Hevea brasiliensis biosynthesis

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. 

Poly(cii-l,4-isoprene) is the polymer that gives natural rubber a very competitive share of the elastomer market. In Hevea brasiliensis latex, this polymer is stored in so-called rubber particles , which make up 20 to 45% of the latex volume. Much work has been undertaken to understand the biosynthesis pathway of this very-long-chain polymer, which stands out through having almost 100% cis double bonds. 

The initiation step consists in the formation of IPP (12), and its isomer DMAPP (13). The conventional metabolic pathway to form these two molecules is called the acetate/mevalonate pathway (MVA pathway) in which three molecules of acetyl-CoA (3) condense successively to form 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) (5), which leads to a key intermediate molecule, namely mevalonic acid (MVA) (6). The latter is further phosphorylated and decarboxylated to form the IPP molecule (12). In Hevea brasiliensis, this cytosolic pathway was described by Lynen and Lebras" " in the early 1960s and reviewed more recently by Kekwick and Ohya." Most experimental validations were obtained by observing the incorporation of radioactive tracers, such as [2- C] MVA and [3- C]HMG-CoA. The incorporation of [ ClIPP into rubber was found to be much faster than that of [2- C] MVA. This was assumed to be due to slow conversion of MVA into IPP." Another explanation might be that the MVA pathway was not exclusive for IPP biosynthesis. Indeed less than 10 years ago, a new, mevalonate-independent, IPP biosynthesis pathway was discovered by Rohmer." This plastidic DXP-MEP pathway initiates with a... 

Chromenes, Benzofurans, Benzopyrans, and Precocenes Polyterpenes Biosynthesis Biological Activity Natural Rubber Hevea brasiliensis Guayule Gutta Percha Chicle... 

The biosynthesis of rubber may be divided into three steps (1) initiation, which requires an allylic diphosphate molecule, (2) elongation, in which IPP units are added to a Z-l,4-polyisoprene chain, and (3) termination, in which the polymer is released from the rubber transferase enzyme (Cornish, 1993). In plants, the elongation of Z-l,4-polyiso-prene (natural rubber) requires a small -allylic diphosphate initiator (less than or equal to C20). Famesyl pyrophosphate (FPP) is an effective initiator of polyisoprene biosynthesis (Light et al, 1989) further, because only one molecule of FPP is needed for each molecule of rubber formed, small traces of this substance that are inadvertently present complicate biosynthetic studies. The E-allylic diphosphates are hydrophilic cytosolic compounds, whereas Z-l,4-polyisoprene is hydrophobic and compartmentalized in subcellular rubber particles. A soluble E-prenyl transferase from the latex of Hevea brasiliensis serves as a famesyl diphosphate synthase and plays no direct role in elongation of Z-l,4-polyisoprene (Cornish, 1993). Because the hydro-phobic rubber molecule is produced inside a rubber particle but is formed from hydrophilic precursors from the cytoplasm, the polymerization reaction must take place at the particle surface. 

Biosynthesis. The initiating species in C. biosynthesis in Hevea brasiliensis is a derivative of trans-tram-famesyl diphosphate (i.e. a C15 allylic diphosphate), which is presumably modified at the methyl carbon of the dimethylallyl group. [Y. Tanaka etal. Initiation of Rubber Biosjmthesis in Hevea brasiliensis Characterization of Initiating Species by Structural Analysis Phytochemistry 41 (19 ) 1501-1505]... 

PURIFICATION AND CHARACTERISATION OF AN INHIBITOR OF RUBBER BIOSYNTHESIS FROM C-SERUM OF HEVEA BRASILIENSIS LATEX... 

A proteinaceous inhibitor of rubber biosynthesis was purified from the C-serum of Hevea brasiliensis latex. The protein inhibited the incorporation of isopentenyl diphosphate into rubber. Purification was achieved by employing three column chromatography methods Sephadex G-150 gel filtration, DEAE-Cellulose ion exchange chromatography and Phenyl Sepharose CL-4B hydrophobic interaction chromatography. 21 refs. 

Chemically the NR produced by the Hevea brasiliensis species is almost pure cw-l,4-polyisoprene. So far none of the manufacturers of synthetic cis-, A-polyisoprene is able to achieve more than 95% of the cis isomer in their commercial products. Jitladda and coworkers have conducted extensive studies on the molecular structures of the NR molecules found in Hevea and correlated these to the biosynthesis of this in the trees. This is intrinsically linked to the end groups of the rubber molecules involving the phospholipids and proteins that are linked to the charging mechanism at the latex particle surface and hence has a direct impact on the stability of the latex. 

Tanaka, Y., Aik-Hwee, E., Ohya, N., Nishiyama, N., Tangpakdee, J., Kawahara, S., Wititsuwan-nakul, R., Initiation of rubber biosynthesis in Hevea brasiliensis characterization of initiating species by structural analysis. Phytochemistry 1996, 41 (6), 1501-1505. 

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