What is oxidative phosphorylation? Definition, localization and steps of the cellular biochemical process known as oxidative phosphorylation.
Oxidative Phosphorylation: What It Is and Where It Occurs
Oxidative phosphorylation is a process that allows energy transduction and constitutes the ATP synthesis pathway through the transfer of electrons to oxygen. Oxidative phosphorylation represents the final stage of all enzymatic reactions that involve the oxidative degradation of carbohydrates, fatty acids and amino acids in aerobic cells. This process occurs in the mitochondria and allows oxygen to be oxidized to water. The mitochondrion, a cellular organelle, is made up of two membranes. The outer membrane is easily permeable to small molecules and ions, while the inner membrane is impermeable to almost all ions and small molecules, allowing passage through it only to chemical species with specific membrane transporters. The membrane contains the components of the respiratory chain responsible for ATP synthesis. The mitochondrial matrix (the space between the two membranes) contains the pyruvate dehydrogenase complex, the enzymes of the citric acid cycle and the fatty acid oxidation pathway.
The electron transport chain
The electron transport chain is the first part of oxidative phosphorylation . Let us start by saying that almost all the electrons involved in the transport are the result of various dehydrogenation phases (pyruvate oxidation, citric acid cycle, β-oxidation of fatty acids, etc.). The electron carriers are, for the most part, integral membrane proteins , containing prosthetic groups capable of accepting and donating one or two electrons, accepting them from the carrier that precedes it and donating them to the donor that follows it, thus establishing the specific transport chain. The electron pathway: ubiquinone , cytochrome and iron -sulfur proteins .
- Ubiquinone can accept a single electron (transforming into a semiquinone radical) or two electrons (therefore completely reducing to ubiquinol UQH2) . Ubiquinone, having very small physical dimensions and being hydrophobic, can freely diffuse in the lipid bilayer of the internal mitochondrial membrane acting as a bridge with the less mobile electron carriers.
- Cytochromes are iron-containing proteins that are responsible for transferring electrons located in the internal mitochondrial membrane; their heme prosthetic group gives them their characteristic red color. There are three classes of cytochromes indicated by the letters a , b, and c, characterized by different modes of light absorption. Of these, cytochromes a and b are tightly bound to the protein; cytochromes of the c group are covalently bound to the Cys residues of the protein. With the exception of mitochondrial cytochrome c, the others are represented by integral membrane proteins.
- Iron-sulfur proteins generally have very simple structures (e.g. a single iron atom coordinated with four sulfur atoms), but can also be very complex and contain up to 4 iron atoms. They are good electron donors as they have standard reduction potential vessels.
In the overall redox reaction, electrons pass from a primary donor (for example, NADH) through flavoproteins, ubiquinone, iron-sulfur proteins, and cytochromes (all immersed in the mitochondrial membrane) to reach oxygen. Each of these complexes represents a fraction of the 4 that make up the respiratory chain.
The complexes associated with the electron transport chain are four:
- Complex I is a complex flavoprotein (containing more than 25 polypeptide chains), the catalyzed reaction is as follows: NADH + H+ +UQ <==> NAD+ + UQH2 . Where ubiquinone accepts two electrons and a proton (in the form of a hydride ion) from NADH and a proton from the aqueous solvent of the mitochondrial matrix, becoming ubiquinol. Ubiquinol is able to diffuse from complex I to complex III where it is oxidized to UQ.
- Complex II – succinate contains two types of prosthetic groups and at least four different proteins. It is assumed that electrons pass from succinate to FAD and from there, through the iron-sulfur centers to ubiquinone. Ubiquinone also receives electrons from other dehydrogenases.
- Complex III contains three different types of cytochromes, an iron-sulfur protein, and at least six other protein subunits. Its function is similar to that of a proton pump, here the protons released from UQH2 are released into the intermembrane space generating a proton gradient.
- Complex IV consists of two cytochromes a and a3, the enzyme complex catalyzes the simultaneous transfer of 4 e- to an oxygen molecule without the formation of intermediates. The same flow of electrons determines the movement of protons from the mitochondrial matrix to the intermembrane space.
ATP synthase
In the respiratory chain described, two electrons pass from NADH to oxygen, releasing a sufficient amount of energy for the synthesis of ATP through the phosphorylation of ADP. We have already observed that the transport of electrons along the respiratory chain is accompanied by an influx of protons through the internal mitochondrial membrane, with the consequent formation of a proton potential and therefore an increase in pH, with the internal wall of the internal mitochondrial membrane being alkaline compared to the external face of the same membrane. This electrochemical energy represents the conservation of a part of the energy released by oxidations and is defined as the proton driving force : to pump protons against the electrochemical gradient, energy must be spent; in this case, by letting the accumulated protons flow spontaneously, an amount of energy equal to that which would have been needed to pump the protons is made available. In detail, from experimental evidence it is believed that for every 2 electrons that pass from NADH to oxygen (through the respiratory chain) 10 protons are pumped out of the matrix. The proton force provides energy for the synthesis of ATP , although it is not yet completely clear how this force is transmitted to ATP synthase. It would seem that the proton force is able to free the active site of the enzyme in which the ATP remains bound after its synthesis, by the enzyme itself, starting from ADP and Pi. The removal of ATP would allow ATP synthase to carry out a new synthesis process, impossible before the removal of the previous synthesized molecule.
