8885dc19690-7503/1/0411:32 AM Page715 6mac76:385 19.2 ATP Synthes 715 Intermembrane Malate- Matrix CH2-C-C00 transporte ooC-CHy-C-Co0 H++ NADH NAdH + h 00C-CHy-C-COo NH3 Oxaloacetate OoC-CH -CH2-C-Co0 Ooc-CH2-CH2-c-Coo- O0C-CH H a-Ketoglutarate a-Ketoglutarate --o Aspartate ooC-CHy-C-CO0 ooC-CHo-C-C00 FIGURE 19-27 Malate-aspartate shuttle. This shuttle for transporting two reducing equivalents to NAD, and the resulting NADH is oxi- reducing equivalents from cytosolic NADH into the mitochondrial ma. dized by the respiratory chain. The oxaloacetate formed from malate trix is used in liver, kidney, and heart. (1 NADH in the cytosol (in- nnot pass directly into the cytosol. 4) It is first transaminated to as. termembrane space) passes two reducing equivalents to oxaloacetate, partate, which can leave via the glutamate-aspartate transporter. producing malate. (2) Malate crosses the inner membrane via the 6 Oxaloacetate is regenerated in the cytosol, completing the cycle malate-d-ketoglutarate transporter. 3 )In the matrix, malate passes Glycolysis NAD+ cytosolic NADH +H hydrogent H CHoO FIGURE 19-28 Glycerol 3-phosphate shuttle. This altemative means of moving reducing equivalents from the cytosol to the phosphate mitochondrial matrix operates in skeletal muscle and the CH,OH mitochondrial rain In the cytosol, dihydroxyacetone phosphate accepts CHOH two reducing equivalents from NADH in a reaction catalyzed dehydrogenase by cytosolic glycerol 3-phosphate dehydrogenase. An isozyme CH20①/EAD of glycerol 3-phosphate dehydrogenase bound to the outer face of the inner membrane then transfers two reducing III equivalents from glycerol 3-phosphate in the intermembrane space to ubiquinone. Note that this shuttle does not involve Matrix
Intermembrane space Oxaloacetate Aspartate Oxaloacetate malate dehydrogenase malate dehydrogenase aspartate aminotransferase aspartate aminotransferase Glutamate -Ketoglutarate Glutamate -Ketoglutarate Glutamate-aspartate transporter Malate– -ketoglutarate transporter 2 3 NAD+ NADH 4 5 Matrix NAD+ H+ + NADH 6 1 Aspartate Malate Malate OH C COO OOC CH2 H NH3 NH3 NH3 NH3 C COO CH2 CH2 H O C COO CH2 CH2 C COO CH2 H C COO CH2 CH2 H C COO CH2 H OH C COO CH2 H O C COO CH2 CH2 O C COO OOC CH2 O C COO OOC CH2 OOC OOC OOC OOC OOC OOC OOC H+ + FIGURE 19–27 Malate-aspartate shuttle. This shuttle for transporting reducing equivalents from cytosolic NADH into the mitochondrial matrix is used in liver, kidney, and heart. 1 NADH in the cytosol (intermembrane space) passes two reducing equivalents to oxaloacetate, producing malate. 2 Malate crosses the inner membrane via the malate–-ketoglutarate transporter. 3 In the matrix, malate passes two reducing equivalents to NAD, and the resulting NADH is oxidized by the respiratory chain. The oxaloacetate formed from malate cannot pass directly into the cytosol. 4 It is first transaminated to aspartate, which 5 can leave via the glutamate-aspartate transporter. 6 Oxaloacetate is regenerated in the cytosol, completing the cycle. 19.2 ATP Synthesis 715 Q Matrix NAD+ NADH + H+ Glycolysis Dihydroxyacetone phosphate Glycerol 3- phosphate FAD FADH2 III CH2OH CH2 CHOH O P P –– – – CH2OH CH2 C O –– – –– – O mitochondrial glycerol 3-phosphate dehydrogenase cytosolic glycerol 3-phosphate dehydrogenase FIGURE 19–28 Glycerol 3-phosphate shuttle. This alternative means of moving reducing equivalents from the cytosol to the mitochondrial matrix operates in skeletal muscle and the brain. In the cytosol, dihydroxyacetone phosphate accepts two reducing equivalents from NADH in a reaction catalyzed by cytosolic glycerol 3-phosphate dehydrogenase. An isozyme of glycerol 3-phosphate dehydrogenase bound to the outer face of the inner membrane then transfers two reducing equivalents from glycerol 3-phosphate in the intermembrane space to ubiquinone. Note that this shuttle does not involve membrane transport systems. 8885d_c19_690-750 3/1/04 11:32 AM Page 715 mac76 mac76:385_reb:
