A compound formed in the mitochondria when an acetyl group (CH3CO–), derived from the breakdown of fats, proteins, or carbohydrates (via glycolysis), combines with the thiol group (–SH) of coenzyme A. Acetyl CoA is a key metabolic building block. It feeds into the Krebs cycle, plays a role in the synthesis and oxidation of fatty acids, and is a component of steroid hormones, plant hormones, certain pigments, and other substances.