Figure 1.

Summary of the Ubiquitin Proteasome Pathway (UPS). Ubiquitin is activated by the E1 ubiquitin-activating enzyme and transferred to an E2 ubiquitinating conjugating enzyme. The E2 then transfers the ubiquitin to a target protein with the assistance of an E3 ubiquitin ligase that recognizes the target protein. The process may then be repeated to form polyubiquitin chains, which bind to the regulatory particle of the 26S proteasome, leading to the degradation of the target protein and the recycling of the ubiquitin units.^{[4a, 5]}