The fourth pass through the beta-oxidation spiral picks up where the last left off with the saturated fatty acid decanoyl-CoA and produces octanoyl-CoA. Four enzymatic steps are required starting with MCAD CoA dehydrogenase (Medium Chain) activity, followed by the enoyl-CoA hydratase activity of crotonase, the 3-hydroxyacyl-CoA dehydrogenase activity of the short chain 3-hydroxyacyl-CoA dehydrogenase (SCHAD), and completed by the ketoacyl-CoA thiolase activity, present in the mitochondrial membrane associated trifunctional protein. Note that the 3-hydroxyacyl-CoA dehydrogenase activity of SCHAD is not actually limited to short chain fatty acids, in fact SCHAD has a broad substrate specificity.