Such direct transfer of high-energy phosphate groups does not occur during electron transport. For example, in the final reaction of glycolysis, the high-energy phosphate of phosphoenolpyruvate is transferred to ADP, yielding pyruvate plus ATP (see Figure 2.32). In the latter cases, a high-energy phosphate is transferred directly to ADP from the other substrate of an energy-yielding reaction. ![]() Importantly, the mechanism by which the energy derived from these electron transport reactions is coupled to ATP synthesis is fundamentally different from the synthesis of ATP during glycolysis or the citric acid cycle. The free energy derived from the passage of electrons through complexes I, III, and IV is harvested by being coupled to the synthesis of ATP. They are then transferred to coenzyme Q and carried through the rest of the electron transport chain as described in Figure 10.8. Electrons from succinate enter the electron transport chain via FADH 2 in complex II. Cytochrome c, a peripheral membrane protein bound to the outer face of the inner membrane, then carries electrons to complex IV ( cytochrome oxidase), where they are finally transferred to O 2 (Δ G°´ = -25.8 kcal/mol). In complex III, electrons are transferred from cytochrome b to cytochrome c-an energy-yielding reaction with Δ G°´ = -10.1 kcal/mol. Coenzyme Q (also called ubiquinone) is a small, lipid-soluble molecule that carries electrons from complex I through the membrane to complex III, which consists of about ten polypeptides. These electrons are initially transferred from NADH to flavin mononucleotide and then, through an iron-sulfur carrier, to coenzyme Q-an energy-yielding process with Δ G°´ = -16.6 kcal/mol. ![]() A fifth protein complex then serves to couple the energy-yielding reactions of electron transport to ATP synthesis.Įlectrons from NADH enter the electron transport chain in complex I, which consists of nearly 40 polypeptide chains ( Figure 10.8). ![]() These carriers are organized into four complexes in the inner mitochondrial membrane. To be harvested in usable form, this energy must be produced gradually, by the passage of electrons through a series of carriers, which constitute the electron transport chain. The transfer of electrons from NADH to O 2 is a very energy-yielding reaction, with Δ G°´ = -52.5 kcal/mol for each pair of electrons transferred. During oxidative phosphorylation, electrons derived from NADH and FADH 2 combine with O 2, and the energy released from these oxidation/ reduction reactions is used to drive the synthesis of ATP from ADP.
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