Hydroxymethylglutaryl-coenzyme A (HMG-CoA) reductase (HMGR)
Hydroxymethylglutaryl-coenzyme A (HMG-CoA) reductase (HMGR) is a tightly regulated enzyme, which catalyzes the synthesis of coenzyme A and mevalonate in isoprenoid synthesis. In mammals, this is the rate limiting committed step in cholesterol biosynthesis. Bacteria, such as Pseudomonas mevalonii, which rely solely on mevalonate for their carbon source, catalyze the reverse reaction, using an NAD-dependent HMGR to deacetylate mevalonate into 3-hydroxy-3-methylglutaryl-CoA. There are two classes of HMGR: class I enzymes which are found predominantly in eukaryotes and contain N-terminal membrane regions and class II enzymes which are found primarily in prokaryotes and are soluble as they lack the membrane region. With the exception of Archaeoglobus fulgidus, most archeae are assigned to class I, based on sequence similarity of the active site, even though they lack membrane regions. Yeast and human HMGR are divergent in their N-terminal regions, but are conserved in their active site. In contrast, human and bacterial HMGR differ in their active site architecture. While the prokaryotic enzyme is a homodimer, the eukaryotic enzyme is a homotetramer.