Cytochromes were first described in 1884 by MacMunn as respiratory pigments. Later, in 1920, Keilin rediscovered these respiratory pigments and gave them the name of Cytochrome, classifying these heme proteins based on the lowest level of the cytochrome energy absorption position.Cytochrome c is a small protein with 104 amino acids located in the intermembrane space of mitochondria of all living beings who do aerobic respiration. Part of its chain is separated by a matrix protease when the polypeptide is inserted into the inner membrane, being anchored in a proper orientation.It is a heteroprotein (protein composed of amino acids and other chemical elements), which besides amino acids, has a heme group (cofactor)that is bound to the cysteines 14:17.It is a hydrophilic protein, highly soluble in water (solubility ~100g /L).The percentage of each type of amino acid present in the protein varies, depending on the species and it is related to their evolutionary proximity. The variation in the primary structure in different species, indirectly reveals their genetic differences since the code for the protein is written in the genes. This protein plays an important role in cellular respiration as it is an electron carrier between complexes III and IV, displacing them to an oxygen molecule (final acceptor), thereby converting molecular oxygen to two molecules of water. In this process, it occurs translocation of protons to the intermembrane space, which help the formation of a chemiosmotic potential used by the ATP synthase for the formation of ATP. It is also responsible for stimulating programmed cell death, or apoptosis, by activating the intrinsic pathway of the process. This leads to activation of caspase 9, which in turn activates caspases 3 and 7, and the target cell dies by apoptosis. Finally, it also promotes the release of calcium stored in the endoplasmic reticulum, increasing the ion concentration in the cytosol.Regarding the formation of cytochromes, they suffer reversible changes in the iron oxidation number, changing between +2 and +3 in a cyclical process. There are three main groups of cytochromes, denominated by the letters a, b and c. They differ in the structure of the prosthetic group (side chain), leading to different absorption spectra, wherein the cytochrome c absorbs the shorter wavelengths.
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