Phospholipids are the major components of the membrane in all type of cells and organelles. They also are critical for cell metabolism, signal transduction, the immune system and other critical cell functions. The biosynthesis of phospholipids is a complex multi-step process with high-energy intermediates. Several enzymes in different metabolic pathways are involved in the initial phospholipid synthesis and its subsequent conversion. While the “Kennedy pathway” is the main pathway in mammalian cells, in bacteria and lower eukaryotes the precursor CDP-DAG is used in the de novo pathway by CDP-DAG alcohol O-phosphatidyl transferases to synthetize the basic lipids. Here we present the high-resolution structures of phosphatidyl serine synthase from Methanocaldococcus jannaschii crystallized in four different states. Detailed structural and functional analysis of the different structures allowed us to identify the substrate binding site and show how CDP-DAG, serine and two essential metal ions are bound and oriented relative to each other. In close proximity to the substrate binding site, two anions were identified that appear to be highly important for the reaction. The structural findings were confirmed by functional activity assays and suggest a model for the catalytic mechanism of CDP-DAG alcohol O-phosphatidyl transferases, which synthetize the phospholipids essential for the cells.

磷脂是所有类型细胞和细胞器中膜的主要成分。它们对于细胞代谢、信号转导、免疫系统和其他关键细胞功能也至关重要。磷脂的生物合成是一个复杂的多步骤过程，需要高能中间体。不同代谢途径中的几种酶参与最初的磷脂合成及其随后的转化。虽然“肯尼迪途径”是哺乳动物细胞中的主要途径，但在细菌和低等真核生物中，CDP-DAG 醇 O-磷脂酰转移酶在从头途径中使用前体 CDP-DAG 来合成基本脂质。在这里，我们展示了来自詹氏甲烷球菌的磷脂酰丝氨酸合酶的高分辨率结构以四种不同的状态结晶。对不同结构的详细结构和功能分析使我们能够识别底物结合位点，并展示 CDP-DAG、丝氨酸和两种基本金属离子如何相互结合和定向。在靠近底物结合位点的地方，鉴定出两个似乎对反应非常重要的阴离子。结构发现通过功能活性测定得到证实，并提出了 CDP-DAG 醇 O-磷脂酰转移酶催化机制的模型，该酶合成细胞必需的磷脂。