CHAPTER 36: OXIDATIVE PHOSPHORYLATION
Chapter 36: Oxidative Phosphorylation
Mitochondrial NADH and FADH2 are energy rich molecules because each contains a pair of electrons that has high transfer potential. When these electrons are transferred in the inner mitochondrial membrane between protein carriers to molecular oxygen (O2), considerable energy is liberated which can be used to generate ATP. This process of oxidative phosphorylation utilizes the electron transport chain (ETC), also known as the respiratory chain, which is the major consumer of O2 in mammalian cells.
Movement of Electrons from Cytoplasmic NADH to the Mitochondrial ETC Intact mitochondrial membranes are impermeable to NADH and NAD+ , and in order for glycolysis to continue, NAD+ must be continually regenerated in the cytoplasm. Therefore, reducing equivalents (i.e., electrons) from NADH, rather than NADH itself, are carried across mitochondrial membranes by either malate (Mal) or glycerol 3-phosphate, thus allowing for cytoplasmic NAD+ reformation, and for NADH and/or FADH2 utilization in the mitochondrial ETC. In the Mal shuttle, reducing equivalents from NADH are accepted by oxaloacetate (OAA), thus forming Mal which crosses mitochondrial membranes via an a-ketoglutarate (a-KG=)-Mal antiporter. Inside mitochondria, NADH is regenerated from Mal, and OAA, which also cannot cross mitochondrial membranes, is returned to the cytoplasm via reversible conversion to aspartate (Asp). The amine group from Asp is transferred to a-KG= in the cytoplasm, thus forming glutamate (Glu), which is returned to mitochondria via an Asp-Glu antiporter. Inside mitochondria, Glu transfers its amine group to OAA, thus reforming Asp and completing the shuttle.
Another carrier of reducing equivalents is glycerol 3-P, which, like Mal and Asp, readily traverses mitochondrial membranes. This shuttle transfers electrons from NADH to dihydroxyacetone phosphate (DHAP), thus forming glycerol 3-P (and NAD+ ) in the cytoplasm. Glycerol 3-P then crosses the outer mitochondrial membrane, and is reoxidized to DHAP by the FAD prosthetic group of glycerol 3-P dehydrogenase. FADH2 is thus formed on the inner mitochondrial membrane, and DHAP diffuses back into the cytosol to complete the shuttle. In some species, activity of this shuttle decreases after thyroidectomy. Although it is present in insect flight muscle, the brain, brown adipose tissue, white muscle tissue and the liver of mammals, in other tissues (e.g., heart muscle), mitochondrial glycerol 3-P dehydrogenase is deficient. It is therefore believed that the malate shuttle is of more universal utility than the glycerol 3-P shuttle, particularly since 3 rather than 2 ATP can be generated per atom of O2 consumed.
Oxidation and Reduction: Oxidation is a process in which electrons are removed, and reduction one in which electrons are gained. When one compound is oxidized, another must be reduced. The ETC functions by passing electrons from compounds with less reductive potential, such as NADH and FADH2, to those with more reductive potential, such as coenzyme Q (CoQ) and cytochrome c (Cyt c). The final recipient of electrons in the respiratory chain is molecular oxygen (O2), which together with hydrogen forms water (H2O).
The ETC contains four protein complexes and two mobile electron carriers. As membrane bound complexes, these carriers (CoQ and Cyt c) accept electrons from the preceding complex, then pass them on to the subsequent complex. Complex I, known as NADH-CoQ reductase, passes two electrons from NADH to CoQ. It contains 25 different proteins including several nonheme iron proteins, and a covalently bound flavin mononucleotide (FMN). Like FAD, FMN is derived from water-soluble vitamin B2 (riboflavin; see Chapter 40). Flavin mononucleotide receives two electrons from NADH, and passes them to a set of iron-sulfur (FeS) proteins (Fe2S2 and Fe4S4; nonheme prosthetic groups), which then pass them on to CoQ. This compound (CoQ), also called ubiquinone, is not firmly attached to any protein, and serves as a lipid-soluble mobile electron carrier.
