SASKATOON, Sask. — Researchers utilizing the Canadian Light Source at the University of Saskatchewan have uncovered a structural vulnerability in the main protease (Mpro) of SARS-CoV-2, revealing a potential new target for antiviral treatments. Mpro is vital for the replication of the virus that causes COVID-19, making it a key focus for drugs such as remdesivir.
“Mpro is a good target for drugs. If we can halt its activity, we can halt viral replication,” says Todd Holyoak, a structural biologist at the University of Waterloo. The protein functions only when two copies are stuck together as a dimer, and it contains an unusually high amount of the amino acid cysteine. Because cysteine is highly susceptible to oxidative damage—a primary mechanism the body uses to combat infection—this abundance should present a liability for the viral protein.
Investigating Oxidative Vulnerability and Protein Structure
Previous research by Holyoak and his colleagues demonstrated that under oxidative conditions, Mpro protects its vulnerable cysteines by temporarily linking C117 and C145, a bond that breaks when conditions improve. To examine further consequences, the researchers engineered altered versions of the protein by replacing one of the cysteines with aspartate, an amino acid mimicking oxidized cysteine.
Disabling C117 caused the Mpro dimer to split into two separate proteins, known as monomers. Utilizing the facility in Saskatoon, the team was able to observe the 3D structure of the Mpro monomer, potentially for the first time, and discovered it possessed a shape significantly different from its structure within a dimer.
New Therapeutic Avenues
Current medications like remdesivir target the active site of the dimer to block it from binding to and cutting up the polyprotein. While effective, this mechanism leaves room for the virus to eventually develop resistance.
Holyoak envisions a different approach that targets the monomer version to prevent the formation of the active dimer. “If we can find molecules that can bind and stabilize the monomer form, it will never even form the dimer,” he says. “That opens up a whole new therapeutic avenue.” The team’s findings have been published in the journal Protein Science.
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