Cornish K
USDA-ARS, Western Regional Research Center, 800 Buchanan Street, Albany, CA 94710, USA.
Phytochemistry. 2001 Aug;57(7):1123-34. doi: 10.1016/s0031-9422(01)00097-8.
This report reviews aspects of the biochemical regulation of rubber yield and rubber quality in three contrasting rubber-producing species, Hevea brasiliensis, Parthenium argentatum and Ficus elastica. Although many similarities are revealed, considerable differences also exist in enzymatic mechanisms regulating biosynthetic rate and the molecular weight of the rubber biopolymers produced. In all three species, rubber molecule initiation, biosynthetic rate and molecular weight, in vitro, are dependent upon substrate concentration 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 transferases as well. All three rubber transferases are capable of producing a wide 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, considerable differences exist between the species with respect to cosubstrate effects, binding constants, effective concentration ranges, and the role of negative 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 genes for transformation of annual crop species. The kinetic data, from the three contrasting rubber-producing species, also were used to develop a model of the rubber transferase active site in which, in addition to separate IPP and allylic-PP binding sites, there exists a hydrophobic region that interacts with the linear portion of allylic-PP initiator proximal to the pyrophosphate. 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 supported by measurements of the rubber particle monolayer membrane using electron paramagnetic resonance spectroscopy.
本报告回顾了三种不同产胶物种，即巴西橡胶树、银胶菊和印度榕中橡胶产量和橡胶质量的生化调控方面。尽管揭示出许多相似之处，但在调节生物合成速率和所产生的橡胶生物聚合物分子量的酶促机制方面也存在相当大的差异。在所有这三个物种中，橡胶分子的起始、生物合成速率和分子量在体外均取决于底物浓度以及异戊烯基焦磷酸（IPP，延伸底物或单体）与法呢基焦磷酸（FPP，一种引发剂）的比例，但这些参数也受橡胶转移酶的内在特性影响。所有这三种橡胶转移酶都能够产生广泛范围的橡胶分子量，这取决于底物浓度，清楚地表明转移酶并非体内产物大小的主要决定因素。然而，尽管存在这些共性，但在共底物效应、结合常数、有效浓度范围以及体外负协同作用的作用方面，不同物种之间存在相当大的差异。银胶菊橡胶转移酶似乎比其他两个物种对其产生的分子量具有更强的控制作用，因此可能为一年生作物物种转化的基因来源提供最佳前景。来自这三种不同产胶物种的动力学数据还被用于构建橡胶转移酶活性位点模型，其中除了单独的IPP和烯丙基-PP结合位点外，还存在一个疏水区域，该区域与靠近焦磷酸的烯丙基-PP引发剂的线性部分相互作用。底物亲和力不断增加，直到活性位点被穿过并到达橡胶颗粒的橡胶内部。动力学数据表明，巴西橡胶树和印度榕中的疏水区域约为1.8纳米长，而银胶菊中仅为1.3纳米。这些估计值得到了使用电子顺磁共振光谱对橡胶颗粒单层膜进行测量的支持。
