Oxidosqualene:protostadienol cyclase (OSPC) from the fungus Aspergillus fumigatus, catalyzes the cyclization of (3S)-2,3-oxidosqualene into protosta-17(20)Z,24-dien-3beta-ol which is the precursor of the steroidal antibiotic helvolic acid. To shed light on the structure-function relationship between OSPC and oxidosqualene:lanosterol cyclase (OSLC), we constructed an OSPC mutant in which the C-terminal residues (702)APPGGMR(708) were replaced with (702)NKSCAIS(708), as in human OSLC. As a result, the mutant no longer produced the protostadienol, but instead efficiently produced a 1:1 mixture of lanosterol and parkeol. This is the first report of the functional conversion of OSPC into OSLC, which resulted in a 14-fold decrease in the V(max)/K(M) value, whereas the binding affinity for the substrate did not change significantly. Homology modeling suggested that stabilization of the C-20 protosteryl cation by the active-site Phe701 through cation-pi interactions is important for the product outcome between protostadienol and lanosterol.
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