Functional Analysis of the DXDDTA Motif in Squalene-Hopene Cyclase by Site-directed Mutagenesis Experiments: Initiation Site of the Polycyclization Reaction and Stabilization Site of the Carbocation Intermediate of the Initially Cyclized A-Ring

In order to clarify the function of the DXDDTA motif in squalene-hopene cyclase and to identify the acidic amino acid residues crucial for the catalysis, site-directed mutagenesis experiments were carried out. The following results were found: (1) residues D374 and D376 work for the initiation of polyolefin cyclization which arises from the proton attack on the terminal double bond; (2) residue D377 stabilizes C-10 carbocation of the initially cyclized A-ring intermediate, leading to subsequent B-ring closure, which was further verified by isolating the partially cyclized monocyclic product; (3) residues D313 and D447 outside the DXDDTA motif were identified as new active sites; (4)the H451 residue is likely to work in the protonated form to enhance the acidity of the carboxyl groups of D374 and/or D376.