Ubiquitin (12-27) is derived from ubiquitin, a protein which is added through a catalytic process to target proteins to initiate processes such as protein degradation, DNA repair, protein kinase activation and vesicle trafficking.

When ubiquitin is added to a target molecule, it is first activated by an ubiquitin-activating enzyme E1 resulting in the formation of the E1-Ub thioester. It is then received by the E2 ubiquitin conjugating enzyme and then transferred onto a lysine residue of the target protein by the E3 ubiquitin ligase.

Ubiquitin plays a major role in protein degradation due to the formation of a polyubiquitin chain. This is produced when the lysine-48 residue on ubiquitin is itself ubiquitinated and sequentially followed by the further addition of ubiquitin molecules. The target protein which now contains the polyubiquitinated chain is recognised by the 26s proteasome and degraded.

Alternatively monoubiquitination signals can initiate processes such as receptor internalisation and DNA repair. Specifically polyubiquitin chains on lysine 63 residues can regulate processes such as protein kinase activation and vesicle trafficking.