The second complex, succinyl-CoQ reductase, serves to bind succinate, FADH2, and enzymes containing bound FADH2 to the rest of the ETC. Two electrons are transferred from FADH2 through several FeS proteins, and finally to CoQ. The FADH2 that enters the ETC at this point is either generated from the mitochondrial succinate to fumarate reaction, or from the glycerol 3-phosphate shuttle. Since this route to CoQ bypasses the first ATP-generating step in NADH oxidation, only 2 (rather than 3) ATP can be generated from FADH2 oxidation.
Cytochrome c reductase is the third complex of the ETC, and it contains an FeS protein as well as cytochromes b and c1 (which are heme-containing proteins). Like the iron in complex I, the heme iron of complex III is reduced from the ferric (Fe+++) to ferrous (Fe++) state. Complex III transfers two electrons from CoQ to Cyt c, the second mobile electron carrier. Unlike the large enzyme complexes, Cyt c is a small heme-containing protein with only 104 amino acid residues. It is loosely attached to the inner mitochondrial membrane rather than being embedded in it. As with the other cytochromes, the iron atom of heme in Cyt c changes from the +3 to +2 state on reduction.
The final complex (IV) is Cyt c oxidase. It transfers a pair of electrons from each of two Cyt c molecules and a copper (Cu)-containing enzyme to O2. Like complex III, the Cyt c oxidase complex contains heme iron as part of cytochromes a and a3. During electron transfer, the copper-containing enzyme changes charge from +2 to +1 on reduction.
Phosphorylation: As electrons are passed between NADH and CoQ, CoQ and Cyt c, and Cyt c and O2, protons are ejected from the inner mitochondrial membrane to re-enter the mitochondrial matrix, and flow through protrusions (F0 and F1) on the inner membrane. The F0 component spans the inner membrane, forming a H+ channel, and the F1 component protrudes into the matrix and contains the active ATP synthase site where ADP and Pi condense to form ATP. Once ATP is produced it moves through the inner mitochondrial membrane in exchange for ADP from extramitochondrial sites via a membrane ADP/ATP antiporter.
Inhibitors and Uncouplers: Inhibitors that arrest cellular respiration may block the ETC at any of 4 sites. The first inhibits Complex I (e.g., barbiturates and the insecticide and fish poison, rotenone); the second Complex II (e.g., malonate, carboxin and TTFA (an Fe-chelating agent)); the third Complex III (e.g., BAL (dimercaprol), and the antibiotic, antimycin); and the fourth Complex IV (e.g., the classic poisons hydrogen sulfide (H2S), carbon monoxide (CO), and cyanide (CN- )). Oligomycin, a compound that blocks movement of H+ through the F0 channel, inhibits ATP synthesis.
The action of uncouplers is to dissociate oxidation from phosphorylation. When this occurs, NADH and FADH2 are oxidized, heat is produced, but none of the energy from oxidation is trapped as ATP. This may be likened unto a car engine in which the clutch has been uncoupled and the engine revved up; a great deal of fuel is burned but, since none of the energy is used for propulsion of the vehicle, all energy escapes as heat. At times this can be metabolically useful, for it is a means of generating heat during hibernation, in the immediate postnatal period, and in animals adapted to the cold.
Examples of exogenous uncouplers are dinitrocresol, pentachlorophenol, m-chlorocarbonyl cyanide phenylhydrazone (CCCP), valinomycin, gramicidin, and the compound that has been studied most, 2,4-dinitrophenol. These compounds allow protons to pass into mitochondria, thereby destroying the proton (i.e., pH) gradient necessary for ATP production. Brown adipose tissue is specialized for heat generation, and contains abundant mitochondria (which impart brown color to the tissue). Large blood vessels of newborns are surrounded by brown adipose tissue, where the oxidation of fatty acids releases heat that helps maintain the temperature of circulating blood.
An inner-membrane protein called thermogenin is a natural uncoupler of oxidative phosphorylation, and acts as a transmembrane H+ transporter. In addition to brown adipose tissue, it is found in muscle-cell mitochondria of seals and other animals adapted to the cold. Like other uncouplers, it short-circuits the proton concentration gradient. Fatty acids can also act as endogenous uncouplers in mitochondria containing thermogenin. In turn, norepinephrine controls the release of fatty acids from (white) adipocytes, and cold stress leads to thyroxine release that also assists in lipolysis, and the uncoupling of oxidation from phosphorylation. Mitochondria containing this protein can thus function as ATP generators, or as miniature furnaces. Additionally, some investigators feel that one function of peroxisomes is heat rather than ATP generation, as a product of long-chain fatty acid oxidation.
