ELECTRON TRANSPORT CHAIN

The electron transport chain is comprised of a series of enzymatic reactions within the inner membrane of the mitochondria, which are cell organelles that release and store energy for all physiological needs.
As electrons are passed through the chain by a series of oxidation-reduction reactions, energy is released, creating a gradient of hydrogen ions, or protons, across the membrane. The proton gradient provides energy to make ATP, which is used in oxidative phosphorylation.

1. ELECTRON TRANSPORT CHAIN PREPARED BY ; MISS RABIA KHAN BABER

2. AIMS AND OBJECTIVES OF PPT

3. OXIDATIVE PHOSPHORYLATION Oxidative phosphorylation is the metabolic pathway in which cells use enzymes to oxidize nutrients, thereby releasing the chemical energy stored within in order to produce adenosine triphosphate. In most eukaryotes, this takes place inside mitochondria. Almost all aerobic organisms carry out oxidative phosphorylation. In Oxidative phosphorylation ATP is formed as a result of the transfer of electrons from NADH or FADH  2  to O  2  by a series of electron carriers.

4. STEPS OF OXIDATIVE PHOSPHORYLATION Within the Electron Transport Chain

5. THE ELECTRON TRANSPORT CHAIN The electron transport chain is comprised of a series of enzymatic reactions within the inner membrane of the mitochondria, which are cell organelles that release and store energy for all physiological needs. As electrons are passed through the chain by a series of oxidation-reduction reactions, energy is released, creating a gradient of hydrogen ions, or protons, across the membrane. The proton gradient provides energy to make ATP, which is used in oxidative phosphorylation.

6. REACTIONS OF THE ELECTRON TRANSPORT CHAIN The reactions of the electron transport chain are carried out by a series of membrane proteins and organic molecules. They are arranged in four complexes. In eukaryotes, the electron transport chain is located in the inner mitochondrial membrane. In prokaryotes, it is located within the plasma membrane. Electrons move through the electron transport chain from a higher to lower energy state. Energy release moves protons through channels in the membrane proteins, moving them into the inner membrane space. This leads to a buildup of positively charged protons, which creates an electrical potential across the membrane.

7. ETC COMPLEXES

8. COMPLEX I-THE NADH DEHYDROGENASE COMPLEX This complex is also known as NADH dehydrogenase complex, consists of 42 different polypeptides, including FMN containing flavoprotein and at least six FeS centers. Complex I is ‘L’ shaped with its one arm in the membrane and another arm extending towards the matrix. During catabolic reaction, NAD +  is reduced to NADH+ H +  and this NADH + H +  feeds electrons and protons at the point of origin in the ETC. Both e –  and protons are transported to FMN which is then reduced to FMNH 2 .

9. FMNH 2  transfers only e –  to FeS center whereas protons are extruded outside the membrane ( intermembrane space), in the process FMNH 2  is oxidized back to FMN. Electrons flow through FeS centers which alternate between reduced (Fe 2+ ) and oxidized (Fe 3+ ) forms. Electrons are finally transferred to ubiquinone , which along with protons obtained by the hydrolysis of water in the matrix site of the membrane is reduced to UQH 2 .

10. COMPLEX II- SUCCINATE DEHYDROGENASE Complex II is also known as succinate dehydrogenase complex. Succinate dehydrogenase complex is located towards the matrix side of the membrane. Succinate is oxidized to fumarate as it transfers two e – s and two protons to FAD. FAD is reduced to FADH 2 . FAD transfers only electrons through FeS center to quinone .

11. COMPLEX III CYTOCHROME C OXIDOREDUCTASE The third complex is composed of cytochrome b, another Fe-S protein, and cytochrome c proteins. Cytochrome proteins have a prosthetic group of heme. The heme molecule is similar to the heme in hemoglobin, but it carries electrons, not oxygen. As a result, the iron ion at its core is reduced and oxidized as it passes the electrons, fluctuating between different oxidation states: Fe ++  (reduced) and Fe +++  (oxidized). Complex III pumps protons through the membrane and passes its electrons to cytochrome c for transport to the fourth complex of proteins and enzymes (cytochrome c is the acceptor of electrons from Q; however, whereas Q carries pairs of electrons, cytochrome c can accept only one at a time).

