04 ETC + OP ai guide PDF

04 ETC Oxidative Phosphorylation 1) Electron Transport Chain Overview Overview of the Electron Transport Chain The Electron Transport Chain (ETC) is a crucial component of cellular respiration, located in the inner mitochondrial membrane. It plays a vital role in the production of ATP through oxida...

04 ETC Oxidative Phosphorylation 1) Electron Transport Chain Overview Overview of the Electron Transport Chain The Electron Transport Chain (ETC) is a crucial component of cellular respiration, located in the inner mitochondrial membrane. It plays a vital role in the production of ATP through oxidative phosphorylation. The process begins with the donation of electrons from reduced substrates, such as NADH and FADH2, which are generated during glycolysis and the citric acid cycle. These electrons are transferred through a series of protein complexes (Complexes I, II, III, and IV) and electron carriers, ultimately reducing oxygen to form water. As electrons flow through the chain, they release energy that is used to pump protons (H+) from the mitochondrial matrix into the intermembrane space, creating an electrochemical gradient. This gradient is essential for ATP synthesis, as it drives the ATP synthase enzyme to convert ADP and inorganic phosphate (Pi) into ATP. Page 1 Oxidative Fuel Production The electron transport chain is part of a larger process that includes glycolysis and the citric acid cycle. These pathways provide the necessary electron carriers (NADH and FADH2) that feed into the ETC. NADH produces approximately 2.5 ATP. FADH2 produces approximately 1.5 ATP. The overall flow of electrons through the ETC is coupled with the pumping of protons, which is critical for establishing the proton gradient needed for ATP synthesis.  Page 14 Mechanism of Proton Pumping The electron transport chain consists of four main complexes: 1. Complex I: Accepts electrons from NADH, pumping 4 H+ into the intermembrane space. 2. Complex II: Accepts electrons from FADH2 but does not pump protons. 3. Complex III: Transfers electrons via the Q cycle, pumping 4 H+ across the membrane. 4. Complex IV: Receives electrons from cytochrome c, reducing O2 to H2O and pumping 2 H+. The total protons pumped are 4 H+ from Complex I, 0 from Complex II, 4 H+ from Complex III, and 2 H+ from Complex IV, contributing to the proton-motive force that drives ATP synthesis. Page 2 Role of ATP Synthase The ATP synthase enzyme utilizes the electrochemical potential created by the proton gradient to synthesize ATP. As protons flow back into the mitochondrial matrix through ATP synthase, the energy released is harnessed to convert ADP and inorganic phosphate into ATP. This process is essential for cellular energy production and is a key aspect of oxidative phosphorylation, which is the final stage of cellular respiration. Summary of the Electron Transport Chain Process Electrons from NADH and FADH2 are transferred through the ETC. Protons are pumped into the intermembrane space, creating a gradient. Oxygen serves as the final electron acceptor, forming water. The proton gradient drives ATP synthesis via ATP synthase. The free energy change associated with the transfer of electrons from NADH to O2 can drive the synthesis of approximately 2.5 ATP per NADH molecule. The overall reaction can be summarized as: NADH + 1/2 O2 + H+ → NAD+ + H2O This process is fundamental to cellular respiration and energy metabolism, highlighting the importance of the electron transport chain in living organisms. 2) Oxidative Phosphorylation Overview of Oxidative Phosphorylation Oxidative phosphorylation is a crucial metabolic process that occurs in the mitochondria, where ATP is produced through the electron transport chain (ETC) and chemiosmosis. This process utilizes the energy released from the transfer of electrons from electron donors, such as NADH and FADH2, to oxygen, the final electron acceptor. The overall reaction can be summarized as: NADH+12O2+H+→NAD++H2OΔ𝐺′∘=−218 kJ mol−1NADH+21​O2​+H+→NAD++H2​OΔG′∘=−218 kJ mol−1\ This energy release drives the synthesis of ATP, making oxidative phosphorylation a vital component of cellular respiration. Page 1 Role of Electron Carriers In oxidative phosphorylation, electron carriers such as NADH and FADH2 play a critical role.  