8885dc19690-7503/1/0411:32 AM Page716 6mac76:385 Chapter 19 Oxidative Phosphorylation and Photophosphorylation ATP synthase carries out "rotational catalysis, CoA (see Table 17-1). A similar calculation can be made in which the flow of protons through Fo causes for the aTP yield from oxidation of each of the amino each of three nucleotide-binding sites in Fito acids(Chapter 18). Aerobic oxidative pathways that cycle from(ADP t Pi-bound to ATP-bound to result in electron transfer to O2 accompanied by oxida- tive phosphorylation therefore account for the vast I ATP formation on the enzyme requires little majority of the ATP produced in catabolism, so the reg energy; the role of the proton-motive force ulation of ATP production by oxidative phosphorylation to push ATP from its binding site on the to match the cells fluctuating needs for AtP is ab synthase. solutely essential. a The ratio of ATP synthesized per -Og reduced to HyO(the P/O ratio) is about 2.5 when elec Oxidative Phosphorylation Is Regulated by Cellular trons enter the respiratory chain at Complex I Energy Needs and 1.5 when electrons enter at coQ The rate of respiration(Oe consumption) in mitochon- nergy conserved in a proton gradient can dria is tightly regulated; it is generally limited by the drive solute transport uphill across a membrane. availability of ADP as a substrate for phosphorylation. Dependence of the rate of O2 consumption on the avail The inner mitochondrial membrane is impermeable to NADH and NAD, but NAI ability of the Pi acceptor ADP (Fig. 19-18b), the ac- ceptor control of respiration, can be remarkable. In equivalents are moved from the cytosol to the some animal tissues, the acceptor control ratio, the matrix by either of two shuttles. NADH ratio of the maximal rate of ADP-induced O, consump equivalents moved in by the malate-aspartate tion to the basal rate in the absence of ADP, is at least shuttle enter the respiratory chain at Complex I and yield a P/O ratio of 2.5; those moved in The intracellular concentration of adP is one meas- by the glycerol 3-phosphate shuttle enter at ure of the energy status of cells. Another, related meas- CoQ and give a p/O ratio of 1.5 ure is the mass-action ratio of the ATP-ADP system [ATP(ADPIPiD. Normally this ratio is very high, so the ATP-ADP system is almost fully phosphorylated 19.3 Regulation of Oxidative When the rate of some energy-requiring process (pro- Phosphorylation tein synthesis, for example)increases, the rate of break down of ATP to ADP and Pi increases, lowering the Oxidative phosphorylation produces most of the ATP mass-action ratio. With more ADP available for oxida- made in aerobic cells. Complete oxidation of a molecule tive phosphorylation, the rate of respiration increases, of glucose to CO, yields 30 or 32 ATP (Table 19-5). causing regeneration of ATP. This continues until the comparison, glycolysis under anaerobic conditions mass-action ratio returns to its normal high level, at (lactate fermentation) yields only 2 ATP per glucose. which point respiration slows again. The rate of oxida Clearly, the evolution of oxidative phosphorylation pro- tion of cellular fuels is regulated with such sensitivity vided a tremendous increase in the energy efficiency of and precision that the [ATPy(adPIPid ratio fluctuates catabolism. Complete oxidation to COg of the coenzyme only slightly in most tissues, even during extreme vari- a derivative of palmitate (16: 0), which also occurs in ations in energy demand. In short, ATP is formed only the mitochondrial matrix, yields 108 ATP per palmitoyl- as fast as it is used in energy-requiring cellular activities TABLE 19-5 ATP Yield from Complete Oxidation of Glucose Direct product Final aiP 2 NADH(cytosolic) 3 or 5 2 ATP Pyruvate oxidation(two per gluc 2 NADH(mitochondrial matrix 6 NADH(mitochondrial matrix two per glucose) 2 FADH2 2 ATP or 2 GTP Total yield per glucose 300r32 The number depends on which shuttle system transfers reducing equivalents into the mitochondrion
