The metabolism of HMB is catalyzed by an uncharacterized enzyme which converts it to   (). HMB-CoA is metabolized by either enoyl-CoA hydratase or another uncharacterized enzyme, producing β-methylcrotonyl-CoA () or hydroxymethylglutaryl-CoA () respectively.  is then converted by the enzyme methylcrotonyl-CoA carboxylase to methylglutaconyl-CoA (), which is subsequently converted to  by methylglutaconyl-CoA hydratase.  is then cleaved into  and acetoacetate by  lyase or used in the production of cholesterol via the mevalonate pathway.