NADH contributes to the production of approximately 2.5 ATP. FADH2 contributes to the production of approximately 1.5 ATP. These carriers donate electrons to the electron transport chain, which consists of a series of protein complexes embedded in the inner mitochondrial membrane. As electrons flow through these complexes, protons (H+) are pumped from the mitochondrial matrix into the intermembrane space, creating a proton gradient. Proton Gradient and ATP Synthesis The proton gradient established by the electron transport chain is essential for ATP synthesis. The inner mitochondrial membrane is selectively permeable, allowing protons to flow back into the matrix through ATP synthase, a process known as chemiosmosis. ATP synthase utilizes the electrochemical potential generated by the proton gradient to convert ADP and inorganic phosphate (Pi) into ATP: ADP+Pi→ATPADP+Pi→ATP This process is driven by the flow of protons back into the mitochondrial matrix, which is facilitated by the ATP synthase enzyme. Page 1 ATP-ADP Translocation The export of ATP and import of ADP across the inner mitochondrial membrane is facilitated by the ATP-ADP translocator, an antiporter that exchanges ATP (-4 charge) for ADP (-3 charge), resulting in a net charge of -1. This process is electrogenic and is driven by the proton gradient established during oxidative phosphorylation. Additionally, inorganic phosphate (Pi) must also be transported back into the mitochondrion, which occurs via a phosphate carrier that operates as a symporter with protons (H+). This coordinated transport is essential for maintaining the efficiency of ATP production in oxidative phosphorylation. Significance of Oxidative Phosphorylation Oxidative phosphorylation is significant for several reasons: 1. ATP Production: It is the primary method of ATP production in aerobic organisms, providing the energy necessary for various cellular processes. 2. Energy Efficiency: The process is highly efficient, with the potential to produce up to 30-32 ATP molecules from one molecule of glucose when combined with glycolysis and the citric acid cycle. 3. Metabolic Integration: It integrates with other metabolic pathways, such as glycolysis and the citric acid cycle, to ensure a continuous supply of electron carriers (NADH and FADH2). 4. Clinical Implications: Dysfunction in oxidative phosphorylation can lead to various metabolic disorders and is associated with conditions such as mitochondrial diseases. 3) Complexes in ETC Complex I: NADH Dehydrogenase Overview of Complex I Complex I, also known as NADH Dehydrogenase, plays a crucial role in the electron transport chain (ETC). 1. Reaction: NADH is oxidized to NAD+ and releases 2 electrons (2e-). 2. Electron Transfer: The electrons are first accepted by Flavin Mononucleotide (FMN). 3. Iron-Sulfur Clusters: The electrons are then carried through 2Fe-2S clusters, which transfer one electron at a time. 4. Coenzyme Q (CoQ): The electrons ultimately reach Coenzyme Q, resulting in the formation of QH2 (ubiquinol). 5. Proton Pumping: For every 2 electrons transferred, 4 protons (H+) are displaced from the mitochondrial matrix to the intermembrane space, contributing to the proton gradient essential for ATP synthesis. Page 6 Page 6 Complex II: Succinate Dehydrogenase Overview of Complex II Complex II, also known as Succinate Dehydrogenase, is another key component of the ETC. 1. Reaction: It catalyzes the conversion of Succinate to Fumarate in the TCA cycle. 2. FADH2 Formation: During this process, FAD is reduced to FADH2, which then releases 2 electrons (2e-). 3. Electron Transfer: The electrons pass through Fe-S clusters to reach Coenzyme Q, forming QH2. 4. Proton Pumping: Notably, Complex II does not pump protons into the intermembrane space, making it less effective in contributing to the proton gradient compared to Complex I and III. 5. Alternative Pathways: Even if Complex I fails, QH2 can still be generated through Complex II, although not as efficiently. Additionally, electrons can also be transferred to CoQ via the Electron-Transferring Flavoprotein (ETFP) or the Glycerol-3-Phosphate Shuttle. Page 8 Page 8 Complex III: Q-Cytochrome C Oxidoreductase Overview of Complex III Complex III, also known as Q-Cytochrome C Oxidoreductase, is a pivotal component of the ETC. 1. Electron Transfer: QH2 binds to Heme bL and then to Heme bH, where it is oxidized back to Q, releasing 2 protons (H+) and 2 electrons (2e-). 2. Electron Pathway: One electron travels through Fe-S clusters to Heme c, which then transfers it to Cytochrome C. The other electron reduces another Q molecule, generating a radical form of Q. 3. Proton Pumping: Complex III pumps 4 protons into the intermembrane space during the Q cycle, contributing to the proton gradient necessary for ATP synthesis. Page 9 Page 9 Complex IV: Cytochrome C Oxidase Overview of Complex IV Complex IV, known as Cytochrome C Oxidase, is the final complex in the electron transport chain. 1. Electron Reception: It requires 4 Cytochrome C molecules to transfer a total of 4 electrons (4e-) to the complex. 