Electron Transport Chain Overview
- The Electron Transport Chain is made of different protein complexes which perform a redox reaction to transfer electrons from electron donor to electron acceptor and also perform the transfer of protons from matrix to intermembrane space.
- The Electron Transport Chain takes place within the mitochondrial matrix.
- The Electron Transport Chain contains different types of electron acceptors such as nicotinamide nucleotides (NAD or NADP) or flavin nucleotides (FMN or FAD), and three types of electron carriers such as ubiquinone, cytochrome, iron-sulfur proteins.
- The NAD or NADP transfers one hydride ion from the substrate to NAD or NADP and forms NADH or NADPH, and it releases another proton from the substrate as a form of hydrogen.
- The oxidized flavin nucleotide can accept one or two electrons.
- In ETC the transfer of electrons can occur via the reduction of Fe3+ to Fe2+ or Via the Hydrogen atom such as H + e- Or via the hydride ions :H-.
- There are four complexes within the ETC such as Complex I, Complex, II, Complex, III, and Complex, IV.
- In the electron transport chain, the electrons are moved towards the high reduction potential from low reduction potential molecules. Those molecules have a high reduction potential; they function as a good electron acceptor and those have a low reduction potential they function as a good electron donor.
The Electron Transport Systems, also known as”the Electron Transport Chain, is an array of reactions which convert redox energy through oxidation of NADH and FADH2 into proton-motive force, which is used to produce ATP by conformational modifications in the ATP synthase complex by an oxidative phosphorylation process.
- Oxidative phosphorylation is the final step in the process of cell respiration.
- This stage is the transfer of electrons between organic molecules and oxygen, while simultaneously releasing energy throughout the process.
- Aerobic respiration’s main electron acceptor is molecular oxygen. In anaerobic respiration , there are additional acceptors, such as sulfur.
- The chain of reactions is vital since it involves the breakdown into ATP to ADP and then resynthesizing this process into ATP which is then using the limited ATPs present within the body around 300 times per day.
- The electron flows take place in four complexes of proteins that are embedded within the mitochondrial inner membrane which is referred to as”the respiratory chain” or electron-transport chain.
- This stage is essential in energy synthesis since every oxidative step in the breakdown of carbohydrates, amino acids, and fats meet at the end of cellular respiration, during where the energy from oxygenation is used to drive the production of ATP.
Electron Transport Chain Location
- Since the citric acid process takes place within mitochondria, electrons with high energy are found in the mitochondria. This is why an electron-transport chain within the eukaryotes is also carried out in mitochondria.
- Mitochondrion is a dual-membraned organelle, which has an outer membrane as well as an inner membrane which is folded into the form of ridges, known as the cristae.
- Two compartments exist inside mitochondria: the matrix and the intermembrane spaces.
- Its membrane outer is impermeable to Ions. It has enzymes essential to citric acid cycles . The inner membrane is inaccessible to ions of various kinds and also contains electron transport chain, and ATP producing enzymes.
- The amount of electron transport chains found in mitochondria depends on the location and the purpose and function of the cell. The liver mitochondria there are 10,000 chains of electron transport in the heart mitochondria, whereas the liver mitochondria have three times as many electron transport chains as the mitochondria of the liver.
- The intermembrane space is home to enzymes like adenylate-kinase and the matrix also contains ATP, ADP, AMP NAD, NADP and other ions, such as Mg2+, Ca2+ and so on.
Components/ Electron carriers of Electron Transport Chain
FMN (Flavin Mononucleotide)
- In the initial stage of the chain of electron transfer that is, the electrons of NADH transfer to flavin Mononucleotide (FMN) which reduces it to FMNH 2..
NAD + H + + FMN – NAD + FMNH 2
- The electron transfer process is catalyzed through the action on NADH dehydrogenase.
- The electrons are then transferred to a variety that are iron sulfur complexes (Fe-S) that have an increased affinity for electrons.
- The Ubiquinone is also known as coenzyme Q, or simply Q.
- It is lipid-soluble benzoquinone which contains a long isoprenoid side chain.
- It is small in size and hydrophobic (due to the presence of benzene rings in it) in nature.
