To further evaluate the two proposed pathways, we synthesized 6F-GGPP (Supplementary Figure ), an excellent substrate for elucidating cyclization pathways . The fluorine atom at C-6, due to its strong electron-withdrawing effect, inhibits 1,6-cyclization in the catalytic pathway of PcTS1. This inhibition is expected to result in the formation of monocyclic products in pathway I or acyclic products in pathway II (Supplementary Figure ). Incubation of 6F-GGPP with PcTS1 followed by HPLC analysis revealed several new peaks (Fig. ), including the main product To enhance our comprehension of the reaction mechanism of biflorane synthase PcTS1 and facilitate a structural comparison with TSs from corals, we intend to elucidate the crystal structures of PcTS1. We initially struggled to obtain the wild-type PcTS1 crystals. So, we expressed and purified a truncated form, PcTS1 , which lacks the first five N-terminal amino acids. For clarity, we will refer to "PcTS1 ΔN1-5 " as "PcTS1" unless stated otherwise. Following activity confirmation of the truncated PcTS1, we performed crystallization experiments (Supplementary Figure ). Luckily, this truncation enabled us to determine the high-resolution 3D structure of the apo form of PcTS1 at 2.5 Å resolution (PDB ID: 8ZWU, Fig. , Supplementary Table ), using the cembrene A synthase from Eleutherobia rubra (PDB ID: 7S5L) as a template . The crystal structure of apo-PcTS1 reveals the classic class I TS α-spiral structure (Fig. ).