2. Copper Involvement: The electrons bind to Copper (Cu) in Subunit II and are then transferred to Subunit I. 3. Water Formation: The electrons, along with 4 protons (H+), are used to reduce O2 to form 2 water molecules (2H2O). 4. Proton Pumping: Complex IV pumps 2 protons into the intermembrane space for every 2 electrons transferred, further contributing to the proton gradient. Page 12 Summary of Proton Pumping in the Electron Transport Chain Summary of Proton Pumping The electron transport chain consists of four main complexes, each contributing to the proton gradient essential for ATP synthesis: Complex I: Pumps 4 protons into the intermembrane space. Complex II: Does not pump protons. Complex III: Pumps 4 protons during the Q cycle. Complex IV: Pumps 2 protons when reducing O2 to water. Overall, the proton-motive force generated by these complexes drives ATP synthesis by ATP synthase. Page 14 4) Electron Carriers Overview of Electron Carriers In the electron transport chain (ETC), several key electron carriers play crucial roles in the transfer of electrons and the generation of ATP. The primary electron carriers include: NADH: Produced during glycolysis and the citric acid cycle, NADH donates electrons to the ETC, ultimately contributing to ATP synthesis. FADH2: Generated in the citric acid cycle, FADH2 also donates electrons to the ETC but at a different point than NADH, resulting in a lower ATP yield. Flavin Mononucleotide (FMN): This carrier can handle either 1 or 2 electrons at a time, facilitating the transfer of electrons from NADH to the next components in the chain. Coenzyme Q (CoQ): Also known as ubiquinone, CoQ can accept either 1 or 2 electrons, allowing it to shuttle electrons between different complexes in the ETC. These carriers are essential for the efficient transfer of electrons, which is coupled to the pumping of protons (H+) across the mitochondrial membrane, creating a proton gradient that drives ATP synthesis. Function of NADH and FADH2 NADH and FADH2 are critical for the electron transport chain: NADH: Each molecule of NADH can generate approximately 2.5 ATP when it donates its electrons to the ETC. FADH2: In contrast, each FADH2 molecule yields about 1.5 ATP. This difference is due to the point at which FADH2 enters the ETC, bypassing Complex I and thus contributing to fewer protons being pumped across the membrane. The overall efficiency of ATP production is influenced by the specific pathways through which these carriers transfer their electrons. Role of FMN and CoQ in Electron Transfer Both FMN and CoQ are vital for the proper functioning of the electron transport chain: Flavin Mononucleotide (FMN) FMN acts as a crucial intermediary that can accept electrons from NADH. It can handle both 1 and 2 electrons, which allows it to effectively transfer electrons to the iron-sulfur (Fe-S) clusters in Complex I. Coenzyme Q (CoQ)  CoQ serves as a mobile electron carrier within the inner mitochondrial membrane. It can also accept either 1 or 2 electrons, facilitating the transfer of electrons from the Fe-S clusters to Complex III. The ability of FMN and CoQ to handle varying numbers of electrons ensures a smooth and efficient flow of electrons through the ETC, which is essential for maintaining the proton gradient necessary for ATP synthesis. Complex II and Its Role in the ETC Complex II, also known as Succinate Dehydrogenase, plays a unique role in the electron transport chain: It catalyzes the conversion of succinate to fumarate in the citric acid cycle, during which FAD is reduced to FADH2. The reaction can be summarized as: FAD + 2e- → FADH2 FADH2 then donates its electrons to the Fe-S clusters within Complex II, which subsequently transfer the electrons to CoQ, forming QH2 (reduced Coenzyme Q). Notably, Complex II does not pump protons into the inner mitochondrial membrane, which means it contributes less to the proton gradient compared to Complex I. However, it provides an alternative pathway for electron transfer, ensuring that QH2 can still be generated even if Complex I is impaired. 5) ETC Inhibitors Inhibitors of the Electron Transport Chain The electron transport chain (ETC) is susceptible to various inhibitors that can disrupt its function. Here are some key inhibitors and their targets: Rotenone: Inhibits Complex I. Amytal: Also inhibits Complex I. Antimycin A: Inhibits Complex III. Cyanide (CN⁻): Inhibits Complex IV. These inhibitors can significantly affect the flow of electrons through the ETC, leading to reduced ATP production. Resilience of Complex II Complex II is unique among the complexes in the electron transport chain because it is not inhibited by rotenone or amytal. This characteristic allows the ETC to continue functioning even when Complex I is inhibited, although the efficiency of the process is reduced. When Complex I is inhibited, electrons can still enter the ETC through Complex II, which receives electrons from FADH₂ (derived from succinate) and transfers them to CoQ.  