- It can accept one electron and form semiquinone radical or either can accept two-electrons and form Ubiquinol.
- Cytochromes are a type of protein that can absorb visible light at different wavelengths, due to the presence of iron containing heme prosthetic groups.
- The mitochondria contain three different types of cytochromes such as cytochrome a (Heme A), cytochrome b (Iron protoporphyrin IX, and cytochrome c (Heme C).
- The Cyt a can absorb 600nm wavelength of light, Cyt b can absorb 560nm, and Cty c can absorb 550nm.
- These are discovered by John S. Rieske, that’s why it is called Rieske iron-sulfur proteins.
- These iron-sulfur (Fe-S) centers range from simple structures with a single Fe atom coordinated to four Cys—SH groups to more complex Fe-S centers with two or four Fe atoms.
Electron Transport Chain Complexes
A set of four complexes of enzymes is found inside the electron transport chain which facilitates the transfer of electrons from various electron carriers to molecular oxygen.
a. Complex I (Mitochondrial complex I)
Complex I of the electron transport chain composed of NADH dehydrogenases as well as the Fe-S center that facilitates an exchange of 2 electrons NADH to the ubiquinone (UQ). In addition, the complex transfers the four H+ ions across the membrane, forming an electron gradient.
NADH + H+ + CoQ – NAD+ + CoQH2
NADH is first converted to NAD+ after the reduction of FMN into FMNH2 via the process of two-step electron transfer. FMNH2 is then converted to FMN and two electrons are transferred to Fe-S-centers and the next step is to convert them to Ubiquinone.
b. Complex II (Mitochondrial complex II)
Complex II is comprised of succinic dehydrogenase(SDH), Fe-S centers, and FAD. The complex of enzymes catalyzes movement of electrons away from other donors such as the fatty acids and glycerol-3-phosphate to ubiquinone by means of Fe-S and FAD centers. The complex is parallel to Complex II. Complex II, but Complex II doesn’t transfer H+ through the cell membrane unlike Complex I.
Succinate + FADH2 + CoQ – Fumarate + FAD+ + CoQH2
c. Complex III (Mitochondrial complex III)
Complex III is composed of cytochrome b and a distinct Fe-S center. The complex of enzymes, called cytochrome reductasecatalyzes exchange of electrons of the reduced CoQH2 to two molecules of cytochrome C. The proton (H+) from the ubiquinone released across the membrane and contribute the proton gradient. The CoQH2 is then oxidized to CoQ and the center of iron (Fe3+) within the cytochrome C decreases into Fe2+.
CoQH2 + 2 cytc c (Fe3+) – CoQ + 2 cytc c (Fe2+) + 4H+
d. Complex IV (Mitochondrial complex IV)
Complex IV is composed of cytochrome a as well as a3, which is known as cytochrome-oxidase. This is the final component of the chain, and is responsible for transfers of electrons from cytochrome C and molecular oxygen (O2) creating water. At the same time four protons are transferred across the membrane to aid in this gradient of proton.
4 cytc c (Fe 2+) + O2 – 4cytc c (Fe3+) + H2O
Electron Transport Chain Products
The Products of Electron Transport Chain are 30-32 ATPs and 44 moles of H2O.
|Stage||Direct products (net)||Ultimate ATP yield (net)|
|Glycolysis||2 ATP||2 ATP|
|2 NADH||3-5 ATP|
|Pyruvate oxidation||2 NADH||5 ATP|
|Citric acid cycle||2 ATP/GTP||2 ATP|
|6 NADH||15 ATP|
|2 FADH2||3 ATP|
Electron Transport Chain Steps and Components
The electron transport chain consists of four different complexes such as Complex I, Complex, II, Complex, III, and Complex, IV. Complex I transfers the electron from NADH to ubiquinone and Complex II transfers the electron from Succinate to Ubiquinone.
Complex III transfers the electron from Ubiquinone to Cytochrome C and then Complex IV completes the process by transferring the electron to the O2 molecules.
- Complex I is known as NADH: Ubiquinone oxidoreductase or NADH:Dehydrogenase.
