Succinate dehydrogenase is involved in two processes, TCA cycle and oxadative phosphorylation. Use the localization, characteristics/properties and functions of the enzyme complex to demonstrate that TCA cycle cannot occur in the absence of oxygen.

Succinate dehydrogenase (SDH) plays a crucial role in both the tricarboxylic acid (TCA) cycle and oxidative phosphorylation, linking these two metabolic pathways. To demonstrate that the TCA cycle cannot occur effectively in the absence of oxygen, we can consider several aspects of SDH's localization, characteristics, properties, and functions.

Localization

1. Mitochondrial Inner Membrane: Succinate dehydrogenase is located in the inner mitochondrial membrane. This unique localization places it at the interface between the TCA cycle and the electron transport chain (ETC), which is where oxidative phosphorylation occurs.

Characteristics/Properties

2. Enzyme Complex: SDH is a flavoprotein and is also part of the larger complex II of the electron transport chain. It catalyzes the oxidation of succinate to fumarate while reducing ubiquinone to ubiquinol.

3. Dependency on Aerobic Conditions: While the TCA cycle itself does not directly require oxygen, the continuation of the cycle heavily relies on the recycling of NAD⁺ and FAD, which are utilized in various reactions within the cycle. These cofactors are regenerated predominantly through the activity of the electron transport chain, particularly in the presence of oxygen.

Functions

4. Link Between TCA Cycle and Electron Transport Chain: SDH plays a double role as it is involved in the TCA cycle by converting succinate to fumarate, and concurrently, it feeds electrons into the electron transport chain. Without oxygen to act as the final electron acceptor in the ETC, the chain becomes backed up, which inhibits NADH and FADH₂ oxidation.

5. Inhibition of TCA Cycle Reaction: When oxygen is scarce, the electron transport chain cannot function properly, which leads to an accumulation of NADH and FADH₂. High levels of these reducing equivalents inhibit enzymes that require NAD⁺ or FAD, including those within the TCA cycle (like isocitrate dehydrogenase and alpha-ketoglutarate dehydrogenase). The inhibition of these enzymes effectively halts the cycle.

Conclusion

In summary, while the TCA cycle can technically initiate without oxygen, its functionality and continuity depend critically on the presence of oxygen due to the need for NAD⁺ and FAD regeneration in the electron transport chain. Succinate dehydrogenase exemplifies this link, as it is involved in both pathways. Without oxygen, the electron transport chain stalls, NAD⁺ and FAD are not replenished, and the TCA cycle cannot proceed effectively, illustrating that aerobic conditions are essential for sustained TCA cycle activity.