This resilience of Complex II is crucial for maintaining some level of ATP production under conditions where other complexes are inhibited. The following diagram illustrates the electron transport chain and highlights the redox potentials at various stages, including the resilience of Complex II: In summary, while inhibitors can severely impact the electron transport chain, Complex II's ability to function independently of Complex I provides a mechanism for continued, albeit less efficient, ATP production. 6) ATP Synthesis Mechanism ATP Synthase Overview ATP synthase is a crucial enzyme in the process of ATP production within the mitochondria. It consists of two main components: the F0 and F1 portions. F0 Portion: This part is embedded in the mitochondrial membrane and is responsible for proton translocation. F1 Portion: This part protrudes into the mitochondrial matrix and is where ATP synthesis occurs. Understanding the structure and function of ATP synthase is essential for grasping how ATP is synthesized during oxidative phosphorylation. Structure of ATP Synthase F1 Portion Composed of a hexagonal array of alpha (α) and beta (β) subunits. Contains a central gamma (γ) stalk that connects to the F0 portion. F0 Portion Contains 8-15 copies of c subunits arranged in a ring around the gamma stalk. Features a b2 pillar that connects to the delta (δ) subunit on top. This unique structure allows ATP synthase to harness the energy from the proton gradient to synthesize ATP. Proton Gradient and ATP Synthesis Proton Translocation Mechanism in F0 1. Protons (H+) bind to aspartic acid (Asp61) residues on the c subunits. 2. The binding of protons causes the c subunit ring to shift 30-40 degrees clockwise. 3. As the ring rotates, it continues to pick up protons until a full turn is completed, at which point the protons are released into the mitochondrial matrix. 4. This rotation is driven by the dissipation of the H+ gradient across the mitochondrial membrane. The rotation of the c subunit ring is crucial as it is mechanically linked to the F1 component, causing it to rotate as well. ATP Synthesis Mechanism in F1 ATP Synthesis Process 1. Open State (O): ADP and inorganic phosphate (Pi) enter the complex. 2. Loose State (L): ADP and Pi are loosely bound, and energy input is indicated. 3. Tense State (T): ADP and Pi are tightly bound, leading to the formation of ATP. 4. ATP is released from the T state, completing the cycle. Each full rotation of the F1 component results in the synthesis of three ATP molecules, one from each beta subunit. The process involves a 120° rotation between states, with ATP synthesis occurring during the transition from the L to T state in the beta subunit. 7) Shuttle Mechanisms Malate-Aspartate Shuttle Mechanism The malate-aspartate shuttle is a crucial biochemical system for transferring reducing equivalents across the mitochondrial membrane. It operates primarily in the liver, heart, and kidney cells, facilitating the transport of electrons from NADH produced in the cytosol into the mitochondria for ATP production. Key Steps in the Malate-Aspartate Shuttle: 1. In the Cytosol: ○ NADH is oxidized to NAD+ while converting oxaloacetate to malate via the enzyme malate dehydrogenase. 2. Transport into the Mitochondrial Matrix: ○ Malate is transported into the mitochondrial matrix through the malate-α-ketoglutarate antiporter. 3. In the Matrix: ○ Malate is converted back to oxaloacetate by malate dehydrogenase, reducing NAD+ to NADH. ○ Oxaloacetate is then converted to aspartate by aspartate aminotransferase, with the conversion of glutamate to α-ketoglutarate. 4. Transport Back to Cytosol: ○ Aspartate is transported out of the matrix via the glutamate-aspartate antiporter.  ○ In the cytosol, aspartate is converted back to oxaloacetate, completing the cycle. Key Enzymes Involved: Malate dehydrogenase Aspartate aminotransferase Two antiporters (malate-α-ketoglutarate and glutamate-aspartate) This shuttle is essential for maintaining the balance of NADH and NAD+ in the cytosol and mitochondria, allowing for efficient ATP production during cellular respiration. Glycerol Phosphate Shuttle The glycerol phosphate shuttle is another important mechanism for transferring reducing equivalents from the cytosol into the mitochondria, primarily utilized in muscle and brain tissues. Key Steps in the Glycerol Phosphate Shuttle: 1. In the Cytosol: ○ Dihydroxyacetone phosphate (DHAP) is converted to glycerol 3-phosphate by the enzyme 3-phosphoglycerol dehydrogenase, with NADH being oxidized to NAD+. 