A review on structure, modifications and structure-activity relation of quercetin and its derivatives.

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 anti-cancer, anti- viral, and antioxidants. Hence, the biosynthesis of novel derivatives is an important field of research. Glycosylation and methylation are two important modification strategies that have been used for a long time and have resulted in many novel metabolites that are not present in natural sources. An overall strategy for the modification of quercetin in E. coli by means of glycosylation, for example, is done by overexpressing respective glycosyltransferases (GTs) in the host and metabolic engineering for increasing nucleoside diphosphate sugar (NDP sugar). Still others have used microorganisms besides E. coli, such as Streptomyces sp. for the biotransformation process. The overall study of the structural activity relationship has provided the information that modification of some residues in quercetin decreased one activity but increased others. This review summarizes all of the information mentioned above.