Pyruvate produced by glycolysis still contains substantial chemical free energy. Under respiratory conditions, many cells convert pyruvate into acetyl-CoA, a two-carbon acyl carrier that can enter the citric-acid cycle.

For one pyruvate, the overall transformation is commonly summarized as

$$\mathrm{pyruvate+CoA+NAD^+\rightarrow acetyl{-}CoA+CO_2+NADH+H^+}.$$

This reaction performs three linked operations.

1. One carbon is removed as carbon dioxide

Pyruvate contains three carbons. Decarboxylation removes one carbon as $\mathrm{CO_2}$, leaving a two-carbon fragment.

Thus

pyruvate: 3 C
   ↓ release CO2
acetyl group: 2 C

For the two pyruvates generated from one glucose, this stage releases two $\mathrm{CO_2}$ molecules in total.

2. The remaining carbon fragment is oxidized

The two-carbon fragment is oxidized while NAD$^+$ is reduced to NADH.

Per pyruvate:

$$1\ \mathrm{NAD^+\rightarrow1\ NADH}.$$

Per glucose, because glycolysis forms two pyruvates:

$$2\ \mathrm{NADH}$$

are produced during pyruvate oxidation.

3. The acetyl group is transferred to CoA

The two-carbon acetyl group is attached to coenzyme A as a thioester, forming acetyl-CoA.

CoA is not consumed permanently. The acetyl group will be transferred into the next pathway and free CoA can later be regenerated.

No ATP is made directly in this step

Pyruvate oxidation does not make ATP by substrate-level phosphorylation. Instead, it captures part of the oxidation free energy in

• reduced NADH;
• the chemically activated acetyl-CoA intermediate.

Compartmentalization in eukaryotic cells

In eukaryotes, glycolysis occurs in the cytosol, whereas pyruvate oxidation commonly occurs in the mitochondrial matrix after pyruvate is transported across mitochondrial membranes.

This compartment boundary is functionally important because acetyl-CoA then enters a mitochondrial citric-acid cycle whose reduced electron carriers feed the inner-membrane respiratory system.

Pyruvate oxidation is therefore a bridge reaction rather than a cycle: it converts the three-carbon endpoint of glycolysis into a two-carbon acetyl carrier while releasing $\mathrm{CO_2}$ and capturing reducing equivalents as NADH.