Protostadienol synthase from Aspergillus fumigatus: Functional conversion into lanosterol synthase

Oxidosqualene:protostadienol cyclase (OSPC) from the fungus Aspergillus fumigatus, catalyzes the cyclization of (3S)-2,3-oxidosqualene into protosta-17(20)Z,24-dien-3 beta-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 11 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. (C) 2009 Elsevier Inc. All rights reserved