In biochemistry, a ligase is an enzyme that can catalyze the joining (ligation) of two molecules by forming a new chemical bond. This is typically via hydrolysis of a small pendant chemical group on one of the molecules, typically resulting in the formation of new C-O, C-S, or C-N bonds. For example, DNA ligase can join two complementary fragments of nucleic acid by forming phosphodiester bonds, and repair single stranded breaks that arise in double stranded DNA during replication.
In general, a ligase catalyzes the following dehydration reaction, thus joining molecules A and B:
A-OH + B-H → A–B + H2O