Notes on Electron Transport Chain

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Electron Transport Chain Equation:

To study electron transport, let us start with glucose oxidation. The equation for this process is:

Glucose + 6O2 = 6CO2 + 6H2O + 2823 kJ/mol

To closely see electron transport, this equation can be broken down further:

Glucose + 6O2 = 6CO2 + 24H+ + 24e-


6O2 + 24H+ + 24e- = 12H2O

Electron Transport Chain Equation Explained:

• The electron transfer process connecting the above two equations is a multi-step process that harnesses the liberated free energy to form ATP.

• The 24 electrons produced are absorbed by coenzymes NAD+ and FAD to form NADH and FADH2; these electrons are then transferred to O2.

• These electrons then pass to the electron transport chain and participate in redox reactions before reducing O2 to H2O.

• Thus, protons are expelled from the mitochondrion; the free energy stored in the change in pH drives the synthesis of ATP.

• The free energy necessary to generate ATP is extracted from the energy released when NADH and FADH2 release electrons (oxidation) by the electron transport chain.

• Electrons are carried from Complexes I and II to Complex III by coenzyme Q (CoQ), and from Complex III to Complex IV by the peripheral membrane protein cytochrome.

• As electrons are shuttled between the four complexes, they lost energy is harnessed to generate ATP.

Inside the Mitochondrion?

• All the above steps take place in the mitochondrion.

• We focus on the transport of NADH across the inner mitochondrial membrane.

• The inner mitochondrial membrane lacks a NADH transport protein.

• Only electrons from cytosolic NADH are transported into the mitochondrion by a cytoplasmic "shuttle" system.

• This shuttle system is called malate-aspartate shuttle, which functions in the heart, liver, kidney; mitochondrial NAD+ is reduced by cytosolic NADH.

Mitochondrial NAD+ Reduction by Cytosolic NADH:

Phase A:

• Oxaloacetate reduced to NAD+ and malate; malate transported to mitochondrial matrix; malate then oxidized to yield NADH and oxaloacetate.

Phase B:

• Oxaloacetate converted to aspartate; aspartate transported from matrix to cytosol; aspartate then converted to oxaloacetate in cytosol.

Functions of the four Complexes:

Complex I:

Catalyzes the oxidation of NADH by CoQ.

Complex III:

Catalyzes oxidation of CoQ (reduced) by cytochrome c.

Complex IV:

Catalyzes oxidation of cytochrome c (reduced) by O2, the terminal electron acceptor of the electron transport process.

Complex II:

Catalyzes the oxidation of FADH2 by CoQ.

Energy gains:

2.5 ATPs per NADH.

Additional Readings:

Basic Biochemistry

1. Nucleic Acid Structure and Organization
2. DNA Replication and Repair
3. Transcription and RNA Processing
4. Genetic Code, Mutations, and Translation
5. Genetic Regulation
6. Recombinant DNA
7. Amino Acids, Proteins, Enzymes
8. Hormones
9. Vitamins
10. Energy Metabolism
11. Glycolysis and Pyruvate Dehydrogenase
12. Citric Acid Cycle and Oxidative Phosphorylation
13. Glycogen, Gluconeogenesis, and Hexose Monophosphate Shunt
14. Lipid Synthesis and Storage
15. Lipid Mobilization and Catabolism
16. Amino Acid Metabolism Disorders
17. Purine and Pyrimidine Metabolism
18. Electron Transport
19. Citric Acid Cycle and Glyoxylate Cycle
20. Glycolysis
21. Pyruvate Metabolism
22. Mitochondrial ATP formation
23. Gluconeogenesis
24. Glycogen Metabolism
25. Nitrogen Fixation (Metabolism) reactions, and Heme Metabolism
26. Amino Acid Metabolism
27. What is Medium Chain Acyl-CoA Dehydrogenase Deficiency (MCADD)?

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