Electron transport chain | Biochemistry

ETC is in the inner mitochondrial membrane. Inner membrane is impermeable to most small ions including H+, Na+ and K+ and small molecules such as ATP, ADP, pyruvate. To carry them across the inner membrane carriers or transport systems are required. The mitochondrial matrix includes the enzymes for oxidation of pyruvate, amino acids, beta oxidation of fatty acids and TCA cycle. The synthesis of glucose, urea and heme occur partially in the matrix. It also contains NAD+, FAD, ADP and Pi and mitochondrial DNA, RNA and ribosomes.

The ETC is composed of five complexes I to V. It accounts for the greatest use of oxygen by the body. The ETC is also known as the respiratory chain.

Complex I is NADH Dehydrogenase, it has a tightly bound molecule of FMN and has iron and Sulphur atoms.

Complex II is Succinate Dehydrogenase. It is linked to FAD.

CoQ (Coenzyme Q or Ubiquinone) accepts hydrogen atoms from FMNH2 and FADH2 and transfers electrons to Complex III.

Complex III (cyt bc1) and Complex IV (cyt a and a3). Cytochromes have a heme group (porphyrin ring plus iron). Electrons are passed from CoQ to Complex III, then Cyt c, then Complex IV (cytochrome oxidase, has copper atoms) which then directly reacts with oxygen to form water.

Complex V is enzyme ATP Synthase. Also called F1/F0 ATPase. F0 domain spans the inner mitochondrial membrane and F1 appears as a sphere that protrudes into the mitochondrial matrix. Also called ATP Synthase. The chemiosmotic hypothesis suggests that protons re-enter the matrix by passing through pores on F0 driving rotation of F0 causing conformational changes in F1 that allows it to bind ADP and Pi to form ATP.

Proton Pump: Electron transport is coupled to phosphorylation of ADP by the pumping of protons (H+) across the inner membrane from the matrix to the intermembranous space at Complexes I, III and IV. This creates an electrical gradient with more positive charges on the outside of the membrane and a pH gradient with the outside at a lower pH than the inside of the membrane. The energy generated by this gradient is used to drive ATP synthesis thus coupling oxidation to phosphorylation.

Inhibitors of ETC: They block electron transfer in the ETC at different locations.

i) Amytal and Rotenone (block electron transfer from FMN at Complex I to CoQ)

ii) Antimycin A (blocks Cyt bc1 i.e. C III to cyt c)

iii) Cyanide, CO and Sodium Azide block C IV to O2.

Oligomycin: binds to F0 on C V and closes the H+ channel preventing the reentry of protons into the mitochondrial matrix. Electron transport stops.

Uncouplers: Present in the inner membrane of mammals and humans. Create a proton leak allowing protons to reenter mitochondrial matrix without energy being captured as ATP. The energy is released as heat called thermogenesis. UCP 1 present in brown fat is seen in neonates. Synthetic uncouplers also increase the permeability of the inner membrane to protons e.g. 2,4 DNP, aspirin and salicylates in high doses causing fever in toxicity.

Incomplete reduction of oxygen to water creates reactive oxygen species (ROS) such as O2- superoxide, H2O2, OH - which damage DNA and proteins and cause lipid peroxidation. Enzymes such as superoxide dismutase or SOD, glutathione peroxidase and catalase are cellular defenses against ROS.

The inner mitochondrial membrane lacks an NADH transporter. Shuttles are utilized to transport NADH across the inner membrane. The Glycerol phosphate shuttle results in the formation of 2 ATPs for each NADH oxidized while the malate - aspartate shuttle results in 3 ATPs per NADH oxidized.

13 of the approximately 120 polypeptides for oxidative phosphorylation are coded by mitochondrial DNA and are synthesized in the mitochondria. Mitochondrial DNA has a 10 times higher mutation rate than nuclear DNA resulting in mitochondrial myopathies and Leber’s hereditary optic neuropathy with bilateral loss of central vision due to retinal degeneration.

In the intrinsic pathway of apoptosis pores are formed in the outer mitochondrial membrane causing the release of cyt c from mitochondria into the cytoplasm which activates proteolytic caspases leading to cell death.