What is the first substrate formed in the Citric Acid Cycle?

The first substrate formed in the Citric Acid Cycle is citrate. This occurs when acetyl-CoA, which is derived from carbohydrates, fats, and proteins, condenses with oxaloacetate to produce citrate. This reaction is catalyzed by the enzyme citrate synthase. The formation of citrate marks the initiation of the cycle, setting the stage for subsequent reactions that will ultimately lead to the regeneration of oxaloacetate and the production of energy-rich molecules, such as ATP, NADH, and FADH2, which are crucial for cellular metabolism.

The other options represent substrates formed later in the cycle: isocitrate is produced from citrate via an isomerization reaction, alpha-ketoglutarate is generated from isocitrate through oxidative decarboxylation, and succinyl-CoA is formed from alpha-ketoglutarate in a subsequent step. Understanding the sequence of substrate formation is key to grasping how the cycle operates and contributes to energy production within the cell.