Thousands of enzymes have been discovered, isolated and studied.
Most of these enzymes have been classified into different groups based on the type of reactions they catalyse. Enzymes are divided into 6 classes each with 4-13 subclasses and named accordingly by a four-digit number.
Oxidoreductases/dehydrogenases: Enzymes which catalyse oxidoreduction between two substrates S and S’ e.g., S reduced + S’ oxidised ⎯ →⎯ S oxidised + S’ reduced.
Transferases: Enzymes catalysing a transfer of a group, G (other than hydrogen) between a pair of substrate S and S’ e.g., S - G + S’ ⎯ →⎯ S + S’ - G
Hydrolases: Enzymes catalysing hydrolysis of ester, ether, peptide, glycosidic, C-C, C-halide or P-N bonds.
Lyases: Enzymes that catalyse removal of groups from substrates by mechanisms other than hydrolysis leaving double bonds.
Isomerases: Includes all enzymes catalysing inter-conversion of optical, geometric or positional isomers.
Ligases: Enzymes catalysing the linking together of 2 compounds, e.g., enzymes which catalyse joining of C-O, C-S, C-N, P-O etc. bonds.