The Ubiquitin Proteasome Pathway (UPP) is the cell’s principle mechanism for protein catabolism. The UPP has been shown to have significant involvement in the regulation of critical cellular processes such as transcription, oncogenesis, cell cycle progression, development, growth, selective elimination of abnormal proteins, and antigen processing.The covalent bond between ubiquitin and substrate proteins requires the activation of the C-terminal carboxyl group of Ub to be activated. In this reaction, ubiquitin is linked to the Ubiquitin Activating Enzyme (E1) through a thioester bond between its C-terminal Gly and the active site cysteine residue in the enzyme (E1-S-Ub). E1 hydrolyzes ATP to AMP and PPi with<br>the intermediate formation of an E1-bound ubiquitin adenylate. The activated Ub is then transferred from E1 to any of a family of Ubiquitin Carrier Protein Enzymes (E2s) in a similar thioester linkage (E2-S-Ub). Ubiquitin can then be transferred directly to the ε-Lys of a substrate from E2 enzymes. Other conjugation reactions require the involvement of a third enzyme, Ubiquitin Ligase (E3). E3 binds to substrate proteins and also to an E2 to form an E2-E3- substrate complex. It is the recognition and formation of said complex that has the highest level of substrate specificity for the conjugation cascade.