2. Transport to the Inner Mitochondrial Membrane: ○ Glycerol 3-phosphate is then transported to the inner mitochondrial membrane. 3. In the Mitochondria: ○ Glycerol 3-phosphate is oxidized back to DHAP by flavoprotein dehydrogenase, transferring electrons to FAD to form FADH2.  ○ The electrons from FADH2 are then passed to the electron transport chain, contributing to ATP production. Key Enzymes Involved: 3-phosphoglycerol dehydrogenase (cytosolic) Flavoprotein dehydrogenase (mitochondrial) The glycerol phosphate shuttle is particularly important in tissues where rapid ATP production is necessary, such as during exercise in muscle cells. 8) Fructose Metabolism Fructose Metabolism Overview Fructose metabolism occurs primarily in the liver and muscle tissues, where fructose is converted into intermediates that can enter glycolysis, the central pathway for glucose metabolism. In muscle tissue, fructose is phosphorylated by hexokinase using ATP to form fructose-6-phosphate, which then enters glycolysis directly. In contrast, in the liver, fructose is phosphorylated by fructokinase to form fructose-1-phosphate. This compound is then split by fructose-1-phosphate aldolase into two key intermediates: dihydroxyacetone phosphate (DHAP) and glyceraldehyde. Glyceraldehyde is subsequently phosphorylated by glyceraldehyde kinase to form glyceraldehyde-3-phosphate, which also enters glycolysis. DHAP can be converted to glycerol-3-phosphate by glycerol phosphate dehydrogenase, involving the reduction of NAD+ to NADH. The following diagram illustrates these pathways: Page 4 This diagram highlights the distinct pathways for fructose metabolism in muscle (yellow box) and liver (beige box), as well as the entry point into glycolysis (blue box). Key Enzymes in Fructose Metabolism 1. Hexokinase: Catalyzes the phosphorylation of fructose to fructose-6-phosphate in muscle tissue. 2. Fructokinase: Catalyzes the phosphorylation of fructose to fructose-1-phosphate in liver tissue. 3. Fructose-1-phosphate Aldolase: Splits fructose-1-phosphate into dihydroxyacetone phosphate and glyceraldehyde. 4. Glyceraldehyde Kinase: Converts glyceraldehyde into glyceraldehyde-3-phosphate. 5. Glycerol Phosphate Dehydrogenase: Converts dihydroxyacetone phosphate to glycerol-3-phosphate, involving NADH and NAD+ in the process. Metabolic Pathway Integration The metabolism of fructose is crucial for integrating dietary sugars into the energy production pathways of the body. In muscle tissue, the conversion of fructose to fructose-6-phosphate allows it to enter glycolysis, contributing to ATP production during muscle contraction. In the liver, the conversion of fructose to intermediates like glyceraldehyde-3-phosphate and glycerol-3-phosphate allows for both energy production and the synthesis of lipids, highlighting the liver's role in metabolic regulation. Visual Representation of Pathways The following image illustrates the conversion between dihydroxyacetone phosphate and glycerol-3-phosphate, emphasizing the role of these intermediates in metabolic pathways: Page 4 This diagram shows the enzymatic reactions involved in the conversion of DHAP to glycerol-3-phosphate and vice versa, illustrating the interconnectedness of metabolic pathways. 9) Mitochondrial Dysfunction Overview of MERRF Overview of Myoclonic Epilepsy with Ragged Red Fibers (MERRF) Myoclonic Epilepsy with Ragged Red Fibers (MERRF) is a mitochondrial disorder that is maternally inherited, meaning that the mitochondrial DNA (mtDNA) is passed down from mothers to their offspring. MERRF is characterized by disruptions in the synthesis of proteins involved in oxidative phosphorylation, which is crucial for ATP production in cells. This dysfunction leads to a variety of clinical symptoms. Clinical Features of MERRF Clinical Features of MERRF Individuals with MERRF may experience a range of clinical features, including: Myoclonic seizures: Sudden, brief involuntary muscle jerks. Hearing loss: Impaired auditory function. Lactic acidosis: An accumulation of lactic acid in the body, often due to anaerobic metabolism. Exercise intolerance: Difficulty in performing physical activities due to muscle weakness or fatigue. Poor night vision: Impaired vision in low-light conditions. These symptoms arise from the accumulation of inactive mitochondria in muscle fibers, which can be visually identified as ragged red fibers when stained with Gomori trichrome stain. Mitochondrial Dysfunction in MERRF Mitochondrial Dysfunction in MERRF The underlying cause of MERRF is mitochondrial dysfunction, which affects the electron transport chain (ETC) and oxidative phosphorylation processes. This dysfunction leads to: Impaired ATP production: Due to the disruption in the synthesis of proteins necessary for the electron transport chain. Accumulation of inactive mitochondria: These clumps of inactive mitochondria are characteristic of MERRF and contribute to the clinical symptoms observed in affected individuals. The image above shows a cross-section of a mitochondrion, highlighting the structural abnormalities associated with MERRF. 