Abstract：
Salicylic acid (SA) plays a crucial role in plant defense against biotrophic and semi-biotrophic pathogens. In Arabidopsis (Arabidopsis thaliana), isochorismate synthase 1 (AtICS1) is a key enzyme for the pathogen-induced biosynthesis of SA via catalytic conversion of chorismate into isochorismate, an essential precursor for SA synthesis. Despite the extensive knowledge of ICS1-related menaquinone, siderophore, tryptophan (MST) enzymes in bacteria, the structural mechanisms for substrate binding and catalysis in plant isochorismate synthase (ICS) enzymes are unknown. This study reveals that plant ICS enzymes catalyze the isomerization of chorismate through a magnesium-dependent mechanism, with AtICS1 exhibiting the most substantial catalytic activity. Additionally, we present high-resolution crystal structures of apo AtICS1 and its complex with chorismate, offering detailed insights into the mechanisms of substrate recognition and catalysis. Importantly, our investigation indicates the existence of a potential substrate entrance channel and a gating mechanism regulating substrate into the catalytic site. Structural comparisons of AtICS1 with MST enzymes suggest a shared structural framework with conserved gating and catalytic mechanisms. This work provides valuable insights into the structural and regulatory mechanisms governing substrate delivery and catalysis in AtICS1, as well as other plant ICS enzymes.
摘要：
水杨酸（SA）在植物防御生物营养和半生物营养病原体中起着至关重要的作用。在拟南芥（拟南芥）中，异氯酸盐合成酶1（AtICS1）是通过将氯酸盐催化转化为异氯酸盐来诱导病原体诱导的SA生物合成的关键酶，SA合成的重要前体。尽管对ICS1相关的甲基萘醌有广泛的了解，铁载体,细菌中的色氨酸（MST）酶，植物异氯酸合酶（ICS）酶中底物结合和催化的结构机制尚不清楚。这项研究表明，植物ICS酶通过镁依赖性机制催化分支酸盐的异构化，AtICS1表现出最显著的催化活性。此外,我们提出了apoAtICS1及其与分支酸盐配合物的高分辨率晶体结构，提供对底物识别和催化作用机制的详细见解。重要的是,我们的调查表明存在潜在的底物入口通道和调节底物进入催化位点的门控机制。AtICS1与MST酶的结构比较表明具有保守的门控和催化机理的共享结构框架。这项工作为AtICS1以及其他植物ICS酶中控制底物递送和催化的结构和调节机制提供了有价值的见解。
