## 1 Introduction
### Biosynthetic Pathways  
It is now accepted that two mechanisms, the mevalonate pathway (MVA) and the methylerythitol phosphate (MEP) pathway, together with glycolysis, convert glucose into the two five-carbon atoms, universal precursors of terpenoids: dimethylallyl diphosphate (DMAPP) and isopentenyl diphosphate (IPP). Both pathways involve an 18-enzyme cascade [2], which can be summarized via the following steps: activation (enzymes 1 and 2 activate the precursor molecule, often through phosphorylation or isomerization); intermediate formation (subsequent enzymes 3-6 modify the precursor by adding or removing functional groups, leading to the formation of key intermediates); cyclization and structural modification (enzymes 7-8 catalyze cyclization and introduce structural changes, refining the molecule's core structure); oxidation and reduction (enzymes 9-11 carry out oxidation and reduction reactions, further modifying the intermediate to approach the final product); stereochemical adjustments (enzymes 13-16 alter the stereochemistry to ensure the correct 3D orientation of the molecule); and final modifications (enzymes 17 and 18 add groups like acetyl or sugar moieties, stabilize the product, and prepare it for release). For the MVA pathway, the starting primary metabolite, the acetyl-CoA, is converted to acetyl-SCoA, which goes through a series of Claisen reactions to condense three molecules of acetyl-SCoA to mevalonic acid. Claisen reactions result in a C-C bond between a single ester and one carbonyl compound, and in this case, one acetyl-CoA molecule serves as an electrophile at C-1 and the other as the equivalent of a C-2 carbonation. Both hydroxyl groups of the mevalonic acid undergo phosphorylation followed by decarboxylation with elimination of a phosphate group, which directly generates IPP and its isomer DMAPP. Both IPP and DMAPP can also be formed through the methylerythitol phosphate (MEP) pathway in plastids, including the chloroplast in plants, using the pyruvate and glyceraldehyde-3-phosphate from glycolysis to produce deoxyxylulose-5 phosphate, which is reduced to MEP. A series of reactions convert MEP into a mixture of IPP and DMAPP. This pathway is of special relevance to bacteria [3; 4]. While C5 units derived from the MVA pathway are used in the formation of compounds such as sesquiterpenes, sterols, and triterpenes, C5 units of the MEP pathway are used in the production of isoprene, monoterpenes, diterpenes, chlorophylls, and carotenoids. Figure 1: Importance of terpenoids in plant and human welfare and potential industrial fields of application. The condensation of DMAPP and IPP in head-to-tail mode (nucleophilic attack of IPP's terminal double bond on the especially reactive allylic carbon of DMAPP) provides a 10-carbon skeleton of geranyl diphosphate (GPP), which is the immediate precursor of all monoterpenes. A further IPP addition to GPP's allylic carbon leads to the formation of farnesyl pyrophosphate (FPP), which is the precursor of sesquiterpenes (15 carbons) and another IPP addition yields geranylgeranyldiphosphate (GGPP) as a precursor of diterpenes (20 carbon). All these additions are \"head-to-tail\", connecting C1 from one unit to C4 of the next unit. Triterpenes (30 carbons) and tetraterpenes (40 carbons) result from the dimerization of FPP and GGPP, respectively, in a \"tail-to-tail\" fashion condensation. Monoterpenoids and sesquiterpenoids show an enormous variety of structures due to the instability of the carbocations resulting from the removal of the terminal diphosphate group from GPP and FPP, respectively. Due to cyclase enzymes, which catalyze the formation of a cyclic compound, GPP can undergo isomerization to nervIPP or rearrangement to linalylPP, from which cyclic structures may occur (Figure 2). Monoterpencyclase enzymes use GPP as a substrate and typically catalyze a single cyclization reaction. However, sometimes, you look at more than one product. These enzymes require Mg\({}^{2+}\) or Mn\({}^{2+}\) as a cofactor.