Max Planck Institute for Mathematics in the Sciences, Inselstraße 22, 04103 Leipzig, Germany.
Department of Biology, University of Victoria, PO Box 1700 STN CSC, Victoria, BC, V8W 2Y2, Canada.
J Theor Biol. 2019 Feb 7;462:158-170. doi: 10.1016/j.jtbi.2018.11.005. Epub 2018 Nov 6.
In higher plants, the amino acid phenylalanine is a substrate of both primary and secondary metabolic pathways. The primary pathway that consumes phenylalanine, protein biosynthesis, is essential for the viability of all cells. Meanwhile, the secondary pathways are not necessary for the survival of individual cells, but benefit of the plant as a whole. Here we focus on the monolignol pathway, a secondary metabolic pathway in the cytosol that rapidly consumes phenylalanine to produce the precursors of lignin during wood formation. In planta monolignol biosynthesis involves a series of seemingly redundant steps wherein shikimate, a precursor of phenylalanine synthesized in the plastid, is transiently ligated to the main substrate of the pathway. However, shikimate is not catalytically involved in the reactions of the monolignol pathway, and is only needed for pathway enzymes to recognize their main substrates. After some steps the shikimate moiety is removed unaltered, and the main substrate continues along the pathway. It has been suggested that this portion of the monolignol pathway fulfills a regulatory role in the following way. Low phenylalanine concentrations (viz. availability) correlate with low shikimate concentrations. When shikimate concentratios are low, flux into the monolignol pathway will be limited by means of the steps requiring shikimate. Thus, when the concentration of phenylalanine is low it will be reserved for protein biosynthesis. Here we employ a theoretical approach to test this hypothesis. Simplified versions of plant phenylalanine metabolism are modelled as systems of ordinary differential equations. Our analysis shows that the seemingly redundant steps can be sufficient for the prioritization of protein biosynthesis over the monolignol pathway when the availability of phenylalanine is low, depending on system parameters. Thus, the phenylalanine precursor shikimate may signal low phenylalanine availability to secondary pathways. Because our models have been abstracted from plant phenylalanine metabolism, this mechanism of metabolic signalling, which we call the Precursor Shutoff Valve (PSV), may also be present in other biochemical networks comprised of two pathways that share a common substrate.
在高等植物中，氨基酸苯丙氨酸是初级和次级代谢途径的底物。消耗苯丙氨酸的初级途径——蛋白质生物合成，对所有细胞的存活都是必不可少的。与此同时，次级途径对于单个细胞的存活并不是必需的，但对植物整体有利。在这里，我们专注于木质素单体途径，这是细胞质中的一种次级代谢途径，它在木材形成过程中迅速消耗苯丙氨酸产生木质素的前体。在植物体内，木质素单体生物合成涉及一系列看似冗余的步骤，其中莽草酸，一种在质体中合成的苯丙氨酸前体，暂时与途径的主要底物连接。然而，莽草酸在木质素单体途径的反应中没有催化作用，只需要途径酶来识别它们的主要底物。经过一些步骤后，莽草酸部分不变地被去除，主要底物继续沿着途径前进。有人提出，木质素单体途径的这一部分通过以下方式发挥调节作用。低苯丙氨酸浓度（即可用性）与低莽草酸浓度相关。当莽草酸浓度较低时，通过需要莽草酸的步骤限制木质素单体途径的通量。因此，当苯丙氨酸浓度较低时，它将被保留用于蛋白质生物合成。在这里，我们采用理论方法来检验这一假设。植物苯丙氨酸代谢的简化版本被建模为常微分方程系统。我们的分析表明，当苯丙氨酸的可用性较低时，根据系统参数，看似冗余的步骤可以足以优先考虑蛋白质生物合成而不是木质素单体途径。因此，苯丙氨酸前体莽草酸可能会向次级途径发出低苯丙氨酸可用性的信号。由于我们的模型是从植物苯丙氨酸代谢中抽象出来的，这种代谢信号的机制，我们称之为“前体截止阀”（PSV），也可能存在于其他由两个共享共同底物的途径组成的生化网络中。