12. COMPLEX IV – CYTOCHROME C OXIDASE It is also called as cytochrome oxidase . Cytochrome c undergoes oxidation in the side of the membrane facing the intermembrane space and O 2  is reduced in the matrix side of the membrane to H 2 O. Complex IV consists of iron containing heme-a and heme-a 3 . Along with iron atoms, cytochrome oxidase also consists of Cu A and Cu B. Cu A is closely but not intimately associated with heme ‘a’ and Cu B is intimately associated with heme a 3 . Electrons from cytochrome c flows to Cu A and then to heme ‘a’ and then to heme a 3  and then to Cu B and then finally to Oxygen. Cytochrome c —> Cu A —–> Heme a—–> heme a3—->Cu B—> O2

13. The copper atoms interconvert between cuprous (reduced) and cupric (oxidized). Electrons from Cu B and heme a 3  is transferred to O 2. Two protons are supplied from the matrix side forming OH –  and OH – . Now, addition of two more proton from matrix side resulting in formation of two molecule of water (2H 2 O).

14. CHEMIOSMOSIS In chemiosmosis , the free energy from the series of redox reactions is used to pump hydrogen ions (protons) across the membrane. The uneven distribution of H +  ions across the membrane establishes both concentration and electrical gradients, owing to the hydrogen ions’ positive charge and their aggregation on one side of the membrane. ELECTROCHEMICAL GRADIENT An electrochemical gradient is a gradient of electrochemical potential, usually for an ion that can move across a membrane.

15. SYNTHESIS OF ATP AND ROLE OF ATP SYNTHASE Chemiosmotic theory given by Peter Mitchell (1961) in the widely accepted mechanism of ATP generation. According to this theory electron and proton channel into the membrane from the reducing equivalence flows through a series of electron carriers, electrons flow from NADH through FMN, Q, cytochrome and finally to O 2 . However, proton as they flow through the membrane are extended at different position in the intermembrane space. The extension of protons creates a slight positivity/acidity to the outerside of membrane. Reduction of quinones and O 2  to water requires protons which are provided by the hydrolysis of water in the matrix side of the membrane.

16. This results in accumulation of hydroxyl ion in the inner (matrix) side of membrane resulting in slight negativity/alkalinity in the inner side of the membrane. This creates a charge difference between outer side of the membrane, and inner side of membrane which energizes the membrane. This is electrochemical potential, and this potential along with the pH gradient generates the proton motive force (PMF). This proton motive force tends to drive the proteins through ATP synthase in to the inner side of the membrane, the consequence of which is ATP production.

17. ENERGY CALCULATIONS OF THE ETC GLYCOLYSIS GLUCOSE ----PYRUVATE 2 ATPS 2 NADH2 7 ATP KREB’S CYCLE WHEN GLUCOSE PASSES THROUGH ONE CYCLE 2 GTPS 6 NADH 2 FADH2 20 ATP ELECTRON TRANSPORT CHAIN GLUCOSE --- PYRUVATE PYRUVATE---- ACETYL CO A KREB’S CYCLE 2 NADH 2 NADH 6 NADH 2 FADH2 5 ATPS 5 ATPS 15 ATP 3 ATP 1 NADH CAN PUMP 10 HYDROGEN IONS WHICH CAN PHOSPHORYLATE 1 ADP MOLECULE 1 FADH2 CAN PUMP 6 HYDROGEN IONS IN TOTAL WE HAVE 32 ATPS PER MOLECULE OF GLUCOSE WHICH PASSES THROUGH ALL THESE BIOCHEMICAL REACTIONS

18. summary The electron transport chain is the portion of aerobic respiration that uses free oxygen as the final electron acceptor of the electrons removed from the intermediate compounds in glucose catabolism. The electron transport chain is composed of four large, multiprotein complexes embedded in the inner mitochondrial membrane and two small diffusible electron carriers shuttling electrons between them. The electrons are passed through a series of redox reactions, with a small amount of free energy used at three points to transport hydrogen ions across a membrane.

19. This process contributes to the gradient used in chemiosmosis . The electrons passing through the electron transport chain gradually lose energy, High-energy electrons donated to the chain by either NADH or FADH 2  complete the chain, as low-energy electrons reduce oxygen molecules and form water. The level of free energy of the electrons drops from about 60 kcal/mol in NADH or 45 kcal/mol in FADH 2  to about 0 kcal/mol in water. The end products of the electron transport chain are water and ATP. A number of intermediate compounds of the citric acid cycle can be diverted into the anabolism of other biochemical molecules, such as nonessential amino acids, sugars, and lipids. These same molecules can serve as energy sources for the glucose pathways.

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