Oxidosqualene Cyclase (OSC)

Lanosterol synthase (EC 5.4.99.7) catalyzes the cyclization of (S)-2,3 oxidosqualene to lanosterol in the reaction that forms the sterol nucleus. Baker et al. (1995) cloned and characterized the human gene encoding lanosterol synthase, which they called OSC, and found that it predicts an 83-kD protein of 732 amino acids. The deduced amino acid sequence is 36 to 40% identical to known yeast and plant homologs and 83% identical to rat lanosterol synthase. A yeast lanosterol synthase-deficient mutant was complemented by the human gene, and a cell-free homogenate of the yeast mutant strain expressed in the human gene was shown to convert 2,3-oxidosqualene to lanosterol.  Oxidosqualene cyclase solves this problem by sticking to the membrane in microsomes inside the cell.