Some proteins are inactive first after they have been synthesized. Furin is an enzyme that targets certain sites in certain biomolecules and cuts a piece off, a

Some proteins are inactive first after they have been synthesized. Furin is an enzyme that targets certain sites in certain biomolecules and cuts a piece off, activating these proteins. Apparently some furin is floating around in the human body for this purpose. The article suggest that there is a furin target site (binding site) in the surface "spikes" of the coronavirus. And when furin cuts piece off the spikes, the resulting, or remaining, part then assists the entry of the virus into cells. Here is a review article from 2019 on furin. Apparently several viruses use this "trick" as part of their entry mechanism. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6682551/ https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6682551/ (I have some biology studies from long ago, I am not professional in molecular biology.)