SUMMARY
Chapter 36 discusses oxidative phosphorylation, which is the process of generating ATP using the energy released from the transfer of electrons in the inner mitochondrial membrane. NADH and FADH2 are energy-rich molecules that carry electrons and transfer them to molecular oxygen through the electron transport chain (ETC). The ETC consists of four protein complexes and two mobile electron carriers. Complex I transfers electrons from NADH to coenzyme Q (CoQ), complex II transfers electrons from FADH2 to CoQ, complex III transfers electrons from CoQ to cytochrome c (Cyt c), and complex IV transfers electrons from Cyt c to molecular oxygen. As electrons are passed along the ETC, protons are ejected from the inner mitochondrial membrane, creating a proton gradient that drives ATP synthesis by ATP synthase. Inhibitors can block different sites in the ETC, while uncouplers dissociate oxidation from phosphorylation, resulting in the production of heat instead of ATP. Examples of uncouplers include dinitrocresol, pentachlorophenol, and 2,4-dinitrophenol. Brown adipose tissue contains abundant mitochondria and is specialized for heat generation. Thermogenin, an inner-membrane protein, acts as a natural uncoupler in brown adipose tissue and muscle-cell mitochondria of cold-adapted animals. Fatty acids can also act as endogenous uncouplers.
OUTLINE
I. Introduction
Mitochondrial NADH and FADH2 are energy-rich molecules
Electron transfer in the inner mitochondrial membrane generates ATP
Oxidative phosphorylation utilizes the electron transport chain (ETC)
II. Movement of Electrons from Cytoplasmic NADH to the Mitochondrial ETC
NADH cannot cross mitochondrial membranes
Malate shuttle and glycerol 3-phosphate shuttle transport reducing equivalents across membranes
III. Oxidation and Reduction
Oxidation is the removal of electrons, reduction is the gain of electrons
ETC passes electrons from NADH and FADH2 to compounds with more reductive potential
Molecular oxygen is the final recipient of electrons, forming water
IV. Components of the ETC
Four protein complexes and two mobile electron carriers
Complex I (NADH-CoQ reductase) transfers electrons from NADH to CoQ
Complex II (succinyl-CoQ reductase) transfers electrons from FADH2 to CoQ
Complex III (cytochrome c reductase) transfers electrons from CoQ to Cyt c
Complex IV (cytochrome c oxidase) transfers electrons from Cyt c to O2
V. Phosphorylation
Protons are ejected from the inner mitochondrial membrane during electron transfer
Protons re-enter the mitochondrial matrix through ATP synthase, generating ATP
VI. Inhibitors and Uncouplers
Inhibitors block the ETC at different sites
Uncouplers dissociate oxidation from phosphorylation, generating heat instead of ATP
VII. Examples of Uncouplers
Dinitrocresol, pentachlorophenol, CCCP, valinomycin, gramicidin, 2,4-dinitrophenol
Thermogenin is a natural uncoupler found in brown adipose tissue and muscle-cell mitochondria
VIII. Endogenous Uncouplers
Fatty acids and norepinephrine can act as endogenous uncouplers
Peroxisomes may also play a role in heat generation
Note: This outline provides a concise summary of the main points covered in Chapter 36
QUESTIONS
Flashcard 1:
Question: What is the process of oxidative phosphorylation?
Answer: Oxidative phosphorylation is the process in which electrons are transferred in the inner mitochondrial membrane between protein carriers to molecular oxygen (O2), generating ATP.
Flashcard 2:
Question: How are reducing equivalents from NADH carried across mitochondrial membranes?
Answer: Reducing equivalents from NADH are carried across mitochondrial membranes by either malate (Mal) or glycerol 3-phosphate, allowing for cytoplasmic NAD+ reformation and utilization of NADH and/or FADH2 in the mitochondrial ETC.
