Enzymatic cyclization reactions of geraniol, farnesol and geranylgeraniol, and those of truncated squalene analogs having C20 and C25 by recombinant squalene cyclase

Abstract

The substrate specificity of squalene–hopene cyclase was investigated using the C₁₀–C₂₅ analogs including naturally occurring substances, e.g. geraniol (C₁₀), farnesol (C₁₅) and geranylgeraniol (C₂₀). No cyclization occurred for geraniol, but a significantly high conversion ratio (64%) was observed for farnesol, yielding the cyclic sesquiterpenes consisting of 6/6-fused bicyclic ring systems. Among them, an attractive compound having C₃₀ was produced, in the structure of which acyclic the farnesol unit is linked to the bicyclic skeleton through ether linkage. Conversion of geranylgeraniol was low (ca. 12%). The squalene analogs having C₂₀ and C₂₅ also were cyclized in yields of ca. 33–36%, but the analogs having the methyl group at C(7) and/or at C(11) underwent no cyclization; the large steric bulk size of C(7)–Me and/or C(11)–Me, which is arranged in α-disposition for all the pre-chair conformation, would have interacted repulsively with the cyclase recognition site near to the C(7) and/or C(11), resulting in no construction of the all-chair conformation inside the reaction cavity. A relatively low yield of geranylgeraniol indicated that a less bulky hydrogen atom must be located at C(14) for the efficient polycyclization reaction. The squalene cyclase shows remarkably broad substrate specificity to accept the truncated analogs having carbon-chain lengths of C₁₅–C₂₅ in addition to C₃₀.