Set of reaction that transfers the Hydrogen and electrons from reduced cofactors to Oxygen, obtaining water and using the released energy for the synthesis of ATP
Sometimes, the terms Electron Transport Chain and Oxidative Phosphorylation are used indistinctly to refer to the Respiratory Chain. We prefer to use Respiratory chain for the whole process, in which we can distinguish two different parts:
• Electron transport chain: Set of reaction through which the Hydrogen and electrons are transfer from reduced cofactors to Oxygen, obtaining water and releasing energy. It allows that the energy derived from redox reactions be released little by little with a better use and without damaging the cell. It involves the transfer of electrons from complex I (or II) to Oxygen. (An excellent animation here)
• Oxidative phosphorylation: It’s the biosynthesis of ATP from ADP + (P) using the energy released in the electron transport chain. It occurs in Complex V (ATP synthase). The coupling of ATP synthesis and the energy released in the ETC is indirect (Animation here)
Substances that interfere with the Respiratory Chain function can be classified as:
Substances that bind to some of the components of the ETC blocking its ability to change in a reversible form from an oxidized state to a reduced state. It results in the accumulation of reduced forms before the inhibitor point, and oxidized forms of the components of the ETC ahead the inhibition point. Sites of action of some inhibitors (and some artificial electron acceptors) can be found in this graphic
Since energy is not released, the synthesis of ATP also stops.
Ex: Amital, Rotenone, Antimycin A, CO, Sodium Azide, CN
Inhibitors of Oxidative Phosphorylation:
These compounds bind to the Complex V (ATP synthase), impeding the synthesis of ATP, by inhibiting the return of protons to the matrix. Since this process is coupled to the ETC these inhibitors also stop the function of the ETC.
• Ex Oligomycin
Uncouplers of respiratory chain:
Substances that dissipate the electro-chemical gradient by facilitating the entrance of protons to the matrix using “shortcuts” and by that reason the electron transport takes places but not the oxidative phosphorylation.
• Ex. 2,4 dinitrophenol
• Thermogenin – UCP-1, DNP
Inhibitors of ATP/ADP exchange
ATP is produced in the mitochondria but is used all over the cell, so it should be allowed to go from the production site (mainly, mitochondria) to the locations where it is going to be used. At the same time, the ADP resulting from the use of ATP all over the cell should be allowed to enter the matrix in order to be used in the formation of more ATP. The inhibitors of ATP/ADP exchange inhibit the translocase that allows the entrance of ADP to the matrix and the output of recently synthesized ATP outside of the mitochondria.
• Ex Atractyloside
More information about these substances that interfere with the Respiratory Chain will be included in future posts dedicated to each specific group.