Sulfolipid and Betaine Lipid Head Group Biosynthesis

非磷极性脂质在光合生物的膜中非常丰富。在植物和细菌中，磷酸盐限制增加了非磷脂质与磷脂的比例，并且已经提出了非磷脂质对磷脂的功能性替代。实例是磺基喹诺糖基二酰基甘油（SQDG）取代磷脂酰甘油和甜菜碱脂质二酰基甘油-N，N，N-三甲基高丝氨酸（DGTS）取代磷脂酰胆碱。硫脂存在于所有植物和大多数光合细菌中，而甜菜碱脂质在非种子植物、细菌和真菌中含量丰富。SQDG头基是葡萄糖的磺酸衍生物（磺基奎宁糖）。拟南芥的SQD 1蛋白是参与由UDP-葡萄糖和亚硫酸盐形成硫脂头基的关键酶。蛋白质的晶体结构已经解析，并且可以进行体外测定。这些都是在这个项目中追求的SQD 1反应机制的详细分析的先决条件。此外，SQD 1与其他蛋白质的相互作用及其在体内的真实活性正在研究中。在紫色细菌Rhodobacter sphaeroides中分离到两个甜菜碱脂质生物合成所必需的基因。基于初步的分析，一个基因，btaA，提出了编码S-腺苷甲硫氨酸：二酰基甘油3-氨基-3-羧基丙基转移酶催化甜菜碱脂质生物合成的第一个反应。第二种酶可能编码S-腺苷甲硫氨酸：二酰基甘油高丝氨酸N-甲基转移酶，将二酰基甘油高丝氨酸转化为DGTS。实验正在进行中，以验证这一假设，并更详细地了解这两种酶的反应机制。 磺酸的化学合成是众所周知的，产生了人类在日常生活中遇到的许多化合物。植物的硫脂是天然存在的磺酸的突出实例。糖代谢和硫代谢在硫脂的生物合成中相遇。了解硫脂头基的生物合成将为自然界中磺酸盐的从头形成提供一个模型。本项目的重点是在试管中和植物叶绿体内详细分析拟南芥SQD 1蛋白的功能。该蛋白负责将磺酸引入硫脂生物合成的前体中。甜菜碱脂质生物合成由两种有趣的蛋白质介导。第一种方法不寻常地使用了一种常见的代谢物S-腺苷甲硫氨酸。类似的反应发生在抗生素诺卡地星的生物合成中。阐明这种酶的基本反应机理是目标之一，并可能为有用化合物的合成提供线索。第二种酶使脂质底物的氨基甲基化。计划了解这种酶的机制的实验将为脂质连接的N-甲基化的一般机制提供基本的见解。

Non-phosphorous polar lipids are very abundant in membranes of photosynthetic organisms. In plants and bacteria, phosphate limitation increases the ratio of non-phosphorous lipids to phospholipids and a functional substitution of phospholipids by non-phosphorous lipids has been proposed. Examples are the sulfolipid sulfoquinovosyldiacylglycerol (SQDG) substituting for phosphatidylglycerol and the betaine lipid diacylglycerol-N,N,N-trimethylhomoserine (DGTS) replacing phosphatidylcholine. The sulfolipid is present in all plants and most photosynthetic bacteria while betaine lipids are abundant in non-seed plants, bacteria and fungi. The SQDG head group is a sulfonic acid derivative of glucose (sulfoquinovose). The SQD1 protein of Arabidopsis is the key enzyme involved in the formation of the sulfolipid head group from UDP-glucose and sulfite. The crystal structure for the protein has been solved and an in vitro assay is available. These are the prerequisites for a detailed analysis of the SQD1 reaction mechanism pursued in this project. Furthermore, the interaction of SQD1 with other proteins and its true activity in vivo are under investigation. Two genes essential for betaine lipid biosynthesis in the purple bacterium Rhodobacter sphaeroides have been isolated. Based on preliminary analysis, one gene, btaA, is proposed to encode an S-adenosylmethionine:diacylglycerol 3-amino-3-carboxypropyl transferase catalyzing the first reaction of betaine lipid biosynthesis. The second enzyme presumably encodes an S-adenosylmethionine:diacylgycerolhomoserine N-methyltransferase converting diacylglycerol-homoserine to DGTS. Experiments are underway to test this hypothesis and to understand the reaction mechanism of these two enzymes in greater detail. The chemical synthesis of sulfonic acids is well known, giving rise to numerous compounds encountered by humans during daily life. The sulfolipid of plants is a prominent example of a naturally occurring sulfonic acid. Carbohydrate and sulfur metabolism meet in the biosynthesis of sulfolipid. Understanding the biosynthesis of the sulfolipid head group will provide a model for the de novo formation of sulfonates in nature. This project focuses on the detailed analysis of the function of the Arabidopsis SQD1 protein in the test tube and inside the plant chloroplast. This protein is responsible for the introduction of the sulfonic acid into a precursor of sulfolipid biosynthesis. Betaine lipid biosynthesis is mediated by two interesting proteins. The first makes an unusual use of a common metabolite, S-adenosylmethionine. A similar reaction occurs in the biosynthesis of the antibiotic nocardicin. Elucidating the basic reaction mechanism of this enzyme is one of the goals and may provide clues for the synthesis of useful compounds. The second enzyme methylates the amino group of a lipid substrate. Experiments planned to understand the mechanism of this enzyme will provide fundamental insights into the general mechanism of lipid-linked N-methylation.