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You are watching: The direct energy source that drives atp synthesis during respiratory oxidative phosphorylation is

Cooper GM. The Cell: A molecule Approach. 2nd edition. Sunderland (MA): Sinauer Associates; 2000.


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Most that the usable energy obtained from the malfunction of carbohydrates or fats is obtained by oxidative phosphorylation, i beg your pardon takes place within mitochondria. For example, the failure of glucose through glycolysis and also the citric mountain cycle returns a complete of four molecules the ATP, ten molecules of NADH, and also two molecule of FADH2 (see thing 2). Electrons from NADH and FADH2 are then transferred to molecule oxygen, combination to the development of second 32 to 34 ATP molecule by oxidative phosphorylation. Electron transport and also oxidative phosphorylation are crucial activities of protein complexes in the within mitochondrial membrane, which at some point serve together the significant source of to move energy.


The Electron deliver Chain

During oxidative phosphorylation, electrons derived from NADH and FADH2 incorporate with O2, and the energy released from this oxidation/ reduction reactions is used to journey the synthesis of ATP from ADP. The deliver of electrons from NADH to O2 is a very energy-yielding reaction, through ΔG°´ = -52.5 kcal/mol because that each pair of electron transferred. Come be harvest in usable form, this energy must be created gradually, by the passage of electrons with a series of carriers, which constitute the electron transfer chain. This carriers are organized into 4 complexes in the inside mitochondrial membrane. A fifth protein facility then offer to couple the energy-yielding reactions of electron transfer to ATP synthesis.

Electrons indigenous NADH get in the electron move chain in complex I, which consists of almost 40 polypeptide chains (Figure 10.8). These electrons are initially transferred native NADH to flavin mononucleotide and also then, with an iron-sulfur carrier, come coenzyme Q—an energy-yielding procedure with ΔG°´ = -16.6 kcal/mol. Coenzyme Q (also dubbed ubiquinone) is a small, lipid-soluble molecule the carries electron from facility I with the membrane to complex III, which consists of around ten polypeptides. In facility III, electrons are transferred from cytochrome b come cytochrome c—an energy-yielding reaction with ΔG°´ = -10.1 kcal/mol. Cytochromec, a peripheral membrane protein bound come the outer confront of the inner membrane, then carries electrons to complex IV (cytochrome oxidase), whereby they are finally transferred to O2 (ΔG°´ = -25.8 kcal/mol).


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A unique protein complex (complex II), which consists of four polypeptides, receives electron from the citric mountain cycle intermediate, succinate (Figure 10.9). These electrons room transferred come FADH2, rather than come NADH, and then come coenzyme Q. Native coenzyme Q, electrons space transferred to complicated III and also then to facility IV as already described. In comparison to the move of electron from NADH to coenzyme Q at facility I, the transport of electron from FADH2 come coenzyme Q is not associated with a far-reaching decrease in complimentary energy and, therefore, is no coupled to ATP synthesis. Consequently, the passage of electrons derived from FADH2 through the electron transport chain yields complimentary energy just at complexes III and also IV.


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Figure 10.9

Transport of electron from FADH2. Electron from succinate go into the electron transport chain via FADH2 in complicated II. They space then transferred to coenzyme Q and also carried through the remainder of the electron carry chain as described in figure 10.8. The (more...)


The complimentary energy acquired from the i of electrons v complexes I, III, and IV is harvest by being coupled to the synthetic of ATP. Importantly, the mechanism whereby the energy derived from these electron move reactions is coupled to ATP synthesis is fundamentally different from the synthetic of ATP during glycolysis or the citric acid cycle. In the latter cases, a high-energy phosphate is transferred directly to ADP from the other substrate of an energy-yielding reaction. For example, in the last reaction the glycolysis, the high-energy phosphate of phosphoenolpyruvate is transferred to ADP, yielding pyruvate add to ATP (see number 2.32). Such direct transfer the high-energy phosphate teams does not occur during electron transport. Instead, the energy derived from electron deliver is coupled to the generation of a proton gradient throughout the inside mitochondrial membrane. The potential energy stored in this gradient is then harvested by a 5th protein complex, i beg your pardon couples the energetically favorable circulation of proton back throughout the membrane to the synthetic of ATP.


Chemiosmotic Coupling

The mechanism of coupling electron move to ATP generation, chemiosmotic coupling, is a striking example of the relationship between structure and role in cabinet biology. The theory of chemiosmotic coupling was an initial proposed in 1961 by Peter Mitchell, who said that ATP is produced by the use of power stored in the kind of proton gradients across biological membranes, fairly than by straight chemical move of high-energy groups. Biochemists to be initially very skeptical the this proposal, and the chemiosmotic theory took more than a decade to win basic acceptance in the scientific community. Overwhelming evidence eventually accumulated in its favor, however, and chemiosmotic coupling is now known as a general mechanism the ATP generation, operating not just in mitochondria but additionally in chloroplasts and in bacteria, where ATP is generated via a proton gradient across the plasma membrane.

Electron transport v complexes I, III, and IV is coupled to the move of protons the end of the internal of the mitochondrion (see number 10.8). Thus, the energy-yielding reaction of electron transport are coupled come the deliver of proton from the procession to the intermembrane space, which develops a proton gradient across the within membrane. Complexes I and IV appear to act together proton pumps that transport protons throughout the membrane as a an outcome of conformational transforms induced by electron transport. In complex III, protons space carried across the membrane by coenzyme Q, i m sorry accepts proton from the procession at complexes ns or II and releases them right into the intermembrane an are at facility III. Complexes I and also III each transfer four protons throughout the membrane every pair that electrons. In facility IV, two protons per pair that electrons are pumped across the membrane and another two protons every pair of electrons are combined with O2 to form H2O within the matrix. Thus, the equivalent of 4 protons per pair that electrons are transported out of the mitochondrial matrix at each of these three complexes. This carry of proton from the matrix to the intermembrane an are plays the an important role of convert the energy obtained from the oxidation/reduction reaction of electron move to the potential energy stored in a proton gradient.