■ ATP synthase carries out “rotational catalysis,” in which the flow of protons through Fo causes each of three nucleotide-binding sites in F1 to cycle from (ADP Pi )–bound to ATP-bound to empty conformations. ■ ATP formation on the enzyme requires little energy; the role of the proton-motive force is to push ATP from its binding site on the synthase. ■ The ratio of ATP synthesized per 1 2 O2 reduced to H2O (the P/O ratio) is about 2.5 when electrons enter the respiratory chain at Complex I, and 1.5 when electrons enter at CoQ. ■ Energy conserved in a proton gradient can drive solute transport uphill across a membrane. ■ The inner mitochondrial membrane is impermeable to NADH and NAD, but NADH equivalents are moved from the cytosol to the matrix by either of two shuttles. NADH equivalents moved in by the malate-aspartate shuttle enter the respiratory chain at Complex I and yield a P/O ratio of 2.5; those moved in by the glycerol 3-phosphate shuttle enter at CoQ and give a P/O ratio of 1.5. 19.3 Regulation of Oxidative Phosphorylation Oxidative phosphorylation produces most of the ATP made in aerobic cells. Complete oxidation of a molecule of glucose to CO2 yields 30 or 32 ATP (Table 19–5). By comparison, glycolysis under anaerobic conditions (lactate fermentation) yields only 2 ATP per glucose. Clearly, the evolution of oxidative phosphorylation provided a tremendous increase in the energy efficiency of catabolism. Complete oxidation to CO2 of the coenzyme A derivative of palmitate (16:0), which also occurs in the mitochondrial matrix, yields 108 ATP per palmitoylCoA (see Table 17–1). A similar calculation can be made for the ATP yield from oxidation of each of the amino acids (Chapter 18). Aerobic oxidative pathways that result in electron transfer to O2 accompanied by oxidative phosphorylation therefore account for the vast majority of the ATP produced in catabolism, so the regulation of ATP production by oxidative phosphorylation to match the cell’s fluctuating needs for ATP is absolutely essential. Oxidative Phosphorylation Is Regulated by Cellular Energy Needs The rate of respiration (O2 consumption) in mitochondria is tightly regulated; it is generally limited by the availability of ADP as a substrate for phosphorylation. Dependence of the rate of O2 consumption on the availability of the Pi acceptor ADP (Fig. 19–18b), the acceptor control of respiration, can be remarkable. In some animal tissues, the acceptor control ratio, the ratio of the maximal rate of ADP-induced O2 consumption to the basal rate in the absence of ADP, is at least ten. The intracellular concentration of ADP is one measure of the energy status of cells. Another, related measure is the mass-action ratio of the ATP-ADP system, [ATP]/([ADP][Pi ]). Normally this ratio is very high, so the ATP-ADP system is almost fully phosphorylated. When the rate of some energy-requiring process (protein synthesis, for example) increases, the rate of breakdown of ATP to ADP and Pi increases, lowering the mass-action ratio. With more ADP available for oxidative phosphorylation, the rate of respiration increases, causing regeneration of ATP. This continues until the mass-action ratio returns to its normal high level, at which point respiration slows again. The rate of oxidation of cellular fuels is regulated with such sensitivity and precision that the [ATP]/([ADP][Pi ]) ratio fluctuates only slightly in most tissues, even during extreme variations in energy demand. In short, ATP is formed only as fast as it is used in energy-requiring cellular activities. 716 Chapter 19 Oxidative Phosphorylation and Photophosphorylation Process Direct product Final ATP Glycolysis 2 NADH (cytosolic) 3 or 5* 2 ATP 2 Pyruvate oxidation (two per glucose) 2 NADH (mitochondrial matrix) 5 Acetyl-CoA oxidation in citric acid cycle 6 NADH (mitochondrial matrix) 15 (two per glucose) 2 FADH2 3 2 ATP or 2 GTP 2 Total yield per glucose 30 or 32 TABLE 19–5 ATP Yield from Complete Oxidation of Glucose * The number depends on which shuttle system transfers reducing equivalents into the mitochondrion. 8885d_c19_690-750 3/1/04 11:32 AM Page 716 mac76 mac76:385_reb:
8885dc19690-7503/1/0411:32 AM Page717 6mac76:385 19.3 Regulation of Oxidative Phosphorylation 717 FIGURE 19-29 Structure of Fr-ATPase in a complex with its regulatory protein IF1.(Derived from PDB ID 1OHH Two F, molecules are viewed here as in Figure 19-23c. The inhibitor IF (red)binds to the ap interface of the subunits in the diphosphate (ADP) conformation (aADP and BADP) freezing the two Fi complexes and thereby blocking ATP hydrolysis (and synthesis).