Electron Transport System : Mechanism and Components

Introduction: 

Electron Transport System  

• The electron transport system, or ETS, is the metabolic mechanism of electron transport. 

Glycolysis and the Krebs cycle produce reduced coenzymes such as 10 molecules of NADH⁺, H⁺ ions, 2 molecules of FADH₂, and 4 molecules of ATP. To liberate the energy held in these reduced coenzymes, they must be oxidized.  

• This is made possible by electron and proton transfer from these coenzymes to oxygen via electron carriers found in the inner mitochondrial membrane. The electron transport system, or ETS, is the metabolic mechanism of electron transport. 
• NADH dehydrogenase complex (Complex I), Ubiquinone (Complex Q), Succinate dehydrogenase complex (Complex II), Cytochrome bc1 complex (Complex III), Cytochrome c, Cytochrome c oxidase (Complex IV) are all components of ETS.  

Diagramatic Representation Of Ets 

Mechanism Of Electron Transport System  

• The release of energy from an electron transport system: Electrons move from carrier to carrier in an electron transport system via a sequence of oxidation-reduction processes. During each transmission, some energy is released.  
• According to this hypothesis, the transfer of electrons down an electron transport system via a sequence of oxidation-reduction processes releases energy. This energy enables some carriers in the chain to transfer hydrogen ions (H⁺ or protons) through a membrane.

• As hydrogen ions collect on one side of a membrane, the concentration of hydrogen ions causes an electrochemical gradient or potential difference (voltage) across the membrane. (The fluid on the side of the membrane where the protons concentrate gets a positive charge, while the fluid on the opposite side of the membrane retains a negative charge.)  
• The energetic condition of the membrane as a result of this charge separation is known as proton motive force or PMF.  
• This proton motive force supplies the energy required for ATP synthases, which are also found in the membranes indicated above, to catalyze the synthesis of ATP from ADP and phosphate.  
• This ATP production happens as protons cross the membrane via the ATP synthase complexes and re-enter either the bacterial cytoplasm or the mitochondrial matrix. The energy released by protons as they pass down the concentration gradient through the ATP synthase causes the rotor and rod of the ATP synthase to revolve. 
•  As phosphate is added to ADP to generate ATP, the mechanical energy from this rotation is turned into chemical energy.  

Components Of The Electron Transport Chain  

1. Complex I (NADH dehydrogenase): It comprises FMN, which receives two electrons and one hydrogen ion from two NADH molecules to create the reduced form of FMNH₂; it also contains iron atoms, which aid in the transport of the e and H⁺ to coenzyme Q. 

2. Complex II (Succinate dehydrogenase): This complex contains iron and succinate, and it oxidizes FAD to generate FADH₂. 

3. Coenzyme Q: Accepts electrons from FMNH₂ (complex I) and FADH₂ (complex II) before transferring them to complex III. 

4. Complex III (cytochrome b): It includes the heme group, which takes electrons from coenzyme Q to form Fe²⁺. Electrons are transferred to cytochrome c. 

5. Cytochrome c: It includes the heme group, in which the Fe³⁺ absorbs electrons from complex III to produce Fe²⁺. Transfers electrons to Complex IV. 

6. Complex IV (cytochrome a): This complex comprises the heme group, which receives electrons from cytochrome c to form Fe²⁺. Electrons are transferred to O₂, which is then mixed with hydrogen to generate H₂O. 

7. Complex V (ATP synthase): It features a proton channel that permits protons to enter into the matrix, where they use the proton gradient energy to generate ATP. 

Major Steps In Electron Transport System  

1. Electron transfer from NADH to coenzyme Q: 

• NADH transfers electrons to FMN via the NADH dehydrogenase complex (complex I). The complex is sometimes referred to as NADH: CoQ oxidoreductase.  
• NADH is created via the TCA cycle processes -ketoglutarate dehydrogenase, isocitrate dehydrogenase, and malate dehydrogenase, the pyruvate dehydrogenase reaction that converts pyruvate to acetyl-CoA, -oxidation of fatty acids, and other oxidation reactions. 
• NADH generated in the mitochondrial matrix diffuses to the inner mitochondrial membrane, where it transfers electrons to FMN, a protein that is closely linked to it. 

• FMN transfers electrons to coenzyme Q via a succession of iron–sulfur (Fe–S) complexes, which receive electrons one at a time, creating semiquinone and finally ubiquinol. 
• The energy generated by these electron exchanges is utilized to push protons to the inner mitochondrial membrane’s cytosolic side. 
• ATP is produced as protons flow back into the matrix through the pores of the ATP synthase complex. 

2. Coenzyme Q electron transfer to cytochrome c 

• Coenzyme Q transfers electrons to cytochromes b and c1, which then transmit the electrons to cytochrome c through Fe–S centres. 

• Complex III, also known as the cytochrome b-c1 complex, is the protein complex involved in these transfers. CoQ:C1 oxidoreductase is another name for the complex. 
• Each of these cytochromes has heme as a prosthetic group but distinct Apo-protein. The heme iron can receive one electron in the ferric (Fe³⁺) state and be reduced to the ferrous (Fe²⁺) state. 
• Because cytochromes may only transport one electron at a time, each cytochrome complex must decrease two molecules for every molecule of NADH that is oxidized. 
• The energy released during the electron transport from coenzyme Q to cytochrome c is used to push protons across the inner mitochondrial membrane. 
• ATP is produced as protons flow back into the matrix through the pores of the ATP synthase complex. 

• Electrons from FADH₂ enter the electron transport chain at complex II, which contains succinate dehydrogenase, after being created by processes such as the oxidation of succinate to fumarate. 
• Without the accompanying proton pumping across the inner mitochondrial membrane, Complex II will transport electrons to coenzyme Q.

3. Electron transfer from cytochrome c to oxygen: 

• Cytochrome c sends electrons to the cytochrome aa3 complex, which in turn transmits electrons to molecular oxygen, reducing it to water. 
• This electron transport is catalyzed by cytochrome oxidase (complex IV). Cytochromes a and a3 each have a heme and two distinct copper-containing proteins. 
• Because it takes two electrons to decrease one atom of oxygen, each NADH oxidation converts one-half of O₂ to H₂O. 
• The energy generated by the transport of electrons from cytochrome c to oxygen is utilized to push protons across the inner mitochondrial membrane. ATP is produced as protons return to the matrix.  

ATP Generation in ETS  

• The synthesis of ATP is linked to the transfer of electrons to O₂ via the electron transport chain. The complete procedure is known as oxidative phosphorylation. Protons move via the membrane-bound ATP synthase down their electrochemical gradient. Protons travel through the ATPase, allowing the enzyme to create ATP. 
• The precise quantity of ATP produced by this process is unknown, but current thinking holds that for every pair of electrons that enter the chain from NADH, 10 protons are pushed out of the mitochondria. Given that it takes four protons to pass through the ATPase to produce one ATP, 2.5 moles (10 divided by 4) of ATP may be produced from one mole of NADH. 
• Because electrons from FADH₂ enter the chain via coenzyme Q, avoiding the NADH dehydrogenase step, roughly 1.5 moles of ATP are created for every mole of FADH₂ oxidized