Electron Transport System (ETS) : Definition, steps

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Electron Transport System (ETS) : Definition, steps
Electron Transport System (ETS) : Definition, steps

In Krebs cycle when the process of oxidation taking place, dehydrogenase enzymes release pairs of Hydrogen and electrons from different substrates. Hydrogen and electron pass through different electrons carriers and ultimately react with oxygen (at the end of ETS) and form water molecules. During transfer of hydrogen atoms from one enzyme carrier to another in ETS, large amount of energy is released, which is stored in the pyrophosphate bond of ATP, produced from ADP and inorganic phosphate (iP). The enzymes which participate in this process are called Respiratory Chain Enzymes or Electron Transport System Enzymes or terminal oxidation. All these enzymes are found in F1 particles of mitochondria.

Following are the different steps of respiratory chain-

1. Dehydrogenase enzymes accept hydrogen from different substrates of Krebs cycle. Hydrogen atoms break up into protons (+) and electrons (-).

                                                  2H → 2H+ + 2e-

Proton (hydrogen ions) reduce NAD

                                   2NAD + 2H+  → 2NAD.2H (2 NADH.H)

NADH+ enters into F1 particles of mitochondria.

2. Hydrogen of NADH is accepted by FAD as a result of FAD is reduced to FADH+ and NADH+ is oxidised to NAD.

3. From FAD.2H electrons are accepted by coenzyme (CoQ) biquinone. From CoQ, H ions move to the matrix of mitochondria and electrons are accepted by cytochromes.

4. Cytochromes are iron containing pigments found in all cells in which aerobic respiration takes place. These pigments accept (Fe³+ → Fe²+) and donate (Fe²+ → Fe³+) electrons. They are reduced by accepting electrons and oxidised by donating electrons. (Electron from cytochrome-b to cyto c₁ → to cycto c).

5. From cytochromes c these electrons are taken by cytochrome-a and then to cytochrome a3 (both cyto-a and a3 together are called cytochrome oxidase).

6. These electrons from cytochrome oxidase react with atom of oxygen making it reactive.

7. This activated oxygen react with 2 protons of hydrogen forming molecule of water.
                                                         2H+
                               O + 2e  →  O-   →   H₂O

8. All other cytochromes have Fe only while cyt a3 possesses both Fe and Cu. With Fe it accepts electrons and with Cu it hands over electrons to oxygen.

Electron Transport System (ETS) : Definition, steps



Summary of Electron Transport System
  1. It is made up of coenzymes NAD+ or NADP+, FAD and coenzyme Q and cytochromes b, C₁, C, a and a3.
  2. The transfer of electrons in all compounds except succinic acid takes place first in NAD* or NADP and later on in FAD.
  3. The transfer of electrons from succinic acid takes place directly to the FAD.
  4. 3ATP molecules are produced for each NADH+ H+ or NADPH + H+ molecule.
  5. Only 2ATP molecules are produced for each FADH2 molecule.
  6. The reduction of various cytochromes requires only electrons and no protons.
  7. The formation of one molecule of water requires ½O2 + 2e- + 2H+ while reduction of one molecule of oxygen (O2) requires 4e- + 4H+.
  8. Reduction and oxidation of coenzymes and cytochromes take place in a sequence and stepwise because in electron transport chain they are arranged in a series according to their redox potential. The first coenzyme (NAD+) possesses low redox potential while last cytochrome (Cyt. a3) highest. Thus, the transfer of electrons proceeds from compounds with lower redox potential to compounds with higher redox potential.



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