this transport, each with different efficiencies that may affect the P/O ratio.

• ATP Synthase Efficiency: The number of protons required to turn the c-ring of ATP synthase is usually 8 to 14 per rotation, where a complete rotation generates approximately 3 ATPs. These numbers are approximate, but reflect the efficiency of the F0 complex [Complex V notes].

• Mitochondrial State: The physiological state of the mitochondria and the cell can also impact the P/O ratio.

• Experimental Conditions: Variations in the experimental setup can lead to differing P/O ratios, and it is difficult to determine an exact number for P/O ratios.

Examples and Calculations:

• Oxidation of Glucose:

  • The complete oxidation of one molecule of glucose yields around 30-32 ATP molecules.

  • Glycolysis produces 2 NADH and 2 ATP. The 2 NADH are typically oxidized in the mitochondria using the malate-aspartate shuttle, resulting in 5 ATP (2 NADH x 2.5 ATP/NADH).

• Pyruvate decarboxylation produces 2 NADH and 2 acetyl-CoA molecules resulting in 5 ATP( 2 NADH x 2.5 ATP/NADH).

• The citric acid cycle produces 6 NADH, 2 FADH2 and 2 GTP (which are equivalent to 2 ATP), resulting in 15 ATP (6 NADH x 2.5 ATP/NADH) and 3 ATP (2 FADH2 x 1.5 ATP/FADH2), and 2 ATP.

• So the total ATP produced is 2+5+5+15+3+2 = 32 ATP.

• Considering substrate-level phosphorylation (4 ATP from glycolysis and citric acid cycle), the rest of ATP is generated via oxidative phosphorylation.

• Succinate Oxidation: If intact mitochondria are incubated with succinate, the expected P/O ratio is approximately 1.5, as succinate directly reduces FAD to FADH2, which enters the ETC at Complex II.

Why P/O Ratio is Important:

• Metabolic Efficiency: The P/O ratio provides information on how efficiently energy from the electron transport chain is converted to ATP and is essential for cellular energy metabolism.

• Understanding Disease: Changes in P/O ratios can indicate defects in mitochondrial function or oxidative phosphorylation, which can occur in various diseases.

• Drug Action: Some drugs such as dinitrophenol, act as uncouplers and can affect the P/O ratio.

• Regulation: P/O ratios are influenced by factors such as the availability of ADP, the level of oxygen and the type of fuel that is being used.

Summary: The P/O ratio is a vital measure of the efficiency of ATP synthesis during oxidative phosphorylation. While theoretical ratios exist for NADH and FADH2 (2.5 and 1.5 respectively), the actual P/O ratio can vary based on several factors. Understanding this concept is critical in biochemistry, as it directly relates to how cells generate energy and how disruptions can lead to diseases.

Inhibitors of the Electron Transport Chain (ETC) and Uncouplers of Oxidative Phosphorylation

Both inhibitors and uncouplers are substances that can interfere with the normal processes of the electron transport chain (ETC) and oxidative phosphorylation, ultimately affecting ATP production. However, they have different mechanisms of action and effects on the ETC and ATP synthesis.

Inhibitors of the ETC:

• ETC inhibitors are substances that block the transfer of electrons at specific points within the electron transport chain. This can prevent the creation of the proton gradient and reduce or stop ATP production.

• These inhibitors often bind to specific protein complexes within the ETC, interfering with their function.

• Specific examples of inhibitors and their sites of action:

  • Complex I Inhibitors:

    • Rotenone, an insecticide and fish poison, inhibits the transfer of electrons from NADH reductase to Coenzyme Q.

    • Amobarbital (a barbiturate) and Piericidin A (an antibiotic) also block electron transfer at Complex I.

  • Complex II Inhibitors:

    • Carboxin and Malonate inhibit the transfer of electrons at complex II.

  • TTFA (Thenoyltrifluoroacetone) also inhibits Complex II.

  • Complex III Inhibitors:

    • Antimycin A blocks the transfer of electrons from cytochrome b to cytochrome c.

    • British Anti-Lewisite (BAL), also known as dimercaprol, blocks electron transfer at Complex III .

