Methylmalonyl-CoA mutase and mitochondrial fuel

Inside the mitochondria, adenosylcobalamin partners methylmalonyl-CoA mutase, which converts methylmalonyl-CoA into succinyl-CoA so it can enter the Krebs cycle and make energy. That path is how odd-chain fats, cholesterol and several amino acids are burned. When B12 runs short, methylmalonic acid backs up and can be measured in blood, and the cell loses one of its fuel doors. Low energy on a shortage is not vague tiredness, it is a blocked step in how fuel reaches the furnace.