This reaction happens in the decarboxylation step of the pyruvate dehydrogenase complex (PDC). The PDC is an enzyme complex that converts pyruvate to acetyl-CoA, a key step in cellular respiration.

Here's a simplified breakdown:

1. Pyruvate enters the mitochondria, the powerhouse of the cell.

2. PDC catalyzes the decarboxylation of pyruvate, meaning it removes a carbon dioxide molecule from pyruvate.

3. The remaining two-carbon molecule is then oxidized and combined with coenzyme A (CoA) to form acetyl-CoA.

So, while it might seem confusing that CO2 reacts with pyruvic acid to release CO2, it's important to remember that this is a decarboxylation reaction, meaning a carbon dioxide molecule is removed from pyruvate.