 MCM resides in the mitochondria, where a number of substances, including the branched-chain amino acids isoleucine and valine, as well as methionine, threonine, thymine and odd-chain fatty acids, are metabolized via methylmalonate semialdehyde (MMlSA) or propionyl-CoA (Pr-CoA) to a common compound - methylmalonyl-CoA (MMl-CoA).MCM catalyzes the reversible isomerisation of l‐methylmalonyl‐CoA to succinyl‐CoA, requiring cobalamin (vitamin B12) in the form of adenosylcobalamin (AdoCbl) as a cofactor.As an important step in propionate catabolism, this reaction is required for the degradation of odd-chain fatty acids, the amino acids valine, isoleucine, methionine, and threonine, and cholesterol, funneling metabolites from the breakdown of these amino acids into the tricarboxylic acid cycle.