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| Ubiquitin Enzyme System
Ubiquitin is a small highly conserved protein that is conjugated onto substrates in a process called ubiquitination. Poly-ubiquitination targets the substrate for destruction by the 26S proteasome. Besides ubiquitination-mediated proteasomal degradation, ubiquitin-signaling is also involved in regulating cell-cell interactions, intracellular targeting and gene expression. Ubiquitination is catalyzed in a stepwise manner by the three enzymes: E1 ubiquitin-activating enzyme, E2 ubiquitin-conjugating and E3 ubiquitin-protein ligase. First, E1 catalyzes the ATP-dependent conjugation of a sulfhydryl group to ubiquitin. Then, the ubiquitin is then transferred to the sulfhydryl group of E2 ubiquitin-conjugating enzyme. Finally, E3 catalyzes the transfer of ubiquitin from E2 to the target protein. Successive ubiquitin molecules can be linked to lysine 48 of the attached ubiquitin to form poly-ubiquitin chains, amplifying the protein degradation signal. Dysregulated ubiquitination is linked to various diseases such as Creutzfeldt-Jakob disease (CJD) and cervical cancer where insufficient proteolysis can lead to excess protein loads in neuronal tissue.