Complex II

Definition

Complex II, also known as succinate dehydrogenase, is an essential enzyme complex in the mitochondrial electron transport chain that facilitates the transfer of electrons from succinate to ubiquinone (coenzyme Q). It plays a dual role in both the citric acid cycle, where it converts succinate to fumarate, and in oxidative phosphorylation, where it contributes to ATP production by facilitating electron transfer and proton pumping across the inner mitochondrial membrane.

5 Must Know Facts For Your Next Test

1. Complex II is unique among the electron transport chain complexes because it is the only one that is part of both the citric acid cycle and the respiratory chain.
2. The reaction catalyzed by Complex II does not contribute directly to proton pumping across the mitochondrial membrane, making it different from Complexes I, III, and IV.
3. Complex II consists of four subunits and contains several cofactors including FAD (flavin adenine dinucleotide) and iron-sulfur clusters that are crucial for its enzymatic function.
4. Malonate is a known inhibitor of Complex II, which competes with succinate for binding, effectively disrupting the electron transfer process.
5. Complex II's activity can be influenced by various factors including oxygen availability, substrate concentration, and overall cellular energy demands.

Review Questions

• How does Complex II connect the citric acid cycle to oxidative phosphorylation?
  • Complex II connects the citric acid cycle to oxidative phosphorylation by oxidizing succinate to fumarate within the cycle while simultaneously transferring electrons to ubiquinone in the electron transport chain. This dual function allows for efficient energy conversion, as it links substrate-level phosphorylation occurring in the citric acid cycle with the oxidative phosphorylation process that generates ATP. As a result, Complex II plays a critical role in cellular respiration by integrating these two key metabolic pathways.
• Discuss the structural components of Complex II and their significance in its function.
  • Complex II is made up of four subunits that contain critical cofactors such as FAD and iron-sulfur clusters. The presence of FAD allows Complex II to participate in redox reactions by accepting electrons from succinate. Iron-sulfur clusters play a crucial role in transferring these electrons to ubiquinone. The specific arrangement of these components is vital for ensuring efficient electron transfer, which ultimately contributes to maintaining the flow of electrons through the entire electron transport chain.
• Evaluate the implications of malonate inhibition on cellular respiration and ATP production.
  • Malonate inhibition of Complex II has significant implications for cellular respiration and ATP production. By competing with succinate for binding at Complex II, malonate effectively halts electron transfer at this point in the pathway. This blockage disrupts the entire electron transport chain's functionality, leading to decreased ATP synthesis since fewer electrons reach downstream complexes that generate a proton gradient. As a result, cells may experience reduced energy availability, impacting overall metabolic processes and cellular functions.

Related terms

Succinate: A four-carbon dicarboxylic acid that serves as a substrate for Complex II in the citric acid cycle, being oxidized to fumarate.

Ubiquinone:

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

ATP Synthase: An enzyme complex located in the inner mitochondrial membrane that synthesizes ATP from ADP and inorganic phosphate using the proton gradient generated by the electron transport chain.