ligase, any one of a class of about 50 enzymes that catalyze reactions involving the conservation of chemical energy and provide a couple between energy-demanding synthetic processes and energy-yielding breakdown reactions. They catalyze the joining of two molecules, deriving the needed energy from the cleavage of an energy-rich phosphate bond (in many cases, by the simultaneous conversion of adenosine triphosphate [ATP] to adenosine diphosphate [ADP]). A ligase catalyzing the formation of a carbon-oxygen bond between an amino acid and transfer RNA is called amino acid–RNA ligase. Carbon–nitrogen (C―N) bonds are formed by the action of such enzymes as amide synthetases and peptide synthetases.