Abstractí¬The 3C-like protease (3CLpro) of severe acute respiratory syndrome-associated coronavirus (SARS-CoV) is one of the
most promising targets for discovery of drugs against SARS, because of its critical role in the viral life cycle. In this study, a natural
compound called quercetin-3-b-galactoside was identified as an inhibitor of the protease by molecular docking, SPR/FRET-based
bioassays, and mutagenesis studies. Both molecular modeling and Q189A mutation revealed that Gln189 plays a key role in the
binding. Furthermore, experimental evidence showed that the secondary structure and enzymatic activity of SARS-CoV 3CLpro
were not affected by the Q189A mutation. With the help of molecular modeling, eight new derivatives of the natural product were
designed and synthesized. Bioassay results reveal salient features of the structureĘCactivity relationship of the new compounds: (1)
removal of the 7-hydroxy group of the quercetin moiety decreases the bioactivity of the derivatives; (2) acetoxylation of the sugar
moiety abolishes inhibitor action; (3) introduction of a large sugar substituent on 7-hydroxy of quercetin can be tolerated; (4)
replacement of the galactose moiety with other sugars does not affect inhibitor potency. This study not only reveals a new class
of compounds as potential drug leads against the SARS virus, but also provides a solid understanding of the mechanism of inhibition
against the target enzyme.