Department of Chemistry, The University of Texas at Austin, Austin, Texas 78712, United States.
Division of Chemical Biology and Medicinal Chemistry, College of Pharmacy, The University of Texas at Austin, Austin, Texas 78712, United States.
J Am Chem Soc. 2023 Oct 4;145(39):21361-21369. doi: 10.1021/jacs.3c06354. Epub 2023 Sep 21.
Apramycin is an aminoglycoside antibiotic isolated from and that has found clinical use in veterinary medicine. The apramycin structure is notable for its atypical eight-carbon bicyclic dialdose (octose) moiety. While the apramycin biosynthetic gene cluster () has been identified and several of the encoded genes functionally characterized, how the octose core itself is assembled has remained elusive. Nevertheless, recent gene deletion studies have hinted at an -acetyl aminosugar being a key precursor to the octose, and this hypothesis is consistent with the additional feeding experiments described in the present report. Moreover, bioinformatic analysis indicates that AprG may be structurally similar to GlcNAc-2-epimerase and hence recognize GlcNAc or a structurally similar substrate suggesting a potential role in octose formation. AprG with an extended -terminal sequence was therefore expressed, purified, and assayed in vitro demonstrating that it does indeed catalyze a transaldolation reaction between GlcNAc or GalNAc and 6'-oxo-lividamine to afford 7'--acetyldemethylaprosamine with the same 6'- and 7'- stereochemistry as those observed in the apramycin product. Biosynthesis of the octose core in apramycin thus proceeds in the [6 + 2] manner with GlcNAc or GalNAc as the two-carbon donor, which has not been previously reported for biological octose formation, as well as novel inverting stereochemistry of the transferred fragment. Consequently, AprG appears to be a new transaldolase that lacks any apparent sequence similarity to the currently known aldolases and catalyzes a transaldolation for which there is no established biological precedent.
安普霉素是一种从 和 中分离出来的氨基糖苷类抗生素，在兽医临床上已得到应用。安普霉素的结构以其非典型的八碳双环二酮（辛糖）部分为特征。虽然已经确定了安普霉素生物合成基因簇（），并且对几个编码基因进行了功能表征，但辛糖核心本身是如何组装的仍然难以捉摸。然而，最近的基因缺失研究表明，-乙酰氨基糖可能是辛糖的关键前体，这一假设与本报告中描述的额外的喂养实验一致。此外，生物信息学分析表明，AprG 可能在结构上与 GlcNAc-2-差向异构酶相似，因此可以识别 GlcNAc 或结构类似的底物，这表明它在辛糖形成中可能具有潜在的作用。因此，表达、纯化并在体外进行了 AprG 带有扩展的 -末端序列的酶促反应，结果表明它确实催化了 GlcNAc 或 GalNAc 与 6'-氧-乳清酸之间的反醛醇缩合反应，生成具有与在安普霉素产物中观察到的相同 6'-和 7'-立体化学的 7'--乙酰基去甲基阿普罗胺。因此，安普霉素中辛糖核心的生物合成以 GlcNAc 或 GalNAc 作为两碳供体以 [6 + 2] 方式进行，这在生物辛糖形成中尚未报道过，并且转移片段的反转立体化学也是新的。因此，AprG 似乎是一种新的反醛醇酶，它与目前已知的醛醇酶没有任何明显的序列相似性，并催化一种没有既定生物学先例的反醛醇缩合反应。
