Electron Transport Chain
In non-biologic systems, energy is produced in the form of heat by direct reaction between hydrogen and oxygen, then heat can be transformed into mechanical or electric energy. This process is explosive, inefficient and uncontrolled.
In biologic systems, the cells use electron transport chain to transfer electrons stepwise from substrates to oxygen. Thus producing energy gradually to prevent sudden release of huge amount of energy, which may be wasted or destructive to the cells. This process is stepwise, efficient and controlled. Electron transport chain is a chain of catalysts of increasing redox potential. It collects reducing equivalents (hydrogen atoms and electrons) from substrates transferring it stepwise to be oxidized in a final reaction with oxygen to form water and energy. It is also known as redox chain or respiratory chain. It is simply a chain of hydrogen and electron carriers of increasing redox potential. The electron carriers are found within four membrane-bound enzyme-complexes, which are imbedded in the inner mitochondrial membrane.
Components of the electron transport chain The electron transport chain is formed of: A. Hydrogen and electron carriers B. Four membrane-bound enzyme complexes Hydrogen and electron carriers of the electron transport chain
It is a coenzyme that acts as a hydride carrier as it carries hydride ion (H ). It receives two hydrogen atoms (2H) from substrates as isocitrate, malate, β-hydroxy acyl CoA and β-hydroxy butyrate. Its reduced form (NADH+H+) passes its hydrogen to flavoprotein containing FMN and iron sulfur protein (FeS).
2- Flavoproteins FAD and FMN serve as hydrogen carriers, which are tightly bound to flavoproteins as a manner that prevents its reduced form from reacting with oxygen directly. There are many types of flavoproteins that have a role in electron transport chain
Flavoprotein Fp1 containing FMN receives two hydrogen atoms from reduced NAD passing them to coenzyme Q. Flavoproteins Fp2 containing FAD receive two hydrogen atoms from substrates as succinate, acyl CoA and choline passing them to coenzyme Q.
3- Ubiquinone (Coenzyme Q) Ubiquinones are a group of compounds containing quinone ring but vary according to number of isoprene units at the side chain. The most common ubiquinone is coenzyme Q that has structural similarity to vitamin K. It is a small molecule, which is soluble in lipid, so it is freely mobile in the inner mitochondrial membrane colleting reducing equivalents from the more fixed component of the respiratory chain.
Ubiquinone can carry two hydrogen atoms forming ubiquinol (reduced coenzyme Q) or one hydrogen atom forming semiquinone. So, It forms a bridge between flavoproteins, which can carry 2 hydrogen atoms, and cytochrome b, which can carry one electron only. Reduced coenzyme Q passes the electrons to cytochrome b and releases 2H+ into the mitochondrial matrix The oxidation of ubiquinol involves the successive action of 2 enzymes:
a) - Ubiquinol (coenzyme Q) dehydrogenase which transfers electrons to cytochrome c. It needs cyt b, FeS protein and cyt c1 as coenzymes. b) - Cytochrome oxidase which transfers electrons from cyt c to oxygen. It needs cyt a and cyt a3 as coenzymes.
4- Cytochromes They are electron carriers transferring electrons from coenzyme Q to oxygen. They have given letter designation a, b and c according to their order of discovery. All cytochromes are haemoproteins but they differ in redox potential.
The haeme in cytochromes differs from that of haemoglobin as the iron atom oscillates between oxidation (Fe+3; ferric state) and reduction (Fe+2; ferrous state) during the physiological action of cytochromes, while the iron of haemoglobin remains in the reduced form during its physiological action.
