In the citric acid cycle, the first step is catalyzed by the enzyme citrate synthase, which facilitates the condensation of acetyl CoA and oxaloacetate, resulting in the formation of citrate. This reaction is characterized by a significant negative change in Gibbs free energy (ΔG), indicating that it is a thermodynamically favorable and driving step in the cycle. During this process, water is added, and CoA is released.
The second reaction, carried out by aconitase, involves the conversion of citrate into isocitrate through an enzymatic intermediate. Although this reaction has a positive ΔG under standard conditions, it is readily reversible under cellular conditions, making it a dynamic part of the cycle. A notable feature of citrate is its prochiral nature, meaning it behaves as if it were chiral due to the specific orientation in which aconitase binds to it. Aconitase also contains iron-sulfur complexes, which play a crucial role in its function and regulation. These complexes allow aconitase to respond to iron levels in the body, influencing gene expression by binding to RNA when iron is scarce.
The third reaction is catalyzed by isocitrate dehydrogenase, which converts isocitrate into alpha-ketoglutarate (α-KG). This reaction is significant as it generates NADH and releases carbon dioxide (CO₂), classifying it as an oxidative decarboxylation. The enzyme utilizes manganese as a cofactor, distinguishing it from other dehydrogenases like pyruvate dehydrogenase, which requires thiamine pyrophosphate (TPP), lipoate, FAD, and CoA. It is essential to recognize that not all dehydrogenases share the same cofactor requirements, as seen with isocitrate dehydrogenase.
