## 3 Sesquiterpenol Biosynthesis  
### Biosynthetic Pathway  
Farnesyl pyrophosphate (FPP) serves as the direct biosynthetic precursor for sesquiterpenol synthases in the production of sesquiterpenols. Isopentenyl pyrophosphate (IPP) and dimethylallyl pyrophosphate (DMAPP), the universal precursors for FPP biosynthesis, also constitute the fundamental building blocks for diverse terpenoid compounds [48]. These isoprenoid precursors are naturally derived from two distinct metabolic routes: the mevalonate (MVA) pathway and the methylerythntol phosphate (MEP) pathway, as shown in Figure 2[49]. The MEP pathway exists in bacteria, cyanobacteria, green algae, and plant plastids; while the MVA pathway is found in eukaryotic microorganisms, archaea, and the cytoplasm of some higher plants [50]. In the chassis _Escherichia coli_, terpenoid precursor supply is exclusively mediated through the MEP pathway, which is initiated with glyceraldehyde 3-phosphate (G3P) and pyruvate substrates. The catalytic activities of 1-deoxy-D-xylulose-5-phosphate synthase (DXS) and 1-deoxy-D-xylulose-5-phosphate reductoisomerase (DXR) represent the rate-limiting steps in this pathway [51; 52]. In yeast systems, including _Saccharomyces cerevisiae_ and _Yarrowia lipolytica_, the MVA pathway is used for sesquiterpenol precursor biosynthesis. Acetyl-CoA serves as the precursor for this pathway, with the rate-limiting step is the reduction of 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) to mevalonate by HMG-CoA reductase [51; 52].  
Under the catalysis of farnesyl diphosphate synthase (ERG20), geranyl diphosphate (GPP, the electrophilic partner) and isopentenyl diphosphate (IPP, the nucleophilic partner) generated farnesyl diphosphate (FPP), which serves as the critical precursor. The subsequent transformation of FPP by sesquiterpene synthases yields structurally diverse sesquiterpenoids [53]. Sesquiterpenols, as a prominent subclass of these compounds, exhibit remarkable structural diversity driven by varied cyclization patterns arranged by STS catalysis. The enzymatic process is initiated by FPP-derived carbocation formation, generating (E, E)-germacradienyl cations (C1-C10 closure) and (E, E)-humulyl cations (C1-C11 closure). Alternatively, FPP isomerization produces aryl diphosphate (NPP), which undergoes cyclization to form distinct carbocation intermediates, including bisabolyl cations (C1-C6 closure) and (Z,E)-germacryl cations (C1-C10 closure) [54]. These intermediates undergo further deprotonation or hydration, ultimately yielding cyclic sesquiterpenols. Notably, the farnesyl cation and nerolidyl cation demonstrate alternative stabilization pathways through undergoing deprotonation or hydration, giving rise to acyclic sesquiterpenols, such as farnesol [55] and nerolidol [56] (Figure 3).  
Figure 2: Metabolic pathway of sesquiterpenols biosynthesis. Abbreviation of metabolites: HMG-CoA, 3-hydroxy-3-methylglutaryl-CoA; MVA, mevalonate; MVA-P, 5-phosphomevalonate; MVA-PP, 5-diphosphomevalonate; IPP, isoprene diphosphate; DMAPP, dimethylallyl diphosphate; FPP, farnesyl diphosphate; G3P, glyceraldehyde-3-phosphate; DXP, 1-deoxy-D-xylulose-5-phosphate; MEP, 2-C-Methyl-d-erythnitro1-4-phosphate; CD-ME, 4-(cytidine-5’-diphospho)-2-C-methyl-D-erythriitol; ME-cpp, 2-C-methyl-D-erythriitol-2,4-cyclodiphosphate; ERG10, acetoacetyl-CoA thiolase; ERG13, 3-hydroxy-3-methylglutaryl-CoA synthase; HMGR, 3-hydroxy-3-methylglutaryl-CoA reductase; ERG12, mevalonate kinase; ERG8, phosphomevalonate kinase; ERG19, mevalonate diphosphate decarboxylase; IDI, isopentenyl diphosphate isomerase; DXS, 1-deoxy-D-xylulose-5-phosphate synthase; MCT, 2-C-methyl-D-erythriitol-4-phosphate cytidyltransferase; CMK, 4-diphosphocytidyl-2-C-methyl-D-erythriitol kinase; MDS, 2-C-methyl-D-erythriitol-2,4-cyclodiphosphate synthase; HDS, 4-hydroxy-3-methylbut-2-enyl-diphosphate synthase; HDR, 4-hydroxy-3-methylbut-2-enyl diphosphate reductase; ERG20, geranyl/farnesyl diphosphate synthase; STS, sesquiterpenonal synthase.