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Electron Transport Chain and Its Mechanism

    In this electron transport chain and its mechanism post we have briefly explained about structure of mitochondria, components, enzymes in electron transport chain, and inhibitors. Read on to learn more about electron transport chain reaction!

    Electron Transport Chain Reaction

    To understand biological oxidation and the electron transport chain reaction, an understanding of oxidation, reduction reactions and the biology of mitochondria are essential. In eukaryotic cells, cellular metabolism takes place in major compartments of the cell.

    For instance, glycolysis takes place in the cytosol, whereas oxidation of pyruvate, β oxidation of fatty acids and the citric acid cycle take place in the mitochondrial matrix.

    The mitochondrial electron transport chain reaction (ETC) is a complicated system, where a series of electron carriers are arranged in the inner membrane of the mitochondria in the order of increasing electron affinity. They transfer the electrons derived from reduced coenzymes to oxygen.

    Structure of Mitochondria

    A mitochondrion is about the same size as a bacterium (varying from 1 to 2 m). Mitochondria are thought to have evolved from aerobic bacteria in association with primitive anaerobic eukaryotes during an early stage of development. Endosymbiosis is the term for this type of relationship.

    The mitochondria contain two membranes: a smooth outer membrane and an inner membrane that folds into many cristae that surround the matrix and extend into the interior of the mitochondria.

    The density of the cristae is connected to the cell’s respiratory activity since the respiratory proteins are embedded in the inner membrane. When compared to the liver, the heart muscle possesses more mitochondria due to its severe oxidative metabolism.

    Proteins abundant in the mitochondrial membrane. Small molecules can be transferred between the cytoplasm and the intermembrane gap thanks to porins found in mitochondria’s outer membrane.

    transport

    Electron Transport Chain Reaction and Its Mechanism: Structure of Mitochondria

    With the exception of oxygen, carbon dioxide, and water, the inner mitochondrial membrane is impermeable to tiny molecules. All of the other substrates and products of mitochondrial metabolism must thus be transported across the inner membrane by specific transporters.

    Various protein carriers, mostly cytochromes, are embedded within the inner membrane of mitochondria. Multi-protein complexes are used to structure the respiratory chain. ATP synthase and other enzymes are also found in the inner space.

    In oxidative phosphorylation, the inner membrane plays a crucial function. The matrix of the mitochondrion is similar to a 50% protein gel. Enzymes involved in the oxidation of pyruvate, amino acids, fatty acids, the TCA cycle, as well as NAD+, FAD, ADP, and Pi, are all found in the matrix.

    The energy released during carbohydrate and food oxidation is stored in mitochondria as reducing equivalents (2H), which are retrieved by the respiratory chain for oxidation and linked ATP production.

    Components

    In eukaryotes, the electron transport chain reaction components are arranged into four major protein complexes in the inner mitochondrial membrane. Several proteins and prosthetic groups make up these complexes. Apart from these, two additional molecules in the electron transport chain reaction, coenzyme Q and cytochrome c, perform important roles.

    Electron transport chain

    Electron Transport Chain Reaction and Its Mechanism

    Complex I

    Complex I, also known as NADH-coenzyme Q oxidoreductase, or NADH dehydrogenase, catalyses the transfer of electrons from NADH to Coenzyme Q (CoQ or Ubiquinone UQ), because it is ubiquitous in biological systems). It contains one molecule of flavin mononucleotide (FMN) and 8 iron-sulfur clusters that participate in electron transfer.

    Complex II

    Succinate Dehydrogenase or Succinate–Coenzyme Q Oxidoreductase transfers electrons from succinate to CoQ through succinate dehydrogenase and three tiny hydrophobic subunits.

    Complex III

    Coenzyme QH2 -cytochrome c Oxidoreductase is Complex III. It is responsible for transporting electrons from reduced CoQ to cytochrome c. It has cytochromes b and c1 as well as one [2Fe-2S] cluster.

    Cytochrome C

    The fourth complex is cytochrome c oxidase. It catalyses the one-electron oxidation of four reduced cytochrome c molecules followed by the four-electron reduction of one oxygen molecule, yielding two water molecules.

    The pumping of protons from the matrix across the inner mitochondrial membrane into the intermembrane gap is caused by electron transport via complexes I to IV. ATP synthesis is fuelled by the proton-motive force that is produced. ATP production is catalysed by Complex V.

