Monday, June 2, 2014

Cellular respiration - Complex III

The complex III of the mitochondrial respiratory chain is called bc1 complex or ubiquinone:cytochrome c oxidoreductase. Its main function is to receive electrons from ubiquinone and transfer them to cytochrome c. In fact, complex III will accept electrons from 2 reduced ubiquinone (ubiquinol) molecules, wich means that it will receive 4 electrons. However, one of those ubiquinone molecules (now oxidized) will receive again 2 electrons. Thus, although complex III receives 4 electrons, it will only transfer 2 to cytochrome c. this process is often called Q-cycle, since ubiquinone can also be mentioned as coenzyme Q.
Cytochrome c is a small soluble protein present in the intermembrane space. Its function is to receive electrons from complex III and transfer them to complex IV, that means, it is not part of any complex of the respiratory chain in particular. It has a very important feature, which is the fact that it can only accept one electron, which means that for the complex III to exert its function it needs to transfer 2 electrons of a molecule of ubiquinone (since it is reduced it is known as ubiquinol) to two molecules of cytochrome c. This will have obvious implications for the formation of reactive oxygen species, but I'll leave that subject for a future post ...

From the structural point of view, the complex III presents a dimeric structure composed of two monomers with at least 11 subunits. Of these, three of each of the monomers have a direct role in the transfer of electrons along the complex. Of all the subunits, we should highlight the presence of cytochrome b and Rieske protein, which is a protein with a Fe-S center, more specifically 2Fe-2S.
As it happens with complex I (and IV , as I will develop in a future post), the flow of the electrons through complex III liberates energy, and that energy is used to actively transport H+ from the matrix to the intermembrane space, creating an electrochemical gradient, which subsequently will be involved in the synthesis of ATP. In this case, for each 2 electrons that are transported along complex III, 4H+ are transferred to the intermembrane space.
The complex III is inhibited, for example, by antimycin A. When it is inhibited, it can lead to the leakage of electrons that can reduce molecular oxygen, originating superoxide anion. Therefore, besides the potentially severe consequences associated with the inhibition of the respiratory chain, oxidative stress may also occur.

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