A contributing membrane-bound enzyme that facilitates spike protein entry via ACE-2 receptors is the TMPRSS2 serine protease. This protease primes the coronavirus spike protein for entry by cleaving protein domains that increase the interaction with host cell membranes that result in membrane fusion [28]. It was also suggested that TMPRSS2 cleaves ACE-2 sites that promote the affinity for the viral spike protein, hence enhances cell entry [29].