Dimer-monomer equilibrium of SARS-CoV-2 main protease as affected by small molecule inhibitors: a biophysical investigation

The maturation of coronavirus SARS-CoV-2, which is the etiological agent at the origin of the COVID-19 pandemic, requires a main protease Mpro to cleave the virus-encoded polyproteins. Despite a wealth of experimental information already available, there is wide disagreement about the Mpro monomer-dimer equilibrium dissociation constant. Since the functional unit of Mpro is a homodimer, the detailed knowledge of the thermodynamics of this equilibrium is a key piece of information for possible therapeutic intervention, with small molecules interfering with dimerization being potential broad-spectrum antiviral drug leads. In the present study, we exploit small angle x-ray scattering (SAXS) to investigate the structural features of the SARS-CoV-2 Mpro monomer-dimer equilibrium, by revealing the corresponding equilibrium dissociation constant and the associated thermodynamic parameters. SAXS is also used to study how the Mpro dissociation process is affected by small inhibitors selected through combinatorial design. Our results show that a clear picture connecting the ability of inhibitors to disrupt the Mpro dimerization with the loss of catalytic activity cannot be provided, thus highlighting the possible role of allosteric effects for the regulation of Mpro functionality.