The final stage of cellular respiration – oxidative phosphorylation – requires the presence of the oxygen and special enzymes to successfully perform all needed reactions. Mansilla et al. (2018) state that “the respiratory chain is composed of several multiprotein complexes, which are involved in the transfer of electrons and the translocation of H+ for ATP synthesis through oxidative phosphorylation” (p. 662). The first part of the phosphorylation is the electron transport chain, ETC – with it, electrons can be transferred between molecules, and the released energy supplies the electrochemical gradient. After that, the energy of this gradient is used to synthesize ATP – it is called chemiosmosis, the second part of phosphorylation. During the reactions in the ETC, oxygen, accepting protons and electrons from the oxidized substrate, is reduced to various end products, depending on the number of electrons attached to its molecule. However, there is one reaction that uses the most of the absorbed oxygen – the astounding 90% – and it is considered the main biological oxidation pathway. This process is localized on the inner mitochondrial membrane.
A special transmembrane protein complex participates in the final stage of phosphorylation – the enzyme cytochrome c oxidase or Complex IV. The Complex IV transfers electrons from cytochrome c to oxygen, thus, oxygen gets reduced to water. Cytochrome c-oxidase carries two copper-containing centers (CuA and CuB), as well as hemes a and a3 for carrying out redox reactions, through which electrons finally enter oxygen. When O2 is reduced, a strong basic O2– anion is formed, which binds two protons and transforms into a water molecule. The electron flux is coupled with the formed Complexes I, III, and IV by a proton gradient. During the reaction, four more protons are transferred from the stroma to the intermembrane space. As a result, there are few protons in the stroma, and, on the contrary, there are many in the intermembrane space, thus, a proton gradient arises. After all respective reactions are performed and the necessary energy is gathered, the protein complex ATP-synthase, which is often called the respiratory Complex V, uses the energy of this gradient to synthesize ATP in the mitochondria.
Reference
Mansilla, N., Racca, S., Gras, D. E., Gonzalez, D. H., & Welchen, E. (2018). The Complexity of Mitochondrial Complex IV: An Update of Cytochrome c Oxidase Biogenesis in Plants. International Journal of Molecular Sciences, 19(3), 662. Web.