electron transport chain.pptx

electron transport chain

1. Electron transport chain (ETC) ETC, also called electron-transporting system (ETS), respiratory chain, hydrogen-carrying chain, or oxidation reduction chain , is a complex multi-enzyme chain which transports electrons or energised hydrogen from a donor molecule to molecular oxygen . In prokaryotes, it is located in the plasma membrane , and in eukaryotes in the inner mitochondrial membrane . There will be several thousand electron-transport systems in each mitochondrion.

2. Mitochondrial ETC is characterised by a series of oxidation-reduction (redox) reactions when there is a transfer of electrons from a donor (reductant) to an acceptor (oxidant). The potentiality of a reductant to pass electrons to an oxidant is called oxidation-reduction potential, or redox potential . The oxidant and the reductant, operating in a redox reaction, together form a redox couple, or redox pair .

3. The redox reactions of glyclolysis and Krebs cycle are dehydrogenation reactions . So, each redox reaction releases a pair of hydrogen atoms . FAD and NAD accept these hydrogen atoms and get reduced to FADH, and NADH + H+ respectively. Reduced FAD and NAD (i.e., FADH, and NADH + H+) ultimately reduce molecular oxygen to form H₂O (molecular oxygen serves as the terminal electron acceptor or oxidising agent in aerobic respiration). This is accomplished by the step-by-step transfer of hydrogen or electrons through a chain of intermediate electron-carriers, known as electron transporting system (ETS) or respiratory chain .

4. In aerobic cells, the catabolic pathways of carbohydrates, fats and amino acids ultimately converge into the respiratory chain. Along this chain energised electrons flow from organic molecules to molecular oxygen, yielding energy . This energy is recovered and used for the phosphorylation of ADP to synthesise ATP . Thus, respiratory chain functions as the final common route for the flow of all electrons from different fuels to molecular oxygen .

5. Components of the respiratory chain The major components of the respiratory chain consist of several intermediate carriers of hydrogen or electron . They include NAD, flavoproteins, non- haem metalloproteins (Fe-S proteins), ubiquinone or co-enzyme Q (UQ or CoQ ) and cytochromes . Each member of the chain accepts electrons from the preceding member and then passes them on to the next member in a specific sequence .

6. Flavoproteins are conjugate proteins whose prosthetic group is a flavin molecule , either FMN or FAD. Flavoproteins and NAD serve as hydrogen - carriers in cellular respiration. NAD can accept only one hydrogen atom and get reduced to NADH+H+ . Flavin nucleotides can accept either one hydrogen to yield the semiquinone forms (FMNH or FADH) , or two hydrogens to yield FMNH₂ or FADH₂ . Ubiquinone (UQ), or co-enzyme Q( CoQ ), is a hydrophobic , fat-soluble and electroncarrying benzoquinone . It can accept either one electron to become a semiquinone radical (UQH) or two electrons to become ubiquinol (UQH₂).

7. Cytochromes are red or brown iron-containing haemoproteins (iron-porphyrin haemoproteins ), which serve as the electron-transporting enzymes in cellular respiration and photosynthesis . Their characteristic strong colours are produced by the heme prosthetic group . They are universally present in all aerobic cells and are located in the plasma membrane of bacteria , thylacoid membrane of chloroplasts and the inner membrane of mitochondria . In cellular respiration, they receive electrons from ubiquinone and transport them in a relay process to molecular oxygen .

8. All about 50 odd kinds of cytochromes have so far been identified. They fall under three main classes, namely a, b and c. Five members of these usually exist in a chain, called cytochrome series or respiratory assembly . They are arranged in the order cytochromes b-c₁-c-a- a 3 . Only the last member of the chain, namely a 3 can donate electrons directly to molecular oxygen. So, it alone is directly oxidised by oxygen. Therefore, it is referred to as cytochrome oxidase . Just as UQ, cyt.c also is a mobile carrier of electrons.

9. Iron- sulphur proteins are iron-containing and electron-transferring mitochondrial proteins . In them, iron is present not in the heme state (as in cytochromes), but in association with inorganic sulphur atom and also with the sulphur atom of the cysteine residue of the protein. The Fe-S centre may be simple with a single Fe atom, or complex with 2 or 4 Fe atoms. In the oxidised state, all iron atoms are in the ferric (Fe³+) state . In the reduced state, one of them becomes ferrous (Fe²+) .

10. Organization of the respiratory chain The mitochondrial respiratory chain is organized as four electron-carrier complexes , all embedded in the inner mitochondrial membrane. Complex I catalyzes the transfer of electrons from NADH to UQ , complex II catalyzes the transfer from succinate to UQ , complex III catalyzes the transfer from UQ to cytochrome c , and complex IV completes the sequence by transferring electrons from cytochrome c to O₂ .

