The MCCC2 gene encodes the beta subunit of a mitochondrial enzyme called the 3-methylcrotonoyl-CoA carboxylase or 3-MCC. The 3-MCC enzyme has six pairs of beta subunits with larger alpha subunits. It has an important role in leucine and isovaleric acid catabolism and catalyzes the carboxylation of 3-methylcrotonyl-CoA to form 3-methylglutaconyl-CoA. Defects in the 3-MCC enzyme result in building up leucine byproducts in the body, causing the characteristics of 3-MCC deficiency. These characteristics can range from mild to life-threatening; that include feeding difficulties, recurrent episodes of vomiting and diarrhea, lethargy, and hypotonia.
