Complex II

Definition

Complex II, also known as succinate dehydrogenase, is an essential component of the electron transport chain located in the inner mitochondrial membrane. It plays a dual role in cellular respiration by participating in both the citric acid cycle and the electron transport chain, facilitating the transfer of electrons from succinate to ubiquinone while also contributing to the generation of ATP through oxidative phosphorylation.

5 Must Know Facts For Your Next Test

1. Complex II is unique as it is the only enzyme that is part of both the citric acid cycle and the electron transport chain.
2. It catalyzes the oxidation of succinate to fumarate, which is coupled with the reduction of ubiquinone to ubiquinol.
3. Complex II does not pump protons across the inner mitochondrial membrane, unlike other complexes, resulting in less contribution to the proton gradient.
4. The activity of Complex II can be influenced by various factors, including substrate availability, enzyme inhibition, and redox state.
5. Deficiencies or dysfunctions in Complex II are linked to various metabolic disorders and can contribute to conditions such as cancer and neurodegenerative diseases.

Review Questions

• How does Complex II participate in both the citric acid cycle and the electron transport chain?
  • Complex II acts as a key enzyme in the citric acid cycle by catalyzing the conversion of succinate to fumarate while simultaneously being a component of the electron transport chain. During this process, it transfers electrons from succinate to ubiquinone, contributing to ATP production. This dual role allows Complex II to integrate metabolic pathways, linking substrate oxidation directly to energy generation.
• What is the significance of Complex II not pumping protons across the inner mitochondrial membrane compared to other complexes?
  • The lack of proton pumping by Complex II means it contributes less to the establishment of the proton gradient used for ATP synthesis during oxidative phosphorylation. This characteristic differentiates it from other complexes like Complex I and III, which actively transport protons. Consequently, Complex II's role emphasizes its function in substrate oxidation rather than in creating an electrochemical potential for ATP production.
• Evaluate how dysfunctions in Complex II can impact cellular metabolism and contribute to disease states.
  • Dysfunctions in Complex II can disrupt cellular metabolism by impairing both the citric acid cycle and oxidative phosphorylation processes. This can lead to reduced ATP production and increased reactive oxygen species (ROS) generation, which are harmful to cells. Such imbalances are linked to various diseases, including cancer, where altered energy metabolism supports rapid cell proliferation, and neurodegenerative diseases, where energy deficits compromise neuronal function.

Related terms

Ubiquinone: A lipid-soluble electron carrier that accepts electrons from Complex II and passes them to Complex III in the electron transport chain.

Citric Acid Cycle: A series of enzymatic reactions occurring in the mitochondrial matrix that generates electron carriers and ATP from the oxidation of acetyl-CoA.

Oxidative Phosphorylation: The metabolic process occurring in the inner mitochondrial membrane, where ATP is produced using energy derived from electrons transferred through the electron transport chain.