O-Acetylation of the capsular polysaccharide (CPS) of Neisseria meningitidis serogroup A (NmA) is critical for the induction of functional immune responses, making this modification mandatory for CPS-based anti-NmA vaccines. Using comprehensive NMR studies, we demonstrate that O-acetylation stabilizes the labile anomeric phosphodiester-linkages of the NmA-CPS and occurs in position C3 and C4 of the N-acetylmannosamine units due to enzymatic transfer and non-enzymatic ester migration, respectively. To shed light on the enzymatic transfer mechanism, we solved the crystal structure of the capsule O-acetyltransferase CsaC in its apo and acceptor-bound form and of the CsaC-H228A mutant as trapped acetyl-enzyme adduct in complex with CoA. Together with the results of a comprehensive mutagenesis study, the reported structures explain the strict regioselectivity of CsaC and provide insight into the catalytic mechanism, which relies on an unexpected Gln-extension of a classical Ser-His-Asp triad, embedded in an α/β-hydrolase fold.

脑膜炎奈瑟菌A 血清群(NmA) 荚膜多糖 (CPS) 的O-乙酰化对于诱导功能性免疫反应至关重要，因此基于 CPS 的抗 NmA 疫苗必须进行这种修饰。通过全面的 NMR 研究，我们证明O-乙酰化稳定了 NmA-CPS 不稳定的异头磷酸二酯键，并且由于酶促转移和非酶促酯迁移而分别发生在N-乙酰甘露糖胺单元的 C3 和 C4 位。为了阐明酶促转移机制，我们解析了APO 和受体结合形式的胶囊O -乙酰转移酶 CsaC 的晶体结构，以及作为与 CoA 复合物捕获的乙酰酶加合物的 CsaC-H228A 突变体的晶体结构。结合全面诱变研究的结果，报告的结构解释了 CsaC 的严格区域选择性，并提供了对催化机制的深入了解，催化机制依赖于嵌入 α/ 的经典 Ser-His-Asp 三联体的意外 Gln 延伸β-水解酶折叠。