Ligases

Ligases (EC 6.-.-.-) form a major class of enzymes that catalyze the ligation (i.e. linking together) of two molecules with concomitant hydrolysis of the pyrophosphate bond in adenosine 5'-triphosphate (ATP) or a similar triphosphate, forming C–C , C–O , C–S , P–O or C–N bonds[1]. Originally, biochemical nomenclature distinguished synthetases and synthases. Under the original definition, synthases do not use energy from nucleoside triphosphates (such as ATP, GTP, CTP, TTP, and UTP), whereas synthetases do use nucleoside triphosphates. It is also said that a synthase is a lyase (a lyase is an enzyme that catalyzes the breaking of various chemical bonds by means other than hydrolysis and oxidation, often forming a new double bond or a new ring structure) and does not require any energy, whereas a synthetase is a ligase (a ligase is an enzyme that binds two chemicals or compounds) and thus requires energy. However, the Joint Commission on Biochemical Nomenclature (JCBN) dictates that "synthase" can be used with any enzyme that catalyses synthesis (whether or not it uses nucleoside triphosphates), whereas "synthetase" is to be used synonymously.