Isopentenylpyrophosphate formation

Even though E. coli is a very well-studied bacterium, many interesting mechanistic problems in cofactor biosynthesis in this organism remain unsolved. The mechanisms for the formation of the nicotinamide ring of NAD, the pyridine ring of pyridoxal, the pterin system of molybdopterin, and the thiazole and pyrimidine rings of thiamin are unknown. The sulfur transfer chemistry involved in the biosynthesis of lipoic acid, biotin, thiamin and molybdopterin is not yet understood. The formation of the isopentenylpyrophosphate precursor to the prenyl side chain of ubiquinone and menaquinone does not occur by the mevalonate pathway. None of the enzymes involved in this alternative terpene biosynthetic pathway have been characterized. The aim of this review is to focus attention on these unsolved mechanistic problems. 

Two compounds common in plant metabolism are believed to be precursors of isoprenoid cytokinins in plants adenosine-5 -monophosphate (AMP) and A -isopentenylpyrophos-phate (iPP). As a final product of the mevalonate pathway, the latter substance serves also as a precursor for a wide spectrum of metabolites including some other plant hormones, as abscisic acid, gibberellins and brassinosteroids. The hypothetical scheme of reactions resulting in the formation of iPA, Z and DHZ is given in Fig. 2. The enzyme of entry into isoprenoid cytokinin formation is A -isopentenylpyrophosphate 5 -AMP-A -iso-pentenyltransferase (EC 2.5.1.8, trivially named cytokinin synthetase ). This enzyme activity was first detected in a cell-free preparation from the slime mould Dictyostelium discoideum [7,8]. Later the enzyme from higher plants (cytokinin-independent tobacco callus [9,10] and immature Zea mays kernels [11]) was described and the data were recently summarised in [12], The enzyme is very specific as far as the substrate is concerned [13,14] only the nucleotide AMP can be converted and only iPP (with a double bond in A position) may function as a side chain donor. 

In addition to free cytokinins, cytokinin moieties also occur as constituents of some tRNA species of a wide range of organisms including plants [57]. They are located at the strategic 37 position adjacent to the 3 -end of the anticodon [58]. In contrast to the formation of free cytokinins the biosynthetic pathways of tRNA cytokinins are well understood. The first step in their formation is post-transcriptional isopentenylation of Ade using iPP and unmodified tRNA as substrates. This reaction is catalysed by A -isopentenylpyrophosphate tRNA-A -isopentenyltransferase (EC 2.5.1.8) which was partially purified from yeast [59], E. coli [60] and com [61]. This enzyme is encoded by... 

Gibberellin biosynthesis is initiated by the activation of MVA, followed by its conversion to isopentenylpyrophosphate (IPP). Stepwise condensation leads to the formation of trans-geranylpyrophosphate (GPP), trans-farnesylpyrophosphate (FPP) and trans-geranylgeranylpyrophosphate (GGPP). 

Biosynthesis. In Claviceps purpurea, and presumably also in higher plants, E.a. are biosynthesized from tryptophan and isopentenylpyrophosphate (see Terpenes). Synthesis proceeds via 4-dimethylallyl-tiyptophan, which is converted into the alkaloid chanoclavine (by hydroxylation, methylation, decarboxylation and formation of a new C-C bond). All the other clavine alkaloids and the lysergic acid derivatives are derived from chanoclavine. The peptide moieties of the ergotamine and ergotoxin alkaloids are formed by a multienzyme complex (Fig. 2). E.a. are prepared from the sclerotia of rye, previously inoculated with Claviceps and they are also produced by culture of the fungus on artificial growth media. 

Fig. 94. Formation of all-trans-prenyl pyrophosphates 1 Isopentenylpyrophosphate A-isomerase 2 prenyltransferase...

Natural rubber can be regarded as a 1,4-addition polymer of isoprene. The basic building block of five carbons for the polymerization is 3-isopentenyIpyro-phosphate (3-IPP). The first reaction is an enzymatic isomerization of the olefin of 3-IPP to 2-isopentenylpyrophosphate (2-IPP). The carbon-carbon bond formation between these two pyrophosphates initiates the polymerization in which the pyrophosphate group acts as a leaving group. The isoprene units of natural rubber are all linked in a head-to-tail fashion and all of the double bonds have a cis-structure. The stereocontrol of the formation of the cis-unit is achieved by the function of the elongation factor which combines with the famesyl pyrophosphate (FPP) synthase [16]. 

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