 On the other hand, the third carbon of propionyl-CoA can fit into the active site of Gen5 with the correct orientation.In the neonatal developmental stages, propionic acidemia, which is a medical issue defined as the lack of propionyl-CoA carboxylase, can cause impairment, mental disability, and numerous other issues.This is caused by an accumulation of propionyl-CoA because it cannot be converted to methylmalonyl-CoA.