Cathepsin L, transmembrane peptidase/serine subfamily member 2/4, and other host proteases in COVID-19 pathogenesis – with impact on gastrointestinal tract

Figure 1 Alternations in spike proteins conformation upon binding to ACE 2. Spike S1 subunit contains receptor binding domain, that has to change from “down-conformation” state to “up-conformation” state to be accessible for ACE2. Changes in S1 subunit trigger conformational changes in S2 subunit, causing exposition of hydrophobic domain, changing it from “pre-fusion” to “post-fusion” state. This enables fusion of the virus with host membrane (after Zhu et al[7]).