Chen Meng, Guo Zhengyan, Sun Jinyuan, Tang Wei, Wang Min, Tang Yue, Li Pengwei, Wu Bian, Chen Yihua
State Key Laboratory of Microbial Resources & CAS Key Laboratory of Microbial Physiological and Metabolic Engineering, Institute of Microbiology, Chinese Academy of Sciences, Beijing 100101, China.
University of Chinese Academy of Sciences, Beijing 100049, China.
Acta Pharm Sin B. 2023 Feb;13(2):765-774. doi: 10.1016/j.apsb.2022.05.031. Epub 2022 Jun 3.
l-Heptopyranoses are important components of bacterial polysaccharides and biological active secondary metabolites like septacidin (SEP), which represents a group of nucleoside antibiotics with antitumor, antifungal, and pain-relief activities. However, little is known about the formation mechanisms of those l-heptose moieties. In this study, we deciphered the biosynthetic pathway of the l,l-gluco-heptosamine moiety in SEPs by functional characterizing four genes and proposed that SepI initiates the process by oxidizing the 4'-hydroxyl of l-glycero--d-manno-heptose moiety of SEP-328 () to a keto group. Subsequently, SepJ (C5 epimerase) and SepA (C3 epimerase) shape the 4'-keto-l-heptopyranose moiety by sequential epimerization reactions. At the last step, an aminotransferase SepG installs the 4'-amino group of the l,l-gluco-heptosamine moiety to generate SEP-327 (). An interesting phenomenon is that the SEP intermediates with 4'-keto-l-heptopyranose moieties exist as special bicyclic sugars with hemiacetal-hemiketal structures. Notably, l-pyranose is usually converted from d-pyranose by bifunctional C3/C5 epimerase. SepA is an unprecedented monofunctional l-pyranose C3 epimerase. Further and experimental studies revealed that it represents an overlooked metal dependent-sugar epimerase family bearing vicinal oxygen chelate (VOC) architecture.
L-庚吡喃糖是细菌多糖和生物活性次级代谢产物（如杀隔菌素，SEP）的重要组成部分，杀隔菌素是一类具有抗肿瘤、抗真菌和止痛活性的核苷类抗生素。然而，关于这些L-庚糖部分的形成机制知之甚少。在本研究中，我们通过对四个基因进行功能表征，破译了SEP中L,L-葡糖-庚糖胺部分的生物合成途径，并提出SepI通过将SEP-328（）的L-甘油-D-甘露庚糖部分的4'-羟基氧化为酮基来启动这一过程。随后，SepJ（C5差向异构酶）和SepA（C3差向异构酶）通过连续的差向异构化反应塑造4'-酮-L-庚吡喃糖部分。在最后一步，氨基转移酶SepG安装L,L-葡糖-庚糖胺部分的4'-氨基以生成SEP-327（）。一个有趣的现象是，具有4'-酮-L-庚吡喃糖部分的SEP中间体以具有半缩醛-半酮结构的特殊双环糖形式存在。值得注意的是，L-吡喃糖通常由双功能C3/C5差向异构酶从D-吡喃糖转化而来。SepA是一种前所未有的单功能L-吡喃糖C3差向异构酶。进一步的和实验研究表明，它代表了一个被忽视的具有邻位氧螯合（VOC）结构的金属依赖性糖差向异构酶家族。
