acetyl

Meaning: The acetyl group is CH₃–C(=O)–, the acyl group derived from acetic acid. In metabolism it is carried on coenzyme A as acetyl-CoA, a thioester whose hydrolysis releases a large amount of free energy, so the acetyl group transfers easily. Acetyl-CoA comes from pyruvate (pyruvate dehydrogenase complex, mitochondrial matrix), β-oxidation of fatty acids, and ketogenic amino acids. It can enter the citric acid cycle by condensing with oxaloacetate, supply fatty acid and cholesterol synthesis, or form ketone bodies in the liver. Acetylation of histone lysines is another use.

Equation: Pyruvate + CoA + NAD⁺ → acetyl-CoA + CO₂ + NADH, where CoA is coenzyme A and each term is an amount in mol.

On the MCAT: Expect questions on this link reaction. Pyruvate dehydrogenase is inhibited by high acetyl-CoA and NADH, and acetyl-CoA activates pyruvate carboxylase in gluconeogenesis. Fatty acids give acetyl-CoA, but animals cannot make net glucose from it, because the cycle loses two carbons as CO₂ for the two that enter.

Common confusion: Acetyl, acetate and acetyl-CoA are not interchangeable; the acetyl group is the fragment, and acetyl-CoA is its activated carrier. Also, the two carbons of an acetyl group entering a turn of the cycle are not the two released as CO₂ in that same turn; those come from oxaloacetate.