God help anyone that relies on this book for anything of substance. Of course Rand Paul in the book argues that the coronavirus was bioengineered from gain of function research and Fauci covered it up. Derp.
Wonder how he got these definitions?
Furin is an enzyme. It acts within the cell to activate proteins by "cutting" peptide bonds. This is after translation in the cell.
It can also do this for viral proteins in the cell. Influenza and coronavirus undergo furin cleavage. In the case of the coronavirus, the furin cleaves the spike protein at the furin cleavage site separating the spike protein into S1 and S2 subunits and putting the spike protein into an "open" binding formation before it leaves the cell.
There are other cleavage sites with the coronavirus. TMPRSS2 is another enzyme that helps the coronavirus fuse with cells by cleaving the S2 subunit. These viruses need to be "cut" to fuse with cells.
Interestingly hydroxychloroquine inhibits another enzyme that cleaves the coronavirus, but this pathway is not the main way the coronavirus gets into the cell. So that's why hydroxychloroquine is not very effective against the virus, among other issues with it.