The enzymatic activities of wild type 3CLpro (●), S139A (■), S144A (▲), and S147A (♦) were determined at 25°C with an enzyme concentration of 1 μM in 10mM NaCl, 10mM NaPi, 1mM EDTA, 1mM TCEP (pH 7.4). The initial velocity, which was measured as arbitrary fluorescence units per second, is plotted as a function of substrate concentration.