11. Complex I (NADH to UQ)Also known as NADH dehydrogenase complex. It is composed of as many as 40 polypeptide sub-units , and its prosthetic groups include FMN and Fe-S . The whole complex remains embedded in the inner mitochondrial membrane in such a way that its NADH-binding site faces the mitochondrial matrix. This arrangement enables the complex to interact with the NADH produced by the dehydrogenases in the mitochondrial matrix. The overall reaction catalyzed by the complex is as follows: NADH + H+ + UQ ↔ NAD+ + UQH₂ (ubiquinol)

12. In this reaction, oxidized UQ accepts hydride ions (two electrons and one proton) from NADH and a proton from the aqueous medium of the matrix . During this, the enzyme complex transfers a pair of reducing equivalents from NADH to its prosthetic group FMN. Through the Fe-S of the complex, electrons pass from FMN to UQ . The ubiquinol (UQH₂), formed in the reaction, soon diffuses from complex I to complex III and then gets oxidised to UQ . The electron flow from UQ through complex I is always accompanied by the outward flow of protons from the mitochondrial matrix to the intermembrane space .

13. Complex II (Succinate to UQ) Also called succinate dehydrogenase (the only membrane-bound enzyme of the TCA cycle). It is formed of at least 4 different protein units , and its prosthetic groups include FAD and Fe-S with 4 Fe atoms . In this complex, electrons are believed to pass from succinate to UQ through FAD and Fe-S .

14. [ Not all substrates for mitochondrial dehydrogenases pass electrons into the respiratory chain through complex II; some bypass complex II . An example is the first step in the betaoxidation of fatty acyl-CoA, catalysed by the flavoprotein acyl-CoA dehydrogenase . In this case, electrons are first transferred from the substrate to the FAD of the dehydrogenase , then from FAD to the electron-transferring flavoprotein (ETFP), then from ETFP to the enzyme ETFP uniquinone dehydrogenase, and finally from this enzyme to UQ ].

15. Complex III (UQ to cytochrome c) Also called UQ-cytochrome c oxidoreductase or cytochrome b-c1 complex .  It consists of Cyt.b (b562 and b566), Cyt.c , and up to 10 protein units . Its prosthetic groups include hemes and Fe-S . The flow of electrons through complex III ultimately results in the oxidation of UQ and the reduction of cytochrome c .

16. Complex III serves as a "proton pump". The protons, produced during the oxidation of UQH2 to UQ, are released to the intermembrane space of the mitochondrion. This produces a difference in proton concentration across the mitochondrial membrane, called proton gradient . This is very significant in the synthesis of ATP.

17. Complex IV (reduction of O₂) Also called cytochrome oxidase . It consists of cytochromes a and a3, and its prosthetic groups include hemes and the copper ions CuA and CuB . These copper ions are very vital in the transfer of electrons to O2 , without generating incompletely reduced, highly reactive and seriously harmful intermediates, such as H₂O₂ or hydroxyl free radicals. Electron flow from Cyt c to O₂ along complex IV causes a net movement of protons from the intermembrane space. Thus, complex IV functions as a proton pump that generates a proton-motive force.

18. On the whole, it becomes clear that the combined functioning of the electron carrier complexes I, III and IV results in the transfer of electrons from NADH to O₂ and the combined action of complexes II, III & IV brings about the transfer of electrons from succinate to O₂ . Under normal cellular conditions, the mitochondrial oxidation of NADH or succinate yields more free energy than what is actually required for the synthesis of one ATP (51.8 kJ/ mol).

19. Functioning of the respiratory chain In the respiratory chain, the constituent molecules are arranged in the order of increasing redox potential, decreasing electron pressure and diminishing free energy level (redox potential or oxidation - reduction potential is the tendency or potential to donate or accept electrons). So, along the chain there would be a step-by-step flow of electrons from a most negative initial donor (e.g., NAD) to the most positive terminal acceptor (molecular oxygen) through a series of increasingly more positive intermediate carriers.

20. The electrons, entering the respiratory chain, are energy-rich. However, as they flow down, they lose free energy. Much of this energy gets conserved in ATP through the coupled synthesis of ATP from ADP and Pi . The rest is lost as heat. In the respiratory chain, enzyme-bound hydrogen is used as the fuel for energy generation. These hydrogen atoms are available from the dehydrogenation reactions of substrate oxidation (glycolysis, conversion of pyruvic acid to acetyl CoA, oxidation of fatty acids and amino acids, Krebs cycle).

21. From the hydrogen-donating substrates, hydrogen atoms are collected by dehydrogenases and then supplied to the respiratory chain through two coenzyme channels, namely NAD and FAD (in some synthetic pathways a third channel also operates, namely NADP). These co-enzymes serve as the acceptors, carriers and donors of hydrogen, or as a hydrogen-carrying system . The hydrogen atoms they carry soon donate their electrons to the electron-transport chain and become H+ ions. These H+ ions soon escape to the aqueous medium.

