- The rate-limiting step in the biosynthesis of cholesterol is catalyzed by the enzyme HMG-CoA reductase.
- This enzyme converts HMG-CoA (3-hydroxy-3-methylglutaryl-CoA) into mevalonate, a crucial early step in the cholesterol synthesis pathway.
- Because this step controls the flux through the entire biosynthetic route, HMG-CoA reductase is tightly regulated by multiple mechanisms including feedback inhibition by cholesterol, phosphorylation by kinases, and control at the level of gene expression.
- This regulation ensures cellular cholesterol levels remain balanced.

In contrast, other enzymes listed serve different functions:
- Squalene synthase catalyzes a later step where two molecules of farnesyl pyrophosphate condense to form squalene.
- Acetyl-CoA carboxylase is involved in fatty acid synthesis, not cholesterol biosynthesis.
- Lanosterol demethylase participates in the demethylation of lanosterol during cholesterol synthesis, but this step occurs downstream of mevalonate formation and is not rate-limiting.

Therefore, HMG-CoA reductase represents the key regulatory and rate-limiting enzyme in cholesterol biosynthesis.

Reference: Harper’s Illustrated Biochemistry, 31st Edition, Chapter 23/Pages 478-480