Flashcard 3:
Question: What are the four protein complexes in the electron transport chain (ETC)?
Answer: The four protein complexes in the ETC are NADH-CoQ reductase (Complex I), succinyl-CoQ reductase (Complex II), cytochrome c reductase (Complex III), and cytochrome c oxidase (Complex IV).
Flashcard 4:
Question: What is the role of ATP synthase in phosphorylation?
Answer: ATP synthase is the enzyme responsible for the synthesis of ATP. It contains the active site where ADP and Pi condense to form ATP.
Flashcard 5:
Question: What is the function of uncouplers in cellular respiration?
Answer: Uncouplers dissociate oxidation from phosphorylation, allowing for the oxidation of NADH and FADH2 but preventing the trapping of energy as ATP.
Movement of Electrons from Cytoplasmic NADH to the Mitochondrial ETC
Malate Shuttle
Glycerol 3-P Shuttle
Oxidation and Reduction
Electron transfer from compounds with less reductive potential to those with more reductive potential
Final recipient of electrons is molecular oxygen
Components of the Electron Transport Chain
Complex I: NADH-CoQ reductase
Complex II: Succinyl-CoQ reductase
Complex III: Cytochrome c reductase
Complex IV: Cytochrome c oxidase
Phosphorylation
Proton ejection and flow through ATP synthase
ATP production and exchange with ADP
Inhibitors and Uncouplers
Inhibitors of ETC complexes
Uncouplers that dissociate oxidation from phosphorylation
Exogenous and Endogenous Uncouplers
Examples of exogenous uncouplers
Thermogenin as a natural uncoupler
Fatty acids as endogenous uncouplers
Heat Generation in Brown Adipose Tissue
Role of mitochondria and thermogenin
Release of fatty acids and uncoupling of oxidation from phosphorylation
Peroxisomes and Heat Generation
Potential function of peroxisomes in heat generation
Malate Shuttle
Oxaloacetate (OAA)
α-ketoglutarate (α-KG)
Aspartate (Asp)
Glutamate (Glu)
Glycerol 3-P Shuttle
Dihydroxyacetone phosphate (DHAP)
Glycerol 3-P dehydrogenase
FADH2
Components of the Electron Transport Chain
Flavin mononucleotide (FMN)
Iron-sulfur (FeS) proteins
Coenzyme Q (CoQ)
Cytochrome c (Cyt c)
Heme-containing proteins
Read and understand the concept of oxidative phosphorylation and its importance in generating ATP.
Focus on the movement of electrons from cytoplasmic NADH to the mitochondrial ETC.
Study the Mal shuttle and the glycerol 3-P shuttle for transferring reducing equivalents across mitochondrial membranes.
Take notes on the key points and mechanisms involved in electron transfer.
Review the concepts of oxidation and reduction and their significance in the ETC.
Understand the transfer of electrons from compounds with less reductive potential to those with more reductive potential.
Study the role of coenzyme Q (CoQ) and cytochrome c (Cyt c) as mobile electron carriers.
Learn about the four protein complexes in the ETC and their functions in electron transfer.
Focus on the process of phosphorylation and how it is coupled with electron transfer in the ETC.
Understand the movement of protons across the inner mitochondrial membrane and their role in ATP synthesis.
Study the structure and function of ATP synthase and its active site.
Learn about the membrane ADP/ATP antiporter and the exchange of ATP and ADP.
Explore the inhibitors that can block cellular respiration at different sites in the ETC.
Study the effects of inhibitors on Complex I, Complex II, Complex III, and Complex IV.
Understand the action of oligomycin in inhibiting ATP synthesis.
Learn about uncouplers and their role in dissociating oxidation from phosphorylation.
Study examples of exogenous uncouplers and their impact on ATP production.
Focus on the role of thermogenin as a natural uncoupler of oxidative phosphorylation.
Understand its function in brown adipose tissue and muscle-cell mitochondria.
Study the effects of fatty acids as endogenous uncouplers in mitochondria containing thermogenin.
Explore the regulation of fatty acid release and the role of norepinephrine and thyroxine.
