Complex II

Definition

Complex II, also known as succinate dehydrogenase, is a vital enzyme complex in the electron transport chain that facilitates the transfer of electrons from succinate to ubiquinone (coenzyme Q). This process is integral for cellular respiration, linking the citric acid cycle and oxidative phosphorylation. Complex II plays a crucial role in energy production by contributing to the proton gradient necessary for ATP synthesis, showcasing its importance in metabolic pathways.

5 Must Know Facts For Your Next Test

1. Complex II is unique as it is the only enzyme complex in the electron transport chain that is not involved in pumping protons across the inner mitochondrial membrane.
2. It contains four subunits and two main prosthetic groups: FAD (flavin adenine dinucleotide) and iron-sulfur clusters, which facilitate electron transfer.
3. Complex II catalyzes the oxidation of succinate to fumarate, a reaction that is coupled with the reduction of FAD to FADH2.
4. The FADH2 produced by Complex II feeds electrons directly into the electron transport chain at a lower energy level compared to NADH from Complex I.
5. Deficiencies or mutations in Complex II are linked to various diseases, including certain types of cancer and mitochondrial disorders.

Review Questions

• How does Complex II connect the citric acid cycle to the electron transport chain, and what is its significance?
  • Complex II connects the citric acid cycle to the electron transport chain by oxidizing succinate to fumarate while reducing FAD to FADH2. This reaction is crucial because it allows electrons derived from the citric acid cycle to enter the electron transport chain, thus contributing to ATP production. Its significance lies in its role as a link between these two critical metabolic pathways, facilitating efficient energy conversion.
• Evaluate the differences in electron transfer between Complex I and Complex II within the electron transport chain.
  • Complex I transfers electrons from NADH to ubiquinone while actively pumping protons across the mitochondrial membrane, generating a proton gradient. In contrast, Complex II transfers electrons from FADH2 to ubiquinone without pumping protons. As a result, while both complexes contribute to the overall electron flow within the chain, Complex I contributes more significantly to establishing the proton motive force due to its proton-pumping capability, leading to more ATP production than Complex II.
• Assess how defects in Complex II can impact cellular respiration and overall energy metabolism.
  • Defects in Complex II can severely disrupt cellular respiration by impairing the electron flow within the electron transport chain. Since Complex II plays a key role in transferring electrons from succinate-derived FADH2, any dysfunction can lead to reduced ATP production and increased oxidative stress due to potential accumulation of reactive oxygen species. This not only affects energy metabolism at a cellular level but may also contribute to various pathologies such as mitochondrial diseases and certain cancers due to compromised metabolic efficiency.

Related terms

Ubiquinone:

Also known as coenzyme Q, ubiquinone is a lipid-soluble electron carrier that transports electrons between complexes in the electron transport chain.

Citric Acid Cycle: A series of enzymatic reactions occurring in the mitochondrial matrix that generates energy through the oxidation of acetyl-CoA derived from carbohydrates, fats, and proteins.

Proton Gradient:

An electrochemical gradient created by the movement of protons across the inner mitochondrial membrane, essential for ATP synthesis during oxidative phosphorylation.