Oxidative Phosphorylation

What is Oxidative Phosphorylation?

• Also known as Electron Transport Chain or ETC, oxidative phosphorylation is a metabolic pathway in which nutrients get oxidized with the help of enzymes and release of the chemical energy of molecular oxygen that can be used to produce adenosine triphosphate (ATP). Oxidative phosphorylation occurs inside the mitochondria in most of the eukaryotes and almost all the aerobic organisms carry out this process. 

• During the oxidative phosphorylation process, the transfer of electrons takes place from electron donors to electron acceptors like oxygen where redox reactions take place. Redox reactions lead to the formation of ATP which is formed as a result of energy stored in the relatively weak double bond of Oxygen. 

• In eukaryotes, a series of protein complexes catalyze the redox reactions and the proteins are present within the inner membranes of the cell’s mitochondria whereas in prokaryotes the proteins are present in the cell’s intermembrane space. These sets of proteins linked with each other are called electron transport chains. 

• Eukaryotic oxidative phosphorylation involves five protein complexes, namely Complex I known as NADH Dehydrogenase, complex II known as Succinate Dehydrogenase, Complex III known as Cytochrome C Oxidoreductase, complex IV known as Cytochrome Oxidase and complex V known as ATP Synthase. On the other hand, in prokaryotes, there are different kinds of enzymes and many varieties of electrons donors and acceptors are involved. 

• Oxidative phosphorylation is a vital part of metabolism as it generates reactive oxygen species such as hydrogen peroxide and superoxide. It also leads to the propagation of free radicals, cell damage, diseases and aging. The enzymes involved in this metabolic pathway are also an interest for studying many drugs and poisons inhibitions through their activities.

• Oxidative phosphorylation is the terminal process of cellular respiration in Eukaryotes and yields high ATP.  

• The two important substances to begin oxidative phosphorylation are the electron carriers from glycolysis, preparatory step and krebs cycle which are 10 NADH and 2FADH2.

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Electron Transport System and Oxidative Phosphorylation

Let’s understand the mechanism of oxidative phosphorylation or ETC.. Here, electrons are being transferred through the chain of protein complexes in an electron transport system or oxidative phosphorylation. The electrons getting transferred come through the carriers obtained from glycolysis, preparatory step and krebs cycle. The ultimate destination of electrons is Oxygen where electrons reduce oxygen to form H20. Therefore, oxygen is called the final electron acceptor.  During this process, hydrogen ions are pumped out with whose help ATP are generated.

Ubiquinone (Coenzyme CO10) is a mobile protein that floats in the inner mitochondrial membrane and carries electrons through the different complexes. 

Complex I 

Important structure within complex I is FMN (Flavin Mononucleotide), and also this protein complex is the center for Iron and Sulphur (Fe and S).  Reaction taking place in this complex can be represented as:

Complex II

One of its features is that it extends from the flavin and iron-sulphur redox cofactors in the membrane extrinsic domain to the b heme cofactors and quinone in the membrane domain.

Complex III

It has three important structures namely cytochrome b, Rieske iron-sulphur proteins and cytochrome c. The most important one of them is cytochrome c which is a mobile protein in the intermembrane space and attached to complex III. 

Complex IV

It has an extremely complicated structure and contains 13 subunits, two heme groups and multiple metal ion cofactors including 3 atoms of Cu, 1 Mg and 1 Zn atom.

Complex V

It is ATP Synthase that consists of many subunits of which major ones are F0 and F1 subunits. F1 comprises alpha and beta subunits, gamma and epsilon subunits. F0 subunit consists of C10 , a, b2 subunits. ATP synthase makes ATP when hydrogen ions pass through this complex.  

Oxidative Phosphorylation Steps

The major oxidative phosphorylation steps taking place in mitochondria include:

1. Delivery of Electrons by NADH and FADH2

Transfer of electrons takes place from NADH and FADH2 to the molecules found near the beginning of the oxidative phosphorylation in mitochondria. After the transfer of electrons, they get reduced to NAD+ and FAD respectively and are further utilized in the other steps of cellular respiration.

2. Electron Transport and Pumping of Protons

The electrons jump from a higher energy level to a lower energy level, thereby releasing energy. Some of the energy is utilized to move the electrons from the matrix to the intermembrane space. And here, therefore, an electrochemical gradient is established.

3. Splitting of Oxygen to form Water

The electrons are now transferred to the oxygen molecule which splits into half and uptakes Hydrogen ions to form water.

4. ATP Synthesis

The Hydrogen ions (H+) pass through an enzyme called ATP synthase while flowing back into the  matrix. As a result, it controls the flow of protons to synthesize ATP.