Rubber transferase enzyme

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. 

The building block (monomer) of NR is also IPP (Figure 4.6). At the physiological pH = 7.4 IPP is a stable dianion with potassium as counterion. The bios5mthesis of NR is catalyzed by the rubber transferase enzyme, cis-prenyl transferase. 

This enzyme [EC 2.5.1.20], also known as rubber allyl-transferase and rubber transferase, catalyzes the reaction of poly-dx-polyprenyl diphosphate (or, rubber particles) with isopentenyl diphosphate to produce a poly-cK-... 

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]. 

The exact structure of the rubber tramferase, a cis-prenyl transferase enzyme, remains imidentified because it is bound to the outer membrane of the latex particle, stabilized by a phospholipid monolayer (Figure 4.23). The stmcture of the latex is similar to milk native Meso-... 

In plants, cellulose is synthesized by rosette terminal complexes (RTCs). The RTCs are hexameric protein structures containing the cellulose synthase enzyme. This enzyme synthesizes the individual cellulose chains. Each RTC is located at the interface of the cell membrane, similarly to the membrane-bound rubber transferase. Each RCT will polymerize a polysaccharide chain. This sounds a little bit complicated, but we are going to break down the synthesis into the... 

The Z-1,4-prenyl transferase is associated with rubber particles in Hevea brasiliensis and other rubber-producing plants. This enzyme is membrane bound (Cornish, 1993). This enzyme requires Mg or Mn ", but will not synthesize mbber unless a second component of the system, an allylic diphosphate, is present (Fig. 18.7). 

NR is the major component of latex, the main product of the rubber tree. Even if Hevea brasiliensis is widely recognized as the major eommercial produeer, latex can be produced by different plants, in which up to 12,000 species were found to be latex producers. Nevertheless, important proteins or protein complexes occur in all species. Indeed, latex is synthesized from hydrophilic substrates by specific enzymes, mainly cis-prenyl transferases, which remain closely linked to the rubber particle interior. Moreover, as NR latex is not a homogeneous fluid, the dispersion of proteins is not homogeneous either. Thus the purification of the NR remains low, and the final product often contains up to 6% of non-rubber components the overall protein content can reach 2% of latex fresh weight. ... 

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