Flavonoid-mediated inhibition of SARS coronavirus 3C-like protease expressed in Pichia pastoris

The 3C-like protease (3CLpro) of severe acute respiratory syndrome associated coronavirus (SARS-CoV) is vital for SARS-CoV replication and is a promising drug target. Recombinant 3CLpro was expressed in Pichia pastoris GS115 as a 42 kDa protein that displayed a K m of 15 ± 2 μM with Dabcyl-KTSAVLQSGFRKME-Edans as substrate. Purified 3CLpro was used for inhibition and kinetic assays with seven flavonoid compounds. The IC50 of six flavonoid compounds were 47–381 μM. Quercetin, epigallocatechin gallate and gallocatechin gallate (GCG) displayed good inhibition toward 3CLpro with IC50 values of 73, 73 and 47 μM, respectively. GCG showed a competitive inhibition pattern with K i value of 25 ± 1.7 μM. In molecular docking experiments, GCG displayed a binding energy of −14 kcal mol−1 to the active site of 3CLpro and the galloyl moiety at 3-OH position was required for 3CLpro inhibition activity.