Ligases

In biochemistry, ligase (from the Latin verb ligāre &mdash; "to bind" or "to glue together") is an enzyme that can catalyze the joining of two large molecules by forming a new chemical bond, usually with accompanying hydrolysis of a small chemical group dependent to one of the larger molecules or the enzyme catalyzing the linking together of two compounds, e.g., enzymes that catalyze joining of C-O, C-S, C-N, etc. In general, a ligase catalyzes the following reaction:


 * Ab + C → A–C + b

or sometimes


 * Ab + cD → A–D + b + c

where the lowercase letters signify the small, dependent groups.

Nomenclature
The common names of ligase enzymes often include the word "ligase," such as DNA ligase, an enzyme commonly used in molecular biology laboratories to join together DNA fragments. Other common names for ligases include synthetases, because they are used to synthesize new molecules.

Note that, 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.

Classification
Ligases are classified as EC 6 in the EC number classification of enzymes. Ligases can be further classified into six subclasses:


 * EC 6.1 includes ligases used to form carbon-oxygen bonds
 * EC 6.2 includes ligases used to form carbon-sulfur bonds
 * EC 6.3 includes ligases used to form carbon-nitrogen bonds (including argininosuccinate synthetase)
 * EC 6.4 includes ligases used to form carbon-carbon bonds
 * EC 6.5 includes ligases used to form phosphoric ester bonds
 * EC 6.6 includes ligases used to form nitrogen-metal bonds