Size-exclusion chromatography of Mpro. The enzyme in phosphate-buffered saline buffer was applied to a preequilibrated Superose 12 10/300 GL column. The Mpro and its mutants, without or with preincubation in 600 μM substrate, are wild-type (solid circles), wild-type with substrate (open circles), R298A (solid triangles), R298A with substrate (open triangles), R298A/Q299A (solid squares), and R298A/Q299A with substrate (open squares).