SARS CoV main proteinase: The monomer-dimer equilibrium dissociation constant

The SARS coronavirus main proteinase (SARS CoV main proteinase) is required for the replication of the severe acute respiratory syndrome coronavirus ( SARS CoV), the virus that causes SARS. One function of the enzyme is to process viral polyproteins. The active form of the SARS CoV main proteinase is a homodimer. In the literature, estimates of the monomer-dimer equilibrium dissociation constant, K-D, have varied more than 650000-fold, from < 1 nM to more than 200 mu M. Because of these discrepancies and because compounds that interfere with activation of the enzyme by dimerization may be potential antiviral agents, we investigated the monomer-dimer equilibrium by three different techniques: small-angle X-ray scattering, chemical cross-linking, and enzyme kinetics. Analysis of small-angle X-ray scattering data from a series of measurements at different SARS CoV main proteinase concentrations yielded K-D values of 5.8 +/- 0.8 mu M (obtained from the entire scattering curve), 6.5 +/- 2.2 mu M (obtained from the radii of gyration), and 6.8 +/- 1.5 mu M (obtained from the forward scattering). The K-D from chemical cross-linking was 12.7 +/- 1.1 mu M, and from enzyme kinetics, it was 5.2 +/- 0.4 mu M. While each of these three techniques can present different, potential limitations, they all yielded similar K-D values.