The MCEE protein (methylmalonyl-CoA difunctionalase) is a key metabolic enzyme located in the mitochondria. Its core function is to convert the (R)-methylmalonyl-CoA, which cannot be utilized by the downstream processes, into the (S)-configuration that can be recognized, thereby ensuring the normal progression of branched-chain amino acid and odd-chain fatty acid catabolism. The final product, succinyl-CoA, can enter the tricarboxylic acid cycle for energy production. If the gene encoding this protein undergoes mutation, it will lead to the accumulation of methylmalonyl-CoA and its derivatives, causing an autosomal recessive genetic disease - methylmalonic acidemia. Patients may experience severe metabolic disorders, neurotoxicity, and multi-organ damage. Therefore, MCEE plays a crucial role in maintaining energy metabolic homeostasis and preventing genetic metabolic diseases.