The conversion of HMG-CoA to mevalonate by HMG-CoA reductase is the rate-limiting step of cholesterol biosynthesis and is under strict regulatory control (see Figure 1 ). HMGR is the target of compounds that are effective in lowering serum cholesterol levels. Human HMG-CoA reductase consists of a single polypeptide chain of 888 amino acids. The amino-terminal residues are membrane bound and reside in the endoplasmic reticulum membrane, while the catalytic site of the protein resides in its cytoplasmic, soluble carboxy-terminal portion. A linker region connects the two portions of the protein.
Polyketides are structurally a very diverse family of natural products with diverse biological activities and pharmacological properties.  They are broadly divided into three classes: type I polyketides (often macrolides produced by multimodular megasynthases ), type II polyketides (often aromatic molecules produced by the iterative action of dissociated enzymes ), and type III polyketides (often small aromatic molecules produced by fungal species). Polyketide antibiotics , antifungals , cytostatics , anticholesteremic , antiparasitics , coccidiostats , animal growth promoters and natural insecticides are in commercial use. [ citation needed ]