Review the concept of peroxisomes and their potential role in heat generation.
Chapter 36: Oxidative Phosphorylation
Mitochondrial NADH and FADH2 are energy rich molecules because each contains a pair of electrons that has high transfer potential. When these electrons are transferred in the inner mitochondrial membrane between protein carriers to molecular oxygen (O2), considerable energy is liberated which can be used to generate ATP. This process of oxidative phosphorylation utilizes the electron transport chain (ETC), also known as the respiratory chain, which is the major consumer of O2 in mammalian cells.
Movement of Electrons from Cytoplasmic NADH to the Mitochondrial ETC Intact mitochondrial membranes are impermeable to NADH and NAD+ , and in order for glycolysis to continue, NAD+ must be continually regenerated in the cytoplasm. Therefore, reducing equivalents (i.e., electrons) from NADH, rather than NADH itself, are carried across mitochondrial membranes by either malate (Mal) or glycerol 3-phosphate, thus allowing for cytoplasmic NAD+ reformation, and for NADH and/or FADH2 utilization in the mitochondrial ETC. In the Mal shuttle, reducing equivalents from NADH are accepted by oxaloacetate (OAA), thus forming Mal which crosses mitochondrial membranes via an a-ketoglutarate (a-KG=)-Mal antiporter. Inside mitochondria, NADH is regenerated from Mal, and OAA, which also cannot cross mitochondrial membranes, is returned to the cytoplasm via reversible conversion to aspartate (Asp). The amine group from Asp is transferred to a-KG= in the cytoplasm, thus forming glutamate (Glu), which is returned to mitochondria via an Asp-Glu antiporter. Inside mitochondria, Glu transfers its amine group to OAA, thus reforming Asp and completing the shuttle.
Another carrier of reducing equivalents is glycerol 3-P, which, like Mal and Asp, readily traverses mitochondrial membranes. This shuttle transfers electrons from NADH to dihydroxyacetone phosphate (DHAP), thus forming glycerol 3-P (and NAD+ ) in the cytoplasm. Glycerol 3-P then crosses the outer mitochondrial membrane, and is reoxidized to DHAP by the FAD prosthetic group of glycerol 3-P dehydrogenase. FADH2 is thus formed on the inner mitochondrial membrane, and DHAP diffuses back into the cytosol to complete the shuttle. In some species, activity of this shuttle decreases after thyroidectomy. Although it is present in insect flight muscle, the brain, brown adipose tissue, white muscle tissue and the liver of mammals, in other tissues (e.g., heart muscle), mitochondrial glycerol 3-P dehydrogenase is deficient. It is therefore believed that the malate shuttle is of more universal utility than the glycerol 3-P shuttle, particularly since 3 rather than 2 ATP can be generated per atom of O2 consumed.
Oxidation and Reduction: Oxidation is a process in which electrons are removed, and reduction one in which electrons are gained. When one compound is oxidized, another must be reduced. The ETC functions by passing electrons from compounds with less reductive potential, such as NADH and FADH2, to those with more reductive potential, such as coenzyme Q (CoQ) and cytochrome c (Cyt c). The final recipient of electrons in the respiratory chain is molecular oxygen (O2), which together with hydrogen forms water (H2O).
The ETC contains four protein complexes and two mobile electron carriers. As membrane bound complexes, these carriers (CoQ and Cyt c) accept electrons from the preceding complex, then pass them on to the subsequent complex. Complex I, known as NADH-CoQ reductase, passes two electrons from NADH to CoQ. It contains 25 different proteins including several nonheme iron proteins, and a covalently bound flavin mononucleotide (FMN). Like FAD, FMN is derived from water-soluble vitamin B2 (riboflavin; see Chapter 40). Flavin mononucleotide receives two electrons from NADH, and passes them to a set of iron-sulfur (FeS) proteins (Fe2S2 and Fe4S4; nonheme prosthetic groups), which then pass them on to CoQ. This compound (CoQ), also called ubiquinone, is not firmly attached to any protein, and serves as a lipid-soluble mobile electron carrier.
