Complex III

Definition

Complex III, also known as the cytochrome bc1 complex, is a crucial enzyme in the electron transport chain that facilitates the transfer of electrons from ubiquinol (QH2) to cytochrome c. This complex plays an essential role in oxidative phosphorylation by contributing to the generation of a proton gradient across the inner mitochondrial membrane, which is vital for ATP production. By shuttling electrons and pumping protons into the intermembrane space, Complex III links the oxidation of NADH and FADH2 to the synthesis of ATP through chemiosmosis.

5 Must Know Facts For Your Next Test

1. Complex III consists of multiple subunits and contains several redox-active cofactors, including heme groups and an iron-sulfur cluster.
2. The Q cycle mechanism in Complex III allows for efficient electron transfer and proton translocation, enhancing the overall efficiency of oxidative phosphorylation.
3. Complex III contributes to reactive oxygen species (ROS) production as a byproduct of electron transport, which can affect cellular signaling and health.
4. Inhibition of Complex III can result in decreased ATP production and can be caused by specific inhibitors such as antimycin A.
5. Complex III is located in the inner mitochondrial membrane, where it participates in both energy production and the regulation of metabolic pathways.

Review Questions

• How does Complex III contribute to the generation of a proton gradient during oxidative phosphorylation?
  • Complex III contributes to the proton gradient by transferring electrons from ubiquinol to cytochrome c while simultaneously pumping protons from the mitochondrial matrix into the intermembrane space. This process not only facilitates electron transfer but also increases the concentration of protons outside the matrix, creating a potential energy difference. This proton motive force is essential for ATP synthesis as protons flow back into the matrix through ATP synthase.
• Evaluate the significance of the Q cycle mechanism in Complex III for overall cellular respiration efficiency.
  • The Q cycle mechanism is significant because it allows Complex III to effectively harness energy from electrons transferred from ubiquinol while maximizing proton translocation. By facilitating two rounds of electron transfer per molecule of ubiquinol, it boosts ATP yield during oxidative phosphorylation. This mechanism ensures that more protons are pumped into the intermembrane space than would occur with a straightforward electron transfer process, ultimately increasing the efficiency of cellular respiration.
• Analyze how inhibitors targeting Complex III might affect cellular metabolism and energy production.
  • Inhibitors targeting Complex III, such as antimycin A, disrupt the normal flow of electrons through the electron transport chain. This blockage reduces proton pumping into the intermembrane space, resulting in a diminished proton motive force and consequently lower ATP production via ATP synthase. The impaired function of Complex III can lead to increased reactive oxygen species (ROS) generation and may contribute to metabolic disorders or cellular dysfunctions due to insufficient energy supply.

Related terms

Ubiquinone: A lipid-soluble electron carrier that accepts electrons from Complex I and II and transfers them to Complex III.

Cytochrome c: A small heme protein that carries electrons from Complex III to Complex IV in the electron transport chain.

Proton Motive Force:

The force generated by the proton gradient across the mitochondrial membrane, driving ATP synthesis through ATP synthase.