22. NAD-linked (or NADP-linked) dehydrogenase removes two hydrogen atoms from the substrate. One of them is transferred to NAD as a hydride ion (:H), and the other one appears in the medium as H+. Reduced substrate + NAD+ ↔ Oxidised substrate + NADH+H+ FAD-linked dehydrogenase also removes two hydrogen atoms from the substrate. In most cases, both of them will be accepted by FAD, yielding FADH₂. Reduced substrate + FAD ↔ Oxidised substrate + FADH₂

23. From the co-enzyme channels, electrons are funnelled to molecular oxygen through a chain of electron-carriers. They include flavoproteins, non-heme iron- sulphur (Fe-S) proteins, ubiquinone or coenzyme Q (UQ or CoQ ) and cytochromes . In the respiratory chain, electrons are transferred in three ways: ( i ) as electrons (ii) as hydrogen atom (H+ + e-) and (iii) as hydride ion (:H-) which bears two electrons. During the flow of electrons, every member of the electron-transporting chain gets alternately reduced and oxidised in a cyclic manner.

24. In this process, their iron atom oscillates between ferric (Fe³+) and ferrous (Fe2+) states. At the initial end, this redox reaction chain is linkded to dehydrogenation reactions, and at its terminal end to molecular oxygen. At the same time, some of its intermediate reactions are coupled with the phosphorylation of ADP. Dehydrogenation reactions maintain a steady flow of hydrogen , and phosphorylation reactions ensure a constant synthesis of ATP .

25. The coupling of the redox reactions of the respiratory chain with the phosphorylation of ADP constitutes a reaction complex, known as oxidative phosphorylation or respiratory chain phosphorylation (the coupling enzymes and the phosphorylation enzymes are located in the mitochondrial oxysomes ). The biochemical reactions in which electrons are transferred from one molecule to the next in the electron-transport chain are called oxidation-reduction reactions or redox reactions . In them, the electron-donating molecule is called the reducing agent or the reductant and the electron accepting molecule is called the oxidising agent or oxidant .

26. A reducing and an oxidising agent together function as a conjugate redox pair, just as an acid and its base together form a conjugate acid-base pair . Each member of the cytochrome series alternately serves as an oxidising agent and a reducing agent . The oxidation of hydrogen in the respiratory chain is completed in several sequential steps.

27. The major steps are the following: (I) Dehydrogenation reactions remove hydrogen atoms from the substrate . In carbohydrate catabolism , there are six such reactions, one in glycolysis, one in the oxidation of pyruvic acid to acetyl CoA and four in each turn of the TCA cycle . (ii) The hydrogen acceptors NAD and FAD collect the hydrogen atoms in pairs and get reduced to NADH + H+ and FADH₂ respectively . In carbohydrate catabolism, FAD receives hydrogen from one reaction, and NAD from the remaining five reactions.

28. (iii) In the hydrogen acceptors, hydrogen atoms undergo ionisation and get split up to protons and electrons . Soon, each hydrogen acceptor gets re- oxidised by transferring electrons to the first member of the cytochrome chain through certain intermediate carriers. The intermediate carriers include FMN-containing proteins (flavoproteins), Fe-S-proteins and CoQ for the transfer of electrons from NADH ; Fe-S proteins and CoQ for the transfer from FADH₂ . Protons are simultaneously released from the hydrogen acceptors to the medium.

29. (iv) Every member of the cytochrome chain first accepts electrons from the preceding member and then passes them on to the next member. Thus, a series of redox reactions take place in the cytochrome chain. The last member of the chain passes electrons directly to molecular oxygen . This reduces and excites oxygen, and results in the formation of hydroxyl (OH-) ions . Hydroxyl ions combine with the protons (H+) in the medium and form H₂O.

30. Biogenesis of mitochondria ( mitochondriogenesis )

31. Three mechanisms have been proposed to account for the formation of mitochondria. They are: ( i ) self-duplication of pre-existing mitochondria (ii) de novo origin and (iii) transfor mation of non-mitochondrial systems to mitochondria. Of these, the self-duplication hypothesis is most widely accepted now.

32. (a) Denovo origin of mitochondria This hypothesis holds that mitochondria are formed from cytoplasmic vesicles. During this, the vesicles form buds which are soon covered by membrane. The buds grow in size, undergo internal compartmentalisation by the formation of cristae, and transform to mitochondria. The current view is that mitochondria and chloroplasts are never formed de novo, but are formed from pre-existing ones.

33. (b) Transformation of non-mitochondrial systems to mitochondriaAccording to this hypothesis, mitochondria are formed by the infolding of plasma membrane and ER, or by the delamination of nuclear membrane. Though the origin of mitochondria from plasma membrane and ER has been observed in rat liver cells and the nerve fibres of cray fish, it is not the case with most organisms.