The electron transport chain (ETC) is a group of proteins and organic molecules found in the inner membrane of mitochondria. Each chain member transfers electrons in a series of oxidation-reduction (redox) reactions to form a proton gradient that drives ATP synthesis. The importance of ETC is that it is the primary source of ATP production in the body. The electron transport chain’s functioning is somewhat analogous to a slinky toy going down a flight of stairs. Who Discovered the Electron Transport Chain American biochemist, Albert Lehninger, discovered the electron-transport chain in 1961. The complete ETC was found to have four membrane-bound complexes named complex I, II, III, and IV and two mobile electron carriers, namely coenzyme Q and cytochrome c.
Where does the Electron Transport Chain Take Place
What does the Electron Transport Chain Do
How does the Mitochondrial Electron Transport Chain Work
Summary of the Process
Electron Transport Chain in Bacteria
Steps of Mitochondrial Electron Transport
As discussed above, the entire process of the electron transport chain involves four major membrane proteins that function together in an organized fashion to accomplish ATP synthesis. The events of the electron transport chain are detailed below: Complex I: (NADH-CoQ oxidoreductase) – Transfer of Electrons from NADH to Coenzyme Q It is the first complex of the electron transport chain. It is found to be composed of one flavin mononucleotide (FMN) and six-seven iron-sulfur centers (Fe-S) as cofactors. The process starts by catalyzing the oxidation of NADH to NAD+ by transferring the two electrons to FMN, thus reducing it to FMNH2. Each of the two electrons from FMNH2 is relayed through a series of Fe-S clusters and then to a lipid-soluble carrier molecule known as coenzyme Q (ubiquinone). The reduced QH2 freely diffuses within the membrane. The above process allows Complex I to pump four protons (H+) from the mitochondrial matrix to the intermembrane space, establishing the proton gradient. NADH + H+ + CoQ → NAD+ + CoQH2 Complex II: (Succinate dehydrogenase) – Transfer of Electrons from FADH2 to Coenzyme Q It consists of FAD, and several Fe-S centers. Complex II is involved in the oxidation of succinate to fumarate, thus catalyzing FAD reduction to FADH2. Next, the electrons from FADH2 reach coenzyme Q through a series of Fe-S centers. Complex II runs parallel to complex I in the transport chain. However, complex II does not transport protons across the inner mitochondrial membrane, unlike the first complex. Complex II is thus not a part of creating the proton gradient in the ETC. Succinate + FADH2 + CoQ → Fumarate + FAD+ + CoQH2 Thus, CoQ receives electrons from Complex I and Complex II and gets reduced to CoQH2, which then delivers its electrons to the next complex of the chain, called Complex III. Complex III (Cytochrome bc1 complex): Transfer of Electrons from CoQH2 to Cytochrome c It is composed of cytochrome b, c, and a specific Fe-S center, known as cytochrome reductase. Complex III catalyzes the transfer of two electrons from CoQH2 to cytochrome c. This step results in the translocation of four protons similar to complex I across the inner membrane of mitochondria, thus forming a proton gradient. The reduced CoQH2 is thus oxidized back CoQ while the iron center (Fe3+) in the cytochrome c is reduced to Fe2+. CoQH2 + 2 cyt c (Fe3+) → CoQ + 2 cyt c (Fe2+) + 4H+ Cytochrome c thus forms the connection between Complex I, II, and III with complex IV with the help of CoQ. Although CoQ carries pairs of electrons, cytochrome c can only accept one at a time. Complex IV (Cytochrome c oxidase): Transfer of Electrons from Cytochrome c to Oxygen This step is the last complex of the electron transport chain and comprises two cytochromes a, and a3, which are made of two heme groups and three copper ions. Complex IV involves transferring two electrons from cytochrome c to molecular oxygen (O2), the final electron acceptor, thus forming water (H2O). The removal of H+ from the system pumps two protons across the membrane, forming a proton gradient. 4 cyt c (Fe2+) + O2 → 4 cyt c (Fe3+) + H2O
Chemiosmosis and Oxidative Phosphorylation
The proton gradient is formed within the mitochondrial matrix, and the intermembrane space is called the proton motive force. Since protons cannot pass directly through the phospholipid bilayer of the plasma membrane, they need the help of a transmembrane protein called ATP synthase to help their cause. Theoretically, ATP synthase is somewhat similar to a turbine in a hydroelectric power plant, which is run by H+ while moving down their concentration gradient. As ATP synthase turns, it catalyzes the addition of phosphate to ADP, thus forming ATP. This process is called chemiosmosis. Chemiosmosis couples the electron transport chain to ATP synthesis and thus complete the oxidative phosphorylation process.
Chemical Equation
6O2 + C6H12O6 + 38 ADP + 39Pi → 38 ATP + 6CO2 + 6H2O Reactants (Inputs)
NADHFADH2O2
End Products (Outputs)
NAD+FADH2OATP
How Many ATP are Generated in the Electron Transport Chain
Roughly, around 30-32 ATP is produced from one molecule of glucose in cellular respiration. However, the number of ATP molecules generated from the breakdown of glucose varies between species. The number of H+ ions that the electron transport chain pumps differ within them.
Where do the 30-32 ATP Count Come From
From a single molecule of glucose producing two ATP molecules in glycolysis and another two in the citric acid cycle, all other ATPs are produced through oxidative phosphorylation. Based on the experiment, it is obtained that four H+ ions flow back through ATP synthase to produce a single molecule of ATP. After moving through the electron transport chain, each NADH yields 2.5 ATP, whereas each FADH2 yields 1.5 ATP. Given below is a table showing the breakdown of ATP formation from one molecule of glucose through the electron transport chain: As given in the table, the ATP yield from NADH made in glycolysis is not precise. The reason is that glycolysis occurs in the cytosol, which needs to cross the mitochondrial membrane to participate in the electron transport chain. Cells with a shuttle system to transfer electrons to the transport chain via FADH2 are found to produce 3 ATP from 2 NADH. In others, the delivery of electrons is done through NADH, where they produce 5 ATP molecules. ↑ Complex II ↑ Succinate
Inhibitors of Electron Transport Chain
The following are considered to be inhibitors of the electron transport chain:
RotenoneAntimycin ACyanideCarbon Monoxide
Electrons can enter the chain at three different levels: a) at dehydrogenase, b) at the quinone pool, or c) at the cytochrome level. The electrons entering the chain flows through the four complexes with the help of the mobile electron carriers and are finally transferred to an oxygen molecule (for aerobic or facultative anaerobes) or other terminal electron acceptors such as nitrate, nitrite, ferric iron, sulfate, carbon dioxide, and small organic molecules (for anaerobes).