10) ATP-ADP Translocation ATP-ADP Translocation Mechanism The ATP-ADP translocator plays a crucial role in mitochondrial oxidative phosphorylation by facilitating the exchange of ATP and ADP across the inner mitochondrial membrane. Export of ATP: ATP, which carries a net charge of -4, is exported from the mitochondrial matrix. Import of ADP: ADP, with a net charge of -3, is imported into the matrix. This exchange results in a net charge of -1, which is significant for maintaining the electrochemical gradient across the membrane. This process is mediated by the Adenine Nucleotide Translocase, which operates as an antiporter. The translocation of ATP and ADP is an electrogenic process, meaning it is driven by the proton gradient established during the electron transport chain (ETC). This gradient is essential for ATP synthesis. Role of the Proton Gradient The proton gradient is a vital component of oxidative phosphorylation, as it provides the energy necessary for ATP synthesis. Protons (H+) are pumped from the mitochondrial matrix into the intermembrane space during the electron transport process, creating a higher concentration of protons in the intermembrane space compared to the matrix. The pH of the mitochondrial matrix is approximately 7.8, which is higher than the pH of the intermembrane space, which ranges from 7.0 to 7.4. This difference in pH is a direct result of proton pumping. The energy stored in this proton gradient is harnessed by ATP synthase to synthesize ATP from ADP and inorganic phosphate (Pi). The protons flow back into the matrix through ATP synthase, driving the conversion of ADP to ATP. Phosphate Transport In addition to ATP and ADP translocation, the transport of inorganic phosphate (Pi) is also essential for ATP synthesis. The phosphate must return to the mitochondrion, which is facilitated by a phosphate carrier/translocase that operates as a symporter. This means it transports inorganic phosphate along with protons (H+) back into the mitochondrial matrix. This coordinated transport of ATP, ADP, and phosphate is crucial for maintaining the energy balance within the mitochondria and ensuring efficient ATP production during oxidative phosphorylation. Energy Yield from Electron Transport The free energy released during the electron transport from NADH to O2 is substantial and can drive the synthesis of approximately 2.5 ATP molecules per NADH molecule oxidized. The overall reaction can be summarized as follows: NADH+12O2+H+→NAD++H2OΔ𝐺′∘=−218 kJ mol−1NADH+21​O2​+H+→NAD++H2​OΔG′∘=−218 kJ mol−1 This reaction highlights the energy released during the conversion of NADH to NAD+, which is harnessed to pump protons and create the proton gradient necessary for ATP synthesis. Visual Representation of the Process The following diagram illustrates the electron transport chain within the inner mitochondrial membrane, showing the flow of electrons and the associated movement of protons (H+ ions) across the membrane: This image emphasizes the role of the proton gradient in driving ATP production during oxidative phosphorylation. 11) ATP Yield from Glucose Theoretical ATP Yield from Glucose Oxidation In theory, the ATP yield from glucose oxidation can be calculated based on the number of electrons transferred by NADH and FADH2 during the electron transport chain (ETC). NADH: ○ Transfers 2 electrons, resulting in the pumping of 10 protons (H+). ○ This leads to the synthesis of 3 ATP per NADH. FADH2: ○ Transfers 2 electrons, resulting in the pumping of 6 protons (H+). ○ This leads to the synthesis of 2 ATP per FADH2. Thus, the theoretical maximum ATP yield from one molecule of glucose can be summarized as follows: Glycolysis: 2 NADH (cytosolic) → 3 or 5* ATP Glycolysis: 2 ATP → 2 ATP Pyruvate oxidation (two per glucose): 2 NADH (mitochondrial matrix) → 5 ATP Citric acid cycle (per glucose): 6 NADH (mitochondrial matrix) → 15 ATP Citric acid cycle: 2 FADH2 → 3 ATP Citric acid cycle: 2 ATP or 2 GTP → 2 ATP Total yield per glucose: 30 or 32 ATP *Note: The actual yield can vary based on the shuttle system used to transfer reducing equivalents into the mitochondrion. Actual ATP Yield from Glucose Oxidation In practice, the actual ATP yield from glucose oxidation is lower than the theoretical maximum due to various factors: NADH: The actual yield is approximately 2.5 ATP per NADH. FADH2: The actual yield is approximately 1.5 ATP per FADH2. This discrepancy arises from:  Proton leakage: Nonspecific leakage of protons back into the mitochondrial matrix can lead to the production of reactive oxygen species (ROS). Transport mechanisms: The transport of inorganic phosphate (Pi) and ADP into the matrix via the phosphate translocase (a symporter) consumes protons, affecting the overall ATP yield. Specifically, 4 H+ are consumed per ATP synthesized from ADP + Pi. Factors Influencing ATP Yield Several factors can influence the ATP yield from glucose oxidation: Shuttle systems: The type of shuttle system that transfers reducing equivalents into the mitochondrion can affect the number of ATP produced. For example, the malate-aspartate shuttle and the glycerol phosphate shuttle have different efficiencies. Proton leakage: As mentioned, the nonspecific leakage of protons can reduce the efficiency of ATP synthesis and increase ROS production. Transport efficiency: The efficiency of the transport mechanisms for ADP, Pi, and protons can also impact the overall ATP yield. Understanding these factors is crucial for comprehending the complexities of cellular respiration and energy metabolism. 12) Reactive Oxygen Species Formation of Reactive Oxygen Species (ROS) Formation of Reactive Oxygen Species (ROS) Reactive Oxygen Species (ROS) are formed through the process of homolytic cleavage, which involves the breaking of a bond in such a way that each atom retains one of the shared electrons. This results in the formation of radicals, which are atoms, molecules, or ions with unpaired valence electrons. The unpaired electron allows ROS to engage in various chemical reactions. The primary steps in the formation of ROS include: 1. Triplet Oxygen ( 3𝑂23O2​): The process begins with triplet oxygen gaining an electron to form superoxide ( 𝑂2−O2−​). 2. Superoxide to Hydrogen Peroxide: Superoxide can gain another electron and two protons to form hydrogen peroxide ( 𝐻2𝑂2H2​O2​). 3. Hydrogen Peroxide to Hydroxyl Radicals: Hydrogen peroxide can further react to form hydroxyl radicals ( ⋅𝑂𝐻⋅OH) and hydroxide ions ( 𝑂𝐻−OH−**). The effects of ROS include: DNA Damage: ROS can cause mutations and damage to the genetic material. Lipid Peroxidation: They oxidize polyunsaturated fatty acids in lipids, leading to cell membrane damage. Protein Oxidation: ROS can modify amino acids in proteins, affecting their function. Enzyme Deactivation: They can oxidatively deactivate specific enzymes involved in the oxidation of cofactors. Reactions Involving ROS Reactions Involving Reactive Oxygen Species (ROS) Reactive Oxygen Species can participate in various reactions that further propagate oxidative stress. Key reactions include: 1. Fenton Reaction: 𝐹𝑒2++𝐻2𝑂2→𝐹𝑒3++𝑂𝐻−+⋅𝑂𝐻Fe2++H2​O2​→Fe3++OH−+⋅OH This reaction involves ferrous iron (Fe²⁺) reacting with hydrogen peroxide to produce ferric iron (Fe³⁺), hydroxide ions, and hydroxyl radicals. 2. Haber-Weiss Reaction: 𝑂2+𝐻2𝑂2→𝑂2+⋅𝑂𝐻+𝑂𝐻−O2​+H2​O2​→O2​+⋅OH+OH− This reaction describes the interaction between superoxide and hydrogen peroxide, leading to the formation of additional hydroxyl radicals. Antioxidants and Their Role Antioxidants and Their Role in Neutralizing Oxidative Stress Antioxidants are crucial for neutralizing the harmful effects of ROS and maintaining cellular health. They can be classified into several categories: Enzymes: These include superoxide dismutase, catalase, and peroxidases, which catalyze reactions to convert ROS into less harmful substances. Buffering Ions: Elements like iron (Fe) and copper (Cu) can play roles in antioxidant defense. Proteins: Specific proteins such as transferrin, haptoglobin, and emopexin help in binding and neutralizing free radicals. Metallothionein and Ceruloplasmin: These proteins also contribute to antioxidant defense by binding metals and reducing oxidative stress. Vitamins and Cofactors: ○ Vitamin C and Vitamin E are well-known antioxidants that scavenge free radicals.  ○ Beta-carotene and Uric acid also play roles in antioxidant activity. Glutathione: A key antioxidant that exists in reduced form (GSH) and can be regenerated from its oxidized form (GSSG) using NADPH from the pentose phosphate pathway. Key Antioxidant Reactions Superoxide Dismutase Reaction: 2𝑂2−+2𝐻+→𝑂2+𝐻2𝑂22O2−​+2H+→O2​+H2​O2​ This reaction converts superoxide radicals into hydrogen peroxide. Catalase Reaction: 2𝐻2𝑂2→𝑂2+2𝐻2𝑂2H2​O2​→O2​+2H2​O Catalase breaks down hydrogen peroxide into water and oxygen. Peroxidase Reaction: 𝑅+𝐻2𝑂2→𝑅0+2𝐻2𝑂R+H2​O2​→R0+2H2​O This reaction involves the reduction of hydrogen peroxide by various substrates (R). These antioxidant mechanisms are vital for mitigating oxidative damage and maintaining cellular redox balance. 13) Nonshivering Thermogenesis Nonshivering Thermogenesis Overview Nonshivering thermogenesis is a metabolic process that generates heat in organisms, particularly in response to cold temperatures. This process is crucial for maintaining body temperature in endothermic (warm-blooded) animals, especially in newborns and hibernating mammals. In nonshivering thermogenesis, the energy produced from the oxidation of nutrients is not used for ATP synthesis but is instead released as heat. This is primarily facilitated by uncoupling proteins, particularly uncoupling protein 1 (UCP-1), which is found in brown adipose tissue. Role of Uncoupling Proteins (UCP-1) Uncoupling protein 1 (UCP-1) plays a vital role in nonshivering thermogenesis. It is located in the inner mitochondrial membrane and functions by uncoupling the electron transport chain (ETC) from ATP synthesis. Mechanism of UCP-1 Proton Uncoupling: UCP-1 allows protons (H+) to flow back into the mitochondrial matrix without passing through ATP synthase. This process dissipates the proton gradient that is normally used to drive ATP synthesis. Heat Production: As a result of this uncoupling, the energy that would typically be used to produce ATP is instead released as heat, contributing to thermoregulation in the body. Activation of UCP-1 UCP-1 is activated by various factors, including: Norepinephrine: When the body is exposed to cold, norepinephrine is released, which binds to receptors on brown adipose tissue cells. This initiates a signaling cascade that activates UCP-1. Fatty Acids: Free fatty acids released from triglycerides can also stimulate UCP-1 activity, enhancing the thermogenic response. Significance of Nonshivering Thermogenesis Nonshivering thermogenesis is particularly important in certain physiological contexts: Thermoregulation: It helps maintain body temperature in cold environments, especially in infants and small mammals that have a high surface area-to-volume ratio. Energy Expenditure: This process contributes to overall energy expenditure and can play a role in weight management and metabolic health. Adaptation to Cold: Animals that hibernate or live in cold climates rely heavily on nonshivering thermogenesis to survive low temperatures. 14) Reduction Potentials Reduction Potential Overview Reduction potential is a measure of the tendency of a chemical species to acquire electrons and thereby be reduced. It is denoted as Eº' and is expressed in volts (V). The standard reduction potential is crucial in understanding biochemical reactions, particularly in the context of the electron transport chain (ETC) and oxidative phosphorylation. The relationship between Gibbs free energy change (ΔGº') and standard reduction potential is given by the equation: ΔGº' = -nF Eº' where: n = number of moles of electrons transferred F = Faraday's constant (approximately 96.485 kJ/V·mol) Eº' = standard reduction potential A higher Eº' value indicates a stronger tendency for a species to be reduced. Interpreting Standard Reduction Potentials In biochemical reactions, the standard reduction potentials of half-reactions can be compared to determine which species will act as the electron donor and which will act as the electron acceptor. A half-reaction with a more negative reduction potential will donate electrons (oxidation). A half-reaction with a more positive reduction potential will accept electrons (reduction). For example, in the oxidation of NADH to NAD+, NADH donates electrons to O2, which is reduced to H2O. This process is fundamental in cellular respiration and energy production. Standard Reduction Potentials Table The following table lists some biochemically important half-reactions along with their standard reduction potentials: Half-Reaction Eº' (V) 2H⁺ + 2e⁻ → H₂ 0.815 NO₃⁻ + 2H⁺ + 2e⁻ → NO₂⁻ + H₂O 0.420 Cytochrome a (Fe³⁺) + e⁻ → Cytochrome a (Fe²⁺) 0.385  O₂ + 4H⁺ + 4e⁻ → 2H₂O 0.295 Cytochrome c (Fe³⁺) + e⁻ → Cytochrome c (Fe²⁺) 0.235 Ubiquinone + 2H⁺ + 2e⁻ → Ubiquinol 0.045 FAD + 2H⁺ + 2e⁻ → FADH₂ -0.040 NAD⁺ + H⁺ + 2e⁻ → NADH -0.315 This table illustrates the varying tendencies of different molecules to gain electrons, which is essential for understanding their roles in metabolic pathways and the electron transport chain Page 11.