Although all three classes of enzymes have the same function, there is little overlap among their amino acid sequences. For example, only 40 out of 302 amino acid sequences (13%) are the same between monofunctional peroxisomal and mitochondrial enzymes in humans. In fact, in mammals, the peroxisomal enzyme has an extra N-terminal domain that is not present in the mitochondrial counterpart. Also, it has been found to be a subunit of the peroxisomal trifunctional enzyme (pTFE) and contributes only to minor cleavages of the fatty acid chain. In that sense, for many higher organisms, the mitochondrial enzyme is essential for deriving maximum energy from lipids and fueling muscles.