Cysteine proteases work on the similar principle as shown by serine proteases. Cysteine proteases use a thiol group, which is activated by its proximity to a histidine
residue. Therefore, these enzymes have a catalytic diad, not triad as in serine proteases. Propose a mechanism for cysteine protease action on peptide substrate through formation of the acyl-enzyme intermediate (Stop at this point).​