Which compound in the citric acid cycle combines with acetyl CoA to initiate the cycle?

The compound that combines with acetyl CoA to initiate the citric acid cycle is oxaloacetate. In the first step of the cycle, acetyl CoA reacts with oxaloacetate to form citrate. This is a critical reaction because oxaloacetate acts as a four-carbon molecule that accepts the two-carbon acetyl group from acetyl CoA, resulting in the six-carbon compound citrate.

Oxaloacetate is regenerated at the end of the cycle, demonstrating its importance in ensuring the cycle can continue to operate. The continual regeneration of oxaloacetate allows for the sustained processing of acetyl CoA, turning it into energy-rich molecules such as NADH and FADH2 that are utilized in the electron transport chain for ATP production. Thus, the role of oxaloacetate is pivotal for the initiation and continuation of the citric acid cycle.