The International Union of Biochemistry and Molecular Biology (IUBMB) recommends classifying enzymes on the basis of the type of reaction they catalyze. Under this system, hydrogenases are enzymes that belong to the category of Class 1 enzymes known as oxidoreductases. Hydrolases are the Class 3 enzymes per IUBMB guidelines, a major category in themselves.
Hydrogenases are of three types: Nickel-iron (NiFe), iron-iron (Fe-Fe or Fe-only) and Fe-hydrogenase. It was believed that this last class of enzymes did not contain any metal, but in 2000 it was discovered that they too contained iron. Hydrolases can be of several types, such as esterases, phosphatases, peptidases and glycosidases, depending on the type of reaction they catalyze. There are several enzymes within each types. For example, lipase, phospholipase and acetylcholine esterase are all esterases.
The hydrogenase enzyme catalyzes oxidation reduction, or redox, reactions that involve the acceptance of electrons by protons to produce hydrogen. Hydrolase enzymes catalyze reactions that involve hydrolysis of bonds such as the carbon-nitrogen, carbon-oxygen and carbon-carbon bonds. They are also necessary for the cleavage of phosphoric anhydride bonds. The enzymes that catalyze the hydrolysis of amide, ester, peptide and glycosyl bonds perform the additional function of transferring the cleaved group to other acceptor molecules.
The hydrogenase enzymes contain carbonyl and cyanide functional groups that regulate their acidic nature and oxidation reduction potential to enable binding with dihydrogen. Some hydrogenase enzymes catalyze the transfer of hydride from hydrogen to an organic substrate, whereas others bring about electron acceptor reduction to produce hydrogen. Hydrolase enzymes catalyze the breaking of bonds in the presence of water. For example, esterases act to cause cleavage of an ester bond to form an acid and an alcohol.