The second complex, succinyl-CoQ reductase, serves to bind succinate, FADH2, and enzymes containing bound FADH2 to the rest of the ETC. Two electrons are transferred from FADH2 through several FeS proteins, and finally to CoQ. The FADH2 that enters the ETC at this point is either generated from the mitochondrial succinate to fumarate reaction, or from the glycerol 3-phosphate shuttle. Since this route to CoQ bypasses the first ATP-generating step in NADH oxidation, only 2 (rather than 3) ATP can be generated from FADH2 oxidation.
Cytochrome c reductase is the third complex of the ETC, and it contains an FeS protein as well as cytochromes b and c1 (which are heme-containing proteins). Like the iron in complex I, the heme iron of complex III is reduced from the ferric (Fe+++) to ferrous (Fe++) state. Complex III transfers two electrons from CoQ to Cyt c, the second mobile electron carrier. Unlike the large enzyme complexes, Cyt c is a small heme-containing protein with only 104 amino acid residues. It is loosely attached to the inner mitochondrial membrane rather than being embedded in it. As with the other cytochromes, the iron atom of heme in Cyt c changes from the +3 to +2 state on reduction.
The final complex (IV) is Cyt c oxidase. It transfers a pair of electrons from each of two Cyt c molecules and a copper (Cu)-containing enzyme to O2. Like complex III, the Cyt c oxidase complex contains heme iron as part of cytochromes a and a3. During electron transfer, the copper-containing enzyme changes charge from +2 to +1 on reduction.
Phosphorylation: As electrons are passed between NADH and CoQ, CoQ and Cyt c, and Cyt c and O2, protons are ejected from the inner mitochondrial membrane to re-enter the mitochondrial matrix, and flow through protrusions (F0 and F1) on the inner membrane. The F0 component spans the inner membrane, forming a H+ channel, and the F1 component protrudes into the matrix and contains the active ATP synthase site where ADP and Pi condense to form ATP. Once ATP is produced it moves through the inner mitochondrial membrane in exchange for ADP from extramitochondrial sites via a membrane ADP/ATP antiporter.
Inhibitors and Uncouplers: Inhibitors that arrest cellular respiration may block the ETC at any of 4 sites. The first inhibits Complex I (e.g., barbiturates and the insecticide and fish poison, rotenone); the second Complex II (e.g., malonate, carboxin and TTFA (an Fe-chelating agent)); the third Complex III (e.g., BAL (dimercaprol), and the antibiotic, antimycin); and the fourth Complex IV (e.g., the classic poisons hydrogen sulfide (H2S), carbon monoxide (CO), and cyanide (CN- )). Oligomycin, a compound that blocks movement of H+ through the F0 channel, inhibits ATP synthesis.
The action of uncouplers is to dissociate oxidation from phosphorylation. When this occurs, NADH and FADH2 are oxidized, heat is produced, but none of the energy from oxidation is trapped as ATP. This may be likened unto a car engine in which the clutch has been uncoupled and the engine revved up; a great deal of fuel is burned but, since none of the energy is used for propulsion of the vehicle, all energy escapes as heat. At times this can be metabolically useful, for it is a means of generating heat during hibernation, in the immediate postnatal period, and in animals adapted to the cold.
Examples of exogenous uncouplers are dinitrocresol, pentachlorophenol, m-chlorocarbonyl cyanide phenylhydrazone (CCCP), valinomycin, gramicidin, and the compound that has been studied most, 2,4-dinitrophenol. These compounds allow protons to pass into mitochondria, thereby destroying the proton (i.e., pH) gradient necessary for ATP production. Brown adipose tissue is specialized for heat generation, and contains abundant mitochondria (which impart brown color to the tissue). Large blood vessels of newborns are surrounded by brown adipose tissue, where the oxidation of fatty acids releases heat that helps maintain the temperature of circulating blood.
An inner-membrane protein called thermogenin is a natural uncoupler of oxidative phosphorylation, and acts as a transmembrane H+ transporter. In addition to brown adipose tissue, it is found in muscle-cell mitochondria of seals and other animals adapted to the cold. Like other uncouplers, it short-circuits the proton concentration gradient. Fatty acids can also act as endogenous uncouplers in mitochondria containing thermogenin. In turn, norepinephrine controls the release of fatty acids from (white) adipocytes, and cold stress leads to thyroxine release that also assists in lipolysis, and the uncoupling of oxidation from phosphorylation. Mitochondria containing this protein can thus function as ATP generators, or as miniature furnaces. Additionally, some investigators feel that one function of peroxisomes is heat rather than ATP generation, as a product of long-chain fatty acid oxidation.
