ETC 1

The Electron Transport System

Overview

• The Electron Transport System (ETS) is a series of complexes that transfer electrons from NADH/FADH2 to molecular oxygen (O2).

• It plays a vital role in cellular respiration, aiding ATP production.

Iron-Sulfur Clusters

• Essential protein cofactors in electron transport processes.

• Types include [2Fe-2S] and [4Fe-4S] clusters.

• Components: Usually cysteine (Cys) residues bind iron (Fe) and sulfur (S) atoms to form clusters.

• Structure is crucial for their electron transfer roles, influencing redox potential.

Redox Active Prosthetic Groups in Complex I

• Complex I: Contains FMN and iron-sulfur clusters (N1b, N1a, etc.) for electron transfer from NADH to Ubiquinone (UQ).

• FMN (Flavin Mononucleotide): Accepts electrons from NADH and transfers them to the iron-sulfur clusters.

• Electron transfer steps enhance the efficiency of the electron transport chain by linking NADH oxidation with UQ reduction.

• Example Complex: Thermus thermophilus (PDBid 2FUG) provides experimental confirmation of these structures.

Coenzyme Q (Ubiquinone)

• Structure: Consists of an isoprenoid tail making it hydrophobic, facilitating its location within the inner mitochondrial membrane (IMM).

• Function: Acts as an electron carrier, facilitating electron transfer and proton translocation across the membrane.

• Forms: Ubiquinone (oxidized), ubiquinol (reduced) and semiquinone (one electron reduced version).

• Supplementation: Available in forms such as Coenzyme Q10, important for cardiovascular health.

Complex II: Succinate-CoQ oxidoreductase

• Function: Catalyzes the conversion of succinate to fumarate while transferring electrons to CoQ.

• Structure: Contains FAD, iron-sulfur centers, and provides low redox potential changes.

• Significance: Does not pump protons, but connects succinate metabolism with the electron transport chain.

Cytochromes in Electron Transport

• Function: Proteins with heme groups that alternate between oxidation states during electron transport, mainly Fe(II) and Fe(III).

• Types: Heme a, Heme b, and Heme c are key components of various electron transport complexes, facilitating electron transfer through redox reactions.

• Importance: Help in coupling electron transfer to proton translocation, driving ATP synthesis.

Complex IV: Cytochrome c Oxidase

• Function: Catalyzes the reduction of molecular oxygen to water (H2O).

• Components: Contains multiple redox centers including CuA, CuB, heme a, and heme a3.

• Mechanism: Electrons from cytochrome c are transferred to Cu centers and heme groups, ultimately reducing O2 and contributing to proton pumping across the membrane.

The Q Cycle

• Overview: A mechanism to efficiently couple electron transfer to proton translocation in Complex III.

• Process: Involves multiple binding and release of Q and QH2, allowing protons to be pumped into the intermembrane space (IMS).

• Two Steps:

  1. QH2 donates electrons to the Rieske-FeS complex and cytochrome c, moving protons into the IMS.

  2. Subsequent electrons convert semiquinone back to QH2, completing the cycle.

Checkpoints for Review

1. Describe the route of electrons from NADH/FADH2 to O2.

2. Calculate the number of protons pumped per glucose under ideal conditions.

3. Identify prosthetic groups in Complexes I-IV and their electron carrying capabilities.

4. Explain the Q cycle processes, focusing on the two rounds.

5. Outline the proton translocation mechanisms observed in each complex.