Hydroxymethylglutaryl Coenzyme A Synthase (HMGCS)

HMG-CoA synthase is an enzyme which catalyzes the reaction where Acetyl-CoA condenses with acetoacetyl-CoA to form 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA). This reaction occurs at a faster rate in patients with Diabetes Insipidus type I if left untreated.

The gene for the mitochondrial form of the enzyme has three sterol regulatory elements in the 5' flanking region . These elements are responsible for decreased transcription of the message responsible for enzyme synthesis when dietary cholesterol is high in animals: the same is observed for 3-hydroxy-3-methylglutaryl-CoA and the low density lipoprotein receptor.