Quercetin and its derivatives are important metabolites that belong to the flavonol class of flavonoids. Quercetin and some of the conjugates have been approved by the FDA for human use. They are widely distributed among plants and have various biological activities, such as being anticancer, antiviral, and antioxidant. Hence, the biosynthesis of novel derivatives is an important field of research. Glycosylation and methylation are two important modification strategies that have long been used and have resulted in many novel metabolites that are not present in natural sources. A strategy for modifying quercetin in E. coli by means of glycosylation, for example, involves overexpressing respective glycosyltransferases (GTs) in the host and metabolic engineering for increasing nucleoside diphosphate sugar (NDP sugar). Still others have used microorganisms other than E. coli, such as Streptomyces sp., for the biotransformation process. The overall study of the structural activity relationship has revealed that modification of some residues in quercetin decreased one activity but increased others. This review summarizes all of the information mentioned above.

中文翻译：

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槲皮素及其衍生物的结构、修饰及构效关系研究进展。

槲皮素及其衍生物是属于类黄酮的黄酮醇类的重要代谢物。槲皮素和一些偶联物已被 FDA 批准用于人类。它们广泛分布于植物中，具有抗癌、抗病毒、抗氧化等多种生物活性。因此，新型衍生物的生物合成是一个重要的研究领域。糖基化和甲基化是两种重要的修饰策略，长期以来一直在使用，并产生了许多天然来源中不存在的新型代谢物。一种在大肠杆菌中修饰槲皮素的策略例如，通过糖基化，涉及在宿主中过表达相应的糖基转移酶 (GT) 和代谢工程以增加核苷二磷酸糖 (NDP 糖)。还有一些人使用大肠杆菌以外的微生物，例如链霉菌属，用于生物转化过程。对结构活性关系的整体研究表明，槲皮素中某些残基的修饰降低了一种活性，但增加了另一些活性。这篇评论总结了上述所有信息。