JNTU B.Pharmacy II BIOCHEMISTRY Previous Question Paper (October/November-2020 - R17)

The Electron Transport Chain (ETC) is a series of protein complexes and electron carriers located in the inner mitochondrial membrane that facilitate the transfer of electrons from NADH and FADH2 to oxygen, coupled with the synthesis of ATP through oxidative phosphorylation.

The mechanism involves four major protein complexes:

1. Complex I (NADH Dehydrogenase): NADH is oxidized to NAD+, transferring two electrons to FMN and then to Coenzyme Q (Ubiquinone). This process pumps four protons into the intermembrane space.
2. Complex II (Succinate Dehydrogenase): FADH2 is produced in the TCA cycle and transfers electrons directly to Coenzyme Q. No protons are pumped at this complex.
3. Complex III (Cytochrome bc1 Complex): Electrons from reduced Coenzyme Q are transferred to Cytochrome c. This step pumps four protons across the membrane.
4. Complex IV (Cytochrome c Oxidase): Electrons are transferred from Cytochrome c to oxygen, the final electron acceptor, forming water. This complex pumps two protons into the intermembrane space.

The resulting proton motive force, created by the electrochemical gradient across the inner membrane, drives ATP Synthase (Complex V) to phosphorylate ADP into ATP as protons flow back into the matrix.

Regarding energetics, the oxidation of one molecule of NADH yields approximately 2.5 ATP, while the oxidation of one molecule of FADH2 yields approximately 1.5 ATP. The total energy released during electron transfer is captured in the form of the proton gradient, which is then converted into chemical energy stored in the phosphate bonds of ATP.

Oxidative phosphorylation is the process in which ATP is synthesized as electrons are transferred from NADH or FADH2 to oxygen by a series of electron carriers. This process occurs in the inner mitochondrial membrane and is coupled to the generation of a proton gradient.

The mechanism is explained by the Chemiosmotic Hypothesis:

• Electrons from NADH and FADH2 pass through the electron transport chain (ETC) complexes (I, II, III, and IV).
• As electrons move through these complexes, energy is released and used to pump protons (H+) from the mitochondrial matrix into the intermembrane space.
• This creates an electrochemical gradient, known as the proton-motive force, consisting of a pH gradient and an electrical potential across the inner membrane.
• The inner membrane is impermeable to protons, so they can only return to the matrix through the ATP synthase (Complex V) enzyme.
• As protons flow back into the matrix through the F0 subunit of ATP synthase, the energy drives the rotation of the F1 subunit, which catalyzes the phosphorylation of ADP to ATP (ADP + Pi → ATP).

Uncouplers are compounds that disrupt the link between electron transport and ATP synthesis. They work by increasing the permeability of the inner mitochondrial membrane to protons, allowing them to leak back into the matrix without passing through ATP synthase.

Key points regarding uncouplers:

• They dissipate the proton-motive force, causing the energy stored in the gradient to be released as heat rather than being captured as ATP.
• Electron transport continues at a rapid rate because the respiratory control is lost, but ATP production ceases.
• A classic example is 2,4-Dinitrophenol (DNP), which acts as a proton ionophore, carrying protons across the membrane and effectively uncoupling oxidation from phosphorylation.