Acetyl-CoA is an essential cofactor and carrier of acyl groups in enzymatic acetyl transfer reactions. It is formed either by the oxidative decarboxylation of pyruvate in mitochondria, by the oxidation of long-chain fatty acids, or by the oxidative degradation of certain amino acids. Acetyl-CoA is the starting compound for the citric acid cycle (Kreb′s cycle). It is also a key precursor in lipid biosynthesis, and the source of all fatty acid carbons. Acetyl-CoA positively regulates the activity pyruvate carboxylase. It is a precursor of the neurotransmitter acetylcholine. Histone acetylases (HAT) use Acetyl-CoA as the donor for the acetyl group use in the post-translational acetylation reactions of histone and non-histone proteins.