(Parts of IF, that failed to resolve in crystals of Fi are shown in white outline as they occur in crystals of isolated IF1 This complex is stable only at the low glycolysis; when aerobic metabolism resumes, the cytosolic rises, the inhibitor is destabilized, and ATP synt An Inhibitory Protein Prevents ATP Hydrolysis cytochromes, whose heme groups are strong absorbers during Ischemia of visible light The mitochondria of brown fat are like those of other We have already encountered ATP synthase as an ATP. mammalian cells in all respects, except that they have Riven proton pump(see Fig. 11-39; Table 11-3), cat- a unique protein in their inner membrane. Thermo- alyzing the reverse of ArP synthesis. When a cell isis- genin, also called the uncoupling protein (Table 19-4) chemic(deprived of oxygen), as in a heart attack or provides a path for protons to return to the matrix troke, electron transfer to oxygen ceases, and so does without passing through the F F, complex(Fig. 19-30) the pumping of protons. The proton-motive force soon collapses. Under these conditions, the ATP synthase could operate in reverse, hydrolyzing ATP to pump pro- tons outward and causing a disastrous drop in ATP lev- Intermembrane els. This is prevented by a small (84 amino acids) pro- tein inhibitor, IFI, which simultaneously binds to two ATP synthase molecules, inhibiting their ATPase activ- ity(Fig. 19-29). IFi is inhibitory only in its dimeric form, which is favored at pH lower than 6.5. In a cell starved for oxygen, the main source of ATP becomes glycolysis, and the pyruvic or lactic acid thus formed lowers the III H in the cytosol and the mitochondrial matrix. This fa- vors IFI dimerization, leading to inhibition of the ATPase activity of ATP synthase, thereby preventing wasteful hydrolysis of ATP. When aerobic metabolism resumes production of pyruvic acid slows, the ph of the cytosol rises, the IFl dimer is destabilized, and the inhibition of ATP synthase is lifted Uncoupled Mitochondria in Brown Fat Produce Heat There is a remarkable and instructive exception to the general rule that respiration slows when the ATP supply is adequate. Most newborn mammals, including humans H+ ave a type of adipose tissue called brown fat in which ADP +P fuel oxidation serves not to produce ATP but to gener- ate heat to keep the newborn warm. This specialized adipose tissue is brown because of the presence of large ----=---------+H+ numbers of mitochondria and thus large amounts of FIGURE 19-30 Heat generation by uncoupled mitochondria. The un- coupling protein (thermogenin) of brown fat mitochondria, by Uncoupling viding an alternative route for protons to reenter the mitochondrial matrix,causes the energy conserved by proton pumping to be dissi- pated as he Heat
An Inhibitory Protein Prevents ATP Hydrolysis during Ischemia We have already encountered ATP synthase as an ATPdriven proton pump (see Fig. 11–39; Table 11–3), catalyzing the reverse of ATP synthesis. When a cell is ischemic (deprived of oxygen), as in a heart attack or stroke, electron transfer to oxygen ceases, and so does the pumping of protons. The proton-motive force soon collapses. Under these conditions, the ATP synthase could operate in reverse, hydrolyzing ATP to pump protons outward and causing a disastrous drop in ATP levels. This is prevented by a small (84 amino acids) protein inhibitor, IF1, which simultaneously binds to two ATP synthase molecules, inhibiting their ATPase activity (Fig. 19–29). IF1 is inhibitory only in its dimeric form, which is favored at pH lower than 6.5. In a cell starved for oxygen, the main source of ATP becomes glycolysis, and the pyruvic or lactic acid thus formed lowers the pH in the cytosol and the mitochondrial matrix. This favors IF1 dimerization, leading to inhibition of the ATPase activity of ATP synthase, thereby preventing wasteful hydrolysis of ATP. When aerobic metabolism resumes, production of pyruvic acid slows, the pH of the cytosol rises, the IF1 dimer is destabilized, and the inhibition of ATP synthase is lifted. Uncoupled Mitochondria in Brown Fat Produce Heat There is a remarkable and instructive exception to the general rule that respiration slows when the ATP supply is adequate. Most newborn mammals, including humans, have a type of adipose tissue called brown fat in which fuel oxidation serves not to produce ATP but to generate heat to keep the newborn warm. This specialized adipose tissue is brown because of the presence of large numbers of mitochondria and thus large amounts of cytochromes, whose heme groups are strong absorbers of visible light. The mitochondria of brown fat are like those of other mammalian cells in all respects, except that they have a unique protein in their inner membrane. Thermogenin, also called the uncoupling protein (Table 19–4), provides a path for protons to return to the matrix without passing through the FoF1 complex (Fig. 19–30). 