Rubber, biosynthesis

Fig. 2. A section of the isoprenoid pathway illustrating the branch to rubber biosynthesis. Rubber (the product) is synthesized in the cytosol from one molecule of allylic pyrophosphate (the initiator) and many molecules of isopentenyl pyrophosphate (the monomer). Isopentenyl pyrophosphate is produced by the cytosolic mevalonate pathway and by the plastidic methyl-erythritol pathway, as indicated by the shaded boxes. GPP, geranyl pyrophosphate FPP, farnesyl pyrophosphate GGPP, geranylgeranyl pyrophosphate.

The biosynthesis of synthetic natural rubber has been completely determined and appears in Fig. 15.1. Many plants and animals use this same biosynthetic pathway to make hundreds of terpenes and steroids from their common isoprenoid building blocks. 

Rubber is synthesized by plants via a side branch of the isoprenoid pathway by the enzyme rubber transferase (dy-prenyl transferase systematic name poly-dy-polyprenyl-diphosphate isopentenyl-diphosphate polyprenylcistransferase EC 2.5.1.20). Surprisingly, although this process has been studied for decades, due to the labile nature of the rubber transferase and the fact that it is a membrane-associated enzyme present in relatively low abundance, the identification of its protein subunits remain elusive. For some recent reviews on rubber biosynthesis, please refer to [248-251]. 

Rubber is synthesized and sequestered on cytsolic vesicles known as rubber particles. Rubber transferase is localized to the surface of the rubber particles, and biosynthesis is initiated through the binding of an allylic pyrophosphate (APP, a pyrophosphate, produced by soluble trans- rtnyl transferases) primer. Progressive additions of IPP molecules ultimately result in the formation of high molecular weight cjT-l,4-polyisoprene. The rubber transferase also requires a divalent cation, such as Mg + or Mn +, as cofactor. 

The identification and characterization of the genes and enzymes involved in rubber biosynthesis have been slow compared with those involved in the synthesis of other biopolymers. In fact, most of the studies thus far reported begin with rubber particles. 

Although many different APPs are effective initiators of rubber biosynthesis, only IPP can be used as the source of isopentenyl monomer for the c/x-1,4-polymerization of the rubber polymer. 

All rubber transferases exhibit similar kinetic constants and pH optima, and are able to accept a similar range of APPs as initiating substrate [263, 264], In vitro studies have shown that several compounds (DMAPP, GPP, FPP, and GGPP) can initiate rubber biosynthesis, with a faster rate of rubber biosynthesis the longer the APP (up to C15 or C20) [254, 265], Non-natural APPs were also shown to be able to function as a primer for the rubber biosynthesis [266],... 

The cDNAs of the cA-prenyltransferase of H. brasiliensis was successfully identified and expressed in E. coli. The in vitro polymerization of IPP after initiation with FPP using the expressed c/x-prenyltransferase resulted in low degrees of polymerization [267, 268]. After addition of rubber particles to this polymerization, the molecular weight increased tremendously [269], It can be concluded that the rubber particles are essential for rubber biosynthesis. Katarina Cornish established a detailed structural model of the in vivo synthesis of natural rubber in the rubber particle monolayer membrane and partially explained this behavior (see Fig. 12) [251],... 

In addition to its role as an intermediate in cholesterol biosynthesis, isopentenyl pyrophosphate is the activated precursor of a huge array of biomolecules with diverse biological roles (Fig. 21-48). They include vitamins A, E, and K plant pigments such as carotene and the phytol chain of chlorophyll natural rubber many essential oils (such as the fragrant principles of lemon oil, eucalyptus, and musk) insect juvenile hormone, which controls metamorphosis dolichols, which serve as lipid-soluble carriers in complex polysaccharide synthesis and ubiquinone and plastoquinone, electron carriers in mitochondria and chloroplasts. Collectively, these molecules are called isoprenoids. More than... 

Ji, W., Benedict, C.R. and Foster, M.A. (1993) Seasonal variations in rubber biosynthesis, 3-hydroxy-3-methylglutaryl-coenzyme A reductase and rubber transferase activities in Parthenium argentatum in the Chihuahuan Desert. Plant Physiol, 103, 535-42. 

The biogenesis of many cyclic terpenoids requires a cts-double bond in the acyclic precursor. A cis-double bond is frequently incorporated into the terminal isoprenoid unit via an allylic rearrangement mechanism. Alternatively, a cis unit could be incorporated directly (as rubber). Two results this year highlight some features of this problem. Both geraniol (22) and nerol (23) are derived from all-trans units (i.e. retention of the 4i -proton of mevalonate) so that the cis-double bond of nerol is derived by isomerization. The sesquiterpenoid gossypol (34) is derived from c/s,cis-farnesyl pyrophosphate. Hence in its biosynthesis either... 

Squalene and phytosterols are components present in the unsaponifiable lipid fraction of foods (as tocopherols). Squalene is an intermediary in cholesterol biosynthesis, and 33.9-58.4 mg/100 g of it was found in the lipid fraction of quinoa (Jahaniaval et al., 2000 Ryan et ah, 2007) squalene is the biochemical precursor of the whole family of steroids, and besides their effective antioxidant activity, tocotrienols have other important functions, in particular in maintaining a healthy cardiovascular system and a possible role in protection against cancer (Nesaretnam, 2008). Squalene is used as a bactericide and as an intermediate in many pharmaceuticals, organic coloring materials, rubber chemicals, and surface-active agents (Ahamed et ah, 1998). 

Endo, A., Hasumi, K., and Negishi, S. 1985b. Monacolins J and L, new inhibitors of cholesterol biosynthesis produced by Monascus rubber. J. Antibiot. (Tokyo) 38(3), 420 22. 

Archer B.L., Audley B.G., Cockbain E.G., McSweeney G.P. (1963) The biosynthesis of rubber, of mevalonate and isopentenyl pyrophosphate into rubber, by Hevea brasiliensis-lAex fractions. Biochem.J 89, 565—85. 

The biosynthesis of natural rubber has been studied from the viewpoint of an elucidation of initiation and propagation mechanisms mainly by tracer techniques. The steps in the formation of isopentenyl pyrophosphate from acetyl-coA via mevalonate are now well established in the in vitro synthesis of rubber. It has also been confirmed that chain extension occurs on the surface of existing rubber particles by successive additions of isopentenyl pyrophosphate to build up chains of 5000-7000 isoprene units (4,5). The initiation step of rubber formation, however, remains unknown due to the lack of detailed information concerning the direct precursor of the chain extension. 

By analogy with the mechanism of the biosynthesis of farnesyl pyrophosphate and all-trans terpenoids, it was deduced that rubber formation proceeds by the successive addition of isopentenyl pyrophosphate to dimethylallyl pyrophosphate (4). 

Polymers are classified as either natural that resulted from natural biosynthesis, or synthetic. The natural (polysaccharides, proteins, nucleic acids, natural rubbers, cellulose, lignin, etc.) have been used for tens of thousands of years. In Egypt the musical string instruments, papyrus for writing, and styrene [in a tree balsam] for embalming were used 3,000 BC. For millennia shellac has been used in Indian turnery [Chattopadhyaya, 1986]. The natural rubber was used by Olmecs at least 3000 years ago [Stuart, 1993]. 

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