Prokaryotes isoprenoids

Until 1993, all terpenes were considered to be derived from the classical acetate/mevalonate pathway involving the condensation of three units of acetyl CoA to 3-hydroxy-3-methylglutaryl CoA, reduction of this intermediate to mevalonic acid and the conversion of the latter to the essential, biological isoprenoid unit, isopentenyl diphosphate (IPP) [17,18,15]. Recently, a totally different IPP biosynthesis was found to operate in certain eubacteria, green algae and higher plants. In this new pathway glyceradehyde-3-phosphate (GAP) and pyruvate are precursurs of isopentenyl diphosphate, but not acetyl-CoA and mevalonate [19,20]. So, an isoprene unit is derived from isopentenyl diphosphate, and can be formed via two alternative pathways, the mevalonate pathway (in eukaryotes) and the deoxyxylulose pathway in prokaryotes and plant plastids [16,19]. 

One of the more exciting and recent advances in the field of plant biochemistry has been the discovery of the mevalonate-independent pathway for the biosynthesis of isoprenoids (Fig. 10.4). This new pathway, referred to a the methyl-erythritol-phosphate or MEP pathway for the first intermediate committed solely to the biosynthesis of isoprenoids, was first discovered in prokaryotes capable of accumulating hopenes, the equivalent of eukaryotic sterols. 6,17 The MEP pathway has since been confirmed in plants and, not surprisingly, has been localized to chloroplasts.18 Operation of the MEP pathway is intimately related to the reactions of CO2 fixation and photosynthesis, as evidenced by the two immediate precursors pyruvate and phosphoglyceraldehyde for this pathway. Two important features of this pathway are that mevalonate is not an intermediate in the plastidic synthesis of isopentenyl (IPP) and dimethylallyl diphosphate, (DMAPP), and this pathway... 

Of the two existing isoprenoid biosynthetic pathways (Fig. 3), DXP is used by most prokaryotes for production of IPP and dimethylallyl diphosphate (DMAPP) [65,66]. With the available knowledge of the genes involved in the DXP pathway, several groups have studied the impact of changed expression levels of these genes on the production of reporter terpenoids. Farmer and liao reconstructed the isoprene biosynthetic pathway in Escherichia coli (E. colt) to produce lycopene, which was used as an indication... 

The pathways of carbon flow in eukaryotes are more complex than in prokaryotes, mainly because the C2 units produced within mitochondria can be exported (e.g. for lipid biosynthesis) only as part of a citrate unit produced in the citric acid cycle. The branch point in carbon flow to either isoprene or acetate can lead to additional isotopic fractionation. In general, it seems that n-alkyl hpids in a particular organism are depleted in 13C by c. 1.5%o relative to isoprenoids produced from the same substrate (Hayes 1993). In higher plants, the phenolic precursors of lignin derive from glucose (Fig. 2.29), so it is not surprising that the carbon isotopic composition of lignin reflects the major photosynthetic pathway involved (C3 or C4 Benner et al. 1987). 

Carbon isotopic compositions of lipids from cyanobacteria. Although prokaryotic and thus uncompartmentalized, cyanobacteria differ from the organisms just considered in that they are photosynthetic, and thus obtain from light much of the energy required to drive biosynthesis. Moreover, they produce isoprenoidal as well as -alkyl lipids. Sakata et al. (1997) recently reported results of the first investigation of lipid-biosynthetic fractionations in a cultured cyanobacterium, specifically Synechocystis, which uses the... 

Matthew PD, Wurtzel RT (2000) Metabolic engineering of carotenoid accumulation in Escherichia coli by modulation of the isoprenoid precursor pool with expression of deoxyxylulose phosphate synthase. ApplMicrobioIBiotechnol 53 396 Marker M, Bramley PM (1999) Expression of prokaryotic l-deoxy-D-xylulose-5-phosphatases in Escherichia coli increases carotenoid and ubiquinone biosynthesis. FEBS Lett 448 115... 

Fig. 15.2 Biosynthetic pathway of ubiquinone in prokaryotes. Enzyme names and intermediates are inferred from research studies in E. coli. The chain length of isoprenoids tmd ubiquinone pathway intermediates is kept unspecified (n) to generalize the pathway model to encompass all prokaryotes. (1) eiythtose-4-phosphate (2) phosphoenolpyruvate (3) 3-deoxy-ttfabino-heptulosonate 7-phosphate (4) 3-dehydroquinate (5) 3-dehydroshikimate (6) shikimate (7) shikimate-3-phos-phate (8) 5-enolpyruvyl-shikimate-3-phosphate (9) chorismate (10) para hydroxybenzoate (11) pyruvate (12) glyceraldehyde-3-phosphate (13) l-deoxy-D-xyMose-5-phosphate (14) 2-C-methyl-D-eiythrithol-5-phosphate (15) 4-diphosphocytidyl-2C-methyl-D-erythritol (16) 4-diphosphocytidyl-2C-methyl-D-erythritol 2-phosphate (17) 2-C-methyl-D-erythritol-2,4-cyclo-diphosphate (18) l-hydroxy-2-methyl-2-(E)-butenyl 4-diphosphate (19) demethylaUyl diphosphate (20) isopentenyl diphosphate (21) farnesyl diphosphate (22) polyprenyl diphosphate (23) 3-polyprenyl-4-hydrobenzoate (24) 2-polyprenylphenol (25) 2-polypienyl-6-hydtoxyphenol ...

Many yield improvement approaches involve metabolic engineering steps that are devoted to existing plant specialty ingredients like vitamin E (9JO), carotenoids (1IJ2) and flavors and aromas (13). Pathway engineering thereby may involve both plant enzymes as well as prokaryotic and eukaryotic microbial enzymes. These nutritional and dietetic specialties have no bulk feedstock potential, but improvements in terpenoid and isoprenoid biosynthesis may pave the way for the future production of industrial polyisoprenoids like latex and rubber in crops (14). 

Bacterial isoprenoid synthesis - the Rohmer pathway Current biosynthetic evidence indicates that the steps from IPP to isoprenoids in Eubacteria are the same as those in eukaryotes [62-69] (see [70-73] for literature). Especially the incorporation of C-labeled precursors into prokaryotic hopanoids, sterol surrogates in bacterial membranes (see [73] and literature cited therein), or into ubiquinone-8 [70] has revealed that the classic pathway of IPP formation starting from acetyl-CoA via acetoacetyl-CoA, HMG-CoA, MVA, MVA-P, and MVA-PP does not exist in a great variety of bacteria, including E. coli Zymomonas mobilis, Methylobacterium organophilum, Rhodopseudomonas palustris, R. acidophila, Acetobacter aceti ssp. xylinum, but also in the thylakoids of the cyanobacterium Synechocystis sp. [74]. 

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