- The complex I is the largest complex molecule within the ETC. It consists of 42 polypeptide chains, FMN-containing flavoprotein, and 6 iron-sulfur centers.
- Under the electron microscope the Complex I appears in L-shape. The one arm of this L-shape is embedded within the inner membrane and the second arm is extended within the matrix.
- Complex I performs two processes simultaneously such as (a) The exergonic transfer of electrons from NADH to Quinone and a hydrogen atom from matrix, and (b) endergonic transfer of 4 protons from matrix to intermembrane space.
- Due to the transfer of electrons from matrix to intermembrane space, the matrix becomes negatively charged and the intermembrane space becomes positively charged.
- The Cyanide and Carbon monoxide Inhibit cytochrome oxidase; Antimycin A Blocks electron transfer from cytochrome b to cytochrome c1; Myxothiazol, Rotenone, Amytal, and Piericidin A Prevent electron transfer from Fe-S center to ubiquinone.
- Complex II is much smaller and simpler than complex I and it contains 5 prosthetic groups of two different types and 4 protein subunits.
- The four subunits are Subunit A, Subunit B, Subunit C and Subunit D.
- The subunits C and D are integral membrane proteins, these contain heme b protein and ubiquinone binding site.
- Subunit A and B contain a Substrate (Succinate) binding site. Bound FAD, three 2Fe-2S centers.
- In complex II the electrons are transferred from the Substrate binding site to FAD then pass through the Fe-S centers to Ubiquinone.
- Complex III transfers the electron from the ubiquinone to cytochrome C.
- This complex is also known as Cytochrome bc1 complex or Ubiquinone: Cytochrome C oxidoreductase.
- The X-ray crystallography shows the detailed structure of Complex III.
- Complex III is a dimer, containing two Subunits of Cytochrome b, which are surrounding the cavern. Through this quinone can freely move from matrix to intermembrane space by releasing protons and electrons.
- Except transfer of electrons, complex III also transfers 4 proton or Hydrogen molecules from the matrix to intermembrane space.
- In complex III the Ubiquinol releases 2 hydrogen and electrons to form Quinone. Then these 2 hydrogen atoms move toward the intermembrane space and one electron moves towards the Fe-S centers, then cytochrome C1, and finally to Cytochrome C.
- Another electron moves toward the Heme bL, to heme bH and then to Q, which is then formed semiubiquinone. Then semiubiquinone accept 2 hydrogen atom from the matrix and form ubiquinol.
The net redox reaction in complex III is;
QH2 + 2Cyt C (oxidized) + 2H = Q + 2 Cyt C (Reduced) + 4H
- The complex IV transfers the electron from Cytochrome C to oxygen and forms the H2O.
- Complex IV is also known as the Cytochrome oxidase.
- Complex IV contains 13 subunits in mitochondria. In a bacterial cell the complex IV is much simpler and contains three to four subunits.
- In complex IV the 4 electron is transferred from 4 Cytochrome C to a CuA center, then to heme a, to the heme a3–CuB center, and finally to O2.
- During the transfer of four electrons, the complex IV also consumed 4 hydrogen (substrate) atoms to form H2O from O2.
- It also pumps 4 hydrogen atoms from the matrix to intermembrane space.
The net redox reaction in complex IV is;
4Cyt c (reduced) + 8H + O2 = 2H2O + 4 Cyt c (oxidized) + 4 H
Formation of Reactive Oxygen Species (ROS)
During the transfers of electrons through the ETC different highly reactive free radicals are generated which can damage the cells.
The passage of electrons from complex I to QH2 and QH2 to Complex III leads to the formation of an intermediate called Ubiquinol. Ubiquinol with low probability can pass an electron to the O2 and lead to the formation of Superoxide free radical.
This superoxide free radical can form more reactive hydroxyl free radicals which can damage the cells. To prevent the oxidative damage by these radicals the cell releases an enzyme called superoxide dismutase which converts the superoxide into H2O2.
Now H2O2 is converted into H2O with the help of an enzyme called glutathione peroxidase. The glutathione reductase recycles the oxidized glutathione to reduce glutathione with the help of NADPH, which is generated from the pentose phosphate pathway.
