Which enzyme in the Citric Acid Cycle is directly activated by calcium ions (Ca2+)?

Isocitrate dehydrogenase is the enzyme that is directly activated by calcium ions (Ca2+). This enzyme plays a crucial role in the citric acid cycle by catalyzing the oxidative decarboxylation of isocitrate to alpha-ketoglutarate while reducing NAD+ to NADH. The activation of isocitrate dehydrogenase by calcium ions links the citric acid cycle to cellular energy demand; when calcium levels rise, it signals that the cell requires more energy, thus stimulating the cycle to increase the production of NADH and ATP.

In muscle tissues, for example, calcium ions serve as a signal during contraction to enhance energy production through this pathway. This regulatory mechanism is vital because it allows the cell to respond dynamically to changes in metabolic demand, ensuring that energy production is aligned with the physiological needs of the organism.