Department of Molecular Biology and Genetics, Aarhus University, Aarhus C, Denmark.
Department of Biotechnology and Biomedicine, Technical University of Denmark, Kgs. Lyngby, Denmark.
J Biol Chem. 2022 Jul;298(7):102142. doi: 10.1016/j.jbc.2022.102142. Epub 2022 Jun 15.
The bacterial stringent response involves wide-ranging metabolic reprogramming aimed at increasing long-term survivability during stress conditions. One of the hallmarks of the stringent response is the production of a set of modified nucleotides, known as alarmones, which affect a multitude of cellular pathways in diverse ways. Production and degradation of these molecules depend on the activity of enzymes from the RelA/SpoT homologous family, which come in both bifunctional (containing domains to both synthesize and hydrolyze alarmones) and monofunctional (consisting of only synthetase or hydrolase domain) variants, of which the structure, activity, and regulation of the bifunctional RelA/SpoT homologs have been studied most intensely. Despite playing an important role in guanosine nucleotide homeostasis in particular, mechanisms of regulation of the small alarmone hydrolases (SAHs) are still rather unclear. Here, we present crystal structures of SAH enzymes from Corynebacterium glutamicum (RelH) and Leptospira levettii (RelH) and show that while being highly similar, structural differences in substrate access and dimer conformations might be important for regulating their activity. We propose that a varied dimer form is a general property of the SAH family, based on current structural information as well as prediction models for this class of enzymes. Finally, subtle structural variations between monofunctional and bifunctional enzymes point to how these different classes of enzymes are regulated.
细菌严谨反应涉及广泛的代谢重编程，旨在提高应激条件下的长期生存能力。严谨反应的标志之一是产生一组称为“警报素”的修饰核苷酸，这些核苷酸以多种方式影响多种细胞途径。这些分子的产生和降解取决于 RelA/SpoT 同源家族酶的活性，该家族既有双功能（包含合成和水解警报素的结构域），也有单功能（仅由合成酶或水解酶结构域组成）变体，其中双功能 RelA/SpoT 同源物的结构、活性和调节已得到最深入的研究。尽管在鸟苷核苷酸动态平衡中发挥了重要作用，但小警报素水解酶（SAH）的调节机制仍相当不清楚。在这里，我们展示了来自谷氨酸棒杆菌（RelH）和钩端螺旋体（RelH）的 SAH 酶的晶体结构，并表明尽管高度相似，但在底物进入和二聚体构象方面的结构差异可能对调节它们的活性很重要。我们提出，基于当前的结构信息和该酶类的预测模型，多样化的二聚体形式是 SAH 家族的普遍特性。最后，单功能和双功能酶之间的细微结构差异表明了这两类酶是如何被调节的。
