We next pursued the investigations of the enzymatic mechanism of MicA. The 6,10bicyclic eunicellane skeleton can be formed through two possible cyclization pathways. In the first pathway, a 1,10-cyclization is followed by a 1,3-hydride shift and a 1,14-ring closure. This pathway forms eunicellanes from Bnd4 and AlbS (Supplementary Figure ) In the second pathway, a 1,14-cyclization occurs first, followed by two 1,2-hydride transfers and a 1,10-ring closure. The second pathway is followed by coral-derived eunicellane terpene synthases EcTPS1 and BaTC-2 (Supplementary Figure ) . In the coral-derived enzymatic synthesis pathway, it has been hypothesized that prior to cyclization, GGPP undergoes isomerization to form geranyllinalyl diphosphate (GLPP) to explain the formation of 2Z double bond following consistent isomerization mechanisms with polytrichastrene synthase , humulene synthase mutant , and amorpha-4,11-diene synthase . In the present study, we successfully synthesized GLPP as well as 2Z-GGPP (Methods and Supplementary Figure ) and incubated them with MicA in vitro, leading to the production of 1 (Extended Data Fig. ), strongly suggesting that MicA probably follows a similar catalytic route as EcTPS1. Based on these results, two possible cyclization pathways for MicA were proposed, as depicted in Fig. and Extended Data Fig. . Pathway (i) suggests that GGPP isomerizes to GLPP, then to 2Z-GGPP, forming a carbocation which then undergoes a 1,14-cyclization, followed by two consecutive 1,2-hydride shifts, a 1,10-ring closure, and finally deprotonation. Pathway (ii) proposes a 1,10cyclization, 1,3-hydride transfer, a 1,14-ring closure, two 1,3-hydride shifts, and deprotonation. The acquisition of a 14-membered shunt product, β-pinacene (2) that is probably formed by a 1,14-cyclization followed by a 1,2 or 1,3-hydride shift and subsequent deprotonation, suggested that pathway (i) is more favorable than pathway (ii).