# The Terpene Mini-Path, a New Promising Alternative for Ternpenoids Bio-Production
###### Abstract  
Ternpenoids constitute the largest class of natural compounds and are extremely valuable from an economic point of view due to their extended physicochemical properties and biological activities. Due to recent environmental concerns, terpene extraction from natural sources is no longer considered as a viable option, and neither is the chemical synthesis to access such chemicals due to their sophisticated structural characteristics. An alternative to produce terpenoids is the use of biotechnological tools involving, for example, the construction of enzymatic cascades (cell-free synthesis) or a microbial bio-production thanks to metabolic engineering techniques. Despite outstanding successes, these approaches have been hampered by the length of the two natural biosynthetic routes (the mevalonate and the methyl erythroid phosphate pathways), leading to dimethylallyl diphosphate (DMAPP) and isopentenyl diphosphate (IPP), the two common universal precursors of all terpenoids. Recently, we, and others, developed what we called the terpene mini-path, a robust two enzyme access to DMAPP and IPP starting from their corresponding two alcohols, dimethylallyl alcohol and isopentenol. The aim here is to present the potential of this artificial bio-access to terpenoids, either in vitro or in vivo, through a review of the publications appearing since 2016 on this very new and fascinating field of investigation. terpenoids bio-access; terpene mini-path; synthetic biology +
Footnote †: journal: Journal of the Royal Society of London, Series B (Statistical Methodology)  
15 November 20215 November 20 starter unit (DMAPP, geranyl diphosphate GPP, farnesyl diphosphate FPP, geranylgeranyl diphosphate GGPP and the number of extension units added (IPP). A second layer of diversity is then added when the initially formed scaffold is enzymatically decorated with a myriad of oxidases (cytochrome P450, \(\alpha\)-ketoglutarate dependent dioxygenases), glyco- and acyl-transferases, as well as dehydrogenases and halogenases. These two steps of modification, cyclisation and decoration, are, thus, the basis of terpenoids' structural diversity. The first step usually involves a single enzyme, while the second, depending on the final product structure, may involve more than ten enzymes. As a projecting example, we provide here a simplified biosynthesis scheme of taxol (Scheme 1), the whole sequence, particularly during the decoration phase, being actually not entirely known [7]. It should also be noted that DMAPP, as well as diphosphates of higher length, such as GPP, FPP, and GGPP, can be coupled to natural molecules of different biosynthetic origins, such as amino acids or polyphenols, and further processed, offering another source of structural diversity. In the first decade of this century, a masterpiece of work was engaged by the Keasling lab in order to produce artemisinin, a sequiterpenoid with antimalarial activity found in the plant _Artemisia annua_. Impressive titers of amorphadiene (the sesquiterpene hydrocarbon precursor of artemisinin, 40 g/L) and of artemisinin acid (the starting synthon for the chemical access to artemisinin, 25 g/L) were obtained in cultures of recombinant yeasts bearing an optimized MVA pathway [8] at the expense of 150 persons per year of work [9]. One of the difficulties of this approach is that the carbon and energy source of the recombinant microbe is also the biosynthetic source of carbon of the terpenoid of interest, which makes balancing the flux towards the latter quite delicate. Having a look at the biosynthetic scheme of all the terpenes (Scheme 1), it is of note that the road to DMAPP and IPP starting from glucose is not the simplest in terms of chemical modifications. Indeed, not from a biological perspective but from a synthetic chemistry point of view, it is nonsense to start from glucose to access DMAPP or IPP.