Interconversion of IPP and DMAPP

An unusual feature of the 1-DXP pathway is that not all organisms which possess this pathway also possess isoprenyl diphosphate isomerase (IDI), the enzyme which interconverts 

IPP and DMAPP (Ershov etal. 2000). This enzyme, whilst a key feature of the mevalonate pathway, is not vital to the 1-DXP pathway because the final step in the pathway (the reductive dehydration of ( )-4-hydroxy-3-methylbut-2-enyl pyrophosphate or HMBPP, 15) was found to produce IPP and DMAPP in an approximately 6 1 ratio in green plant plastids (Adam et al. 2002). Despite this, IDl is present in plastids. When viral infection was used to silence expression of this enzyme in the tobacco Nicotiana henthamiana the result was an 80% reduction of essential chlorophyll and carotenoid levels in infected leaves. This suggests that IDI, whilst not strictly necessary for viable plants, is highly beneficial to green tissue growth (Page etal. 2004). 

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Recently Fujiwara et al. reported on the in vitro polymerization of trans-polyisoprene using the enzymes isopentenyl diphosphate isomerase (IDI) and fra 3-isoprenyl diphosphate synthase (IDS) [271]. IDI catalyzes the interconversion of IPP and DMAPP. IDS can now catalyze the polymerization of IPP from DMAPP as outlined above for the synthesis of natural rubber, and as outlined in Fig. 13a. However, the condensation process is inhibited due to hydrophobic interaction between IDS and hydrocarbon of the longer products. The hydrophobic chain of the elongating product does not readily protrude into the aqueous phase and it tends to interact with the enzyme. To achieve an efficient in vitro synthesis, the authors used an organic-aqueous two-liquid phase system to successfully synthesize (low molecular weight) fran.y-polyisoprene (see Fig. 13b). 

A central role in the biosynthesis of isoprenoids is filled by the isopentenyl diphosphate-dimethylallyl diphosphate isomerase (IDl) that catalyzes the interconversion of IPP and DMAPP. The necessity for such an enzyme was suggested in the 1950s when only IPP was known as a monomeric isoprenoid precursor, but an allylic diphosphate such as DMAPP was assumed to have the higher intrinsic reactivity for polyisoprenoid synthesis [22, 88, 89]. The first enzymatic isomerization of IPP to DMAPP was observed in 1959 from a cell-free extract of baker s yeast [90, 91]. Two types of IDI with essentially no amino acid sequence or structural similarities are able to catalyze this interconversion by completely different enzyme mechanisms. The well-known IDI-I have been identified in animals, plants, fungi, and bacteria, whereas the IDI-II can be found mainly in archaea but also in some bacteria [92, 93]. 

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