Selectivity in the Cyclization of Citronellal Introduced by Squalene Hopene Cyclase Variants

The squalene hopene cyclase from Alicyclobacillus acidocaldarius (AacSHC) is a highly efficient enzyme catalyst for stereoselective BrOnsted acid catalysis. We engineered AacSHC to catalyze the selective Prins cyclization of citronellal. Four active site variants were identified for the diastereoselective cyclization of (S)-citronellal to stereoisomers (-)-iso-isopulegol, (+)-isopulegol and (-)-neo-isopulegol, respectively. The replacement of active site residues resulted in two triple variants that catalyzed the transformation of (R)-citronellal to give the isomers (+)-neo-isopulegol and (-)-isopulegol with up to >99% de, respectively. The newly designed library of functionally diverse active site geometries exhibits high selective control during citronellal cyclization, leading exclusively to a single diastereomer of the desired isopulegol. Whereas the cyclization of citronellal with chemical catalysts was observed to produce the isopulegol isomer with the lowest energy, the reaction with AacSHC variants proceeded with higher product selectivity. The results of this study show that variants of AacSHC are excellent catalysts for the highly selective formation of isopulegol stereoisomers.