The reactions of beta oxidation and part of citric acid cycle present structural similarities in three of four reactions of the beta oxidation: the oxidation by FAD, the hydration, and the oxidation by NAD+. Each enzyme of these metabolic pathways presents structural similarity. There are at least 25 enzymes and specific transport proteins in the β-oxidation pathway. Of these, 18 have been associated with human disease as inborn errors of metabolism.