Assay Method Information

Assay Name:  SARS-CoV-2 Coronavirus 3C Protease FRET Assay
Description:  Proteolytic activity of SARS-CoV-2 Coronavirus 3CL protease is measured using a continuous fluorescence resonance energy transfer assay. The SARS-CoV-2 3CLpro FRET assay measures the protease catalyzed cleavage of TAMRA-SITSAVLQSGFRKMK-(DABCYL)-OH to TAMRA - SITSAVLQ and SGFRKMK(DABCYL)-OH. The fluorescence of the cleaved TAMRA (ex. 558 nm / em.581 nm) peptide was measured using a TECAN SAFI RE fluorescence plate reader over the course of 10 min. Typical reaction solutions contained 20 mM HEPES (pH 7.0), 1 mM EDTA, 4.0 uM FRET substrate, 4% DMSO and 0.005% Tween-20. Assays were initiated with the addition of 25 nM SARS 3CLpro (nucleotide sequence 9985-10902 of the Urbani strain of SARS coronavirus complete genome sequence (NCBI accession number AY278741)). Percent inhibition was determined in duplicate at 0.001 mM level of inhibitor. Data was analyzed with the non-linear regression analysis program Kalidagraph using the equation:FU =offset+ (limit)(1- e <kobsH)where offset equals the fluorescence signal of the uncleaved peptide substrate, and limit equals the fluorescence of fully cleaved peptide substrate. The kobs is the first order rate constant for this reaction, and in the absence of any inhibitor represents the utilization of substrate. In an enzyme start reaction which contains an irreversible inhibitors, and where the calculated limit is less than 20% of the theoretical maximum limit, the calculated kobs represents the rate of inactivation of coronavirus 3C protease. The slope (kobs/ 1) of a plot of kobs vs. [I] is a measure of the avidity of the inhibitor for an enzyme. For very fast irreversible inhibitors, kobs/l is calculated from observations at only one or two [I] rather than as a slope.
Affinity data for this assay

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