How does acetyl coenzyme A enter the tricarboxylic acid cycle?

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Fluorescent confocal image of HeLa cells with green microtubules outlining cell shapes, red mitochondria, and blue nuclei. — mitochondrion Mitochondria (red) are found throughout the cytoplasm of almost all eukaryotic cells (cell nucleus is shown in blue; cytoskeleton is shown in yellow).

In the mitochondrion, acetyl coenzyme A (acetyl CoA) enters the tricarboxylic acid (TCA) cycle through a condensation reaction with the four-carbon compound oxaloacetate.12

This initial step is catalyzed by the enzyme citrate synthase. During this reaction, the two-carbon acetyl group from acetyl CoA is transferred to oxaloacetate to form citrate, while coenzyme A (CoA-SH) is released. By combining the two-carbon acetyl fragment with the four-carbon acceptor, the cell is able to more effectively dismember and oxidize the molecule to harvest energy in subsequent steps of the cycle.12