Oxidative Phosphorylation Electron Transport Chain

Need help preparing for the Bio/Bio Chemistry section of the MCAT? MedSchoolCoach expert, Ken Tao, will teach everything you need to know about Oxidative Phosphorylation Electron Transport Chain of Cellular Respiration - a key component of the biochemistry section of the MCAT. Watch this Part 1 of a 3 part series to get all the mcat study tips you need to do well on this section of the exam!
Through glycolysis and the citric acid cycle, the cell can produce ATP and GTP for energy. However, the bulk of energy production occurs through oxidative phosphorylation. Oxidative phosphorylation uses the NADH and FADH2 produced by glycolysis and the Krebs cycle to create a proton gradient that is used to produce ATP. Oxidative phosphorylation is a two-step process. In the first step, the electron transport chain is used to produce a proton gradient. In the second step, ATP synthase will use the proton gradient to produce ATP.
Electron Transport Chain
In the electron transport chain (ETC), the electrons from NADH and FADH2, which were produced in glycolysis and the citric acid cycle, are passed through a series of electron carriers and oxygen to form water. In this way, the electron transport chain is a series of oxidation reactions that function to release energy from NADH and FADH2. This process takes place in the inner mitochondrial membrane.
Moreover, the energy released is captured in the form of a proton gradient. As the electrons are traveling through electron carriers in the intermembrane space, protons are being pumped from the mitochondrial matrix into the intermembrane space. There are four complexes in the ETC - Complex I, Complex II, Complex III, Complex IV - as well as two pathways. One pathway involves NADH and uses Complex I, III, and IV. The other pathway involves FADH2and uses Complex II, III, and IV.
Complex I is also known as NADH dehydrogenase. It is at this point that NADH enters into the ETC. Complex I will oxidize NADH to NAD+. Furthermore, it will use these electrons to reduce the coenzyme ubiquinone to ubiquinol. Ubiquinone is a lipid-soluble electron carrier that is also known as coenzyme Q. In this way, ubiquinone is the oxidized form of coenzyme Q, while ubiquinol is the reduced form of coenzyme Q. Some sources will say electrons from NADH are transferred to coenzyme Q in Complex I, which is the same thing as saying NADH transfers electrons to ubiquinone.
Complex II is also known as succinate dehydrogenase. As previously mentioned, succinate dehydrogenase is the only enzyme that participates in both the citric acid cycle and the electron transport chain. In the citric acid cycle, succinate dehydrogenase catalyzes the oxidation of succinate to fumarate and the reduction of FAD to FADH2. In the electron transport chain, succinate dehydrogenase oxidizes FADH2 to FAD and reduces ubiquinone to ubiquinol.
Complex III is also known as coenzyme Q: cytochrome c - oxidoreductase. In this complex, ubiquinol is oxidized to ubiquinone, and two molecules of cytochrome c(Fe3+) are reduced to cytochrome c(Fe2+). From the change in charge, it is clear that each cytochrome c molecule is accepting one electron. This fact is noteworthy because it means that cytochrome c is a one-electron carrier, unlike all of the other electron carriers in the electron transport chain. Also, one electron out of the two is accepted by a cytochrome c molecule, meaning that two cytochrome c molecules per ubiquinol are necessary for this step to occur. Lastly, four protons will be transferred from the mitochondrial matrix into the intermembrane space helping to generate the proton gradient.
The last complex is Complex IV, also called cytochrome c oxidase. In this complex, two molecules of cytochrome c will be reduced from Fe2+ plus to Fe3+. These two electrons will then be used to reduce oxygen to water. In this way, oxygen is the final electron acceptor. Also, four protons are transferred across the intermembrane space contributing to the proton gradient. As mentioned previously, this proton gradient will be used by ATP synthase to produce ATP.
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