Classification of Enzymes

Enzymes are classified into six major groups based on their overall mode of action - the type of chemical transformation they catalyse - regardless of what specific substrate they act on. Oxidoreductases catalyse oxidation and reduction (redox) reactions, following the general scheme A(reduced) + B(oxidised) <=> A(oxidised) + B(reduced); dehydrogenases are a typical example. Transferases transfer a group of atoms from one molecule to another, following the scheme A-B + C <=> A + C-B; examples include transaminases and phosphotransferases. Hydrolases hydrolyse (break down) a substrate by the addition of a water molecule, following the scheme A-B + H2O <=> A-H + B-OH; most digestive enzymes belong to this class. Isomerases control the conversion of one isomer of a molecule to another, essentially transferring a group of atoms from one part of the same molecule to another, following the scheme A-B-C <=> A-C-B; isomerase enzymes themselves are the example given. Lyases break a chemical bond WITHOUT the addition of water, following the scheme A-B <=> A + B; decarboxylase enzymes (which remove a CO2 group) are a typical example. Ligases form new chemical bonds using ATP as a source of energy, following the scheme A + B + ATP <=> A-B + ADP + Pi; DNA ligase, whic …