# The Terpene Mini-Path, a New Promising Alternative for Ternpenoids Bio-Production
## 3 One Enzyme Access to DMAPP/IPP  
### The Isopentenyl Phosphate Kinase Enzyme  
Before being recognized as part of a modified mevalonate pathway in archaea [20], isopentenyl phosphate kinase (IPK) was first detected in _E. coli_ and in _Mentha piperta_[21] and was believed at the time to be the last enzymatic step of the not-yet-elucidated MEP pathway [21]. Using the recombinant and partially purified _E. coli_ IPK (IPK\({}_{E\!c}\)), it was demonstrated that the best tested enzyme substrate in the presence of ATP was isopentenyl phosphate (IP) generating isopentenyl diphosphate (Scheme 3).  
The enzyme proved also to catalyze the phosphorylation of IOH with a non-negligible relative activity of 44% when compared to IP. Low activity towards DMAOH and no activity towards DMAP were noticed [21]. Although not recognized as such at the time, this pioneering work was the first to name a putative enzyme involved in the transformation of IOH to IPP and thus demonstrated the potential of a very short enzymatic access to terpenoids starting from IOH. With the demonstration that the last step in the MEP pathway was independent of IP, no further work, to the best of our knowledge, was devoted to the physiological role of _E. coli_ IPK\({}_{Ec}\) in the following years. Building on the extensive whole genome sequencing effort done at the beginning of the century, it was quickly recognized that numerous archaebacteria genomes lack the two final genes of the classical MVA pathway, that is the genes coding for phosphomevalonate kinase and diphosphomevalonate decarboxylase (Scheme 3). Using Methanolcodecoccus jannaschini and Haloferax volcanicii as model organisms, it was demonstrated that these two enzymes are replaced by a phosphomevalonate decarboxylase [22] and an IPK [23] within archaea. Thus, at least in archaea, the physiological role of IPKs was assessed as being part of an alternate MVA pathway (Scheme 3). Later, it was recognized by bioinformatic studies that each sequenced plant genome possessed an IPK encoding gene [24]. It is, therefore, relevant to consider what the physiological role of this enzyme is in plants. Recently, some studies have demonstrated a potential physiological role of plant IPKs in conjunction with hydrolases from the Nudx family, both enzymes being supposed to be involved in the regulation of DMAPP/DMAP and IPP/IP ratios in plant cytosol ([25], reviewed in [24]). Indeed, the ability of DMAP and IP to competitively inhibit farnesyl diphosphate synthase and the demonstrated presence of both IPK and phosphatase acting specifically on DMAPP and IPP but not on DMAP or IP led to a regulatory scheme [25]. This latter describes the central role of the DMAP/IP pair in the regulation of the biosynthesis of terpenoids through the cytosolic MVA pathway, at least in plants (Scheme 4).  
Thus, at the beginning of the century, one natural enzyme, IPK, was recognized as widely distributed in the tree of life with different physiological roles and catalyzing the phosphorylation of IP into IPP. This enzyme was also proved to be promiscuous enough to catalyze the transformation of DMAP into DMAPP [26]. In the perspective of building a short enzymatic access to DMAPP/IPP from DMAOH/IOH, the second phosphorylation  
Figure 4: Potential physiological role of IPKs (and of Nudx) in the regulation of sesquiterpenoids biosynthesis in plants. IDI stands for isopentenyl diphosphate isomerase. Nudx stands for Nudx phosphatase. FPPS stands for farnesyl diphosphate synthase (adapted from [25]).  
step looked thus to be secure thanks to the IPK. From the work on the IPK\({}_{Ec}\)[21], it was even conceivable to establish a one enzyme double phosphorylation of at least IOH into IPP thanks to IPK.