(A) Addition of 750 nM SARS-CoV-2 H41A (orange points) and H163A (grey points) 3CLpro mutant proteins significantly increases net A625/A525 ratio compared with untreated (blue points) protease reactions with SARS-CoV-2 3CLpro (error bars represent means ± standard deviation; n = 3 independent experiments; **** P < 0.0001 by two-way ANOVA Dunnett's Multiple Comparisons Test compared with Untreated sample); (B) Addition of 750 nM SARS-CoV-2 H41A 3CLpro mutant protein (triangular points and dashed lines) significantly increases net A625/A525 ratio compared with untreated (circular points and solid lines) protease reactions with SARS-CoV-2 3CLpro (blue points) but not with OC43 3CLpro (orange points) (error bars represent means ± standard deviation; n = 4 independent experiments; **** P < 0.0001 by two-way ANOVA Tukey's Multiple Comparisons Test compared with — H41A 3CLpro sample). A representative visual image from independent replicate experiments is shown in a panel beside the graph.