Phosphatidylserine synthase (PssA) is essential in the biosynthesis of phosphatidylethanolamine, a major phospholipid of bacterial membranes. A peripheral membrane protein PssA can associate with the cellular membrane in its active state or exist in the cytosol in an inactive form. The membrane-bound enzyme acts on cytidine diphosphate diacylglycerol (CDP-DG) to form cytidine monophosphate and a covalent intermediate, which is subsequently targeted by serine to produce phosphatidylserine. Here, we present two crystal structures of Escherichia coli PssA, one complexed with CDP-DG and the other without. The lipid-bound structure mimics the Michaelis complex before the formation of a covalent intermediate, revealing key determinants for substrate recognition and catalysis. Notably, membrane-free PssA is in a monomer-dimer equilibrium, with only the monomer capable of associating with the membrane, suggesting a regulatory mechanism for phospholipid biosynthesis dependent on the oligomerization state of the enzyme.

中文翻译：

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大肠杆菌中磷脂酰丝氨酸合酶催化膜缔合的结构基础


磷脂酰丝氨酸合酶 （PssA） 在磷脂酰乙醇胺的生物合成中是必需的，磷脂酰乙醇胺是细菌膜的一种主要磷脂。外周膜蛋白 PssA 可以在其活性状态下与细胞膜结合，或以非活性形式存在于胞质溶胶中。膜结合酶作用于胞苷二磷酸甘油二酯 （CDP-DG） 形成胞苷一磷酸和共价中间体，随后被丝氨酸靶向产生磷脂酰丝氨酸。在这里，我们介绍了大肠杆菌 PssA 的两种晶体结构，一种与 CDP-DG 复合，另一种则没有。脂质结合结构模拟共价中间体形成之前的 Michaelis 复合物，揭示了底物识别和催化的关键决定因素。值得注意的是，无膜 PssA 处于单体-二聚体平衡状态，只有单体能够与膜结合，这表明磷脂生物合成的调节机制取决于酶的寡聚化状态。