Acetaldehyde is a two carbon compound participating in the reactions below:

1. Pyruvate <=> Acetaldehyde + CO2 (catalyzed in yeast by Pyruvate Decarboxylase).

2. Ethanol + NAD⁺ <=> Acetaldehyde + NADH (catalyzed by Alcohol Dehydrogenase)

3. Threonine <=> Acetaldehyde + Glycine (catalyzed by Threonine Aldolase)