English: A comparison of the two mechanisms used for proteolysis. enzyme is shown in black, substrate protein in red and water in blue.The top panel shows 1-step hydrolysis where the enzyme uses an acid to polarise water which then hydrolyses the substrate. The bottom panel shows 2-step hydrolysis where a residue within the enzyme is activated to act as a nucleophile (Nu) and attack the substrate. This forms an intermediate where the enzyme is covalently linked to the N-terminal half of the substrate. In a second step, water is activated to hydrolyse this intermediate and complete catalysis. Other enzyme residues (not shown) donate and accept hydrogens and electrostatically stabilise charge build-up along the reaction mechanism.