Isoprenoids higher plants

Rohmer, M., The discovery of a mevalonate-independent pathway for isoprenoid biosynthesis in bacteria, algae and higher plants, Nat. Prod. Rep., 16, 565, 1999. 

LICHTENTHALER, H.K., SCHWENDER, J., DISCH, A ROHMER, M., Biosynthesis of isoprenoids in higher plant chloroplasts proceeds via a mevalonate-independent pathway, FEBSLett., 1997, 400, 271-274... 

Lichtenthaler HK, Rohmer M, Schwender J (1997) Two independent biochemical pathways for isopentenyl diphosphate and isoprenoid biosynthesis in higher plants. Physiol Plantarum... 

The mevalonate-independent pathway is present in most bacteria and all phototropic organisms. In higher plants and most algae both pathways run independently. The mevalonate pathway is located in the cytoplasm and is responsible for the biosynthesis of most sesquiterpenoids. The mevalonate-independent pathway, in contrast, is restricted to the chloroplasts where plastid-related isoprenoids such as monoterpenes and diterpenes are biosynthesised via this pathway [43-45]. Figure 4.2 illustrates the interrelationships of both biosynthetic pathways connected to Fig. 4.1 [46]. 

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]. 

Sesquiterpenoids (Sq) are Cj5 compounds formed by the assembly of three isoprenoid units. They are found in many living systems but particularly in higher plants. There are a large number of sesquiterpenoid carbon skeletons, which arise from the common precursor, farnesyl diphosphate, by various modes cyclization followed, in many cases, by skeletal rearrangement. 

The biosynthetic pathway producing isoprenoid cytokinins has been identified, whereas that of aromatic cytokinins is poorly characterized.363 385 Two distinct pathways for isoprenoid cytokinin biosynthesis have been described, and each pathway employs a different type of isopentenyltransferase at the initial step. The major pathway in higher plants, which is catalyzed by IPT, is conjugation of adenine nucleotide and DMAPP or 4-hydroxy-3-methyl-2-( )-butenyl diphosphate (HMBDP) (Figure 14). In the less frequently used pathway, cytokinins are formed by degradation of prenylated tRNAs. The initial prenylation reaction of tRNA is catalyzed by tRNA-isopentenyltransferase (tRNA-IPT) (Figure 14). 

Figure 15 Current model of isoprenoid cytokinin biosynthesis pathway in higher plants. tZRDP, tZ riboside 5 -diphosphate tZRTP, tZ riboside 5 -triphosphate DZRMP, DZ riboside 5 -monophosphate cZRMP, cZ riboside 5 -monophosphate ...

Like chlorophyll, plastoquinone A has a nonpolar terpenoid or isoprenoid tail, which can stabilize the molecule at the proper location in the lamellar membranes of chloroplasts via hydrophobic reactions with other membrane components. When donating or accepting electrons, plastoquinones have characteristic absorption changes in the UV near 250 to 260, 290, and 320 nm that can be monitored to study their electron transfer reactions. (Plastoquinone refers to a quinone found in a plastid such as a chloroplast these quinones have various numbers of isoprenoid residues, such as nine for plastoquinone A, the most common plastoquinone in higher plants see above.) The plastoquinones involved in photosynthetic electron transport are divided into two categories (1) the two plastoquinones that rapidly receive single electrons from Peso (Qa and Qb) and (2) a mobile group or pool of about 10 plastoquinones that subsequently receives two electrons (plus two H+ s) from QB (all of these quinones occur in the lamellar membranes see Table 5-3). From the plastoquinone pool, electrons move to the cytochrome b f complex. 

Figure 6 Isoprenoids from bacteria (bacteriohopanepolyols, 20 pentalenolactone, 21 ubiquinone, 22 menaquinone, 23) and from higher plants (phytol, 24 p-carotene, 25 ginkgolide, 26 sterols, 27).

A far more complex situation arises in higher plants that use both the pathways in parallel.53 With hindsight, it is even obvious that the belated discovery of the deoxyxylulose pathway can be traced to a significant extent to the very occurrence of both the pathways in plants. More specifically, due to metabolite exchange between the two pathways that is the subject of this chapter, it appears likely that labeled mevalonate can contribute at least some label to most if not all plant isoprenoids hence, it was easy to jump to the conclusion — fallacious as we now know — that all plant isoprenoids are invariably biosynthesized from mevalonate. 

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