Similarities and differences in rubber biochemistry among plant species

This report reviews aspects of the biochemical regulation of rubber yield a nd rubber quality in three contrasting rubber-producing species, Hevea bras iliensis, Parthenium argentatum and Ficus elastica. Although many similarit ies are revealed, considerable differences also exist in enzymatic mechanis ms regulating biosynthetic rate and the molecular weight of the rubber biop olymers produced. In all three species. rubber molecule initiation, biosynt hetic rate and molecular weight, in vitro; are dependent upon substrate con centration and the ratio of isopentenyl pyrophosphate (IPP, the elongation substrate. or monomer) and farnesyl pyrophosphate (FPP, an initiator), but these parameters are affected by intrinsic properties of the rubber transfe rases as well. All three rubber transferases are capable of producing a wid e range of rubber molecular weight, depending upon substrate concentration, clearly demonstrating that the transferases are not the prime determinants of product size in vivo. However. despite these commonalities, considerabl e differences exist between the species with respect to cosubstrate effects , binding constants, effective concentration ranges, and the role of negati ve cooperativity in vitro. The P. argentatum rubber transferase appears to exert more control over the molecular weight it produces than the other two species and may, therefore, provide the best prospect for the source of ge nes for transformation of annual crop species. The kinetic data, from the t hree contrasting rubber-producing species, also were used to develop a mode l of the rubber transferase active site in which, in addition to separate I PP and allylic-PP binding sites, there exists a hydrophobic region that int eracts with the linear portion of allylic-PP initiator proximal to the pyro phosphate. Substrate affinity increases until the active site is traversed and the rubber interior of the rubber particle is reached. The kinetic data suggest that the hydrophobic region in H. brasiliensis and F. elastica is about 1.8 nm long but only 1.3 nm in P. argentatum. The estimates are suppo rted by measurements of the rubber particle monolayer membrane using electr on paramagnetic resonance spectroscopy. Published by Elsevier Science Ltd.