Cholesterol synthesis is controlled significantly by the hepatic HMG Co A reductase. Hydroxy-3-methylglutaryl coenzyme A (HMG CoA) reductase produces mevalonate, an important intermediate in the synthesis of cholesterol and essential nonsterol isoprenoids. The reductase is subject to a range of forms of feedback control through various mechanisms mediated by sterol and nonsterol end-products of mevalonate metabolism. In an article in Cell Research DeBose-Boyd reviews recent advances on one mechanism for control of reductase, which involves rapid degradation of the enzyme. Accumulation of certain sterols triggers binding of reductase to endoplasmic reticulum (ER) membrane proteins called Insig-1 and Insig-2. Reductase-Insig binding results in recruitment of a membrane-associated ubiquitin ligase called gp78, which initiates ubiquitination of reductase. This ubiquitination is an obligatory reaction for recognition and degradation of reductase from ER membranes by cytosolic 26S proteasomes. Thus, sterol-accelerated degradation of reductase represents an example of how a general cellular process (ER-associated degradation) is used to control an important metabolic pathway (cholesterol synthesis).
DeBose-Boyd (2008 Feedback regulation of cholesterol synthesis: sterol-accelerated ubiquitination and degradation of HMG CoA reductase Cell Research vol 18:609–621

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