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.

中文翻译：

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拟南芥异分支酸合成酶 1 分支酸异构化的结构基础

水杨酸（SA）在植物防御生物营养型和半生物营养型病原体方面发挥着至关重要的作用。在拟南芥 (Arabidopsis thaliana) 中，异分支酸合酶 1 (AtICS1) 是病原体诱导 SA 生物合成的关键酶，通过催化将分支酸转化为异分支酸，异分支酸是 SA 合成的重要前体。尽管人们对细菌中与 ICS1 相关的甲基萘醌、铁载体、色氨酸 (MST) 酶有了广泛的了解，但植物异分支酸合酶 (ICS) 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的异构化，其中 AtICS1 表现出最显着的催化活性。此外，我们还展示了 apo AtICS1 及其与分支酸的复合物的高分辨率晶体结构，为底物识别和催化机制提供了详细的见解。重要的是，我们的研究表明存在潜在的底物进入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，它们具有具有保守的门控和催化机制的共享结构框架。这项工作为 AtICS1 以及其他植物 ICS 酶中控制底物传递和催化的结构和调节机制提供了宝贵的见解。