Staphyloferrin B is a key siderophore secreted by Staphylococcus aureus to acquire ferric ions from a host during infection, and its biosynthetic pathway has been validated to develop efficient antibacterial agents. Herein, we report the crystal structure of AMP-bound SbnC from S. aureus (SaSbnC) as the first representative structure of type B synthetases in the biosynthesis of α-hydroxycarboxylate siderophores. While type B synthetases specifically use α-ketoglutarate (α-KG) as their carboxylic acid substrate, SaSbnC showed unique structural features in the substrate pocket compared with the type A and C synthetases. Screening of α-KG analogues suggested that the hydrogen-bonding interaction between the α-carbonyl group of α-KG and residue Lys552 is a key determinant for the substrate selectivity of type B synthetases. Interestingly, citrate, the product of the tricarboxylic acid cycle and the substrate of type A synthetases, was found to inhibit the activity of SaSbnC with an IC₅₀ value of 83 μM by mimicking α-KG binding, suggesting a potential regulatory role of the tricarboxylic acid cycle, whose activity is under the control of the intracellular iron concentration, to SaSbnC and other type B synthetases. These results provide critical new information to understand the structure, function, and regulation of type B synthetases in the siderophore-based iron acquisition system employed by a large number of pathogenic microbes.

中文翻译：

---

作为代表性的B型铁载体合成酶的SbnC的结构和生化特性。

金黄色葡萄球菌B是金黄色葡萄球菌分泌的关键铁载体，在感染过程中会从宿主获取铁离子，其生物合成途径已被证实可开发有效的抗菌剂。在本文中，我们报道了金黄色葡萄球菌（Sa SbnC）与AMP结合的SbnC的晶体结构，是生物合成α-羟基羧酸铁载体中B型合成的第一个代表性结构。而B型合成酶专门使用α酮戊二酸（α-KG）作为它们的羧酸基，萨与A型和C型合成酶相比，SbnC在底物袋中显示出独特的结构特征。α-KG类似物的筛选表明，α-KG的α-羰基与残基Lys552之间的氢键相互作用是决定B型合成酶底物选择性的关键因素。有趣的是，发现柠檬酸是三羧酸循环与A型合成酶底物的产物，它通过模仿α-KG结合来抑制Sa SbnC的活性，IC ₅₀值为83μM ，暗示了其潜在的调控作用。三羧酸循环，其活性是细胞内的铁浓度的控制下，以萨SbnC和其他B型合成酶。这些结果提供了重要的新信息，以了解由大量病原微生物采用的基于铁载体的铁捕获系统中B型合成酶的结构，功能和调控。