text: 3-Methylcrotonyl-CoA (β-Methylcrotonyl-CoA or MC-CoA) is an intermediate in the metabolism of leucine.
  It is found in mitochondria, where it is formed from isovaleryl-coenzyme A by isovaleryl coenzyme A dehydrogenase. It then reacts with CO to yield 3-Methylcrotonyl-CoA carboxylase.