Free cholesterol can affect cholesterol metabolism in the body by inhibiting cholesterol biosynthesis. Which of the following most closely reflects the mechanism by which cholesterol suppresses its own synthesis?
A) condensation of acetyl-CoA and acetoacetylCoA to form hydroxymethyl glutaryl-CoA
B) cyclizing of squalene to form lanosterol
C) formation of mevalonate from hydroxymethyl glutaryl-CoA
D) kinase that phosphorylates hydroxymethyl glutaryl-CoA reductase
E) reduction of 7-dehydrocholesterol to form cholesterol

Question

Quizplus · Accepted Answer

Cholesterol suppresses its own synthesis by inhibiting the enzyme HMG-CoA reductase, which catalyzes the conversion of HMG-CoA to mevalonate, a key step in cholesterol biosynthesis.  Regulation of HMG-CoA reductase (HMGR) activity is the primary means for controlling the level of cholesterol biosynthesis. This enzyme catalyzes the conversion of mevalonate to hydroxymethyl glutarylCoA. HMGR activity is controlled by 4 distinct mechanisms: feedback inhibition, control of gene expression, rate of enzyme degradation, and phosphorylationdephosphorylation. The first 3 control mechanisms are exerted by cholesterol itself. Cholesterol acts as a feedback inhibitor of preexisting HMGR as well as inducing rapid degradation of the enzyme. In addition, when cholesterol is in excess, the amount of mRNA for HMGR is reduced because of decreased expression of the gene.

Free Cholesterol Can Affect Cholesterol Metabolism in the Body by Inhibiting