Electron Transport Chain

Electron Transport Chain and Oxidative Phosphorylation

Metabolism for energy yield ends at oxidative phosphorylation in case of aerobic organisms. High energy potential molecules like NADH, NADPH and FADH₂ produced in glycolysis, fatty acid oxidation and TCA cycle contain electron pairs with high reducing potentials. In course of transference of these electrons to molecular oxygen and water, a large amount of energy is liberated, which can be utilized to produce ATP.

Oxidative phosphorylation is the process by which ATP is synthesized as a result of the transfer of electrons from NADH, FADH₂ or NADPH to O₂ with the aid of series of electrons. The interaction of those reducing potentials and the terminal electron acceptors (oxygen) in a series of oxidation and reduction reactions that results in the generation of ATP is ultimately known as oxidative phosphorylation.

Oxidation of substrates that releases electrons that are used to phosphorylate ADP and produce ATP is simply called an oxidative phosphorylation. This process is found to be conducted in mitochondrial matrix in eukaryotes an in case of prokaryotes, it is found to be conducted in cytoplasm. It is the major source of ATP in aerobic respiration of all organisms that includes both prokaryotes and eukaryotes.

There are two major things on which oxidative phosphorylation depends. They are:

• Transfer of electron through Electron Transfer Chain and
• Enzyme ATP synthase

Electron Transport chain:

Electron Transport Chain (system) also known as respiratory chain is the series of arrangement of electron carriers through the inner mitochondrial matrix membrane in eukaryotes and in cytoplasm in prokaryotes. These electros carriers pass electrons gained from NADH, FADH₂ and NADPH throughout the chain. During this transfer, there is the stepwise release of energy which is utilized to drive the chemiosmotic synthesis of ATPs with the assistance of enzyme, ATP synthase. In prokaryotes this phenomenal arrangements is found in cytoplasm and in eukaryotes it is inner mitochondrial matrix membrane where this phenomenal arrangement is found. Electron Transfer Chain is not that easy to understand though. It is the complex arrangement of various natured electron carriers in a perfectly arranged manner.

Components of Electron transport chain:

Various electron carrier molecules are arranged in this system that are capable of oxidation and reduction. Especially electron carriers are the large complex protein molecules with an exception of Coenzyme Q. Followings are the electron carrier molecules in the Electron Transport Chain:

• Complex I/ NADH Dehydrogenase
• Complex II/ succinate Dehydrogenase
• Coenzyme Q/ Ubiquinone (Non protein in nature, a lipid soluble benzoquinone with a long isoprenoid side chain )
• Complex III/ Cytochrome bc₁ complex
• Cytochrome c
• Cytochrome IV / cytochrome aa₃ complex

Due to their iron containing haem containing prosthetic group, these are the proteins with strong adsorption capacity.

Reactions of Electron Transport Chain (ETC)

The electron carriers arranged in an array help in the transference of electrons donated by the electron rich reducing potentials, NADH, FADH₂ and NADPH in the electron transport chain. Electrons are transferred from one electron carrier to another with the simultaneous oxidation and reduction of the carriers. During this course of electron transfer there is the generation of energy which is utilized to generate ATP by the oxidative phosphorylation of ADP.

• NADH and FADH₂ after getting reduced to NADH and FADH₂ get involved in electron transport chain and get oxidized themself donating the pairs of electrons and reduce O₂ to water.
• The free electrons are transferred by the reduced coenzyme, NADH to NADH dehydrogenase which is the first complex of the electron transport chain. In the similar condition succinate dehydrogenase complex acts as the electron accepter of FADH₂, which is the second complex of the ETC.
• These complexes (complex I and II) now donate the electron pairs to the Coenzyme Q (CoQ). Electrons transferred by both complex I and II are accepted by CoQ in all biological systems CoQ is ubiquitous and hence is called ubiquinone.
• Then ubiquinone transfer the electrons (reducing potentials) to the cytochromes. In this course of reaction, electrons are transferred along the electron chain through ubiquinone to cyt bc I complex and from bc I complex to cyt₃ complex through cyt C. cytochromes are chemically the iron containing proteins with the porphyrin ring which is known as haem. The iron in this ring participate in electron transfer with the simultaneous change in the valency. Cytochromes can transfer electrons but not H⁺. These H⁺ go into the solutions and are utilized to adjust the protein gradient across the membrane (in case of eukaryotes) and also for the reduction of O₂ to water. Cytochromes are of various types, for example; a₁, a₂, a_3, b_1, c₁ These cytochromes form tight complexes with each other, for example; cyt bc₁ complex which is complex III and cyt aa₃ complex IV in ETC.
• From the last cytochrome, i.e. cyt IV electrons are transferred to oxygen (terminal electron accepter) and water is reduced to H₂ In the course of reduction of O₂ to water, the protons from the solution is utilized.

This way the reaction in ETC completed. Electron is sequentially transferred from one complex to other and in between these transfer, there is the oxidation of coenzymes, i.e. NADH, FADH₂ and NADPH and reduction of the electron accepting complexes. Hence the overall ETC reaction is a typical redox reaction. During the transfer of electron, there is the release of energy which is utilized to generate ATP by the oxidative phosphoryl transfer to ADP.

Complex V/ATP Synthase:

Last but not the least complex V is the final multiprotein complex, called ATP synthase that is needed for the oxidative phosphorylation. This complex is also called F₀F₁ complex. In this complex, the F₀ part lies at the inner membrane of mitochondria of eukaryotes and the inner face of the plasma membrane in prokaryotes. The special feature of this portion is that it contains a channel through which protons are passed. On other the hand H₁ part also known as head part of complex V consists of a pool of ADP and inorganic phosphate (Pi). This is actually which catalyzes the phosphorylation of ADP to yield n ATP.

Functions of Electron Transport Chain (ETC)

It is at Electron Transport chain where the actual production of energy-rich molecule, ATP takes place via oxidative phosphorylation. Apart from ATP synthesis, ETC has some other important functions that are vitally important to regulate rest of metabolic activities, some of which are as follows:

• In the case of bacterial locomotion, proton motive force is needed for flagellar rotation. This protein motive force is generated due to the proton gradient across the membrane. ETC provides this proton gradient in the cell.
• The proton gradient provided by ETC is also utilized for the generation of heat in brown fat.
• From ETC, regeneration of NAD⁺, FAD and NADP are regenerated that are again recycled for the oxidation of energy-rich molecules.
• ETC produces reactive oxygen species by the partial oxidation of oxygen molecule.