HMG-CoA synthase

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). There are two different forms of the enzyme; in humans the cytosolic form has only 60.6% amino acid identity with the mitochondrial from of the enzyme.

The cytosolic enzymes has a key role in the production of cholesterol and other isoprenoids:

The mitochondrial form is used for the production of ketone bodies:

Regulation
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.