19.3 Regulation of Oxidative Phosphorylation 717 FIGURE 19–29 Structure of bovine F1-ATPase in a complex with its regulatory protein IF1. (Derived from PDB ID 1OHH) Two F1 molecules are viewed here as in Figure 19–23c. The inhibitor IF1 (red) binds to the interface of the subunits in the diphosphate (ADP) conformation (ADP and ADP), freezing the two F1 complexes and thereby blocking ATP hydrolysis (and synthesis). (Parts of IF1 that failed to resolve in crystals of F1 are shown in white outline as they occur in crystals of isolated IF1.) This complex is stable only at the low cytosolic pH characteristic of cells that are producing ATP by glycolysis; when aerobic metabolism resumes, the cytosolic pH rises, the inhibitor is destabilized, and ATP synthase becomes active. I II III IV Cyt c Uncoupling protein (thermogenin) ADP + Pi ATP H+ H+ H+ Fo F1 Intermembrane space Matrix Heat FIGURE 19–30 Heat generation by uncoupled mitochondria. The uncoupling protein (thermogenin) of brown fat mitochondria, by providing an alternative route for protons to reenter the mitochondrial matrix, causes the energy conserved by proton pumping to be dissipated as heat. 8885d_c19_690-750 3/1/04 11:32 AM Page 717 mac76 mac76:385_reb:
8885dc19690-7503/1/0411:32 AM Page718 6mac76:385 718 Chapter 19 Oxidative Phosphorylation and Photophosphorylation FIGURE 19-31 Regulation of the ATP-producing pathways. This di- agram shows the interlocking regulation of glycolysis, pyruvate ox dation, the citric acid cycle, and oxidative phosphorylation by the rel- ative concentrations of ATP, ADP, and AMP, and by NADH. High [ATPI Glucose 6-phosphat (or low [ADP] and [AMPI) produces low rates of glycolysis, pyruvate oxidation, acetate oxidation via the citric acid cycle, and oxidative (AMP,ADP phosphorylation. All four pathways are accelerated when the use of phosphofructokinase- ATP and the formation of ADP, AMP, and Pi increase. The interlock ATP citrate ing of glycolysis and the citric acid cycle by citrate, which inhibits Fructose 1, 6-bisphosphate glycolysis, supplements the action of the adenine nucleotide system In addition, increased levels of NADH and acetyl-CoA also inhibit the oxidation of pyruvate to acetyl-CoA, and a high [NADHI/INAD] ra- ultiste tio inhibits the dehydrogenase reactions of the citric acid cycle(see Fig16-18) Phosphoenolpyruvate pyruvate kinase O ATP NADH Pyruvate ATP-Producing Pathways Are Coordinately Regulated 0↓的 AMP. ADP NAD+ The major catabolic pathways have interlocking and ATP NADH concerted regulatory mechanisms that allow them to Acetyl-CoA function together in an economical and self-regulating (ADP manner to produce ATP and biosynthetic precursors ate synthase The relative concentrations of atP and ADP control not only the rates of electron transfer and oxidative phos Citrate phorylation but also the rates of the citric acid cycle pyruvate oxidation, and glycolysis(Fig. 19-31). When ever ATP consumption increases, the rate of electron Isocitrate transfer and oxidative phosphorylation increases. Si multaneously, the rate of pyruvate oxidation via the cit- Citric ric acid cycle increases, increasing the flow of electrons 一8③mm into the respiratory chain. These events can in turn evoke an increase in the rate of glycolysis, increasing the rate of pyruvate formation. When conversion of ADP to ATP lowers the ADP concentration, acceptor control slows electron transfer and thus oxidative phosphoryla tion Glycolysis and the citric acid cycle are also slowed blister because ATP is an allosteric inhibitor of the glycolytic enzyme phosphofructokinase-l(see Fig. 15-18) and of pyruvate dehydrogenase(see Fig. 16-18) Oxaloacetate Phosphofructokinase-1 is also inhibited by citrate the first intermediate of the citric acid cycle. When the P cycle is"idling, citrate accumulates within mitochon- Oxidative NADH dria, then spills into the cytosol. When the concentra- )Respiratory chain tions of both ATP and citrate rise, they produce a con- NAD 20 certed allosteric inhibition of phosphofructokinase-1 