Complex III

Definition

Complex III, also known as cytochrome bc1 complex, is a crucial component of the electron transport chain located in the inner mitochondrial membrane. It plays a key role in transferring electrons from ubiquinol (QH2) to cytochrome c, while simultaneously contributing to the proton gradient that drives ATP synthesis during oxidative phosphorylation. This complex is vital for cellular respiration and energy production.

5 Must Know Facts For Your Next Test

1. Complex III consists of multiple subunits and contains heme groups that are essential for its electron transfer function.
2. The mechanism of electron transfer in Complex III involves the Q-cycle, which allows for efficient coupling of electron transfer to proton translocation.
3. For every two electrons transferred through Complex III, four protons are pumped from the mitochondrial matrix into the intermembrane space, contributing to the proton gradient.
4. Complex III is sensitive to inhibitors such as antimycin A, which can block its activity and disrupt the electron transport chain.
5. The dysfunction of Complex III has been linked to various mitochondrial diseases and is associated with aging and neurodegenerative disorders.

Review Questions

• How does Complex III contribute to the overall process of ATP synthesis during oxidative phosphorylation?
  • Complex III contributes to ATP synthesis by facilitating the transfer of electrons from ubiquinol to cytochrome c while simultaneously pumping protons into the intermembrane space. This action creates a proton gradient across the inner mitochondrial membrane, known as the proton motive force. This gradient is essential for ATP synthase to produce ATP as protons flow back into the mitochondrial matrix.
• Discuss the significance of the Q-cycle mechanism in Complex III's function within the electron transport chain.
  • The Q-cycle mechanism is significant because it enhances the efficiency of electron transfer in Complex III while also linking it to proton pumping. By using two molecules of ubiquinone (Q) in different states, Complex III effectively couples the oxidation of QH2 with the reduction of cytochrome c. This allows for a greater number of protons to be translocated into the intermembrane space compared to the number of electrons transferred, maximizing energy output for ATP synthesis.
• Evaluate how inhibitors like antimycin A affect Complex III and overall cellular respiration.
  • Inhibitors like antimycin A specifically target Complex III, blocking its electron transfer activity and halting the flow of electrons through the electron transport chain. This disruption prevents proton pumping into the intermembrane space, ultimately diminishing the proton motive force needed for ATP synthesis. As a result, cellular respiration is impaired, leading to decreased energy production and potential cell death if not remedied.

Related terms

Ubiquinone: Also known as coenzyme Q, ubiquinone is a lipid-soluble molecule that transports electrons between Complex I or II and Complex III in the electron transport chain.

Cytochrome c: A small heme protein that transfers electrons from Complex III to Complex IV in the electron transport chain, playing a vital role in cellular respiration.

Proton Motive Force: The electrochemical gradient created by the movement of protons across the inner mitochondrial membrane, which drives ATP synthesis during oxidative phosphorylation.