MECHANISMS OF BIOSYNTHESIS OF BRANCHED-CHAIN SUGARS

描述：支链糖是一类重要的糖， 在自然界中广泛存在的碳水化合物。 它们在形式上是由共同的 通过取代糖上的氢或羟基， 具有烷基侧链的仲碳原子。 这种取代 通常会导致细胞生物学功能的关键改变， 产生的糖，也引起其新陈代谢的根本变化。 特别值得注意的是在许多抗生素中发现的支链糖 这些不寻常的糖在赋予 这些天然产物的最佳活性。 虽然生物 支链糖的重要性是公认的，但知之甚少 关于生物合成途径导致两种 支链糖、耶尔森氏糖A和二氢链霉菌糖。 耶尔森氏菌A是 在耶尔森氏菌的脂多糖中发现的一种免疫显性糖 假结核VI，二氢链霉菌糖是其结构成分 分离自灰色链霉菌的链霉素抗生素。 重点将 酶催化的机理研究， 两种情况下的分支链构建步骤。 拟议的实验 包括：1）克隆编码目的酶的基因， 这些基因在E.大肠杆菌，以产生催化活性蛋白; 2）开发 测定靶酶活性的适当方法; 3） 目的酶的纯化及其理化性质的研究 4）制备替代底物作为机械探针， 研究这些酶促反应; 5）阐明详细的反应 这些酶的作用机制。 了解的分子基础， 这些支链糖的生物合成形成不仅有助于 描述了化学转化是如何通过酶催化 这些转换，但也将提供宝贵的知识，设计 控制和/或模拟其生产和生物学特性的方法 活动 由于糖残基在确定 抗生素的生物活性已经得到很好的确立，并且一些 参与抗生素生物合成的糖基转移酶 已经显示出具有稍微放松的底物特异性， 预计从这些研究中获得的新见解也将奠定 基因转移实验的基础，以产生新的或杂交的 抗生素

DESCRIPTION: The branched-chain sugars are an important class of carbohydrates found widely in nature. They are formally derived from common sugars by replacement of either a hydrogen or a hydroxyl group on a secondary carbon atom with an alkyl side chain. Such a substitution generally causes a critical alteration of the biological function of the resulting sugar, and also induces a fundamental change in its metabolism. Particularly notable are the branched-chain sugars found in many antibiotics in which these unusual sugars play an indispensable role in conferring optimal activity on these natural products. Although the biological importance of branched-chain sugars is well recognized, little is known about the biosynthesis pathways leading to the formation of two branched-chain sugars, yersiniose A and dihydrostreptose. Yersiniose A is an immunodominant sugar found in the lipopolysaccharide (LPS) of Yersinia pseudotuberculosis VI, and dihydrostreptose is a structural component of streptomycin antibiotics isolated from Streptomyces griseus. Emphasis will be placed on the mechanistic studies of enzymes catalyzing the branched-chain construction steps in both cases. The proposed experiments include: 1) to clone the genes encoding the target enzymes and express these genes in E. coli to give catalytically active proteins; 2) to develop appropriate methods to assay the activity of the target enzymes; 3) to purify the target enzymes and characterize their physical and biochemical properties; 4) to prepare alternative substrates as mechanistic probe to study these enzymatic reactions; 5) to elucidate the detailed reaction mechanisms of these enzymes. An understanding of the molecular basis of the biosynthetic formation of these branched-chain sugars will not only aid in delineating how chemical transformations are effected by enzymes catalyzing these conversions, but will also provide invaluable knowledge for designing approaches to control and/or mimic their production and biological activities. Since the significance of sugar residues in determining the biological activity of the antibiotics has been well established, and some of the glycosyl transferases involved in the biosynthesis of antibiotics have been shown to have somewhat relaxed substrate specificity, it is expected that the new insights gained from these studies will also lay groundwork for gene transfer experiments to produce novel or hybrid antibiotics.