ADP+ P: ATP that is greater than the sum of their individual effects slowing glycolysis As a result of this short-circuiting of protons, the en- SUMMARY 19.3 Regulation of Oxidative ergy of oxidation is not conserved by ATP formation but Phosphorylation is dissipated as heat, which contributes to maintaining the body temperature of the newborn. Hibernating an a Oxidative phosphorylation is regulated by imals also depend on uncoupled mitochondria of brown cellular energy demands. The intracellular [ADPI fat to generate heat during their long dormancy(see and the mass-action ratio [ATP(ADPIPiD are Box17-1) measures of a cell's energy status
As a result of this short-circuiting of protons, the energy of oxidation is not conserved by ATP formation but is dissipated as heat, which contributes to maintaining the body temperature of the newborn. Hibernating animals also depend on uncoupled mitochondria of brown fat to generate heat during their long dormancy (see Box 17–1). ATP-Producing Pathways Are Coordinately Regulated The major catabolic pathways have interlocking and concerted regulatory mechanisms that allow them to function together in an economical and self-regulating manner to produce ATP and biosynthetic precursors. The relative concentrations of ATP and ADP control not only the rates of electron transfer and oxidative phosphorylation but also the rates of the citric acid cycle, pyruvate oxidation, and glycolysis (Fig. 19–31). Whenever ATP consumption increases, the rate of electron transfer and oxidative phosphorylation increases. Simultaneously, the rate of pyruvate oxidation via the citric acid cycle increases, increasing the flow of electrons into the respiratory chain. These events can in turn evoke an increase in the rate of glycolysis, increasing the rate of pyruvate formation. When conversion of ADP to ATP lowers the ADP concentration, acceptor control slows electron transfer and thus oxidative phosphorylation. Glycolysis and the citric acid cycle are also slowed, because ATP is an allosteric inhibitor of the glycolytic enzyme phosphofructokinase-1 (see Fig. 15–18) and of pyruvate dehydrogenase (see Fig. 16–18). Phosphofructokinase-1 is also inhibited by citrate, the first intermediate of the citric acid cycle. When the cycle is “idling,” citrate accumulates within mitochondria, then spills into the cytosol. When the concentrations of both ATP and citrate rise, they produce a concerted allosteric inhibition of phosphofructokinase-1 that is greater than the sum of their individual effects, slowing glycolysis. SUMMARY 19.3 Regulation of Oxidative Phosphorylation ■ Oxidative phosphorylation is regulated by cellular energy demands. The intracellular [ADP] and the mass-action ratio [ATP]/([ADP][Pi ]) are measures of a cell’s energy status. 718 Chapter 19 Oxidative Phosphorylation and Photophosphorylation Glucose Pi Glycolysis Glucose 6-phosphate Fructose 1,6-bisphosphate AMP, ADP ATP, citrate hexokinase phosphofructokinase-1 multistep Phosphoenolpyruvate ADP ATP, NADH Pyruvate AMP, ADP, NAD ATP, NADH Acetyl-CoA ADP ATP, NADH Citrate ADP ATP pyruvate kinase -Ketoglutarate ATP, NADH Succinyl-CoA Oxaloacetate pyruvate dehydrogenase complex citrate synthase ADP, Pi Respiratory chain H2O ADP Pi ATP O2 1 2 NADH NAD isocitrate dehydrogenase multistep -ketoglutarate dehydrogenase Citric acid cycle Oxidative phosphorylation FIGURE 19–31 Regulation of the ATP-producing pathways. This diagram shows the interlocking regulation of glycolysis, pyruvate oxidation, the citric acid cycle, and oxidative phosphorylation by the relative concentrations of ATP, ADP, and AMP, and by NADH. High [ATP] (or low [ADP] and [AMP]) produces low rates of glycolysis, pyruvate oxidation, acetate oxidation via the citric acid cycle, and oxidative phosphorylation. All four pathways are accelerated when the use of ATP and the formation of ADP, AMP, and Pi increase. The interlocking of glycolysis and the citric acid cycle by citrate, which inhibits glycolysis, supplements the action of the adenine nucleotide system. In addition, increased levels of NADH and acetyl-CoA also inhibit the oxidation of pyruvate to acetyl-CoA, and a high [NADH]/[NAD] ratio inhibits the dehydrogenase reactions of the citric acid cycle (see Fig. 16–18). 8885d_c19_690-750 3/1/04 11:32 AM Page 718 mac76 mac76:385_reb:
