Cellular respiration - Cytochrome c

The cytochrome c is a small protein with 104 amino acids and a mass of about 12 kDa (12.233 kDa in humans). As a consequence of its small size, it is highly conserved among different mammalian species; for example, the human cytochrome c is identical to the chimpanzee! It is a heteroprotein because beyond its amino acid, it contains also an heme group as a cofactor, which is bound to cysteines 14 and 17. It is a hydrophilic protein, highly soluble (solubility ~100 g/L), which is located in the mitochondrial intermembrane space, where it plays a key role in the mitochondrial respiratory chain, though it does not belong to any of the four complexes.
The function of the cytochrome c is to receive electrons from the complex III, and deliver them to the complex IV.  

To acomplish this function, its heme group, as any heme group, has an iron ion that can oscillate between two different oxidation states (Fe²⁺ and Fe³⁺). Since it has only 1 heme group, it can carry only one electron at a time. This feature has two very important consequences:

1. To deliver the 2 electrons from NADH or FADH₂ to O₂ in cellular respiration, it is required 2 molecules of cytochrome c.
2. O2, which is the final electron acceptor of the complex IV, receives one electron at a time, which means that it converted to, even temporarily (in most situations!), a free radical, which potentiates the oxidative stress.
Other functions less characterized of citocromoc are its involvement in catalytic hydroxylation reactions, aromatic oxidation and peroxidation. Also, it appears to be important to the catalytic activity of the nitrite reductase enzyme.

Finally, a very important characteristic of cytochrome c is that it can function as an activator of the intrinsic pathway of programmed cell death, a process referred to as apotose. Soon I will post more information on this subject...