*4.1. Organisms and Chemicals*

The organism *S. cerevisiae* CICC1060 was purchased from the China Center of Industrial Culture Collection (CICC, Beijing, China). *S. cerevisiae* CICC1060 was cultured with YPD medium (tryptone 20 g/L, yeast extract 10 g/L, and glucose 20 g/L) at 30 ◦C for 24 h. The pEASY-E1 expression vector from TransGen Biotech Co., Ltd (Beijing, China) was used for overexpression of the enzyme OYE2y, and the *E. coli* strain BL21(DE3) was used as the host. *E. coli* cultures were grown routinely in Luria Bertani (LB) medium at 37 ◦C for 12 h.

The standards (*S*)-citronellal, (*R*)-citronellal, and (*S*/*R*)-citronellal were obtained from Sigma-Aldrich (Shanghai) Trading Co., Ltd. (Shanghai, China). Other chemicals of analytical grade were purchased from Sangon Biotech Co. Ltd (Shanghai, China) or Shanghai Jingchun Reagent Co., Ltd (Shanghai, China). The site-directed mutagenesis kit and the restriction enzyme *Dpn* I were obtained from Vazyme Biotech Co., Ltd. (Nanjing, China). KOD DNA polymerase was purchased from TransGen Biotech Co., Ltd (Beijing, China). The Ni-NTA-HP resin column for protein purification was obtained from GE Healthcare Life Sciences (Shanghai, China).
