oxidoreductases

Oxidation-reduction reaction (ex: lactate dehydrogenase)

transferases

group transfer (ex: nucleoside monophosphate kinase)

Hydrolases

hydrolysis reactions (transfer of functional groups to water) (ex: chymotrypsin)

Lyases

Addition or removal of groups to form double bonds (ex: fumarase)

Isomerases

isomerization (intramolecular group transfer) (ex: triose phosphate isomerase)

Ligases

ligation of two substrates at the expense of ATP hydrolysis (ex: aminoacyl-tRNA synthetase)