Cytochrome c

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Cytochrome c heme shown in red.

Cytochrome c, or cyt c (horse heart: PDB 1HRC) is a small heme protein found loosely associated with the inner membrane of the mitochondrion. It is a soluble protein, unlike other cytochromes, and is an essential component of the electron transfer chain. It is capable of undergoing oxidation and reduction, but does not bind oxygen. It transfers electrons between Complexes III and IV.

Cytochrome c is a highly conserved protein across the spectrum of species, found in plants, animals, and many unicellular organisms. This, along with its small size (molecular weight about 12,000 daltons), makes it useful in studies of evolutionary divergence. Its primary structure consists of a chain of 100 amino acids. The cytochrome c molecule has been studied for the glimpse it gives into evolutionary biology. Both chickens and turkeys have the identical molecule (amino acid for amino acid) within their mitochondria, whereas ducks possess molecules differing by one amino acid. Similarly, both humans and chimpanzees have the identical molecule, while rhesus monkeys possess mitochondria differing by one amino acid. Cytochrome c can catalyze several reactions such as hydroxylation and aromatic oxidation, and shows peroxidase activity by oxidation of various electron donors such as 2,2-azino-bis(3-ethylbenzthiazoline-6-sulphonic acid) (ABTS), 2-keto-4-thiomethyl butyric acid and 4-aminoantipyrine. Cytochrome c is also suspected to be the functional complex in so called LLLT: Low Level Laser Therapy. In LLLT laser light on the wavelength of 670 nanometer penetrates wounded and scarred tissue in order to increase cellular regeneration. Light of this wavelength appears capable of increasing activity of cytochrome c, thus increasing metabolic activity and freeing up more energy for the cells to repair the tissue.

Cytochrome c is also an intermediate in apoptosis, a controlled form of cell death used to kill cells in the process of development or in response to infection or DNA damage. Cytochrome c was identified in 1996 by Xiaodong Wang and coworkers as a critical protein in the apoptotic pathway. Cytochrome c is released by the mitochondria in response to pro-apoptotic stimuli. The sustained elevation in calcium levels precedes cyt c release from the mitochondria. The release of small amounts of cyt c leads to an interaction with the IP3 receptor (IP3R) on the endoplasmic reticulum (ER), preventing calcium inhibition of ER calcium release. The overall increase in calcium triggers a massive release of cyt c, which then acts in the positive feedback loop to maintain ER calcium release through the IP3Rs. This explains how the ER calcium release can reach pathologic levels. This release in turn activates caspase 9, a cysteine protease. Caspase 9 can then go on to activate caspases 3 and 7, which are responsible for destroying the cell from within.