Functional analysis of Phe605, a conserved aromatic amino acid in squalene-hopene cyclases

Incubation of squalene with the site-directed mutant F605A of squalene-hopene cyclase from Alicyclobacillus acidocaldarius yielded many triterpenes consisting of the 6/6/5-fused tri-, 6/6/6/5-fused tetra-, and 6/6/6/6/5-fused pentacyclic skeletons, the function of F605 being assignable for facilitating the ring expansion and for stabilizing the hopanyl C22-cation, possibly via cation-pi interactions.