HMG-CoA (or 3-hydroxy-3-methylglutaryl-coenzyme A) is an intermediate in the mevalonic acid pathway of cholesterol biosynthesis.
It is formed from acetyl CoA and acetoacetyl CoA by HMG-CoA synthase.
HMG-CoA reductase converts it into mevalonic acid. Also, HMG-CoA lyase breaks it into acetyl CoA and acetoacetate.
It is also an intermediate in the metabolism of leucine. Its immediate precursor is 3-methylglutaconyl CoA.