E3 ligase
Ubiquitination requires the sequential action of 3 enzymes: an activating enzyme (E1), a conjugating enzyme (E2), and a ligase (E3). E3 ligase is the third enzyme in a series responsible for ubiquitylation of target proteins.
E3 enzymes provide platforms for binding E2 enzymes and specific substrates, thereby coordinating ubiquitylation of the selected substrates.