8885dc19690-7503/1/0411:32 AM Page719mac76mac76:385 19.4 Mitochondrial Genes: Their Origin and the Effects of Mutations 719 a In ischemic (oxygen-deprived) cells, a protein bryo derives all its mitochondria from the mother's egg inhibitor blocks ATP hydrolysis by the ATP The rare disease Leber's hereditary optic neuropa- synthase operating in reverse, preventing a thy (lhon affects the central nervous system, in drastic drop in ATPl cluding the optic nerves, causing bilateral loss of vision a In brown fat, which is specialized for the in early adulthood. A single base change in the mito production of metabolic heat, electron transfer chondrial gene ND4 (Fig. 19-32a) changes an Arg is uncoupled from ATP synthesis and the residue to a His residue in a polypeptide of Complex I energy of fatty acid oxidation is dissipated as and the result is mitochondria partially defective in elec- tron transfer from NADH to ubiquinone. Although these mitochondria can produce some ATP by electron trans- AtP and adp concentrations set the rate fer from succinate, they apparently cannot supply suf- of electron transfer through the respiratory ficient ATP to support the very active metabolism chain via a series of interlocking controls on neurons. One result is damage to the optic nerve, lead respiration, glycolysis, and the citric acid cycle ing to blindness. a single base change in the mitochon- drial gene for cytochrome b, a component of Complex Ill, also produces Lhon, demonstrating that the pathol- 19.4 Mitochondrial Genes: Their Origin and ogy results from a general reduction of mitochondrial the effects of mutations function, not specifically from a defect in electron trans fer through Complex I Myoclonic epilepsy and ragged-red fiber dis- ouble-stranded dNa molecule. Each of the hundreds ease ( MErRF) is caused by a mutation in the mito- or thousands of mitochondria in a typical cell has about chondrial gene that encodes a transfer RNA specific for ive copies of this genome. The human mitochondri lycine (lysyl-tRNA). This disease, characterized by un- chromosome(Fig. 19-32)contains 37 genes(16, 569 bp), controllable muscular jerking, apparently results from including 13 that encode subunits of proteins of the defective production of several of the proteins whose respiratory chain (Table 19-6); the remaining genes synthesis involves mitochondrial tRNAS Skeletal mus code for rRNa and trNa molecules essential to the cle fibers of individuals with merrF have abnormally protein-synthesizing machinery of mitochondria. About shaped mitochondria that sometimes contain paracrys 900 different mitochondrial proteins are encoded by nu- talline structures(Fig. 19-32b) Mutations in the mito- clear genes, synthesized on cytoplasmic ribosomes, then chondrial lysyl-trna gene are also one of the causes of imported and assembled within the mitochondria adult-onset (type in diabetes mellitus. Other mutations (Chapter 27 in mitochondrial genes are believed to be responsible for the progressive muscular weakness that character Mutations in Mitochondrial Genes Cause izes mitochondrial myopathy and for enlargement and Human Disease deterioration of the heart muscle in hypertrophic cardio myopathy. According to one hypothesis on the progres- a growing number of human diseases can be at- sive changes that accompany aging, the accumulation tributed to mutations in mitochondrial genes. of mutations in mitochondrial dNa during a lifetime of of these diseases, those known as the mitochon- exposure to DNA-damaging agents such as O2(see drial encephalomyopathies, affect primarily the brain below) results in mitochondria that cannot supply suf- and skeletal muscle (both heavily dependent on an ficient ATP for normal cellular function. Mitochondrial abundant supply of ATP). These diseases are invariably disease can also result from mutations in any of the 900 inherited from the mother, because a developing em- nuclear genes that encode mitochondrial proteins. L TABLE 19-6 Respiratory Proteins Encoded by Mitochondrial Genes in Humans Number Number of subunits encoded of subunits by mitochondrial DNA I NADH dehydrogenase >43 7 ll Succinate dehydrogenase Ill Ubiquinone: cytochrome C oxidoreductase Iv Cytochrome oxidase 13 V ATP synthase 2