SUMMARY
Chapter 36 discusses oxidative phosphorylation, which is the process of generating ATP using the energy released from the transfer of electrons in the inner mitochondrial membrane. NADH and FADH2 are energy-rich molecules that carry electrons and transfer them to molecular oxygen through the electron transport chain (ETC). The ETC consists of four protein complexes and two mobile electron carriers. Complex I transfers electrons from NADH to coenzyme Q (CoQ), complex II transfers electrons from FADH2 to CoQ, complex III transfers electrons from CoQ to cytochrome c (Cyt c), and complex IV transfers electrons from Cyt c to molecular oxygen. As electrons are passed along the ETC, protons are ejected from the inner mitochondrial membrane, creating a proton gradient that drives ATP synthesis by ATP synthase. Inhibitors can block different sites in the ETC, while uncouplers dissociate oxidation from phosphorylation, resulting in the production of heat instead of ATP. Examples of uncouplers include dinitrocresol, pentachlorophenol, and 2,4-dinitrophenol. Brown adipose tissue contains abundant mitochondria and is specialized for heat generation. Thermogenin, an inner-membrane protein, acts as a natural uncoupler in brown adipose tissue and muscle-cell mitochondria of cold-adapted animals. Fatty acids can also act as endogenous uncouplers.
OUTLINE
I. Introduction
Mitochondrial NADH and FADH2 are energy-rich molecules
Electron transfer in the inner mitochondrial membrane generates ATP
Oxidative phosphorylation utilizes the electron transport chain (ETC)
II. Movement of Electrons from Cytoplasmic NADH to the Mitochondrial ETC
NADH cannot cross mitochondrial membranes
Malate shuttle and glycerol 3-phosphate shuttle transport reducing equivalents across membranes
III. Oxidation and Reduction
Oxidation is the removal of electrons, reduction is the gain of electrons
ETC passes electrons from NADH and FADH2 to compounds with more reductive potential
Molecular oxygen is the final recipient of electrons, forming water
IV. Components of the ETC
Four protein complexes and two mobile electron carriers
Complex I (NADH-CoQ reductase) transfers electrons from NADH to CoQ
Complex II (succinyl-CoQ reductase) transfers electrons from FADH2 to CoQ
Complex III (cytochrome c reductase) transfers electrons from CoQ to Cyt c
Complex IV (cytochrome c oxidase) transfers electrons from Cyt c to O2
V. Phosphorylation
Protons are ejected from the inner mitochondrial membrane during electron transfer
Protons re-enter the mitochondrial matrix through ATP synthase, generating ATP
VI. Inhibitors and Uncouplers
Inhibitors block the ETC at different sites
Uncouplers dissociate oxidation from phosphorylation, generating heat instead of ATP
VII. Examples of Uncouplers
Dinitrocresol, pentachlorophenol, CCCP, valinomycin, gramicidin, 2,4-dinitrophenol
Thermogenin is a natural uncoupler found in brown adipose tissue and muscle-cell mitochondria
VIII. Endogenous Uncouplers
Fatty acids and norepinephrine can act as endogenous uncouplers
Peroxisomes may also play a role in heat generation
Note: This outline provides a concise summary of the main points covered in Chapter 36
QUESTIONS
Flashcard 1:
Question: What is the process of oxidative phosphorylation?
Answer: Oxidative phosphorylation is the process in which electrons are transferred in the inner mitochondrial membrane between protein carriers to molecular oxygen (O2), generating ATP.
Flashcard 2:
Question: How are reducing equivalents from NADH carried across mitochondrial membranes?
Answer: Reducing equivalents from NADH are carried across mitochondrial membranes by either malate (Mal) or glycerol 3-phosphate, allowing for cytoplasmic NAD+ reformation and utilization of NADH and/or FADH2 in the mitochondrial ETC.
