All energy yielding catabolic pathways of carbohydrates, fats and amino acids come to an end at the final stage of cellular respiration known as oxidative phosphorylation. In the process of oxidative phosphorylation, the energy derived from the oxidative steps that take place in catabolic process is utilized for the synthesis of ATP (Adenosine triphosphate). ATP is considered as currency of energy of the cell. In eukaryotes, oxidative phosphorylation takes place in mitochondria of cell and this process occurs in mesosome of bacterial cell. Oxidative phosphorylation involves the reduction of O2 to H2O with electrons denoted by NADH and FADH2. The hypothesis of Peter Mitchell in 1961 leads to the way for better understanding of ATP synthesis in mitochondria. Peter Mitchell hypothesis also known as chemiosmotic theory stated that transmembrane differences in proton concentration are the source for energy extracted from the biological catabolic reactions.

Mechanism of Oxidative phosphorylation

Oxidative phosphorylation process has three aspects:

(a)The downhill flow (along the concentration gradient) of electrons through the membrane bound complex in mitochondria.

(b) The energy generated by this exergonic flow of electrons is used for the uphill transportation (against concentration gradient) of protons across proton-impermeable mitochondrial membrane producing transmembrane electrochemical gradient.

(c) This transmembrane electrochemical gradient provides free energy for the synthesis of ATP catalyzed by a membrane protein complex known as ATP synthase that articulates proton flow to phosphorylation of ATP.

Electron transfer reaction in mitochondria

Eugene Kennedy and Albert Lehninnger discovery that mitochondria is the site of oxidative phosphorylation provide an insight into the process of oxidative phosphorylation. Mitochondria membrane has one outer membrane and one inner membrane. The outer membrane of mitochondria is permeable to small molecules and ions. The inner membrane is impermeable to small molecules, ions and possesses membrane bound respiratory chain complexes and ATP synthase which are elucidated below:

Complex I: It is also known as NADH; ubiquinone oxidoreductase or NADH dehydrogenate. This complex is a large enzyme composed of 42 different polypeptide chains including an FMN containing flavorprotein and 6 iron sulphur centers. Complex I catalyses the transfer of electron from NADH to ubiquinone, a mobile electron carrier, meanwhile 4 protons from the matrix of mitochondria move to the inter membrane space through mitochondria inner membrane.

Complex II : This is also known as succinate dehydrogenate complex. It catalyses the transfer of electrons from FADH2 to ubiquinone.

Complex III: It is commonly known as cytochrome bc1 complex or ubiquinone; cytochrome C oxidoreductase. It catalyses the transfer of electron from ubiquinol to cytochrome C which is a mobile electron carrier and 4 protons pass from matrix of mitochondria into inter membrane space through mitochondrial inner membrane.

Complex IV: It is also known as cytochrome C oxidase. It catalyses the transfer of electron from cytochrome C to molecular Oxygen reducing it to water and 2 Protons move from matrix of mitochondria into the inter membrane space through mitochondrial inner membrane.

Proton Motive Force:

For each pair of electron transfer to Oxygen, 4 protons are moved by Complex I, 4 protons by Complex III and 2 protons by Complex IV. This creates a Proton concentration gradient across the inner mitochondrial membrane which is known as Chemical Potential energy and also forms the electrical potential energy that results from the separation of charge (H+). Both chemical potential energy and electrical energy are the component of electro chemical gradient. This sort of energy stored in electro chemical gradient is termed as Proton Motive Force.

ATP synthase

The energy in form of proton motive force is used for synthesis of ATP from ADP and Pi by ATP synthase. This complex is also known as F0-F1 complex. F0 and F1 have stoichiometry of ab2C9-12 and α3β3γδε respectively. ATP synthase is divided into two functional units one is termed as Rotator having stoichiometry γδC9-12 and other one is Stator containing α3β3δab2. The movement of proton from this complex causes rotator to spin in one direction and stator in opposite direction which produces torque. This torque is transmitted to β subunit of F1 domain where it is utilized for binding of ADP + Pi, formation of ATP from ADP and release of tightly bound ATP from β subunit. Total four electrons the required for synthesis of one ATP.

P/O ratio

This ratio is used to calculate the number of ATP produced when one atom of oxygen is reduced to water molecule. P/O ratio is about 2.5 when electron enters through complex I in oxidative phosphorylation process and 1.5 when enters through compexII.

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