Ligase

Overview
In biochemistry, a ligase (from the Latin verb ligāre &mdash; "to bind" or "to glue together") is an enzyme that can catalyse the joining of two large molecules by forming a new chemical bond, usually with accompanying hydrolysis of a small chemical group pendant to one of the larger molecules. Generally ligase catalyses the following reaction:


 * Ab + C → A–C + b

or sometimes


 * Ab + cD → A–D + b + c

where the lower case letters signify the small, pendant 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 synthetase, because they are used to synthesize new molecules, or carboxylase when they are used to add carbon dioxide to a molecule.

Note that "synthetase" should not be confused with synthases, as synthases do not use adenosine triphosphate and belong to the lyase group, while synthetases do use adenosine triphosphate (ATP).

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