BIOSYNTHESIS OF MEMBRANE GLYCOLIPIDS IN RHIZOBIUM

脂多糖(LPS)是革兰氏阴性菌外表面的一种重要糖脂，包括共生生物豆根瘤菌。在大肠杆菌中，内毒素的脂A锚是氨基葡萄糖的六酰化二糖，在1和4‘位带有磷酸部分。大肠杆菌生长所需的最低脂多糖由脂类A和两个额外的糖组成。新出现的基因组序列表明，在大肠杆菌中制造类脂A的酶存在于大多数其他革兰氏阴性细菌中。脂蛋白A(通常被称为内毒素)也是导致革兰氏阴性脓毒症临床并发症的内毒素的活性成分。脂类A结构的微小改变可以对发病机制产生深远的影响。一些类脂A实际上是有效的内毒素拮抗剂。与大肠杆菌相比，豆芽假单胞菌脂蛋白A和核心区的化学结构非常不寻常。豆腐病菌类脂A缺少和4‘位的磷酸盐，但在4’位用半乳糖醛酸修饰。它被一种特殊的28碳脂肪酸酰化，并含有2-脱氧-2-氨基葡萄糖酸代替近端的氨基葡萄糖。豆类根瘤杆菌脂多糖的结构表明存在新的酶来产生不同的类脂A和核心物种。现在已经确定，脂肪A生物合成的前七种酶在大肠杆菌和豆类杆菌中实际上是相同的。这种差异出现在该途径的后期阶段。到目前为止，豆类根霉特有的酶包括4‘-磷酸转移酶、1-磷酸转移酶、具有自己的酰基载体蛋白的长链酰基转移酶和三种不同的核心糖基转移酶。豆类根瘤菌系统的特征将为深入了解类脂A分子的功能，包括在植物共生过程中的特殊作用提供机会，并为创造具有有趣的佐剂或拮抗活性的新型类脂A杂交种提供机会。在人类病原体中发现了豆腐杆菌类脂A的一些结构特征。嗜肺军团菌类脂A含有C28链，而牙龈卟啉单胞菌和幽门螺杆菌类脂A缺乏4‘-磷酸。在未来的资助期间，具体目标是：i)克隆豆类根霉C28酰基转移酶；ii)分析类脂A4‘-磷酸/磷酸转移酶，特别是其合成PtdIns-4-P的能力；iii)确定豆类根霉类脂A近端单位多样性的酶基础；以及iv)鉴定含有豆类根霉脂多糖独特核心糖基的酶。

Lipopolysaccharides (LPSs) are remarkable glycolipids that comprise the outer surfaces of Gram-negative bacteria, including the symbiotic organism, Rhizobium leguminosarum. In Escherichia coli, the lipid A anchor of LPS is a hexa-acylated disaccharide of glucosamine, bearing phosphate moieties at positions 1 and 4'. The minimal LPS required for a growth of E.coli consists of lipid A and two extra sugars. Emerging genomic sequences indicate that the enzymes that make lipid A in E. coli are present in most other Gram-negative bacteria. Lipid A (often termed endotoxin) is also the active component of LPS responsible for the clinical complications of Gram-negative sepsis. Minor modifications in the structure of lipid A can have profound effects on pathogenesis. Some lipid A analogs are actually potent endotoxin antagonists. Compared to E. coli, the chemical structures of the lipid A and core domains of R. leguminosarum LPS are very unusual. R. leguminosarum lipid A lacks the and 4'phosphates, but is modified with galacturonic acid at position 4'. It is acylated with a peculiar 28 carbon fatty acid, and contains 2-deoxy-2-aminogluconate in place of the proximal glucosamine. The structure of R. leguminosarum LPS indicates the existence of novel enzymes for generating diverse lipid A and core species. It is now established that the first seven enzymes of lipid A biosynthesis are in fact the same in E. coli and R. leguminosarum. The differences arise in the later stages of the pathway. To date, enzymes identified as unique to R. leguminosarum include a 4'-phosphatase that is also a phosphotransferase, a 1-phosphatase, a long chain acyltransferase with its own acyl carrier protein, and three distinct core glycosyltransferases. Characterization of the R. leguminosarum system should provide insights into the function of lipid A-like molecules, including special roles during symbiosis in plants, and affords the opportunity to create novel lipid A hybrids that may have interesting adjuvant or antagonist activities. Some structural features of R. leguminosarum lipid A are seen in human pathogens. Legionella pneumophila lipid A contains a C28 chain, while Porphyromonas gingivalis and Helicobacter pylori lipid A lack the 4' phosphate. In the coming grant period, the specific aims are: I) cloning of the C28 acyltransferase of R. leguminosarum; II) analysis of the lipid A 4'-phosphatase/phosphotransferase, especially its ability to synthesize PtdIns-4-P; III) determination of the enzymatic basis for proximal unit diversity in R. leguminosarum lipid A; and IV) characterization of enzymes that incorporate the unique inner core sugars of R. leguminosarum LPS.