Mevalonate plant carotenoid precursor

It is generally agreed that mevalonate is the precursor to sterols in higher plants as well as in animals and is also the precursor to plant carotenoids. However, it is poorly incorporated into monoterpenes and into some diterpenes such as those of the taxane group.26 27 The alternative glyceraldehyde 3-... 

The true carotenoids and xanthophylls have 40 carbon atoms this corresponds to 8 isoprene residues. Like squalene, they are constructed symmetrically and it is assumed that they arise by head-to-head condensation of two C20 precursors. It has been found that mevalonic acid is one important precursor the appropriate enzyme systems from plants can convert mevalonate to carotenoids. The tetra-... 

In addition to steroids, the carotenoids and the various terpenoids are formed by analogous routes in plants and microorganisms. The side chains of the lipoid quinones (vitamins E and K, and ubiquinone) also arise in this fashion in some mammalian organisms. Furthermore, many microorganisms can synthesize from mevalonate or similar precursors the C skeleton of the branched-chain amino acids (leucine, valine, isoleucine), which are essential for man. 

The incorporation of label from mevalonate into ABA, a sesquiterpenoid, has been demonstrated in different parts of plants ( . . 41). This indicates that ABA can be synthesized throughout the plant. In addition to the direct incorporation of three isoprene units, derived from mevalonate, into ABA, an indirect biosynthetic pathway via carotenoids has been proposed. This idea stems from the finding that xanthophylls, in particular violaxanthin, can either photochemically or enzymatically be converted to the neutral inhibitor xanthoxin (42) (Figure 3). When labeled xanthoxin was fed in the transpiration stream to bean or tomato shoots, ca. 10% was converted to ABA over an 8-hr period (43). However, the importance of the biosynthetic route to ABA via xanthophylls and xanthoxin in normal metabolism remains to be established, and most of the evidence favors the direct synthesis route via a precursor (see 2). 

The importance of terpenoids to life is highlighted by the fact that two separate pathways have been found to produce the terpenoid precursor C5 units isopentenyl diphosphate (IDP) and dimethylaUyl diphosphate (DMADP). The mevalonic acid (MVA) pathway is functional in archae, animals and fungi, 2-C-methyl-D-erythritol-4-phosphate (MEP) pathway is found in green algae, and terpenoids are produced by both pathways in bacteria and plants [2]. In plants the MVA pathway is active in the cytosol and it provides C5 units for sesquiterpene, triterpene and polyterpene biosynthesis whereas the MEP pathway occurs in plastids and produces C5 units for isoprene, monoterpenes, diterpenes and carotenoids [1]. Recent reports have indicated metabolic crosstalk between biosynthesis pathways and e.g., the homoterpene DMNT may originate from both pathways. 

Terpenoid biosynthesis is now well established in plants, which the major precursor isopentenyl diphosphate (IPP) can be synthesized in the plastids through 1-deoxy-D-xylulose-5-phosphate (DXP) pathway, generating monoterpenes, diterpenes, and carotenoids, and in the cytosol through the mevalonic acid (MVA) pathway, forming sesquiterpenes, triterpenes, and steroids [9, 13]. 

The NR molecule is widely accepted to be synthesized by the successive addition of IPP to dimethylallyl diphosphate (DMAPP) to build up isoprene units in the cis configuration. Initial biochemical studies presumed that IPP, a precursor of rubber molecules from higher plants, is synthesized via the mevalonate (MVA) pathway,which is operated in the cytoplasm from acetyl-Co A. Later, a non-mevalonate pathway called the methylerythritol phosphate (MEP) pathway has also been considered as responsible for IPP synthesis in the plastids from the condensation of pyruvate and glyceraldehyde-3-phos-phate. Mevalonate and l-deoxy-o-xylulose-5-phosphate (DXP) are intermediates in MVA and MEP pathways, respectively. DMAPP is produced by the isomerization of IPP, catalysed by IPP isomerase, from both pathways. The gene expression of DXP synthase in Hevea latex suggested that the MEP pathway for IPP synthesis is involved in rubber biosynthesis.However, an experiment that involved feeding 600 RRIM seedlings with [l- C] 1-deoxy-o-xylulose triacetate, an intermediate derivative of the MEP pathway, indicated that the MEP pathway is involved only in carotenoid biosynthesis but not in the case of rubber biosynthesis. ... 

Carotenoids are synthesized from the basic C5 terpenoid precursor isopentenyl pyrophosphate (IPP) and dimethylallyl diphosphate (DMAPP). These precursors can be obtained from two distinct pathways the mevalonate pathway (MVA) and the non-MVA pathway also known as 2-C-methyl-D-erythritol 4-phosphate (MEP) pathway [84, 85]. All eukaryotes use the MVA pathway, whereas plant plastids and most bacteria use the MEP pathway [86,87]. Some bacteria also use the MVA pathway [84]. In the MEP pathway, the first step in IPP biosynthesis is the formation of l-deoxy-D-xylulose-5-phosphate (DXP) from pyruvate and glyceraldehyde-3-phosphate catalyzed by DXP synthase (Figure 10.7). DXP is then reduced to MEP by DXP reductase. Additional MEP pathway enzymes are then used in subsequent reactions for converting MEP to IPP, which is isomer-ized to DMAPP by the enzyme IPP isomerase. The MVA pathway begins with the conversion of three molecules of acetyl-CoA to MVA through acetoacetyl-CoA... 

---