■ In ischemic (oxygen-deprived) cells, a protein inhibitor blocks ATP hydrolysis by the ATP synthase operating in reverse, preventing a drastic drop in [ATP]. ■ In brown fat, which is specialized for the production of metabolic heat, electron transfer is uncoupled from ATP synthesis and the energy of fatty acid oxidation is dissipated as heat. ■ ATP and ADP concentrations set the rate of electron transfer through the respiratory chain via a series of interlocking controls on respiration, glycolysis, and the citric acid cycle. 19.4 Mitochondrial Genes: Their Origin and the Effects of Mutations Mitochondria contain their own genome, a circular, double-stranded DNA molecule. Each of the hundreds or thousands of mitochondria in a typical cell has about five copies of this genome. The human mitochondrial chromosome (Fig. 19–32) contains 37 genes (16,569 bp), including 13 that encode subunits of proteins of the respiratory chain (Table 19–6); the remaining genes code for rRNA and tRNA molecules essential to the protein-synthesizing machinery of mitochondria. About 900 different mitochondrial proteins are encoded by nuclear genes, synthesized on cytoplasmic ribosomes, then imported and assembled within the mitochondria (Chapter 27). Mutations in Mitochondrial Genes Cause Human Disease A growing number of human diseases can be attributed to mutations in mitochondrial genes. Many of these diseases, those known as the mitochondrial encephalomyopathies, affect primarily the brain and skeletal muscle (both heavily dependent on an abundant supply of ATP). These diseases are invariably inherited from the mother, because a developing embryo derives all its mitochondria from the mother’s egg. The rare disease Leber’s hereditary optic neuropathy (LHON) affects the central nervous system, including the optic nerves, causing bilateral loss of vision in early adulthood. A single base change in the mitochondrial gene ND4 (Fig. 19–32a) changes an Arg residue to a His residue in a polypeptide of Complex I, and the result is mitochondria partially defective in electron transfer from NADH to ubiquinone. Although these mitochondria can produce some ATP by electron transfer from succinate, they apparently cannot supply sufficient ATP to support the very active metabolism of neurons. One result is damage to the optic nerve, leading to blindness. A single base change in the mitochondrial gene for cytochrome b, a component of Complex III, also produces LHON, demonstrating that the pathology results from a general reduction of mitochondrial function, not specifically from a defect in electron transfer through Complex I. Myoclonic epilepsy and ragged-red fiber disease (MERRF) is caused by a mutation in the mitochondrial gene that encodes a transfer RNA specific for lycine (lysyl-tRNA). This disease, characterized by uncontrollable muscular jerking, apparently results from defective production of several of the proteins whose synthesis involves mitochondrial tRNAs. Skeletal muscle fibers of individuals with MERRF have abnormally shaped mitochondria that sometimes contain paracrystalline structures (Fig. 19–32b). Mutations in the mitochondrial lysyl-tRNA gene are also one of the causes of adult-onset (type II) diabetes mellitus. Other mutations in mitochondrial genes are believed to be responsible for the progressive muscular weakness that characterizes mitochondrial myopathy and for enlargement and deterioration of the heart muscle in hypertrophic cardiomyopathy. According to one hypothesis on the progressive changes that accompany aging, the accumulation of mutations in mitochondrial DNA during a lifetime of exposure to DNA-damaging agents such as O2 (see below) results in mitochondria that cannot supply sufficient ATP for normal cellular function. Mitochondrial disease can also result from mutations in any of the 900 nuclear genes that encode mitochondrial proteins. ■ 19.4 Mitochondrial Genes: Their Origin and the Effects of Mutations 719 Number Number of subunits encoded Complex of subunits by mitochondrial DNA I NADH dehydrogenase 43 7 II Succinate dehydrogenase 4 0 III Ubiquinone:cytochrome c oxidoreductase 11 1 IV Cytochrome oxidase 13 3 V ATP synthase 8 2 TABLE 19–6 Respiratory Proteins Encoded by Mitochondrial Genes in Humans 8885d_c19_690-750 3/1/04 11:32 AM Page 719 mac76 mac76:385_reb: