Hydrolases represent the largest class of all known enzymes. They catalyze the hydrolytic cleavage of carbon-oxygen (C-O), carbon-nitrogen (C-N), carbon-carbon (C-C), and phosphoric anhydride (P-O) bonds. Classification for these enzymes are generally done first by the nature of the bond hydrolysed, then by the nature of the substrate, and lastly by the enzyme. The systematic name always includes hydrolase, however, the common name is formed by the name of the substrate with the suffix -ase. It is know that an enzyme's substrate ending with -ase refers to a hydrolytic enzyme. This class of enzymes tend to pose a problem sometimes during the classification process. Since some of the enzymes have a tendency to have a wide variety of specificity. It is not easy to decide if two are the same or if they should be listed under different entries.

There are a number of hydrolases that not only catalyze the hydrolytic removal of a group from the substrate, but a transfer of that group to acceptor molecules. This characteristic yields the possibility that some hydrolytic enzymes could be classified as transferases because there is the movement of a specific group to water. However, the hydrolytic reaction with water as the acceptor was discovered before the transferase reaction causing it to be considered the main physiological function of the enzyme. Therefore these enzymes are classified as hydrolases rather than transferases.

The active sites of hydrolases are different even though the same mechanism is being used. There are four main types of active sites which are based on the amnio acids present and the mechanism of action. The first type contains aspartic or glutamic acid residues which is found in lysozyme and pepsin like enzymes. The active site relies on the presence of two or more carboxyl groups. One of the carboxyl groups becomes protonated and acts as general acid. The other carboxyl group is depronated and serves as a base. The acid (electrophile) activates the substrate where as the base (nucleophile) activates water. Enzymes that catalyze the hydrolysis of ester, amide and glycosidic bonds use this type of active site. The second type has an imidazole group in the active site to active water using its carboxyl group. The catalytic mechanism includes the acylation of the hydroxyl group of a thiol, usually on a serine within the active site. followed by the formation of acyl-enzyme intermediates. The serine proteases such chymotrypsin are found within this active site group. The third version of active sites include zinc (Zn), cobalt (Co) or nickel (Ni) metal ions for the activation of water and substrate. These metal ions usually contain histidine (imidazole) and aspartic (glutamic) acid carboxyl groups as their ligands. The metal ions can either act as electrophilic agents that activate the attacked reaction center or as electrophilic activators of water molecules that generate hydroxyl cations. Alkaline phosphatase is a primary example of this active site mechanism. The final active site category also includes metal ions. These ion are of maganese (Mn) or magnesium (Mg). They also serve as electrophilic agents that activate the substrate. However, they induce an electrodensity deficit at the reaction center due to their potency. Inorganic pyrophosphatase utilizes this reaction mechanism.

This website will allow you to investigate three proteins within this enzyme class. Three different active site mechanisms will be explored along with information on the folding topologies, cellular location and function.