MECHANISMS OF BIOSYNTHESIS OF BRANCHED-CHAIN SUGARS

描述：支链糖是一类重要的糖类 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 通过取代糖上的氢基或羟基 带有烷基侧链的二次碳原子。这样的替代 通常会导致脑细胞生物学功能的严重改变 由此产生的糖，也会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 在这个过程中，这些不寻常的糖类起着不可或缺的作用 这些天然产物的最佳活性。尽管生物学上 支链糖的重要性众所周知，但知之甚少 关于导致两种物质形成的生物合成途径 支链糖，叶斯尼糖A和二氢链糖。叶尔西尼奥斯A是 耶尔森氏菌脂多糖中的一种免疫优势糖 假结核VI，二氢链糖是一种结构成分 从灰色链霉菌中分离出链霉素类抗生素。强调将 被放在催化酶的机理研究上 在这两种情况下，支链构造都是步骤。拟议中的实验 包括：1)克隆编码目标酶的基因并表达 这些基因在大肠杆菌中产生催化活性蛋白；2)开发 合适的方法测定目标酶的活性；3) 目的酶的纯化及其物理和生化特性研究 性能；4)制备替代底物作为机械探头 研究这些酶反应；5)阐明详细的反应 这些酶的作用机制。对分子基础的理解 这些支链糖的生物合成不仅有助于 描述了酶是如何影响化学转化的 这些转换，但也将为设计提供宝贵的知识 控制和/或模仿其生产和生物的方法 活动。由于糖类残留物在确定 抗生素的生物活性已经得到了很好的证实，一些 参与抗生素生物合成的糖基转移酶 已经被证明有一些松弛的底物专一性，它是 预计从这些研究中获得的新见解也将 基因转移实验产生新的或杂交的基础 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.