Because protons room electrically charged particles, the potential energy stored in the proton gradient is electric as well as chemical in nature. The electrical component synchronizes to the voltage difference across the inner mitochondrial membrane, v the matrix of the mitochondrion an adverse and the intermembrane space positive. The corresponding complimentary energy is provided by the equation


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where F is the Faraday constant and ΔV is the membrane potential. The additional totally free energy corresponding to the difference in proton concentration across the membrane is offered by the equation


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where i and o refer, respectively, come the proton concentrations inside and also outside the mitochondria.

In metabolically active cells, protons are generally pumped the end of the procession such the the proton gradient throughout the within membrane synchronizes to about one pH unit, or a tenfold lower concentration that protons within mitochondria (Figure 10.10). The pH of the mitochondrial matrix is therefore about 8, contrasted to the neutral pH (approximately 7) that the cytosol and intermembrane space. This gradient additionally generates an electric potential of roughly 0.14 V across the membrane, through the procession negative. Both the pH gradient and the electrical potential drive protons ago into the procession from the cytosol, therefore they combine to kind an electrochemical gradient throughout the inside mitochondrial membrane, equivalent to a ΔG of roughly -5 kcal/mol every proton.


Figure 10.10

The electrochemical nature the the proton gradient. Since protons are positively charged, the proton gradient established throughout the inside mitochondrial membrane has actually both chemical and also electric components. The chemistry component is the proton concentration, (more...)


Because the phospholipid double layer is impermeable to ions, protons room able to overcome the membrane just through a protein channel. This restriction allows the energy in the electrochemical gradient to it is in harnessed and converted come ATP together a an outcome of the action of the fifth facility involved in oxidative phosphorylation, complicated V, or ATP synthase (see figure 10.8). ATP synthase is organized right into two structurally unique components, F0 and F1, which are connected by a slender stalk (Figure 10.11). The F0 portion spans the inside membrane and also provides a channel v which protons are able come flow earlier from the intermembrane space to the matrix. The energetically favorable return of proton to the procession is coupled to ATP synthetic by the F1 subunit, i m sorry catalyzes the synthetic of ATP from ADP and also phosphate ion (Pi). Thorough structural studies have established the system of ATP synthase action, which entails mechanical coupling between the F0 and also F1 subunits. In particular, the flow of protons v F0 cd driver the rotation of F1, i beg your pardon acts as a rotating motor to journey ATP synthesis.


Figure 10.11

Structure the ATP synthase. The mitochondrial ATP synthase (complex V) is composed of 2 multisubunit components, F0 and F1, i beg your pardon are linked by a slim stalk. F0 spans the lipid bilayer, developing a channel through which protons can cross the membrane. (more...)


It appears that the flow of four protons back throughout the membrane through F0 is forced to journey the synthetic of one molecule of ATP through F1, constant with the proton move at complexes I, III, and also IV every contributing sufficient totally free energy come the proton gradient to drive the synthetic of one ATP molecule. The oxidation that one molecule of NADH hence leads to the synthesis of 3 molecules the ATP, whereas the oxidation of FADH2, i m sorry enters the electron deliver chain at facility II, yields only two ATP molecules.


Transport of Metabolites across the inner Membrane

In addition to driving the synthesis of ATP, the potential energy stored in the electrochemical gradient drives the deliver of small molecules into and out that mitochondria. For example, the ATP synthesized in ~ mitochondria needs to be exported to the cytosol, when ADP and Pi have to be imported native the cytosol for ATP synthesis to continue. The electrochemical gradient created by proton pumping provides energy required because that the move of this molecules and also other metabolites that have to be focused within mitochondria (Figure 10.12).


Figure 10.12

Transport the metabolites throughout the mitochondrial within membrane. The deliver of little molecules throughout the inside membrane is mediated through membrane-spanning transfer proteins and also driven through the electrochemical gradient. Because that example, ATP is exported indigenous (more...)


The move of ATP and ADP across the within membrane is mediated by one integral membrane protein, the adenin nucleotide translocator, i m sorry transports one molecule the ADP right into the mitochondrion in exchange for one molecule of ATP moved from the mitochondrion come the cytosol. Because ATP carries more negative charge than ADP (-4 contrasted to -3), this exchange is driven by the voltage ingredient of the electrochemical gradient. Because the proton gradient creates a hopeful charge on the cytosolic side of the membrane, the export of ATP in exchange for ADP is energetically favorable.

The synthesis of ATP within the mitochondrion needs phosphate ion (Pi) and also ADP, for this reason Pi must likewise be imported indigenous the cytosol. This is mediated by one more membrane move protein, i beg your pardon imports phosphate (H2PO4-) and also exports hydroxyl ions (OH-). This exchange is electrically neutral because both phosphate and also hydroxyl ions have actually a fee of -1. However, the exchange is driven by the proton concentration gradient; the higher pH in ~ mitochondria synchronizes to a higher concentration the hydroxyl ions, favoring your translocation come the cytosolic next of the membrane.

Energy from the electrochemical gradient is likewise used to drive the carry of various other metabolites right into mitochondria. Because that example, the import of pyruvate indigenous the cytosol (where it is produced by glycolysis) is mediated by a transporter the exchanges pyruvate for hydroxyl ions. Other intermediates the the citric mountain cycle space able come shuttle between mitochondria and also the cytosol by comparable exchange mechanisms.

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