Several enzymes catalyse mechanistically related reactions which involvethe highly complex cyclic rearrangement of squalene or its 2,3 oxide.Lanosterol synthase (5.4.99.7) (oxidosqualene--lanosterol cyclase) catalyzes the cyclization of (S)-2,3-epoxysqualene to lanosterol, the initial precursor of cholesterol, steroid hormones and vitamin D in vertebrates and of ergosterol in fungi (gene ERG7). Cycloartenol synthase (5.4.99.8) (2,3-epoxysqualene--cycloartenol cyclase), is a plant enzyme that catalyzes the cyclization of (S)-2,3-epoxysqualene to cycloartenol, and hopene synthase (5.4.99) (squalene--hopene cyclase), is a bacterial enzyme that catalyzes the cyclization of squalene into hopene, a key step in hopanoid (triterpenoid) metabolism.These enzymes are evolutionary related [1] proteins of about 70 to 85 kD. This highly conserved region is rich in aromatic residues and is located in the C-terminal section.

1. Isolation of an Arabidopsis thaliana gene encoding cycloartenol synthase by functional expression in a yeast mutant lacking lanosterol synthase by the use of a chromatographic screen. Proc. Natl. Acad. Sci. U.S.A. 90, 11628-32

Several enzymes catalyse mechanistically related reactions which involvethe highly complex cyclic
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