MCEE catalyzes the interconversion of D- and L-methylmalonyl-CoA during the degradation of branched chain amino acids, odd chain-length fatty acids, and other metabolites. This protein deficiency is an autosomal recessive inborn error of amino acid metabolism, involving valine, threonine, isoleucine and methionine. This organic aciduria may present in the neonatal period with life-threatening metabolic acidosis, hyperammonemia, feeding difficulties, pancytopenia and coma.