Inhibition of SARS-CoV 3CL protease by flavonoids

Jo, Seri; Kim, Suwon; Shin, Dong Hae; Kim, Mi-Sun

To independently confirm the inhibitory activity of flavonoids, a general tryptophan-based assay method was employed. Tryptophan was well known to emit its fluorescence. Therefore, if tryptophan is positioned adequately in proteins, the change of fluorescence intensity can reflect the binding state of chemicals and be used to judge interaction between proteins and chemicals. The SARS-CoV 3CLpro contains one tryptophan residue (Trp31) at the catalytic domain. Its backbone lines up at the entrance of the active site. Therefore, its fluorescence change can reflect environmental variation of the active site. The catalytic domain of SARS-CoV 3CLpro used in this study displays a fluorescence peak at 340 nm after the tryptophan excitation wavelength of 290 nm. We monitored the change of the fluorescence intensities depending on the presence or absence of all flavonoids. Since each compound in the flavonoid library was almost non-fluorescent under the experiment condition, a change of fluorescence intensity reflects interactions between the catalytic domain and chemicals. Intriguingly, the three inhibitory compounds obviously reduced the fluorescence intensity of the catalytic domain compared with others ( Figure 3 ). The decreased emission intensity confirmed the complex formation between the catalytic domain and inhibitory compounds.