(A) 1 µg purified recombinant SARS-CoV-2 and OC43 3CLpro and HRV3C protein was separated by SDS–PAGE and total protein visualised with Coomassie Blue stain; (B) Recombinant SARS-CoV-2 (blue bars) and OC43 (orange bars) 3CLpro show a concentration dependent increase in net A625/A525 ratio but recombinant HRV-3C (grey bars) shows no significant increase in net A625/A525 ratio above background levels (error bars represent means ± standard deviation; n = 3 independent experiments; **** P < 0.0001, *** P < 0.0005 by two-way ANOVA Tukey's Multiple Comparisons Test). A representative visual image from independent replicate experiments is shown in a panel below the graph.