InterPro

There are two distinct classes of hydroxymethylglutaryl-coenzyme A (HMG-CoA) reductase enzymes: class I consists of eukaryotic and most archaeal enzymes (EC:1.1.1.34), while class II consists of prokaryotic enzymes (EC:1.1.1.88) [PMID: 10068515, PMID: 15535874].

Class I HMG-CoA reductases catalyse the NADP-dependent synthesis of mevalonate from 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA). In vertebrates, membrane-bound HMG-CoA reductase is the rate-limiting enzyme in the biosynthesis of cholesterol and other isoprenoids. In plants, mevalonate is the precursor of all isoprenoid compounds [PMID: 15535874]. The reduction of HMG-CoA to mevalonate is regulated by feedback inhibition by sterols and non-sterol metabolites derived from mevalonate, including cholesterol. In archaea, HMG-CoA reductase is a cytoplasmic enzyme involved in the biosynthesis of the isoprenoids side chains of lipids [PMID: 10600463]. Class I HMG-CoA reductases consist of an N-terminal membrane domain (lacking in archaeal enzymes), and a C-terminal catalytic region. The catalytic region can be subdivided into three domains: an N-domain (N-terminal), a large L-domain, and a small S-domain (inserted within the L-domain). The L-domain binds the substrate, while the S-domain binds NADP.

Class II HMG-CoA reductases catalyse the reverse reaction of class I enzymes, namely the NAD-dependent synthesis of HMG-CoA from mevalonate and CoA [PMID: 15028676]. Some bacteria, such as Pseudomonas mevalonii, can use mevalonate as the sole carbon source. Class II enzymes lack a membrane domain. Their catalytic region is structurally related to that of class I enzymes, but it consists of only two domains: a large L-domain and a small S-domain (inserted within the L-domain). As with class I enzymes, the L-domain binds substrate, but the S-domain binds NAD (instead of NADP in class I).