- In anaerobic respiration Nitrate, Sulfate, Co2 functions as a final electron acceptor in ATC.
- Anaerobic respiration produces less energy or ATP as compared to aerobic respiration.
- The aerobic respiration is performed by few bacteria, archaea, and some eukaryotic microbes.
- Paracoccus denitrificans performs both anaerobic respiration in presence of O2 and anaerobic respiration in absence of O2.
- The anaerobic respiration carried out different enzymes such as nitrate reductase (Nar), nitrite reductase (Nir), nitric oxide reductase (Nor), and nitrous oxide reductase (Nos) for the formation of gaseous Nitrogen from Nitrate.
- Nitrite is reduced to nitric oxide (NO) by the periplasmic enzyme nitrite reductase. Nitric oxide reductase catalyzes the formation of nitrous oxide (N20) from NO. It is part of the membrane-bound cytochrome b complex. Finally, the periplasmic enzyme nitrous oxide reductase catalyzes the formation of N2 from N20.
Why is the energy yield or ATP yield in Anaerobic respiration is Low?
The final electron acceptor of anaerobic respiration is nitrate, which has a low positive reduction potential as compared to oxygen(electron acceptor of aerobic respiration). The difference in standard reduction potential of NADH and Nitrate is lower than the difference of reduction potential between the NADH and O2.
That’s why the energy yield of anaerobic respiration is low, because the energy yield is directly related to the magnitude of reduction potential difference.
Alternative Respiratory Pathways of Plants
- The plant mitochondria contain an alternative pathway for the transfer of electrons from ubiquinone to O2.
- They contain an alternative NADH dehydrogenase, it is insensitive to the complex I inhibitor rotenone that’s why it is called rotenone insensitive NADH dehydrogenase complex. This complex transfers the electrons from NADH to Ubiquinone directly.
- Except alternative NADH dehydrogenase, the plant contains another NADH dehydrogenase at the external face of the inner membrane, which functions the transfer of electrons from the NADPH or NADH to the matrix and then to Ubiquinone.
- Both these external NADH dehydrogenase and alternative NADH dehydrogenase bypass the complex I.
- Now the electron is transferred to O2 via the cyanide-resistant alternative oxidase and generates H2O. In this process energy is not conserved as ATP but is released as heat.
- A skunk cabbage can use the heat to melt snow, produce a foul stench, or attract beetles or flies.
ATP Synthase Definition
- The synthesis of ATP is accomplished by the ATP synthase. ATP synthase is made of two structural subunits called F1 and F0.
- These ATP synthases are found in mitochondria, chloroplasts, and bacteria.
- The F1 complex is spherical in shape and attached on the mitochondria inner membrane with the help of stalk.
- F1 complex is composed of three α subunits which are alternate with three β subunits, and it also contains a γ subunit which is located at the center.
- The γ subunit is extended from the F1 to F0 complex and it can rotate.
- A stalk (γ and ε subunit) connects the F1 complex and F0 complex and serves as a proton channel.
- The F0 complex is composed of one a subunit, two b sunits and 8 -12 c subunits.
- A stator arm is embedded within the membrane and also attached with the F1 complex. This stator arm is made of one a subunit, two b sunit and δ subunit.
- A ring of c subunits in F0 is connected with the stalk (γε). When the ring turns it results in the rotation of Shaft or stalk (γε).
Binding-change model for ATP synthase/ How does ATP synthase produce ATP?
- The binding change mechanism is a widely accepted model of ATP synthesis. This simplified drawing of the model shows the three catalytic β subunits and the γ subunit, which is located at the center of the F1 complex.
- The β subunit contains three catalytic sites such as βE, βDP, and βTP. The βE site is open conformation which does not bind with the nucleotides.
- When the γ subunit rotates it results in conformational change in catalytic sites of β subunits. In one turn of γ subunit the βE is changed to βHC as a result the ADP and Pi bind to this catalytic site.
- The 2nd turn of γ subunit results in a three conformational change in catalytic site of β subunits.
- The βTP is changed to βE as a result the ATP is released from the ATP synthase.
- The βDP is changed into The βTP as results the ATP is generated.
- The βHC is changed to The βDP, where ADP is bounded.