    Reactions

    All components of the respiratory chain are proteins, with the exception of coenzyme Q. These proteins can be enzymes, such as dehydrogenases, or they can include iron in the form of an iron-sulfur core. They may also contain iron covalently bound to a porphyrin ring, as in cytochromes, or copper, as in the cytochrome a + a3 complex.

    NADH

    Dehydrogenases convert NAD+ to NADH. Two hydrogen atoms are removed from the substrate. Several reactions of the citric acid cycle, fatty acid oxidation, and other processes are important producers of NADH. NAD+ receives both electrons and one proton, resulting in NADH and a free proton H+.

    Electron transport chain reaction

    Enzymes in Electron Transport Chain Reaction

    NADH dehydrogenase

    NADH dehydrogenase receives the NADH and H+ produced. This complex I, a big protein complex of 46 polypeptides, is buried in the mitochondrial inner membrane and contains a flavin mononucleotide molecule attached to it (FMN).

    Electron transport chain

    NADH dehydrogenase complex

    FMN is converted to FMNH2 by NADH dehydrogenase. Four protons are released into the intermembrane space when two electrons are sequentially transferred to the iron sulphur core, which is made up of iron atoms complexed with an equal amount of sulphide ions. The purpose of the NADH dehydrogenase complex is to absorb high-energy electrons from NADH.

    Succinate Reductase Complex

    This complex II comprises the succinate dehydrogenase enzyme, which is involved in the TCA cycle and converts succinate to fumarate, forming FADH2. FADH2 remains in the complex and donates two electrons to a succession of Fe-S clusters before transferring them to ubiquionone. This complex is not a proton pump; it does not transport hydrogen ions across the mitochondrial membrane from the matrix to the intermembrane space.

    Electron transport chain

    Succinate dehydrogenase complex

    Coenzyme Q

    This complex II comprises the succinate dehydrogenase enzyme, which is involved in the TCA cycle and converts succinate to fumarate, forming FADH2. FADH2 remains in the complex and donates two electrons to a succession of Fe-S clusters before transferring them to ubiquionone. This complex is not a proton pump; it does not transport hydrogen ions across the mitochondrial membrane from the matrix to the intermembrane space.

    Cytochrome Reductase

    Electrons from reduced ubiquinone (UQH2) are passed to cytochrome c via Complex III. Cytochromes are proteins with heme prosthetic group. All cytochromes have the tendency to act as electron carriers.

    Once the electron is accepted, the iron atom of heme group changes from Fe3+ to Fe2+ state. The pathways of electrons transfer from UQH2 to cytochrome c is quite complex.

    Overview of reactions in complex III

    Cytochrome C Oxidase

    Cytochrome c oxidase of Complex IV catalyzes the one electron oxidation of four consecutive reduced cytochrome c molecules and the concomitant four electron reduction of one oxygen molecule. Cytochrome c oxidase contains two cytochromes (a and b).

    Cyt a is paired with copper atom, CuA and cyt a3 is paired with a different copper atom, CuB. Each of the reduced Cytc molecules donates two electrons, one at a time to CuA.

    The electrons are further transferred to cyt a, cyt a3 , CuB. The transfer of four electrons from cyt c converts oxygen and four protons to two molecule of water.

    Overview of reactions in Complex IV

    Inhibitors of ETC

    1. Electron transport chain reaction may be blocked by many inhibitors. Electron transport chain reaction inhibitors act by binding a suitable area in the Electron transport chain reaction. They prevent electrons from being passed from one carrier to the next one. Each inhibitor binds a particular carrier For example, rotenone and amytal inhibits complex I at NADH dehydrogenase and prevent NADH oxidation.

    2. Antimycin A and dimercaprol inhibit Electron transport chain reaction at complex III. Poisons such as hydrogen sulphide, Cyanide, azide and carbon monoxide inhibit Complex IV. Oligomycin is a ATP synthase inhibitor. Many of the details of respiratory chain were obtained using inhibitors of electron transport chain reaction.

    3. The correct order of the Electron transport chain reaction components was determined using inhibitors. Using oxygen electrode, the extent of electron transport was measured. During this process, when electron transport is inhibited, oxygen consumption is diminished.

    Further Readings

    Reference