  • Complex IV Inhibitors:

    • Cyanide, Hydrogen Sulfide (H2S) and Sodium Azide inhibit the transfer of electrons to molecular oxygen in Complex IV.

    • Carbon Monoxide (CO) inhibits cytochrome oxidase by binding to Complex IV, preventing the terminal transfer of electrons to molecular oxygen.

  • ATP Synthase Inhibitors:

    • Oligomycin and Venturicidin block the movement of protons through the F0 portion of ATP synthase, preventing ATP synthesis.

  • ATP-ADP Transporter Inhibitor:

    • Atractyloside inhibits the transporter of ADP into and ATP out of the mitochondrion.

• Effects of ETC inhibitors:

  • Inhibitors cause a build-up of reduced components before the inhibited site and an accumulation of oxidized components after the inhibited site in the ETC.

  • Ultimately, the proton gradient is not established or maintained, leading to a decrease or cessation of ATP synthesis.

  • Inhibition of the ETC reduces oxygen consumption in normal coupled mitochondria.

• Clinical Significance:

  • Many of these inhibitors are toxic and can cause severe health issues or death due to the disruption of cellular respiration.

  • Some inhibitors are used as therapeutic drugs, but they may cause side effects.

Uncouplers of Oxidative Phosphorylation:

• Uncouplers are substances that dissociate or "uncouple" the synthesis of ATP from electron transport, causing the electron transport to continue without ATP production.

• These molecules are typically amphipathic with a hydrophobic character and a dissociable proton.

• They increase the permeability of the inner mitochondrial membrane to protons, allowing protons to flow back into the matrix without passing through the ATP synthase.

• By disrupting the proton gradient, uncouplers eliminate the electrochemical potential that normally drives ATP synthesis.

• Examples of uncouplers:

  • 2,4-Dinitrophenol (DNP) is a commonly used uncoupler that increases the permeability of the inner mitochondrial membrane to protons, thus collapsing the proton gradient.

  • Dicumarol, FCCP (carbonyl cyanide-p-trifluoromethoxyphenylhydrazone) are also uncouplers.

  • Thermogenin (or uncoupling protein) is a physiological uncoupler found in brown adipose tissue that generates heat.

    • Uncoupling proteins are endogenous uncouplers, which can generate heat by uncoupling the processes.

  • Valinomycin is an ionophore, a lipophilic molecule that facilitates the transport of specific cations, such as K+, across biological membranes.

  • High doses of aspirin can act as an uncoupler.

  • Bongkrekate, a bacterial toxin, also blocks the ADP-ATP antiport.

• Effects of uncouplers:

  • Uncouplers cause an increase in oxygen consumption and electron transport, as the system tries to compensate for the disrupted proton gradient.

  • ATP synthesis is inhibited, and the energy released by electron transport is dissipated as heat.

  • The rate of respiration becomes uncontrolled since the rate is no longer limited by the concentration of ADP or Pi.

• Clinical Significance:

  • Uncouplers can cause hyperthermia (elevated body temperature) due to the dissipation of energy as heat.

  • Some uncouplers such as DNP were previously used as weight-loss agents, but were discontinued due to reports of fatal overdoses.

  • Thermogenin is important in maintaining body temperature.

  • Uncoupling proteins are important in generating heat, particularly in newborns and hibernating animals.

Key Differences between ETC Inhibitors and Uncouplers:

• Mechanism of Action:

  • Inhibitors block electron transfer at specific sites in the ETC.

  • Uncouplers disrupt the proton gradient across the inner mitochondrial membrane.

• Effect on Oxygen Consumption:

  • Inhibitors typically reduce oxygen consumption in normal coupled mitochondria.

  • Uncouplers increase oxygen consumption.

• Effect on ATP Synthesis:

  • Inhibitors directly block ATP synthesis due to the lack of a proton gradient.

  • Uncouplers inhibit ATP synthesis by dissipating the proton gradient.

Summary Both inhibitors of the ETC and uncouplers of oxidative phosphorylation can disrupt the normal process of ATP synthesis. Inhibitors act by blocking electron flow at specific points in the ETC, while uncouplers disrupt the proton gradient necessary for ATP synthase function. Both result in decreased ATP production.