Flashcard 3:
Question: What are the four protein complexes in the electron transport chain (ETC)?
Answer: The four protein complexes in the ETC are NADH-CoQ reductase (Complex I), succinyl-CoQ reductase (Complex II), cytochrome c reductase (Complex III), and cytochrome c oxidase (Complex IV).
Flashcard 4:
Question: What is the role of ATP synthase in phosphorylation?
Answer: ATP synthase is the enzyme responsible for the synthesis of ATP. It contains the active site where ADP and Pi condense to form ATP.
Flashcard 5:
Question: What is the function of uncouplers in cellular respiration?
Answer: Uncouplers dissociate oxidation from phosphorylation, allowing for the oxidation of NADH and FADH2 but preventing the trapping of energy as ATP.
Movement of Electrons from Cytoplasmic NADH to the Mitochondrial ETC
Malate Shuttle
Glycerol 3-P Shuttle
Oxidation and Reduction
Electron transfer from compounds with less reductive potential to those with more reductive potential
Final recipient of electrons is molecular oxygen
Components of the Electron Transport Chain
Complex I: NADH-CoQ reductase
Complex II: Succinyl-CoQ reductase
Complex III: Cytochrome c reductase
Complex IV: Cytochrome c oxidase
Phosphorylation
Proton ejection and flow through ATP synthase
ATP production and exchange with ADP
Inhibitors and Uncouplers
Inhibitors of ETC complexes
Uncouplers that dissociate oxidation from phosphorylation
Exogenous and Endogenous Uncouplers
Examples of exogenous uncouplers
Thermogenin as a natural uncoupler
Fatty acids as endogenous uncouplers
Heat Generation in Brown Adipose Tissue
Role of mitochondria and thermogenin
Release of fatty acids and uncoupling of oxidation from phosphorylation
Peroxisomes and Heat Generation
Potential function of peroxisomes in heat generation
Malate Shuttle
Oxaloacetate (OAA)
α-ketoglutarate (α-KG)
Aspartate (Asp)
Glutamate (Glu)
Glycerol 3-P Shuttle
Dihydroxyacetone phosphate (DHAP)
Glycerol 3-P dehydrogenase
FADH2
Components of the Electron Transport Chain
Flavin mononucleotide (FMN)
Iron-sulfur (FeS) proteins
Coenzyme Q (CoQ)
Cytochrome c (Cyt c)
Heme-containing proteins
Read and understand the concept of oxidative phosphorylation and its importance in generating ATP.
Focus on the movement of electrons from cytoplasmic NADH to the mitochondrial ETC.
Study the Mal shuttle and the glycerol 3-P shuttle for transferring reducing equivalents across mitochondrial membranes.
Take notes on the key points and mechanisms involved in electron transfer.
Review the concepts of oxidation and reduction and their significance in the ETC.
Understand the transfer of electrons from compounds with less reductive potential to those with more reductive potential.
Study the role of coenzyme Q (CoQ) and cytochrome c (Cyt c) as mobile electron carriers.
Learn about the four protein complexes in the ETC and their functions in electron transfer.
Focus on the process of phosphorylation and how it is coupled with electron transfer in the ETC.
Understand the movement of protons across the inner mitochondrial membrane and their role in ATP synthesis.
Study the structure and function of ATP synthase and its active site.
Learn about the membrane ADP/ATP antiporter and the exchange of ATP and ADP.
Explore the inhibitors that can block cellular respiration at different sites in the ETC.
Study the effects of inhibitors on Complex I, Complex II, Complex III, and Complex IV.
Understand the action of oligomycin in inhibiting ATP synthesis.
Learn about uncouplers and their role in dissociating oxidation from phosphorylation.
Study examples of exogenous uncouplers and their impact on ATP production.
Focus on the role of thermogenin as a natural uncoupler of oxidative phosphorylation.
Understand its function in brown adipose tissue and muscle-cell mitochondria.
Study the effects of fatty acids as endogenous uncouplers in mitochondria containing thermogenin.
Explore the regulation of fatty acid release and the role of norepinephrine and thyroxine.
Review the concept of peroxisomes and their potential role in heat generation.
