Three enzymes act in sequence to ubiquitinate targets. The E1 enzyme is the activating enzyme, to which ubiquitin is attached in an ATP-dependent reaction by a thioester bond (shown in red). The E2 enzyme is the conjugating enzyme, to which the ubiquitin is transferred from the E1. The E3 is the ubiquitin ligase, which directly or indirectly catalyzes the transfer of the ubiquitin to the target protein (the substrate), with the formation of an isopeptide bond (shown in black).
Dikic and Robertson BMC Biology 2012 10:22 doi:10.1186/1741-7007-10-22