Complex II

Definition

Complex II, also known as succinate dehydrogenase, is an enzyme complex in the electron transport chain that plays a crucial role in cellular respiration by facilitating the transfer of electrons from succinate to ubiquinone (coenzyme Q). It is unique because it is the only complex that is directly involved in both the citric acid cycle and the electron transport chain, linking these two metabolic pathways.

5 Must Know Facts For Your Next Test

1. Complex II catalyzes the oxidation of succinate to fumarate, while simultaneously reducing ubiquinone to ubiquinol.
2. It does not pump protons across the mitochondrial membrane, making it different from Complex I and III, which contribute to creating a proton gradient.
3. Complex II contains both iron-sulfur clusters and flavin adenine dinucleotide (FAD), which are essential for its electron transfer capabilities.
4. This complex allows for a direct connection between the citric acid cycle and oxidative phosphorylation, enhancing overall metabolic efficiency.
5. Deficiencies or mutations in Complex II are linked to certain mitochondrial diseases and can lead to issues with energy production in cells.

Review Questions

• How does Complex II contribute to both the citric acid cycle and the electron transport chain?
  • Complex II serves as a crucial link between the citric acid cycle and the electron transport chain by oxidizing succinate to fumarate while reducing ubiquinone to ubiquinol. This dual role allows it to facilitate electron transfer from substrates produced during cellular respiration directly into the electron transport chain, making it integral for efficient energy production. Its unique position underscores the interconnectedness of these two vital metabolic pathways.
• Evaluate the differences between Complex II and other electron transport chain complexes regarding proton pumping and metabolic roles.
  • Unlike Complexes I and III, which actively pump protons into the intermembrane space contributing to the proton gradient necessary for ATP synthesis, Complex II does not have this proton-pumping function. Its primary role is focused on the oxidation of succinate and providing electrons to ubiquinone. This distinction affects how energy is harnessed from substrates in cellular respiration, as Complex II relies on substrates from the citric acid cycle rather than participating in generating a proton motive force.
• Discuss how mutations in Complex II can affect cellular metabolism and lead to diseases.
  • Mutations in Complex II can disrupt its function in electron transfer, leading to reduced efficiency in cellular respiration and energy production. Such impairments can manifest in various mitochondrial diseases characterized by energy deficits in high-energy demanding tissues like muscle and brain. This can result in clinical symptoms ranging from fatigue and muscle weakness to more severe neurological issues, highlighting the importance of Complex II's proper function in maintaining overall metabolic health.

Related terms

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

Succinate: Succinate is a four-carbon dicarboxylic acid that serves as a substrate for Complex II during the citric acid cycle, being oxidized to fumarate while transferring electrons.

Electron Transport Chain: The electron transport chain is a series of protein complexes located in the inner mitochondrial membrane that facilitate the transfer of electrons through redox reactions, ultimately leading to ATP synthesis.