Electron Transport Chain: Complex III and the Q Cycle

Overview

In this lecture, we discussed the specifics of Complex III in the Electron Transport Chain (ETC) and the Q cycle mechanism by which electrons are transferred to cytochrome c.

Recap of Complex I and II

• Complex I: Transfers high-energy electrons from NADH to ubiquinone, reducing it to ubiquinol (QH2).
• Complex II: Transfers high-energy electrons from FADH2 to ubiquinone, also resulting in QH2.

Complex III: Introduction

• Complex III Names:
  • Q cytochrome C oxidoreductase
  • Cytochrome reductase
• Structure: Composed of 11 polypeptide chains.
• Function: Transfers electrons from ubiquinol (QH2) to cytochrome c.

Major Components of Complex III

1. Cytochrome c1: Contains a single heme group for electron transfer.
2. Cytochrome b: Contains two different heme groups for electron attachment.
3. Rieske center: Contains a 2Fe-2S group that binds and transfers electrons.

The Q Cycle

• The process where electrons are transferred from QH2 to cytochrome c through Complex III. It is composed of two half cycles.

First Half Cycle

1. Initiation: Ubiquinol (QH2) binds to Complex III.
2. Proton Pumping: Two H+ ions are pumped into the intermembrane space of the mitochondria.
3. Electron Pathways:
   • One electron moves to the Rieske center → heme group of cytochrome c1 → cytochrome c (cytochrome c can only bind one electron).
   • Second electron moves through cytochrome b → partially reduces ubiquinone to form semiquinone radical ion (Q-) with a negative charge.
4. Cytochrome c: Reduced cytochrome c detaches and travels to Complex IV.

Second Half Cycle

1. Repeat Process: A second QH2 binds to Complex III.
2. Proton Pumping and Electron Transfer: Similar pathways as the first half cycle.
3. Ubiquinol Regeneration: Second electron reduces semiquinone (Q-) and two protons from the mitochondrial matrix form QH2, which reenters the inner membrane.

Key Points

1. Electron Carriers: Ubiquinone can carry 2 electrons, cytochrome c can carry only 1 electron.
2. Proton Pumping: Four H+ ions are pumped into the intermembrane space for each Q cycle (2 from each QH2 molecule).
3. Electron Recycling: The second pathway recycles electrons via cytochrome b to regenerate ubiquinol.
4. Reduced Cytochrome c: Two cytochrome c molecules are reduced per Q cycle.
5. Proton Uptake: Two protons are absorbed from the mitochondrial matrix in the second half cycle, contributing to the proton gradient for ATP synthesis.

Summary

• Primary Purpose: The Q cycle efficiently transfers and recycles electrons while contributing to the proton gradient essential for ATP synthesis via ATP synthase.
• Next Lecture: We'll explore how reduced cytochrome c interacts with Complex IV.