## 3 Precursor Geranyl Pyrophosph Production  
Prenylation of OA requires the supply of the C10 isoprenoid GPP. Isoprenoids, or terpenoids, are the largest class of natural products involved in both the primary and specialized metabolism in all three kingdoms of life, and all of them derive from simple C5 building unit isopentenyl pyrophosphate (IPP) and its _cis_-isomer dimethylallyl pyrophosphate (DMAPP) [82].Based on the number of isoprene units in the structure, primary isoprenoids can be classified into monoterpene (C10), sesquiterpene (C15), diterpene (C20), triterpene (C30), and other higher terpen with more isoprene units, such as natural rubber which comprises tens of thousands of isoprene units. Isoprenoids are often cyclized by terpene synthases and further decorated to have impressive complexity, which is the driving force for their diversity and exciting bioactivities in nature. Simple isoprenoids also participate in central cell metabolism such as tRNA genesis (uracil modification), mitochondrial electron transport (_via_ ubiquinone synthesis), and protein glycosylation and cell wall biosynthesis (_via_ dolicol or undecarprenol synthesis). As a result their dynamics in cells are tightly regulated. IPP and DMAPP are produced either from the mevalonate (MVA) pathway or the 2-C-methyl-D-erythnitol 4-phosphate (MEP) pathway, named after respective signature intermediates (Figure 2). The MVA pathway is mostly found in the cytosol of eukaryotes, whereas the MEP pathway is found in most prokaryotes and the plastids of plants. In plants, GPP is synthesized primarily via the plastidial MEP pathway since the GPPSs are localized in plastids. In contrast, the biosynthesis of farnesyl pyrophosphate (FPP) that gives rise to all sesqui- and triterpenoids (e.g., sterols) in plants is primarily localized in the cytosol where the FPP synthases (FPPS) are found. For the popular heterologous production systems; the prokaryote _E. coli_ contains the MEP but not the MVA pathway, whereas the eukaryote yeasts have the MVA but not the MEP pathway. In the past two decades, extensive studies have been put into improving the isoprenoid production in microorganisms for their commercial uses in pharmaceutical and chemical industries. While several key points are explained below, more details are found in many excellent reviews [82; 83; 84; 85]. In principle, engineering the isoprenoid production includes overexpressing the respective pathway enzymes, reducing the product feedback inhibition by enzyme engineering or bypassing these steps with alternative enzymes, reducing the consumption of the intermediates from competing metabolism by gene deletions, and other considerations such as stoichiometry, cell growth and process engineering. HMG CoA synthase (HMGS) and HMG CoA reductase (HMGR) are well-known rate-limiting enzymes in the MVA pathway, which are regulated both transcriptionally and post-translationally, whereas the gateway enzyme 1-deoxy-D-xylulose-5-phosphate synthase (DXPS) in the MEP pathway is inhibited by IPP. Partial or full MVA pathways have been frequently expressed in _E. coli_ to circumvent MEP pathway regulation. In yeast, very commonly the HMGS is expressed as a \(N\)-terminal truncated version to remove the inhibitory domain. Overexpression of the gateway enzyme acetoacetyl CoA thiolase (ACCT) is also used to boost flux into the formation of acetoacetyl CoA in the MVA pathway (Figure 2). Many successes have been made in production of a number of isoprenoid compounds such as the sesquiterpene artemisinin acid in yeast (25g/L) and its precursor amorpha 1,4-diene in _E. coli_ (27 g/L) with fed-batch bioreactor reaching industrial levels [86; 87]. It should be noted that monoterpene production such as GPP in yeast remains challenging compared with the high titers of sesquiterpenes that are achievable in the same system [88]. _S. cerevisiae_ barely produces monoterpene nor does it contain a GPPS [89]. Its FPPS ERG20 and its products (ergosterols, dolichols etc.) are essential for yeast survival.