What compound starts and ends the TCA cycle?

The compound that starts and ends the TCA cycle is oxaloacetate. This four-carbon molecule combines with acetyl CoA, which is derived from carbohydrates, fats, and proteins, to initiate the cycle and form citrate. Throughout the sequence of reactions in the TCA cycle, oxaloacetate undergoes various transformations as it facilitates the processing of acetyl CoA, ultimately leading to the regeneration of oxaloacetate. This regeneration is vital as it allows the cycle to continue processing additional molecules of acetyl CoA.

Understanding the role of oxaloacetate is crucial in biochemistry and metabolism, as it highlights the cyclical nature of the TCA cycle and its importance in energy production through aerobic respiration. The other compounds listed do not fulfill this dual role of both initiating and concluding the cycle, which uniquely characterizes oxaloacetate's function.