Kroll Kourtney, Holland Cynthia K, Starks Courtney M, Jez Joseph M
Department of Biology, Washington University in St. Louis, St. Louis, MO 63130, U.S.A.
Sequoia Sciences, St. Louis, MO 63114, U.S.A.
Biochem J. 2017 Nov 1;474(22):3705-3717. doi: 10.1042/BCJ20170549.
Plants, fungi, and bacteria synthesize the aromatic amino acids: l-phenylalanine, l-tyrosine, and l-tryptophan. Chorismate mutase catalyzes the branch point reaction of phenylalanine and tyrosine biosynthesis to generate prephenate. In , there are two plastid-localized chorismate mutases that are allosterically regulated (AtCM1 and AtCM3) and one cytosolic isoform (AtCM2) that is unregulated. Previous analysis of plant chorismate mutases suggested that the enzymes from early plants (i.e. bryophytes/moss, lycophytes, and basal angiosperms) formed a clade distinct from the isoforms found in flowering plants; however, no biochemical information on these enzymes is available. To understand the evolution of allosteric regulation in plant chorismate mutases, we analyzed a basal lineage of plant enzymes homologous to AtCM1 based on sequence similarity. The chorismate mutases from the moss/bryophyte (PpCM1 and PpCM2), the lycophyte (SmCM), and the basal angiosperm (AmtCM1 and AmtCM2) were characterized biochemically. Tryptophan was a positive effector for each of the five enzymes examined. Histidine was a weak positive effector for PpCM1 and AmtCM1. Neither tyrosine nor phenylalanine altered the activity of SmCM; however, tyrosine was a negative regulator of the other four enzymes. Phenylalanine down-regulates both moss enzymes and AmtCM2. The 2.0 Å X-ray crystal structure of PpCM1 in complex with the tryptophan identified the allosteric effector site and reveals structural differences between the R- (more active) and T-state (less active) forms of plant chorismate mutases. Molecular insight into the basal plant chorismate mutases guides our understanding of the evolution of allosteric regulation in these enzymes.
植物、真菌和细菌能合成芳香族氨基酸：L-苯丙氨酸、L-酪氨酸和L-色氨酸。分支酸变位酶催化苯丙氨酸和酪氨酸生物合成的分支点反应以生成预苯酸。在植物中，有两种定位于质体且受别构调节的分支酸变位酶（AtCM1和AtCM3）以及一种不受调节的胞质同工型（AtCM2）。先前对植物分支酸变位酶的分析表明，早期植物（即苔藓植物/苔藓、石松类植物和基部被子植物）中的酶形成了一个与开花植物中发现的同工型不同的进化枝；然而，关于这些酶的生化信息尚无可用资料。为了解植物分支酸变位酶中别构调节的进化，我们基于序列相似性分析了与AtCM1同源的植物酶的一个基部谱系。对来自苔藓植物（PpCM1和PpCM2）、石松类植物（SmCM）和基部被子植物（AmtCM1和AmtCM2）的分支酸变位酶进行了生化特性分析。色氨酸是所检测的五种酶中每一种的正效应物。组氨酸是PpCM1和AmtCM1的弱正效应物。酪氨酸和苯丙氨酸均未改变SmCM的活性；然而，酪氨酸是其他四种酶的负调节因子。苯丙氨酸下调苔藓植物的两种酶以及AmtCM2。PpCM1与色氨酸复合物的2.0 Å X射线晶体结构确定了别构效应物位点，并揭示了植物分支酸变位酶的R态（活性更高）和T态（活性更低）形式之间的结构差异。对基部植物分支酸变位酶的分子洞察有助于我们理解这些酶中别构调节的进化。
