Trifunctional enzyme subunit alpha, mitochondrial also known as hydroxyacyl-CoA dehydrogenase/3-ketoacyl-CoA thiolase/enoyl-CoA hydratase (trifunctional protein), alpha subunit is a protein that in humans is encoded by the HADHA gene. Mutations in HADHA have been associated with trifunctional protein deficiency or long-chain 3-hydroxyacyl-coenzyme A dehydrogenase deficiency. This gene encodes the alpha subunit of the mitochondrial trifunctional protein, which catalyzes the last three steps of mitochondrial beta-oxidation of long chain fatty acids. The enzyme converts medium- and long-chain 2-enoyl-CoA compounds into the following 3-ketoacyl-CoA when NAD is solely present, and acetyl-CoA when NAD and CoASH are present. The alpha subunit catalyzes this reaction, and is attached to HADHB, which catalyzes the last step of the reaction.