Cytochrome c is a water soluble, peripheral membrane protein. It is relatively mobile. It is associated with iron sulfur protein in addition to the haeme group. Cytochrome a3 contains copper in addition to the haeme group. N.B. The mobile components of the electron transport chain include coenzyme Q and cytochrome c. They collect reducing equivalents from the other fixed components. 5- Iron sulfur protein It is an additional component found in the electron transport chain. It is also called FeS or none-haeme iron. It consists of a cluster of cysteine residues which complex iron through covalent bonds with the sulfur of cysteine. It is associated with the flavoproteins and cytochrome b. The sulfur and iron are thought to take part in the oxidation-reduction mechanism between flavoproteins and coenzyme Q as the iron atom in these complexes oscillates between oxidation and reduction that allows them to either give up or accept electrons. Enzyme Complexes of the Electron Transport Chain The enzymes of the electron transport chain are organized in the inner mitochondrial membrane in the form of four enzyme complexes. The four enzyme complexes of the electron transport chain are: Complex I: NADH dehydrogenase (NADH-ubiquinone oxidoreductase) It is a flavoprotein that contains FMN as well as FeS protein as coenzymes It transfers hydrogen atoms from NADH+H+ to ubiquinone. Complex II: Succinate dehydrogenase (succinate-ubiquinone oxidoreductase). It is a flavoprotein that contains FAD as well as FeS protein as coenzymes It transfers hydrogen atoms from succinate to ubiquinone Complex III: Ubiquinol dehydrogenase (ubiquinol-cytochrome c oxidoreductase). It transfers electrons from ubiquinol to cytochrome c using cyt b and cyt c1 as coenzymes Complex IV: Cytochrome oxidase (cytochrome-oxygen oxidoreductase) It transfers electrons from cytochrome c to oxygen. It needs cyt a and cyt a3 as coenzymes.
The enzyme complexes of the electron transport chain are shown in the following figure
Intermembrane space Complex 1 Complex III Complex IV Complex V Complex II ATP synthase NAD Succinate Fumarate Enzyme complexes of the electron transport chain
In addition to these four enzyme complexes, their is a fifth complex (complex V) which is the ATP synthase that is responsible for biosynthesis of ATP from ADP and inorganic phosphate. Sequence of events in the electron transport chain The following diagram shows the sequence of events that occurs in the electron transport chain Isocitrate Succinate
β-hydroxy acyl CoA
β-hydroxy butyrate Flavoprotein (FAD) FeS Flavoprotein (FMN), FeS Sequence of events in the electron transport chain
The hydrogen atoms produced from oxidation of substrates can enter the chain through
FAD or NAD . The hydrogen atoms are then successively transferred through the respiratory chain to oxygen to produce water and energy. NAD+ collects the reducing equivalents from substrates as isocitrate, malate, β-hydroxy acyl CoA and β-hydroxy butyrate, while FAD collects the reducing equivalents from substrates as succinate, acyl CoA and choline.
The initial oxidation of NADH+H is catalyzed by a membrane bound NADH dehydrogenase (complex I). The electrons are then passed to coenzyme Q.
Electrons from FADH2 are passed to coenzyme Q by enzyme complex II. Ubiquinol (reduced coenzyme Q) is oxidized by ubiquinol dehydrogenase (Complex III). The 2 hydrogen atoms are removed from ubiquinol but they cannot be transferred to cytochrome b as cytochromes can accept or transfer only electrons. So at this step the two hydrogen atoms liberated from coenzyme Q will be ionized giving 2 hydrogen ions and 2 electrons. The hydrogen ions will be liberated into the mitochondrial matrix and the 2 electrons will then reduce the iron in cyt b. The electrons will be successively transferred to cyt c1, cyt c, cyt a and cyt a3
Lastly, electrons are transferred to oxygen by cytochrome oxidase and ionic oxygen (O ) will be produced. Being negatively charged, ionic oxygen attracts 2 hydrogen ions from the mitochondrial matrix to form water.
O x id iz e d O x id iz e d c o e n z ym e Q c yt b (2 F e +2) c yt c 1 (2 F e + 3) c yt c (2 F e +2) N A D lin k e d d e h yd ro g e n a s e s O x id iz e d O x id iz e d c o e n z ym e Q c yt b (2 F e +3) c yt c 1 (2 F e + 2) c yt c (2 F e + 3) O x id iz e d c yt c (2 F e + 2) c yt a (2 F e + 3) c yt a 3 (2 F e + 2) O x id iz e d O x id iz e d c yt c (2 F e + 3) c yt a (2 F e + 2) c yt a 3 (2 F e +3) S e q u e n c e o f e ve n ts in th e e le c tro n tra n s p o rt c h a in
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