English: Differences in cysteine and serine proteolysis mechanisms. The protease (black) performs a nucleophilic attack on peptide substrate (red) to form a tetrahedral intermediate. This breaks down by ejection of the first product, the substrate C-terminus, to form the acyl-enzyme intermediate. Water replaces the first product and hydrolysis occurs via a second tetrahedral intermediate to regenerate free enzyme. Indicated differences are (a) the deprotonated cysteine, (b) aspartate (grey) not present in all cysteine proteases, (c) concerted deprotonation of serine, (d) aspartate hydrogen bonding, (e) amide protonation of the first leaving group, (f) alcohol protonation of the serine leaving group.